CL1.2.6 | Dynamics of Glacials, Deglaciations, and the ocean's role in CO2 changes during quaternary glacial cycles
EDI
Dynamics of Glacials, Deglaciations, and the ocean's role in CO2 changes during quaternary glacial cycles
Co-organized by CR1/OS1
Convener: Ruza Ivanovic | Co-conveners: Markus Adloff, Etienne LegrainECSECS, Svetlana RadionovskayaECSECS, Himadri SainiECSECS, Madison ShankleECSECS
Orals
| Fri, 19 Apr, 08:30–12:30 (CEST), 14:00–15:45 (CEST)
 
Room 0.14
Posters on site
| Attendance Thu, 18 Apr, 16:15–18:00 (CEST) | Display Thu, 18 Apr, 14:00–18:00
 
Hall X5
Posters virtual
| Attendance Thu, 18 Apr, 14:00–15:45 (CEST) | Display Thu, 18 Apr, 08:30–18:00
 
vHall X5
Orals |
Fri, 08:30
Thu, 16:15
Thu, 14:00
Feedbacks within the Earth’s system involving the global carbon cycle, ice-sheet dynamics and oceanic circulation played a significant role in shaping the timing and amplitude of Quaternary deglaciations and their preceding glacial periods, as well as abrupt millennial-scale variability within the Last Glacial Cycle. For example, the deep ocean likely played a key role in modulating changes in atmospheric CO2; and ice sheet evolution exerts a strong control on atmosphere and ocean circulation. However, the precise combination of mechanisms and feedbacks responsible for glacial-interglacial and millennial-scale climate transitions remains unresolved. This session invites contributions from studies that provide an improved understanding of the processes and feedbacks occurring during glacial periods and deglaciations during the past 2.6 Ma. This includes new palaeo records, data syntheses and numerical simulations examining climate, the global carbon cycle, continental ice-sheets, ocean circulation, and sea-level.

Orals: Fri, 19 Apr | Room 0.14

Chairpersons: Ruza Ivanovic, Svetlana Radionovskaya
08:30–08:35
08:35–08:55
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EGU24-9143
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solicited
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On-site presentation
Jimin Yu, Robert Anderson, Zhangdong Jin, Xuan Ji, David Thornalley, Lixin Wu, Nicolas Thouveny, Yanjun Cai, Liangcheng Tan, Fei Zhang, Laurie Menviel, Jun Tian, Xin Xie, Eelco Rohling, and Jerry McManus

Ice-core measurements show diverse atmospheric CO2 variations – increasing, decreasing or remaining stable – during millennial-scale North Atlantic cold periods called stadials. The reasons for these contrasting trends remain elusive. Ventilation of carbon-rich deep oceans can profoundly affect atmospheric CO2, but its millennial-scale history is poorly constrained. In this study, I will show a high-resolution deep-water acidity record from the Iberian Margin in the North Atlantic, a unique setting that allows us to construct a robust chronology for confident comparisons between marine and ice-core records. The new data combined with ice-core CO2 records reveal multiple ocean ventilation modes involving an interplay of the two polar regions, rather than by the Southern Ocean alone. These modes governed past deep-sea carbon storage and thereby atmospheric CO2 variations on millennial timescales. Overall, our record suggests a bipolar control on millennial atmospheric CO2 changes during the past glacial cycle.

How to cite: Yu, J., Anderson, R., Jin, Z., Ji, X., Thornalley, D., Wu, L., Thouveny, N., Cai, Y., Tan, L., Zhang, F., Menviel, L., Tian, J., Xie, X., Rohling, E., and McManus, J.: Bipolar control on millennial atmospheric CO2 changes over the past glacial cycle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9143, https://doi.org/10.5194/egusphere-egu24-9143, 2024.

08:55–09:05
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EGU24-5549
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On-site presentation
Peter Köhler, Luke Skinner, and Florian Adolphi

Carbon cycle models used to interpret the IntCal20 compilation of atmospheric Δ14C have so far neglected a key aspect of the millennial-scale variability connected with the thermal bipolar seesaw: changes in the strength of the Atlantic meridional overturning circulation (AMOC) related to Dansgaard/Oeschger and Heinrich events. Here we implement such AMOC changes in the carbon cycle box model BICYCLE-SE to investigate how model performance over the last 55 kyr is affected, in particular with respect to available 14C and CO2 data. Constraints from deep ocean 14C suggest that the AMOC in the model during Heinrich stadial 1 needs to be highly reduced or even completely shutdown. Ocean circulation and sea ice coverage combined are the processes that almost completely explain modelled changes in deep ocean 14C age, and these are also responsible for a glacial drawdown of ~60 ppm of atmospheric CO2. A further CO2 drawdown of ~25 ppm is caused by the colder ocean surface at the last glacial maximum. We find that the implementation of AMOC changes in the model setup that was previously used for the calculation of the non-polar mean surface marine reservoir age, Marine20, leads to differences of less than ±100 14C years. The representation of AMOC changes therefore appears to be of minor importance for deriving mean ocean radiocarbon calibration products such as Marine20, where atmospheric carbon cycle variables are forced by reconstructions. However, simulated atmospheric CO2 exhibits minima during AMOC reductions in Heinrich stadials, in disagreement with ice core data. This mismatch supports previous suggestions that millennial-scale changes in CO2 were probably mainly driven by biological and physical processes in the Southern Ocean. By modifying the 14C production rate (Q), between one that varies so as to fit simulated atmospheric ∆14C to IntCal20 and an alternative constant Q, we can furthermore show that in our model setup the variability in deep ocean 14C age, especially during the Bølling/Allerød—Younger Dryas—Early Holocene climate transition, has its root cause in the carbon cycle, while a Q that achieves agreement with the IntCal20 atmospheric ∆14C record only enhances deep ocean age anomalies and thus optimizes agreement with the benthic 14C data.

How to cite: Köhler, P., Skinner, L., and Adolphi, F.: Simulated radiocarbon cycle revisited by considering the bipolar seesaw and benthic 14C data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5549, https://doi.org/10.5194/egusphere-egu24-5549, 2024.

09:05–09:15
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EGU24-17778
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ECS
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On-site presentation
Megan Pelly, Madison Shankle, Molly Trudgill, Bruno Millet, Chen Xu, Gwyn Owens, Hermione Owen, Alan Foreman, Thomas Bauska, Andy Ridgwell, Elisabeth Michel, William Gray, Andrea Burke, and James Rae

The ability of the deep ocean to store and exchange large quantities of CO2 with the atmosphere on relatively short timescales means that it is thought to play a key role in dictating glacial-interglacial changes in atmospheric CO2, however records of deep ocean carbon storage and release remain sparse. The Pacific Ocean contains the largest store of carbon in the ocean-atmosphere system. As a result, changes in its circulation dynamics and biogeochemistry have the potential to significantly impact global climate. Despite this, changes in Pacific conditions and carbon storage over the last glacial cycle are poorly constrained.

Here we present new geochemical proxy records from abyssal, deep, and intermediate depths in the Southwestern Pacific to determine the changes in deep ocean carbon storage over the last glacial cycle and the mechanisms involved in driving these changes. Foraminiferal trace element and stable isotope data indicate that increased carbon storage occurred over the course of the last glaciation, promoting a drawdown in atmospheric CO2. The processes involved in driving glacial ocean carbon storage are debated, however proxy data from these sites indicate that changes in circulation dynamics promoting the isolation and expansion of deep Pacific waters was likely a key process involved. Comparison of δ13C data to box model and Earth system model output provides further insight into the physical as well as biogeochemical mechanisms involved and their relative contributions at different stages over the last glacial cycle. This includes the role of Southern Ocean sea-ice expansion, reduced ocean temperatures, and increased Southern Ocean stratification and biological productivity. We find that physical processes dominate the early in the glacial cycle, with biological processes promoting further drawdown as glacial conditions intensify. These results help to improve the understanding of deep ocean carbon cycling over the last glacial cycle and provide a new framework with which to interpret proxy δ13C data.

How to cite: Pelly, M., Shankle, M., Trudgill, M., Millet, B., Xu, C., Owens, G., Owen, H., Foreman, A., Bauska, T., Ridgwell, A., Michel, E., Gray, W., Burke, A., and Rae, J.: Physical and biological controls on deep Pacific carbon storage over the last glacial cycle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17778, https://doi.org/10.5194/egusphere-egu24-17778, 2024.

09:15–09:25
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EGU24-12112
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ECS
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Virtual presentation
Monica Garity and David Lund

Despite decades of research, the cause of glacial-interglacial CO2 cycles is not fully understood, leaving a critical gap in our understanding of Earth’s climate system. One hypothesis is that shoaling of the boundary between Northern Source Water (NSW) and Southern Source Water (SSW) enhanced oceanic carbon sequestration during glacial intervals, resulting in lower atmospheric pCO2. To test this hypothesis, we generated vertical profiles of [CO32-], δ13C, and δ18O using a depth transect of cores from the Brazil Margin, focusing on the two major drops in atmospheric pCO2 during the last glacial cycle at ~115 ka and ~70 ka. Given that [CO32-] is inversely related to the concentration of dissolved inorganic carbon, [CO32-] should decrease if the Atlantic sequestered CO2. We observe no significant change in the [CO32-] across the first decrease in atmospheric pCO2 and no evidence for watermass boundary shoaling in the δ13C and δ18O profiles.  [CO32-] decreased only at ~3600 m, the core site most influenced by SSW.  During the second pCO2 decline at ~70 ka, [CO32-] decreased by ~30 µmol/kg below 2000 m water depth, coincident with marked shoaling in the δ13C and δ18O profiles. The lack of evidence for shoaling and deep Atlantic carbon sequestration at ~115 ka, a time of intermediate ice sheet extent and moderate global cooling, but the clear evidence for shoaling and carbon sequestration at ~70 ka, a time of near glacial maximum ice sheet extent, implies that the deep Atlantic’s capacity to store carbon depends on the Earth’s mean climate state. Our results highlight that distinct mechanisms are necessary to explain the two major drops in atmospheric pCO2 of the last glacial cycle. 

How to cite: Garity, M. and Lund, D.: Investigating oceanic carbon sequestration during glacial inception using vertical profiles of [CO32-], d13C, and d18O from the Southwest Atlantic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12112, https://doi.org/10.5194/egusphere-egu24-12112, 2024.

09:25–09:35
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EGU24-14346
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On-site presentation
Gyana Ranjan Tripathy, Priyasha Negi, Rakesh Kumar Rout, and Ravi Bhushan

Erosion of continental rocks controls nutrient and sediment supply, soil formation and global climate. Intensity of this land-surface process is driven by both climatic (runoff, and temperature) and non-climatic (vegetation, lithology and basin slope) factors. Additionally, climatic-driven fluctuations in sea-level may also influence the exposed land-area, which is available for weathering. The coupling between exposed shelf sediments and weathering, however, has received limited attention. In this contribution, geochemical and Sr-Nd isotopic compositions of a sediment core (VM29-17PC) from the western Bay of Bengal have been investigated to reconstruct weathering and climate interaction during last glacial-interglacial cycle. Radiocarbon dating of foraminifera samples establishes that the core preserves a continuous erosional record for last 35 kyr.  Average Sr-Nd isotopic data for the decarbonated sediments confirm dominant sediment supply from the Higher Himalaya to the core site, with sub-ordinate input from the Deccan region. Temporal changes in the isotopic data hint at a sudden increase in the Himalayan source around 15 kyr BP, which is synchronous with the Bølling-Allerød (B/A) warm phase and the strengthening of the south-west (SW) monsoon. Downcore variation of Chemical Index of Alteration (CIA) and K/Al ratios indicates intensification of chemical weathering around 25 kyr BP. This change in weathering intensity is synchronous to dropping of sea level due to onset of glaciation. This sea-level regression and sudden rise in CaCO3 concentration during this period point to weathering of additional surface exposed in the shelf regions. This enhanced weathering of the shelf sediments may have contributed to the atmospheric CO2 level during the glacial period.

How to cite: Tripathy, G. R., Negi, P., Rout, R. K., and Bhushan, R.: Weathering of shelf sediments exposed during a glacial period: Evidence from geochemistry and Sr-Nd isotopes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14346, https://doi.org/10.5194/egusphere-egu24-14346, 2024.

09:35–09:45
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EGU24-1026
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ECS
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On-site presentation
Luca Castrogiovanni, Pietro Sternai, Claudia Pasquero, and Nicola Piana Agostinetti

Ice core archives allow us to retrieve the atmospheric CO2 concentration of the past 800,000 years characterized by periodically lower and higher CO2 levels corresponding to ice ages and interglacials, respectively. However, there is no broadly accepted consensus regarding the leading drivers of such variability. To a large extent, what prevents us from identifying the mechanisms that underlie changes in atmospheric CO2 concentrations is our inability to split the overall atmospheric CO2 budget into its sources and sinks terms, thereby assessing the fluxes of carbon among different reservoirs. Here, we use a reversible-jump Markov chain Monte Carlo (rj-McMC) algorithm to invert the atmospheric CO2 concentration dataset provided by the EPICA ice core based on a general forward formulation of the geological carbon cycle. We can quantify the most likely temporal variability of atmospheric carbon fluxes in ppm/yr throughout the last 800,000 years. Results suggest that temperature changes have been driving the variations of atmospheric CO2 until the Mid-Brunhes Event (MBE), when the onset of a progressive cyclic increase of  the atmospheric carbon fluxes marks a distinct behavioral change of the climate system. We ascribe such change to mechanisms internal to the Earth system, possibly related to the deglacial triggering of global volcanism and associated feedbacks on climate or a combination of geological, biological, and physical processes. Regardless, our unprecedented quantification of past atmospheric input and output CO2 fluxes provide (1) new constraints for climate carbon cycle and paleoclimate models to assess dominant climate-driving mechanisms, and (2) the benchmark for climate models intercomparison projects and better assessing the anthropogenic perturbation to the geological carbon cycle an associated climatic effect.

 

How to cite: Castrogiovanni, L., Sternai, P., Pasquero, C., and Piana Agostinetti, N.: Input and output fluxes of surface CO2 over the Late Quaternary, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1026, https://doi.org/10.5194/egusphere-egu24-1026, 2024.

09:45–09:55
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EGU24-7296
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On-site presentation
Chen Xu, Jessica Crumpton-Banks, Madison Shankle, Megan Pelly, Hana Jurikova, Jimin Yu, Bradley Opdyke, Claus-Dieter Hillenbrand, Andrea Burke, and James Rae

The critical role of the Southern Ocean in controlling the Pleistocene atmospheric CO2 oscillations is widely acknowledged. However, owing to sampling difficulties surrounding Antarctica, the underlying mechanism and associated pathway of ocean-atmosphere CO2 exchange in the Antarctic Zone (AZ) of the Southern Ocean remains mysterious. Here, we present a new planktonic δ11B record from sediment core PS1506 (68.73°S, 5.85°W) that tracks the pH and surface pCO2 of the AZ over the last 8 glacial cycles. These data are complemented by benthic B/Ca and carbonate preservation indices; due to the location of this core on the continental margin of the eastern Weddell Sea, these data allow us to track the source CO2 chemistry of the dense Antarctic waters that feed the ocean’s lower overturing cell. We find coherent CO2 change between surface and deep waters, indicating persistent formation of AABW that transfers Antarctic surface water signals to depth. Critically, we discover abrupt AZ CO2 decline during glacial onset conditions, coinciding with initial atmospheric CO2 drawdown, highlighting the AZ’s key control on glacial-interglacial CO2 change. After assessing proposed drivers, our findings implicate shifts in Southern Ocean circulation linked to changes in sea ice and/or the Southern Hemisphere westerlies in this glacial onset CO2 change, while productivity, solubility, and sea ice 'lid' effects appear insignificant or counterproductive in this region and time interval. Overall, these reconstructed CO2 system dynamics provide critical insights into Southern Ocean carbon cycling and the associated influence on the atmosphere.

How to cite: Xu, C., Crumpton-Banks, J., Shankle, M., Pelly, M., Jurikova, H., Yu, J., Opdyke, B., Hillenbrand, C.-D., Burke, A., and Rae, J.: The Southern Ocean’s role in the global carbon cycle over the last 800 kyr constrained using reconstructions of the CO2 system, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7296, https://doi.org/10.5194/egusphere-egu24-7296, 2024.

09:55–10:05
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EGU24-14929
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ECS
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On-site presentation
Eva M. Rückert and Norbert Frank

The deep Southern Ocean (SO) circulation is of major significance for the understanding of the ocean´s impact on Earth’s climate as uptake and release of CO2 strongly depend on the redistribution of well and poorly ventilated water masses.

Neodymium isotopes (εNd) preserved in deep sea sediment have proven useful to study the Deep Ocean Circulation and water mass provenance and are of special interest over major climate changes as the Mid Pleistocene Transition (MPT). The MPT marks the change from a 41 ka to a 100 ka glacial-interglacial cyclicity and goes along with a significant intrusion of southern sourced waters (SSW) in the deep North Atlantic.

Here, we present the first millennial resolved authigenic εNd data in the Southern Atlantic spanning across  the MPT of a deep sea sediment core positioned at the polar front. The pre-MPT εNd values of ODP 1093 show a small variability of approx. 2 ε-units around the modern AABW signature of -8. In contrast, the post-MPT εNd variability increases to 6 ε-units with glacial extremes of around -3 – εNd values that can not be found in any Atlantic sourced water mass today!

This supports not only the exsiting hypotesis of stonger SSW export to the North, but rather advocates for a radiogenic  watermass influencing the flow regime in the Atlantic south of the polar front. Increasing ice volume during post-MPT glacials has been argued to lead to a reduced AABW production. Due to continuity of flow, this opens up the possiblity of glacial intrusion through the Drake passage of a water masses likely originating in the Pacific, which would generate  the strongly radiogenic glacial εNd values. At present Pacific deep waters are enriched with respired carbon. Assuming this to hold true in the past, the intrusion of such carbon rich water masses into the deep South Atlantic could further reinforce the strong glacials and the overall global cooling trend after the MPT as suggested previously.

Hence, the SO south of the polar front played a leading role in  reinjecting respired CO2 into the deep Atlantic Ocean and the Atmosphere during climate transitions. 

How to cite: Rückert, E. M. and Frank, N.: Significant change in the flow regime in the deep Southern Ocean through the MPT, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14929, https://doi.org/10.5194/egusphere-egu24-14929, 2024.

10:05–10:15
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EGU24-17827
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ECS
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On-site presentation
Jack Longman, Thomas M. Gernon, Thea K. Hincks, Sina Panitz, and Martin R. Palmer

Reduced ice volume during interglacials is hypothesized to amplify volcanic activity because ice-mass removal reduces pressure on magma chambers (Huybers & Langmuir, 2009). There is some evidence for this process occurring on regional (Maclennan et al., 2002) and perhaps semi-global scales (Kutterolf et al., 2019), but there is a lack of globally representative tephra production records. Therefore, our understanding of the global relationship between glacial-interglacial cycles and volcanism uncertain. As a result, we do not know whether deglaciation directly drives enhanced volcanism, or if the feedbacks are more complex. In this work we use a database of visible tephra layers in marine sediments, and a weighted bootstrap resampling method to develop a record of global tephra (the products of explosive volcanism) production which covers the past million years.

Our results show an intensification of global tephra production around 420 to 400 thousand years ago (ka), which coincides with Marine Isotope Stage (MIS) 11 – the warmest interglacial of the past million years. MIS11 was a period of high sea level (up to 10 m above present) and low ice cover, with Greenland likely largely ice free. We suggest the low ice levels drove enhanced volcanism, and consequently enhanced volcanic carbon dioxide degassing, which in turn drove further ice sheet ablation. This positive feedback may the explain this warmth, and in turn, the Mid-Brunhes transition, which heralded the arrival of generally warmer interglacials after 400 ka. Further, after 400 ka we begin to see cyclicity in the tephra record, mirroring eccentricity forcing seen in ice volume records. More pronounced ice-volcano feedbacks may therefore explain the stronger interglacials of the past 400,000 years.

References

Huybers, P., & Langmuir, C. (2009). Feedback between deglaciation, volcanism, and atmospheric CO2. Earth and Planetary Science Letters, 286(3–4), 479–491.

Kutterolf, S., Schindlbeck, J. C., Jegen, M., Freundt, A., & Straub, S. M. (2019). Milankovitch frequencies in tephra records at volcanic arcs: The relation of kyr-scale cyclic variations in volcanism to global climate changes. Quaternary Science Reviews, 204, 1–16.

Maclennan, J., Jull, M., McKenzie, D., Slater, L., & Grönvold, K. (2002). The link between volcanism and deglaciation in Iceland. Geochemistry, Geophysics, Geosystems, 3(11), 1–25.

 

How to cite: Longman, J., Gernon, T. M., Hincks, T. K., Panitz, S., and Palmer, M. R.: A million-year reconstruction of global volcanism intensity: How does it link to glaciation?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17827, https://doi.org/10.5194/egusphere-egu24-17827, 2024.

Coffee break
Chairpersons: Etienne Legrain, Madison Shankle
10:45–11:05
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EGU24-10190
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solicited
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Virtual presentation
Nikita Kaushal, Heather Stoll, and Carlos Pérez-Mejías

Glacial Terminations represent the largest amplitude climate changes of the last several million years.  Over ~ 10 ky timescale, large northern hemisphere ice sheets retreat and sea level rises, and atmospheric CO2 and global temperatures make a full transition from glacial to interglacial levels.  Several possible orbital-insolation triggers have been described to initiate and sustain glacial Terminations, and feedbacks between ice sheet retreat, ocean circulation and ocean carbon storage are invoked to explain the unstoppable progression. 

Because of the availability of radiocarbon dating, the most recent termination (TI) has been extensively characterized. Yet, it is widely discussed whether this sequence of feedbacks and millennial events, and rate of warming is recurrent over previous Terminations or is unique.  Beyond the limit of radiocarbon dating, the chronologies of climate records from ice and marine cores are often developed by tuning to orbital parameters which limits their use in understanding climate dynamics, particularly the response to orbital forcing.

Speleothems provide absolute age control and high-resolution proxy measurements. This archive therefore provides unique records of climate change across Terminations, and additionally may provide the opportunity to tune ice and marine core archives.  However, speleothem climate signals are encoded in a number of proxies. Unlike proxies in other archives like ice or marine cores, the climatic interpretation of a given proxy can vary quite significantly among different regions.

In this study, we

  • synthesize the available speleothem records providing climate information for Terminations: TII, TIII, TIV and TV.
  • present the records based on the aspect of climate encoded in the available records.
  • examine the effects of different ice volume corrections on the final climate proxy record.
  • evaluate whether there are leads and lags in the manifestation of Terminations across different aspects of the climate systems and different regions.
  • we speculate on suitable tuning targets among marine and ice core proxies, and discuss what model outputs maybe most suitable for comparison.

How to cite: Kaushal, N., Stoll, H., and Pérez-Mejías, C.: Perspective on ice age Terminations from absolute chronologies provided by global speleothem records, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10190, https://doi.org/10.5194/egusphere-egu24-10190, 2024.

11:05–11:15
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EGU24-12199
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ECS
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On-site presentation
Jean-Philippe Baudouin, Nils Weitzel, Lukas Jonkers, Andrew M. Dolman, and Kira Rehfeld

Global mean surface temperature (GMST) is a fundamental measure of climate evolution in both past and present and a key quantity to evaluate climate simulations. However, for paleoclimate periods, its reconstruction hinges on uncertain and indirect observations which are distributed sparsely and non-uniformly in both space and time. Large datasets of homogenised proxy records help to reduce the sparsity. Then, the records can be aggregated in an algorithm retrieving the GMST signal. Here, we build on the algorithm designed in Snyder 2016, and on a recent database of ocean temperature proxy records to reconstruct the GMST evolution over the last glacial cycle (the last 130 thousand years). First, we evaluate the algorithm and quantify the sources of uncertainty. This analysis draws on pseudo-proxy experiments using a range of simulations of the last glacial cycle. We find that the over-representation of some regions (e.g. coasts, the Atlantic), to the detriment of others (e.g. the central Pacific) significantly impacts the reconstructed temperature anomaly and its variations. Additionally, millennial and shorter timescale variability cannot be reconstructed by the algorithm, due to bioturbation and age uncertainty. However, these experiments also demonstrate the ability of our algorithm to reconstruct the amplitude and timing of GMST variations occurring at orbital timescale (>10.000 years). Second, we reconstruct the GMST evolution during the last glacial cycle. We compare our result to previous studies, and discuss the improvements coming from the use of the recent proxy database. The high number of proxy records allow us to additionally investigate smaller regions (e.g. hemisphere) and overall further our understanding of the driver of orbital-scale GMST variability.

How to cite: Baudouin, J.-P., Weitzel, N., Jonkers, L., Dolman, A. M., and Rehfeld, K.: Reconstructing the global mean surface temperature of the last 130 thousand years, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12199, https://doi.org/10.5194/egusphere-egu24-12199, 2024.

11:15–11:25
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EGU24-9708
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ECS
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On-site presentation
Sergio Pérez-Montero, Jorge Alvarez-Solas, Marisa Montoya, and Alexander Robinson

Pleistocene glacial-interglacial variability is still under debate, as the many hypotheses proposed are subject to the models used and assumptions made. The longer time scales involved in glacial cycles make it difficult to use comprehensive climate models because of its large computational cost. In this context, conceptual models are built to mimic complex processes in a simpler and more computationally efficient way. Here we present a conceptual climate-ice sheet model that aims to represent the state-of-the-art physical processes affecting glacial-interglacial variability. Our model was constructed using linear equations that explicitly represent ice-sheet modeling approaches. Preliminary results are consistent with Late Pleistocene variability and point to the existence of nonlinearities related to both ice dynamics and ice aging that determine the timing and shape of deglaciations.

How to cite: Pérez-Montero, S., Alvarez-Solas, J., Montoya, M., and Robinson, A.: Glacial-interglacial variability using a low-complexity, physically based model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9708, https://doi.org/10.5194/egusphere-egu24-9708, 2024.

11:25–11:35
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EGU24-5426
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On-site presentation
Henning Bauch

Pleistocene temperatures correlate well with glacial-interglacial changes in global ice volume. While a discharge of ice-rafter debris (IRD) into the ocean directly reflects the rates of growth and decay (deglaciations) of glacial ice sheet margins at sea level, it is also the result of a rapidly changing global environment which affected both the meridional overturning in the ocean and the patterns in ocean-atmosphere circulation on a regional scale.  Circum-arctic land regions and adjacent ocean basins hold clues of varying ice sheet sizes through time. Understanding these records correctly is therefore an important asset to better appreciate Quaternary climate change also within a much broader global context. Marine sediment core data from the Nordic Seas show a stepwise trend of decreasing fluxes of IRD during major glaciations of the last 500 ka, i.e., marine isotope stages (MIS) 12, 6, and 2. Strongest IRD deposition occurred in MIS 12 (Elsterian), while it was lower in MIS 6 (Saalian) and 2 (Weichselian). These marine results of iceberg discharge rates from the western European margins, in particular, point to significant temporal changes in the ice-sheet coverage over northern Eurasia. Indeed, field data provide evidence for several major pre-Weichselian glaciations. Although their maximum limits were likely asynchronous in certain places, it seems evident that these ice sheets not only pre-date the Saalian time, they also extended much farther south (and east) than at any time later. The stepwise decreases in Eurasian ice-sheet extents during glacial maxima terminated in quite contrasting deglaciations and subsequent interglacial developments. It appears evident that such a systematic change in ice-sheet sizes were the result of specific ocean heat circulation, which influenced the pathways of atmospheric moisture transfer across northern Eurasia.

How to cite: Bauch, H.: Impact of waxing and waning of Northern Ice sheets on Pleistocene climate , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5426, https://doi.org/10.5194/egusphere-egu24-5426, 2024.

11:35–11:45
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EGU24-19780
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On-site presentation
Uta Krebs-Kanzow, Lianne Sijbrandij, Gregor Knorr, Lars Ackermann, Lu Niu, and Gerrit Lohmann

During the last deglaciation large proglacial lakes formed at the base of the retreating northern hemisphere ice sheets. We assess the effect of these ice-contact lakes on regional climate and on the ice sheet surface mass balance components of the adjacent  Laurentide (LIS) and Fennoscandian (FIS) ice sheets,  using an atmosphere general circulation model with a novel extension for proglacial lakes in combination with a surface mass balance scheme for ice sheets, which, for the first time, allows to estimate the effect of the cold surface of these extensive lakes on the surface mass balance of the adjacent ice sheets. In a set of simulations inspired by the  Allerød interstadial around 13000 years before present, we demonstrate that the presence of proglacial lakes significantly reduces summer air temperatures in a larger area around the proglacial lakes and leads to reduced precipitation with increased snow/rain ratio. In consequence surface ablation reduces by 39% over the FIS and 28% over the LIS while accumulation only changes slightly by 1% and -3%  respectively. About one quarter of the response in surface ablation is related to the perennially cold surface of the proglacial lakes.

How to cite: Krebs-Kanzow, U., Sijbrandij, L., Knorr, G., Ackermann, L., Niu, L., and Lohmann, G.: The influence of proglacial lakes on climate and surface mass balance of retreating ice sheets: A study of the Laurentide and Fennoscandian ice sheets at 13 ka BP, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19780, https://doi.org/10.5194/egusphere-egu24-19780, 2024.

11:45–11:55
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EGU24-6815
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On-site presentation
Harunur Rashid, Min Zeng, and Mary Menke

Understanding the impact of freshwater discharge from the late Pleistocene Laurentide Ice Sheet (LIS) to the North Atlantic has been considered pivotal due to its direct regulating influence on the climate of the surrounding continents. Numerous studies using indirect paleo-proxies for iceberg discharge and fine-grained sediment supply have reconstructed the instabilities of the LIS. This study employs direct proxies for iceberg discharge and fine-grained sediment supply using the ice-rafted detritus (IRD) and X-ray fluorescence (XRF) scan combined with published X-ray diffraction (XRD) data from the same samples of the Integrated Ocean Drilling Program (IODP) Site U1313 (41°N; 32.57°W). Prominent Heinrich IRD events (H-events) of the last glacial cycle were accompanied by Ti/Ca and Fe/Ca peaks, consistent with the dolomite and calcite peaks, suggesting their Ordovician and Silurian carbonate rocks source that floor the Hudson Bay and Hudson Strait. However, despite the lack of an IRD/g peak, Ti/Ca and Fe/Ca peaks in H-event 3 suggest the arrival of fine-grained sediments in the southern edge of the IRD belt, most likely by sediment plume. In contrast to the last glacial cycle, IRD/g and Ti/Ca and Fe/Ca peaks, often assigned as Heinrich-like events, were only identified during the cold marine isotope stage (MIS) 6, 8, 10, and 12. The IRD/g, Ti/Ca, and Fe/Ca peaks, in addition to the dolomite and calcite peaks during the MIS 7, suggest a fundamentally different configuration of the LIS compared to the other warm MISs of the last 500 ka. Our data suggest that the LIS-sourced icebergs impacted the northern edge of the subtropical gyre (STG) by directly injecting meltwater and modifying the upper water masses, which most likely resulted in the southward movements of the Polar and Arctic fronts. These frontal movements were accompanied by frequent encroachment of the subpolar to transitional water masses to the STG. The polar water-dwelling planktonic foraminifera Neogloboquadrina pachyderma coupled to the IRD/g or Fe/Ca and Ti/Ca peaks support this hypothesis. The new sedimentological and micropaleontological data suggest that the instability and configuration of the LIS were not uniform during all the warm MISs of the last 500 ka.

How to cite: Rashid, H., Zeng, M., and Menke, M.: Impact of the Laurentide Ice Sheet instabilities on the mid-latitude North Atlantic and subtropical gyre during the last five glacial cycles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6815, https://doi.org/10.5194/egusphere-egu24-6815, 2024.

11:55–12:05
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EGU24-10579
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On-site presentation
Clemens Schannwell, Uwe Mikolajewicz, Marie-Luise Kapsch, and Florian Ziemen

The evolution of the northern hemispheric climate during the last glacial period was shaped by two prominent signals of glacial climate variability known as Dansgaard-Oeschger cycles and Heinrich events. Dansgaard- Oeschger cycles are characterised by a period of rapid, decadal warming of up to 14°C in the high northern latitudes, followed by a more gradual cooling spanning several centuries. Temperature reconstructions from ice cores indicate a dominant recurrence interval of ∼1,500 years for Dansgaard-Oeschger cycles. Heinrich events are quasi-episodic iceberg discharge events from the Laurentide ice sheet into the North Atlantic. The paleo record places most Heinrich events into the cold phase of the millennial-scale Dansgaard-Oeschger cycles. However, not every Dansgaard-Oeschger cycle is accompanied by a Heinrich event, revealing a complex interplay between the two prominent modes of glacial variability that remains poorly understood to this day. Here, we present simulations with a coupled ice sheet-solid earth model to introduce a new mechanism that explains the synchronicity between Heinrich events and the cooling phase of the Dansgaard-Oeschger cycles. Unlike earlier studies, our proposed mechanism does not require a trigger mechanism during the cooling phase. Instead, the atmospheric warming signal associated with the interstadial phase of the Dansgaard-Oeschger cycle causes enhanced ice stream thickening such that a critical ice thickenss and temperature threshold is reached faster, triggering the Heinrich event during the early cooling phase of the Dansgaard-Oeschger cycle. An advantage of our mechanism in comparison to previous theories is that it is not restricted to marine-terminating ice streams, but applies equally to land-terminating ice streams that only become marine-terminating during the actual Heinrich event. Our simulations demonstrate that this mechanism is able to reproduce the Heinrich event characteristics as known from the paleo record under a wide range of forcing scenarios and provides a simple explanation for the observational evidence of synchronous Heinrich events from different ice streams within the Laurentide ice sheet.

How to cite: Schannwell, C., Mikolajewicz, U., Kapsch, M.-L., and Ziemen, F.: A mechanism for reconciling the synchronisation of Heinrich events and Dansgaard-Oeschger cycles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10579, https://doi.org/10.5194/egusphere-egu24-10579, 2024.

12:05–12:15
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EGU24-15214
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Virtual presentation
Matteo Willeit, Daniela Dalmonech, Bo Liu, Tatiana Ilyina, and Andrey Ganopolski

Dansgaard-Oeschger (DO) and Heinrich (H) events are ubiquitous features of glacial climates involving abrupt and large changes in climate over the North Atlantic region, extending also to the Southern Hemisphere through the bipolar seesaw mechanism. Ice core data also indicate that the DO and H events are accompanied by pronounced changes in atmospheric CO2 concentration, but their origin remains uncertain. Here, we use simulations with the fast Earth system model CLIMBER-X, which produces self-sustained DO events as internal variability, to explore the processes involved in the atmospheric CO2 response. While the DO events are internally generated in the model, the Heinrich events are mimicked by adding a freshwater flux of 0.05 Sv over 1000 years in the latitudinal belt between 40°N and 60°N in the North Atlantic.
The simulated Greenland temperature varies by ~7-8°C between stadials and interstadials, with only small differences between H and DO stadials, while Antarctic temperature responds substantially stronger to H than to DO events, broadly in agreement with observations. In the CLIMBER-X simulations, atmospheric CO2 varies by ~5 ppm during DO events, but by ~15 ppm during H events, comparable with ice core data. The peak in CO2 concentrations is delayed by several centuries relative to both the stadial-interstadial transition and the peak in Antarctic temperature. The CO2 rise during the H stadial is driven by ocean outgassing. In contrast, the rapid CO2 increase after the transition to the interstadial results from soil carbon release from high NH latitudes originating from substantial warming.

How to cite: Willeit, M., Dalmonech, D., Liu, B., Ilyina, T., and Ganopolski, A.: Exploring the differing CO2 response to Dansgaard-Oeschger and Heinrich events, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15214, https://doi.org/10.5194/egusphere-egu24-15214, 2024.

12:15–12:25
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EGU24-5280
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ECS
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On-site presentation
Brooke Snoll, Ruza Ivanovic, Lauren Gregoire, Yvan Rome, and Sam Sherriff-Tadano

Romé et al. (2022) present a new set of long-run Last Glacial Maximum experiments with millennial-scale climate oscillations between cold and warm modes. These oscillations are triggered by different snapshots of ice-sheet meltwater derived from the early stages of the last deglaciation. The overall characteristics of the oscillating events share similarities with δ18O records of the last glacial period. We test the robustness of these oscillations under different climate conditions, i.e., changes in atmospheric carbon dioxide concentration and orbital configuration. These experiments were run with intentions to better understand the range of conditions the oscillations can be sustained within the model and provide additional insight into the triggering mechanisms that control abrupt climate changes. The results of our sensitivity analysis show that small changes in carbon dioxide concentrations can impact the periodicity and existence of oscillations. A decrease in carbon dioxide concentration decreases periodicity, and an increase in carbon dioxide concentration increases periodicity, leading to an end of the oscillations. Our results also show that for changes in orbital configuration, an increase in Northern Hemisphere summer insolation decreases periodicity and potentially also amplitude. The results show that small changes in climate conditions can impact the shape and existence of oscillations and how this could relate to the changing periodicity and amplitude of observed Dansgaard-Oeschger events as well as transitions from glacial to interglacial states.

How to cite: Snoll, B., Ivanovic, R., Gregoire, L., Rome, Y., and Sherriff-Tadano, S.: Sensitivity of millennial-scale climate oscillations to boundary conditions in HadCM3 glacial simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5280, https://doi.org/10.5194/egusphere-egu24-5280, 2024.

12:25–12:30
Lunch break
Chairpersons: Markus Adloff, Himadri Saini
14:00–14:20
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EGU24-18082
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ECS
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solicited
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On-site presentation
Bo Liu and Tatiana Ilyina

The ocean plays an essential role in the rise of atmospheric CO2 by about 90 ppmv during the last deglaciation. The deglacial oceanic CO2 outgassing is jointly controlled by the physical, biological and geochemical processes, which affect the variations in ocean circulation, biological carbon pump and alkalinity inventory. Transient simulations of climate-carbon feedback, particularly using the comprehensive Earth System Models, are instrumental tools to quantify the contribution of different processes and their interactions. Nonetheless, knowledge gaps still exist in the deglacial variations of oceanic carbon and nutrient cycling because considerable model uncertainties arise from the choices of model processes and parameters, and the proxy data is too sparse to fully constrain the model outcome.

We conduct transient simulations for the last deglaciation with the Max Planck Institute Earth System Model (MPI-ESM) and examine the impact of different model tuning of the global ocean biogeochemistry component and a sediment module on the deglacial CO2 outgassing. The atmospheric CO2 is prognostically computed for the carbon cycle, considering only the atmosphere and ocean compartments, and it is prescribed for radiation computation. We force the model with reconstructions of atmospheric greenhouse gas concentrations, orbital parameters, ice sheet and dust deposition. In line with the physical ocean component, we account for the automatic adjustment of marine biogeochemical tracers in response to changing bathymetry and coastlines related to deglacial meltwater discharge and isostatic adjustment.

We find the deglacial CO2 outgassing is mainly driven by the sea surface warming in MPI-ESM, whereas variations in surface alkalinity and DIC have a relatively small contribution (~18%). Furthermore, the parameterisation of organic debris remineralisation considerably affects the deglacial increase in the global NPP due to different recycling rates of nutrients in the upper ocean. When a longer lifetime of dissolved organic matter is prescribed, the dissolved organic carbon pool in the glacial ocean increases, further facilitating the glacial ocean carbon sequestration. Including an interactive sediment module strongly impacts surface alkalinity due to input-sedimentation imbalance, affecting air-ocean CO2 flux. Thus, attention has to be given to tuning and adjustments regarding the input-sedimentation imbalance of alkalinity in ESMs to better represent proxy data and the deglacial oceanic CO2 outgassing.

How to cite: Liu, B. and Ilyina, T.: Quantifying the role of ocean biogeochemistry on the deglacial atmospheric CO2 rise using transient simulations with MPI-ESM, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18082, https://doi.org/10.5194/egusphere-egu24-18082, 2024.

14:20–14:30
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EGU24-14882
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ECS
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Virtual presentation
Pauline Depuydt, Stéphanie Duchamp-Alphonse, Nathaelle Bouttes, Chiara Guarnieri, Alice Karsenti, Ji-Woong Yang, Jean-Yves Peterschmitt, and Amaëlle Landais

Measurements of the air trapped in Antarctic ice cores reveal that atmospheric CO2 concentration (pCO2) during the Last Glacial Maximum (LGM) was about 80 ppmv lower than that recorded during the current Holocene interglacial (Bereiter et al., 2015). Studies also show a strong link between pCO2, ice volume and Antarctic temperature, suggesting pCO2 as a forcing or amplifying factor behind glacial/interglacial cycles (Petit et al., 1999; Parrenin et al., 2013). Despite such importance in the global climate changes, mechanisms behind rapid variations in pre-anthropic pCO2 remain elusive. Numerical models emphasized the crucial role of exported marine productivity Pexp, (namely, the Soft Tissue Pump) in such changes. In particular, they feature marine productivity patterns from the Southern Ocean and show that a decrease in Pexp in the Sub-Antarctic zone, linked to a reduction in iron inputs from aeolian dusts, could have increased pCO2 by 20 to 50 ppmv (Köhler and Fischer, 2006; Martínez-Garcia et al., 2009; Lambert et al., 2012). However, these studies are usually compared to proxy data from the Atlantic sector of the Subantarctic Zone i.e., an area under the direct influence of wind fields that makes it possible to test the “Fe-hypothesis” (Martin et al., 1990) but that is not necessarily representative of the entire ocean (e.g. Lambert et al., 2015). Due to a lack of recent Pexp data compilation but also of direct comparisons with model outputs integrating marine biogeochemistry­­, it remains difficult to understand the role marine biological productivity exerted on the carbon cycle and more specifically on the low pCO2 during the LGM.

The aim of this study is to explore Pexp patterns during the LGM compared to the pre-industrial Holocene and understand the mechanisms driving their global changes, in order to try and estimate the contribution of marine productivity to the pCO2 signalbased on (i) a new compilation of Pexp proxy data using the strategy previously proposed by Kohfeld et al. (2005) after Bopp et al., (2003), and (ii) a comparison of these data to outputs from the iLOVECLIM intermediate complexity.

Proxy data show that Pexp is generally higher during the LGM compared to the pre-industrial Holocene. This is particularly the case in the sub-Antarctic and sub-Arctic areas, in the equatorial Atlantic Ocean and in coastal upwelling settings i.e., regions that usually witness higher nutrient content due to revigorated ocean circulation and/or intensified surface winds. Simulations generally confirm such features except from the coastal upwelling and the Southern Ocean, due to a lack of spatial resolution and of aeolian inputs in the model, respectively. However, preliminary results from sensitivity tests show (i) net marine productivity fronts around ~40°N and 45°S due to extended sea ice cover and reduced global temperature, (ii) a decreased Pexp in the Pacific Ocean due to an overall thermohaline circulation slow down and (iii) an increase of Pexp in areas where fertilization by iron-rich dusts is expected (Lambert et al., 2021).

How to cite: Depuydt, P., Duchamp-Alphonse, S., Bouttes, N., Guarnieri, C., Karsenti, A., Yang, J.-W., Peterschmitt, J.-Y., and Landais, A.: Impact of marine productivity on atmospheric pCO2 during the Last Glacial Maximum: a model-data comparison, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14882, https://doi.org/10.5194/egusphere-egu24-14882, 2024.

14:30–14:40
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EGU24-16591
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On-site presentation
Alvaro Fernandez, Laura Rodríguez-Sanz, Victoria Taylor, Nele Meckler, and Francisca Martínez-Ruiz

The last glacial maximum (LGM) is the most recent time period in Earth’s history with a climate that was much colder than the present. Robust temperature reconstructions from this period are needed to improve estimates of Earth's climate sensitivity and aid in future climate change projections. However, reconstructing sea surface temperatures (SSTs) during this period can be challenging due to the various limitations with the commonly used proxies. Here, we present new SST estimates from the Alboran Sea in the Western Mediterranean, an area where existing SST records for the LGM (derived from UK37, TEX86, planktic foraminiferal Mg/Ca) show large disagreements. Our new SST estimates are based on clumped isotope analyses of planktic foraminifera (G. bulloides), the same species as used for the Mg/Ca measurements in this area. Due to the insensitivity of the clumped isotope thermometer to changes in seawater chemistry, our results offer new independent constraints on the range of temperature shifts between glacial and interglacial periods in this area. Our findings are evaluated against existing SST estimates, highlighting the benefits and limitations of different proxy estimates. We find that while all proxies agree on the general millennial scale temperature trends during the period of deglaciation, they diverge in the magnitude of these temperature changes. Temperature reconstructions derived from clumped isotopes align more closely with those based on alkenone and Mg/Ca proxies than with those from TEX86, which show large differences. Our research demonstrates that clumped isotopes are a potentially effective tool to improve the accuracy of climate reconstructions from the LGM and the subsequent deglacial period.

 

 

How to cite: Fernandez, A., Rodríguez-Sanz, L., Taylor, V., Meckler, N., and Martínez-Ruiz, F.: Bridging Proxy Discrepancies: SST Reconstructions from the Alboran Sea During the Last Glacial Maximum and Deglaciation. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16591, https://doi.org/10.5194/egusphere-egu24-16591, 2024.

14:40–14:50
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EGU24-9419
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ECS
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On-site presentation
Hidetaka Kobayashi, Akira Oka, Takashi Obase, and Ayako Abe-Ouchi

Atmospheric carbon dioxide concentrations (pCO2) have increases by approximately  from the Last Glacial Maximum (LGM) to the late Holocene (last deglaciation). These changes in atmospheric greenhouse gases are recognized as climate system responses to gradual changes in insolation. Previous modeling studies have suggested that the deglacial increases in atmospheric pCO2 are primarily attributed to the release of CO2 from the ocean. In addition, it has been suggested that abrupt changes in the Atlantic Meridional Overturning Circulation (AMOC) and associated interhemispheric climate changes are involved in the release of CO2. However, there is still limited understanding in oceanic circulation changes, factors responsible for changes in chemical tracers in the ocean of the last deglaciation and its impact on atmospheric pCO2.

In this study, we investigate the evolution of the ocean carbon cycle during the last deglaciation (21 to 11 ka BP) using three-dimensional ocean fields from the transient simulation of the MIROC 4m climate model, which exhibits abrupt AMOC changes as in reconstructions. We validate the simulated ocean carbon cycle changes and discuss possible biases and missing or underestimated processes in the model by comparing simulated carbon isotope ratios with sediment core data.

The qualitative changes in atmospheric pCO2 are consistent with ice core records: during Heinrich Stadial 1 (HS1), atmospheric  increases by . This is followed by a decrease of  during the Bølling–Allerød (BA) and an increase of  during the Younger Dryas (YD). However, the model underestimates the changes in atmospheric  during these events compared to ice core data. Radiocarbon and stable isotope signatures ( and ) indicate that the model underestimates the activated deep ocean ventilation and reduced efficiency of biological carbon export in the Southern Ocean, and active ventilation in the North Pacific Intermediate Water during HS1. The relatively small changes in simulated atmospheric  changes during HS1 may be attributed to these underestimations of ocean circulation changes. The changes in  associated with strengthening and weakening in the AMOC during the BA and YD are generally consistent with the sediment core record. On the other hand, while the data show a continuous  increase in the deep ocean throughout the YD, the model shows the opposite trend. This suggests that the model simulates excessive weakening of the AMOC during the YD, or limited representations in geochemical processes in the model including marine ecosystem responses and terrestrial carbon storage.

Decomposing the factors behind changes in ocean  reveals that changes in temperature and alkalinity have the main effects on atmospheric  changes. The compensation of the effects of temperature and alkalinity suggests the AMOC changes and associated bipolar climate changes contribute to a slight decrease or increase in atmospheric  during the BA and YD periods, respectively.

How to cite: Kobayashi, H., Oka, A., Obase, T., and Abe-Ouchi, A.: Assessing transient changes in the ocean carbon cycle during the last deglaciation through carbon isotope modeling , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9419, https://doi.org/10.5194/egusphere-egu24-9419, 2024.

14:50–15:00
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EGU24-9411
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ECS
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On-site presentation
Peisong Zheng, Matthew Osman, and Thomas Bauska

The last deglaciation, spanning approximately 23 to 6 thousand years before present (ky BP), represents the most recent period in which Earth’s climate underwent large-scale reorganizations comparable (albeit not strictly analogous) to those projected under future climate changes. However, the precise sequence of events – in particular, the timing and spatial manifestation of the initial warming – remains uncertain. Here we present a new method using Gaussian Mixture Model clustering to objectively decompose a model and proxy-based climate reconstruction (LGMR; Osman et al., 2021) into four patterns of temperature change across the last deglaciation. Broadly speaking, the patterns allow us to delineate the impact of retreating Northern Hemisphere ice sheets, the rise in greenhouse gases, and the influence of the bipolar seesaw. Crucially, our analysis reveals that the high latitudes of the Southern Hemisphere exhibited the earliest signs of warming onset around 21 kyr BP, coincident with a retreat of sea ice across the Southern Ocean. A similar pattern is observed when decomposing a solely model-based climate reconstruction (TraCE-21k; He et al., 2013).  Using a combination of both highly-simplified energy balance-type models and fully-coupled climate models forced with insolation alone, we show that the early warming and sea ice retreat was likely linked to an initial rise in high latitude summertime energy that is dominated by enhanced obliquity-driven forcing. Our findings collectively suggest that insolation dynamics in the Southern Hemisphere were a critical trigger of the Last Deglacial onset and, further, may represent one of the key prerequisites for glacial terminations during the late Pleistocene.

How to cite: Zheng, P., Osman, M., and Bauska, T.: An Early Warming Over the Southern Ocean During the Last Deglaciation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9411, https://doi.org/10.5194/egusphere-egu24-9411, 2024.

15:00–15:10
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EGU24-8715
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ECS
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On-site presentation
Laura Endres, Ruza Ivanovic, Yvan Romé, Julia Tindall, and Heather Stoll

The addition of meltwater from continental ice sheets to the North Atlantic is thought to have played a pivotal role in the reorganization of climate and ocean circulation over the last deglaciation as well as during Heinrich events. This is supported by recent analysis of PMIP and CMIP results, which shows that meltwater addition into the North Atlantic can largely alter global climate, and remains a key uncertainty for both reconstruction and climate projections. 

To date, most model studies of freshwater “hosing” assume a relatively uniform distribution and apply meltwater to a large portion of the North Atlantic basin. However, AMOC weakening is sensitive to the actual input position of the typically cold and non-saline meltwater perturbation, and, on a centennial-millennial timescale, the resulting temperature and salt anomaly will only partially disperse over the entire North Atlantic surface ocean. In contrast, most proxy data sensitive to meltwater record the signal at a specific location. It is unclear if spatial heterogeneity of the ocean’s distribution of the meltwater anomaly may contribute to disagreements between freshwater proxy records and model simulations with freshwater additions tuned to reproduce the record of past AMOC weakenings.

To enhance understanding of the spatial distribution of meltwater anomalies during deglaciations, we present the results of a model sensitivity study using HadCM3 and artificial dye tracers to track the fate of meltwater originating from different Northern ice sheet sectors. We consider different meltwater scenarios consistent with Heinrich Stadial 1 ice sheet reconstructions and compare the results under different AMOC states. The results confirm that, on a centennial timescale, meltwater distribution is not uniform over the North Atlantic Ocean. The emerging patterns expose that the efficiency of a meltwater injection to produce a surface ocean anomaly (in, e.g., salinity or δ18Osw) at a given proxy location differs between different ice sheet sectors by orders of magnitudes. Further, besides the direct effect of meltwater, the sensitivity of climate indicators, such as temperature, to changes in AMOC strength also shows regional discrepancies. 

How to cite: Endres, L., Ivanovic, R., Romé, Y., Tindall, J., and Stoll, H.: Tracking the fate of meltwater from different Northern ice sheet sectors over Heinrich Stadial 1, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8715, https://doi.org/10.5194/egusphere-egu24-8715, 2024.

15:10–15:20
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EGU24-3128
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On-site presentation
Yuchen Sun, Xu Zhang, Gregor Knorr, Martin Werner, Lev Tarasov, and Gerrit Lohmann

Deglacial abrupt warming event is a ubiquitous feature of deglaciations during the Late Pleistocene. Nevertheless, during the last deglaciation an unusually early onset of abrupt Northern Hemisphere warming event, known as Bølling/Allerød (B/A) warming, complicates our understanding of their underlying dynamics, especially due to the large uncertainty in histories of ice sheet retreat and meltwater distributions. Here applying the latest reconstruction of ice sheet and meltwater flux, we conduct a set of transient climate experiments to investigate the triggering mechanism of the B/A warming. We find that the realistic spatial distribution of meltwater flux can stimulate the warming even under a persistent meltwater background. Our sensitivity experiments further show that its occurrence is associated with an orbitally induced climate self-oscillation under the very deglacial climate background related to atmospheric CO2 level and ice sheet configurations. Furthermore, the continuous atmospheric CO2 rising and ice sheet retreating appear to mute the oscillation by freshening the North Atlantic via modulating moisture transport by the Westerly. 

How to cite: Sun, Y., Zhang, X., Knorr, G., Werner, M., Tarasov, L., and Lohmann, G.: Bølling-Allerød warming as a part of orbitally induced climate oscillation , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3128, https://doi.org/10.5194/egusphere-egu24-3128, 2024.

15:20–15:30
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EGU24-6271
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ECS
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On-site presentation
Fernando Sergio Gois Smith, Renata Giulia Lucchi, Monica Bini, Caterina Morigi, Patrizia Ferretti, Laura Bronzo, and Nessim Douss

The Last Glacial Maximum (LGM) was defined based on the low stand sea-level records from the most recent period when global ice sheets reached their maximum volume, between 26,500 and 19,000 years before present. The end of this cold period was the last glacial termination (T1), occurred between 20 and 11.7 ka BP marking the transition to the current interglacial. During T1, the sea level rise responded to a variety of processes although the melting of the world widely distributed ice sheets was initially the main contributor and responsible for abrupt relative sea level rises known as meltwater pulses (MWPs) that deeply changed the Earth’s physiography. MWPs are short-lived global acceleration in sea-level rise resulting from intense glacial melting, surge of large ice streams into oceans and intense iceberg discharge during ice sheet disintegration. Nowadays, the main concerns related to the present fast global climate change is the possibility that sudden drastic ice loss from the Greenland and/or in the West Antarctic Ice Sheet would lead to a new abrupt acceleration of the relative sea level with consequent inundation of vast coastal areas and/or to cause an abrupt slowdown of the Atlantic Meridional Overturning Circulation (i.e. Golledge et al., 2019). To better understand the dynamics and risks associated with the onset of those events, their impact on thermohaline ocean circulation and climate it is pivotal the study of the well-preserved polar marine sediment records of the events occurred during the T1. Here, we present the results of a high-resolution sedimentological, micropaleontological and geochemical investigation of 3 sediment cores collected on the western margin of Svalbard and eastern side of the Fram Strait (Artic). The sediment cores were collected between 1322 m and 1725 m of water depth, in correspondence of the southern IODP sites that will be drilled during the IODP Exp-403 (June-August 2024).

How to cite: Gois Smith, F. S., Lucchi, R. G., Bini, M., Morigi, C., Ferretti, P., Bronzo, L., and Douss, N.: High-resolution sedimentological and palaeoceanographic investigation of the Last Glacial Termination (T1) recorded on the western margin of the Svalbard (Arctic), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6271, https://doi.org/10.5194/egusphere-egu24-6271, 2024.

15:30–15:40
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EGU24-15672
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ECS
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On-site presentation
Thale Damm-Johnsen, Michael J. Bentley, Darren R. Gröcke, Dominic Hodgson, and Erin L. McClymont

Evidence from both marine and ice cores strongly indicates that surface ocean processes influencing air-sea gas exchange of the Southern Ocean played a crucial role in the transition from a glacial to interglacial climate state. However, few archives have been able to reconstruct how high latitude surface ocean processes affected the biogeochemical changes occurring in nutrient utilization, primary productivity, and their effects on carbon sequestering in ecosystems. An opportunity to explore these processes is provided by accumulated snow petrel (Pagodroma nivea) stomach oil deposits, defensively regurgitated by snow petrels at their nest sites. These deposits provide a record of biogeochemical processes in the austral summer, at a high trophic level and integrated over a relatively wide area defined by snow petrel foraging range. Here, we present a joint carbon and nitrogen stable isotope record from stomach oil deposits from the Lake Untersee nunataks in Dronning Maud Land (DML) integrating data from a coastal area of 65-70°S. Our results show a 3‰ offset in δ13C and 4‰ offset in δ15N between LGM and Holocene, indicating that the coastal high latitudes underwent large changes over the deglaciation. The δ15N depletion into the Holocene shows strong similarity to changes occurring in nutrient utilization along the margin of the polar front, indicating that the Southern Ocean high latitudes were not an isolated oasis during the LGM but biogeochemically connected to the surface ocean beyond the summer sea-ice margin. In addition, the presence of stomach oil deposits indicates that open water was present in summer along the coast of DML over both the LGM and Holocene. Such highly productive, open water areas were potentially an important factor in the air-sea gas exchange contributing to the deglacial atmospheric CO2 -rise.

How to cite: Damm-Johnsen, T., Bentley, M. J., Gröcke, D. R., Hodgson, D., and McClymont, E. L.: Carbon and nitrogen stable isotopes across the last deglaciation: perspectives from snow petrel stomach oil deposits, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15672, https://doi.org/10.5194/egusphere-egu24-15672, 2024.

15:40–15:45

Posters on site: Thu, 18 Apr, 16:15–18:00 | Hall X5

Display time: Thu, 18 Apr 14:00–Thu, 18 Apr 18:00
Chairpersons: Markus Adloff, Etienne Legrain
X5.123
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EGU24-7339
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ECS
Yu'ao Zhang and Xu Zhang

Global sea-level changes are significantly associated with variations in Northern Hemisphere ice sheets (NHISs) during the last glacial cycle. However, their responses to glacial millennial-scale climate variability (also known as Dansgaard - Oeschger (DO) cycles), especially during the Marine Isotope Stage 3 (MIS3, ~30-65ka), remains poorly studied, in addition to the contrast lines of geological evidence regarding paleo-sea level changes. Instead of applying Glacial Index Method which overlooks potential tempo-spatial heterogeneity of temperature and precipitation in the northern high latitudes, in this study, we conducted transient PISM ice sheet modeling by imposing full climate forcing derived from fully coupled climate model experiments which are characterized by spontaneous millennial variability. Our results show that control factors of ice volume changes in Laurentide and Eurasian ice sheets are different due to spatially heterogenous climate forcing. During stadial periods, North American Ice sheets is growing because of increased precipitation especially along the margins of the ice sheets, despite spatially heterogenous but trivial changes in the surface air temperature. Meanwhile, dramatic cooling on the southern regions of Eurasian Ice sheets effectively reduces ice loss and hence promote the overall ice growth. In brief, NHIS ice volume grows during stadials while declines during interstadials. We hence propose that stadial-to-interstadial duration ratio is the key to the net change in NHIS volume in a signal DO cycle, providing dynamic understanding of accelerated sea level drop during 40-30ka.

How to cite: Zhang, Y. and Zhang, X.: Millennial-scale Northern Hemisphere ice sheet growth controlled by stadial-versus-interstadial duration, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7339, https://doi.org/10.5194/egusphere-egu24-7339, 2024.

X5.124
|
EGU24-8676
Ruza Ivanovic, Yvan Rome, Lauren Gregoire, Brooke Snoll, Oliver Pollard, and Jacob Perez

In the course of glacial terminations, the increases in greenhouse gas concentrations, summer insolation and the ice sheet demise can trigger episodes of millennial-scale variability. Such variability was observed during the last deglaciation, between 19 ka BP (thousand years ago) and 8 ka BP, in the form of  the abrupt North Atlantic temperature shifts of the Bølling–Allerød Warming (14.5 ka BP) and Younger Dryas (12.900 ka BP).

In some climate models, abrupt climate changes are generated by modifications to the boundary conditions and freshwater discharge. Despite much study, the sensitivity of climate simulations to ice sheet geometry and meltwater is complex and not fully understood, which is a caveat when considering the impact of the rapid demise of the Northern Hemisphere ice sheets during the last deglaciation. In a new set of last glacial maximum HadCM3 simulations that can produce millennial-scale variability, we studied the influence of two ice sheet reconstructions, ICE-6G_C and GLAC-1D, and their associated deglacial meltwater history, on the simulated chain of events of the last deglaciation.

In this experiment, our simulations using the GLAC-1D ice sheet reconstruction produced abrupt climate changes. Triggered by freshwater released close to the Nordic Seas and Iceland Basin deep water formation sites, these simulations display abrupt shifts in the Atlantic Meridional Overturning Circulation (AMOC) that are decoupled from the meltwater flux. In contrast, the reconstructed ICE-6_G ice sheet modifies the North Atlantic wind patterns in the model, preventing convection in the Nordic Seas and intensifying the Iceland Basin deep water formation. As a result, no abrupt climate changes are simulated with ICE-6G_C ice sheets and the AMOC decreases almost linearly with the introduction of freshwater.

The simulations do not capture the timing of the last deglaciation chain of events, but the modelled abrupt changes replicate the main Northern Hemisphere characteristics of the Bølling Warming/Younger Dryas transitions, and are very similar to Dansgaard-Oeschger events.

How to cite: Ivanovic, R., Rome, Y., Gregoire, L., Snoll, B., Pollard, O., and Perez, J.: Abrupt climate changes triggered with GLAC-1D ice sheet, but not with ICE-6G_C, in simulations of the Last Glacial Maximum/Deglaciation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8676, https://doi.org/10.5194/egusphere-egu24-8676, 2024.

X5.125
|
EGU24-17501
|
ECS
Bo Liu, Tatiana Ilyina, Victor Brovkin, Matteo Willeit, Ying Ye, Christoph Völker, Peter Köhler, Malte Heinemann, Takasumi Kurahashi-Nakamura, André Paul, Michael Schulz, Ute Merkel, and Fanny Lhardy

The ocean contained a larger carbon content at the Last Glacial Maximum (LGM, ~21kyr before present) compared to the late Holocene, making a considerable contribution to the deglacial atmospheric CO2 rise of about 90 ppm. Yet, there’s no consensus on the mechanisms controlling the glacial-interglacial changes in oceanic carbon storage due to uncertainties and sparseness of proxy data. Numerical simulations have been widely used to quantify the impact of key factors, such as changes in sea surface temperatures, ocean circulation and biological production, on glacial ocean carbon sequestration. However, the robustness of these findings is subject to further testing due to the differences in process representation, parameterization, model architecture, or external forcing employed by models.

Towards further constraining the LGM ocean carbon cycle, we conducted a multi-model comparison with three comprehensive Earth System Models (Alfred Wegener Institute Earth System Model, AWI-ESM; Community Earth System Model, CESM; Max Planck Institute Earth System Model, MPI-ESM) and one Earth system Model of Intermediate Complexity (CLIMBER-X). We carried out three coordinated experiments with each model: 1) PI (the pre-industrial control simulation), 2) LGM-PMIP (following PMIP4 LGM protocol) and 3) LGM-LowCO2 (as LGM-PMIP, but with boosted alkalinity inventory to lower atmospheric CO2 to about 190 ppm. All experiments were conducted with the prognostic CO2 for the carbon cycle, considering only the atmosphere and ocean reservoirs, and prescribed CO2 for radiative forcing.

All models consistently show that applying the PMIP4 LGM boundary conditions alone leads to only a 5-40 ppm decrease in atmospheric CO2. Globally, the glacial CO2 drawdown in LGM-PMIP is mainly controlled by the enhanced solubility pump. The spatial distribution of the increased glacial DIC depends on the ocean circulation state in each model. In MPI-ESM and CLIMBER-X, the shallower and weaker AMOC facilitates carbon storage in the deep Atlantic. An LGM atmospheric CO2 of 190 ppm can be achieved by boosting alkalinity by 5-8% in scenario LGM-LowCO2. In all models, boosting LGM alkalinity inventory increases DIC in the bottom water. However, comparison to proxy data reveals that the models lack respired carbon, particularly in the deep Pacific. This suggests a need to enhance the glacial biological carbon pump in the models.

How to cite: Liu, B., Ilyina, T., Brovkin, V., Willeit, M., Ye, Y., Völker, C., Köhler, P., Heinemann, M., Kurahashi-Nakamura, T., Paul, A., Schulz, M., Merkel, U., and Lhardy, F.: Constraining glacial ocean carbon cycle – A multi-model study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17501, https://doi.org/10.5194/egusphere-egu24-17501, 2024.

X5.126
|
EGU24-13226
Miho Ishizu, Axel Timmermann, and Kyung-Sook Yun

Sea-ice formation in the Southern Ocean can generate supercooled waters, which can even remain below the in-situ freezing point at depths below 1,000 m. These water masses can play an important role in carbon transport to the abyssal ocean and may have therefore also played an important role in glacial-interglacial CO2 cycles.

To address this question, we examined model outputs from the transient 3 Ma simulation conducted with the CESM1.2 model (Community Earth System Model version 1.2, ~3.75 horizontal resolution. This simulation was driven by time-varying orbital forcing and estimates of atmospheric greenhouse gas concentrations and northern hemispheric ice-sheet orography and albedo. Our analysis shows the presence of large swaths of supercooled glacial deep waters mainly in the northern Pacific. This water is originally formed in the seasonal sea-ice formation regions in the subarctic North Pacific during periods of brine release and rapid mixed layer deepening. During interglacial periods, the volume of supercooled water decreases, which may hint towards a possible positive climate-carbon cycle feedback.

In climate models the freezing condition is usually only applied at the surface. Hence, they are incapable of simulating brinicles – vertical sea-ice structures that can extend from the surface to shallower depths, sometimes even reaching the ocean floor. In my presentation, I will address whether such structures may have played a more prominent role during glacial periods, and whether localized deep ocean freezing may have been a possibility.

How to cite: Ishizu, M., Timmermann, A., and Yun, K.-S.: Super-cooled glacial deep waters, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13226, https://doi.org/10.5194/egusphere-egu24-13226, 2024.

X5.127
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EGU24-4269
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ECS
Himadri Saini, Katrin Meissner, Laurie Menviel, and Karin Kvale

Rising atmospheric CO2 concentration is a major driver of climate change. One of the several processes proposed to explain the lower atmospheric CO2 concentration during the last glacial period is an increase in aeolian iron flux into the Southern Ocean. As the Southern Ocean is a high-nutrient-low-chlorophyll region, increased iron deposition can impact Southern Ocean marine ecosystems,  increase export production, and reduce surface Dissolved Inorganic Carbon (DIC) concentration. Here, we investigate the responses of Southern Ocean marine ecosystems to changes in iron flux and their impact on ocean biogeochemistry and atmospheric CO2 during the last glacial period. We use a recently developed complex ecosystem model that includes four different classes of phytoplankton functional types and fully incorporated iron, silica and calcium carbonate cycles. We show that the changes in atmospheric CO2 are more sensitive to the solubility of iron in the ocean than the regional distribution of the iron fluxes. If surface water iron solubility is considered constant through time, we find a CO2 drawdown of ∼4 to ∼8 ppm. However, there is evidence that iron solubility was higher during glacial times. A best estimate of solubility changing from 1 % during interglacials to 3 % to 5 % under glacial conditions yields a ∼9 to 11 ppm CO2 decrease at 70 ka, while a plausible range of CO2 drawdown between 4 to 16 ppm is obtained using the wider but possible range of 1 % to 10 %. We also show that the decrease in CO2 as a function of Southern Ocean iron input follows an exponential decay relationship, which arises due to the saturation of the biological pump efficiency and levels out at ∼21 ppm in our simulations.

We also investigate the role of iron flux changes on the abrupt atmospheric CO2 increase during Heinrich Stadials, which are associated with a near collapse of the Atlantic Meridional Overturning Circulation (AMOC), a sudden decrease in Greenland temperature and warming in the Southern Ocean. Previous modelling studies have investigated the role of the ocean circulation in driving changes in atmospheric CO2 concentration during these abrupt events, while the role of reduced aeolian iron input during Heinrich stadials remained poorly constrained. We show that a weakened iron fertilisation during Heinrich Stadials can lead to ~6 ppm rise in CO2 out of the total increase of 15 to 20ppm as observed. This is caused by a 5% reduction in nutrient utilisation in the Southern Ocean, leading to reduced export production and increased carbon outgassing from the Southern Ocean.

How to cite: Saini, H., Meissner, K., Menviel, L., and Kvale, K.: Impact of iron fertilisation on Southern Ocean ecosystems and global carbon cycle during the last glacial cycle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4269, https://doi.org/10.5194/egusphere-egu24-4269, 2024.

X5.128
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EGU24-4464
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ECS
Madison Shankle, Graeme MacGilchrist, William Gray, Casimir de Lavergne, Laurie Menviel, Andrea Burke, and James Rae

The Southern Ocean is widely thought to have played a driving role in the atmospheric CO2 fluctuations of the ice ages, ventilating carbon-rich deep waters to the atmosphere during interglacial periods and limiting this CO2 leakage during glacial periods. A more efficient Southern Ocean biological pump during glacial periods is one of the leading hypotheses for how this “leak” might have been stemmed, but the exact dynamics responsible are still debated. Previous hypotheses have invoked reduced upwelling and/or enhanced stratification in reducing the carbon and nutrient supply to the glacial Southern Ocean surface, thus enhancing the net efficiency of its biological pump. Here we consider an alternative, complementary scenario in which the nutrient and carbon content of the upwelled water itself is reduced. Noting the striking similarity between proxy records from the North Pacific and Southern Ocean over the Last Glacial Cycle and given that carbon-rich waters upwelling in the Southern Ocean today are largely fed by the North Pacific, we propose that low-carbon/nutrient glacial Southern Ocean surface waters could have been sourced from a well-ventilated, low-carbon/nutrient glacial North Pacific. We then show in intermediate-complexity Earth system model simulations how a well-ventilated North Pacific can directly reduce the outgassing potential of waters upwelled in the Southern Ocean. While not precluding the possibility of changes to upwelling or mixing, our results demonstrate the ability of changes in the upwelled waters’ carbon content – outside of any changes to Southern Ocean physical dynamics (e.g., upwelling rate) – to change Southern Ocean carbon content and outgassing. This provides a novel mechanism linking Northern Hemisphere climate to Southern Ocean carbon cycling and may thus help explain the cyclic CO2 variations of the ice ages.

How to cite: Shankle, M., MacGilchrist, G., Gray, W., de Lavergne, C., Menviel, L., Burke, A., and Rae, J.: Polar Twins: Glacial CO2 outgassing reduced in the Southern Ocean by upwelling of well-ventilated waters from the North Pacific , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4464, https://doi.org/10.5194/egusphere-egu24-4464, 2024.

X5.129
|
EGU24-4451
Malte Heinemann, Victor Brovkin, Matteo Willeit, Joachim Segschneider, and Birgit Schneider

Despite intense efforts, current generation comprehensive Earth system models have, to our knowledge, not been able to simulate the full extent of the atmospheric pCO2 drawdown (as recorded in ice cores) during the Last Glacial Maximum (LGM). Yet, the intermediate complexity model CLIMBER-2 has successfully been used to simulate not only the LGM drawdown but also the transient evolution of CO2 concentrations during entire glacial–interglacial cycles. To better understand why this is the case, we compare the CLIMBER-2 results to pre-industrial and LGM simulations using two related models with increasing complexity, namely, the recently developed intermediate complexity model CLIMBER-X and the state-of-the-art comprehensive Earth system model MPI-ESM as used in the PalMod project, focusing on ocean carbon cycle changes.

How to cite: Heinemann, M., Brovkin, V., Willeit, M., Segschneider, J., and Schneider, B.: Simulating glacial-interglacial CO2 variations: What's right with CLIMBER?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4451, https://doi.org/10.5194/egusphere-egu24-4451, 2024.

X5.130
|
EGU24-9678
Luke Skinner

Be10 in ice cores provides a uniquely well resolved indication of past radionuclide production rates, with a direct bearing on past radiocarbon production.  In the absence of past carbon cycle perturbations (e.g. involving ocean-atmosphere carbon exchange), Be10-based radiocarbon production rate anomalies should correlate directly with atmospheric radiocarbon anomalies, as confirmed by models.  Over the past ~30ka, Be10-inferred radiocarbon production rates and atmospheric radiocarbon (i.e. Intcal20) both exhibit recurrent millennial anomalies, typically of ~5ka duration.  A correlation between these anomalies breaks down during the deglaciation.  This is intriguing and suggests a mix of millennial carbon cycle and radionuclide production influences. Here, global compilations of marine carbon isotope data (radiocarbon and 𝛿13C) are used to assess the potential contribution of ocean circulation and air-sea gas exchange to the apparent millennial component of variability in Intcal20, and atmospheric CO2. We find that existing marine 𝛿13C data provide strong support for a marine influence on atmospheric radiocarbon. Support from marine radiocarbon data is more complex, due to the influence of ‘attenuation biases’ (arising from radiocarbon production changes), and due to a distinct regionalism in the ocean’s impact on atmospheric radiocarbon, versus atmospheric CO2, with air-sea gas-exchange playing a significant role. Major differences in the long-term evolution of radiocarbon and 𝛿13C across the last deglaciation further point to distinct and independent controls on these isotopes systems, providing clues as to the nature and timing of different carbon cycle processes during deglaciation.

How to cite: Skinner, L.: Globally resolved marine carbon isotope data spanning the last 25ka: what do they tell us about the drivers of atmospheric radiocarbon and CO2 on millennial and deglacial timescales? , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9678, https://doi.org/10.5194/egusphere-egu24-9678, 2024.

X5.131
|
EGU24-18786
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ECS
Svetlana Radionovskaya, Julia Gottschalk, David Thornalley, Mervyn Greaves, and Luke Skinner

Understanding the evolution of deep ocean circulation and chemistry over the last glacial cycle is key to elucidating the ocean’s role in modulating atmospheric CO2 changes on millennial and orbital timescales. MIS 4 is a key paleoclimatic interval of the last glacial inception for assessing the role of the deep-ocean carbon storage in driving atmospheric CO2 levels, because it is characterized by a large decrease of air temperature and a rapid atmospheric CO2 drop of ~40 ppmv, and includes several millennial climatic events, for example Heinrich Stadial 6. Although various paleo proxy records suggest a weakened Atlantic overturning during MIS 4, and particularly HS 6, changes in AMOC strength and the geometric extent of NADW shoaling remain poorly understood. Here, we present deep-water temperature reconstructions based on infaunal benthic foraminiferal Mg/Ca ratios and bottom water oxygen concentration reconstructions using redox-sensitive foraminiferal U/Ca, from the deep North (~2.65km) and South (~3.8km) Atlantic to assess the changes in deep water hydrography and by extension circulation.

Our reconstructed deep-water temperature changes from the Iberian Margin (~2.65 km water depth) suggest a stronger influence of colder southern sourced waters during MIS 4 and particularly during HS 6; and a clear subsurface warming during MIS 5a stadials. Meanwhile, changes in deep-water temperatures in the Atlantic Sector of the Southern Ocean (SO) closely follow variations in Antarctic temperature, atmospheric CO2 and the mean ocean temperature, likely mediated by buoyancy forcing in the SO, which is in turn likely linked to sea-ice expansion at the MIS 5a/4 transition. Together with (arguably smaller) contributions from reduced air-sea gas exchange efficiency in the SO, these combined changes would have lowered atmospheric CO2through more efficient carbon sequestration in an expanded deep Atlantic reservoir during MIS 4, through their impact on the solubility- and soft tissue “pumps” (i.e. the ocean’s disequilibrium and respired carbon budgets). Indeed, bottom water oxygenation reconstructions from the South Atlantic support the conclusion that the Southern Ocean appears to have represented a significant reservoir for sequestering CO2 away from the atmosphere during MIS 4.

How to cite: Radionovskaya, S., Gottschalk, J., Thornalley, D., Greaves, M., and Skinner, L.: Mechanisms of atmospheric CO2 drawdown during Marine Isotope Stage 4 based on Atlantic deep-water temperature and bottom-water oxygenation reconstructions , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18786, https://doi.org/10.5194/egusphere-egu24-18786, 2024.

X5.132
|
EGU24-1157
Xuan Ji and Jimin Yu

One of the critical features of deglaciations is the sudden increase in atmospheric CO2 levels. Regulating the Pleistocene atmospheric CO2 variations requires the involvement of oceanic carbon storage changes. However, the mechanisms and pathways for air-sea carbon exchanges remain elusive, partly resulting from the insufficiency of marine carbonate system proxy data with a robust age control beyond Termination I.

The deglacial CO2 rise toward Marine Isotope Stage (MIS) 9e (Termination IV) started from 197.1 ppm to 300.7 ppm[1], representing the highest natural atmospheric CO2 recorded in the Antarctic ice cores over the past 800 ka[2]. Our high-resolution carbonate system records from the Iberian Margin with a robust age control suggest an expansion of southern-sourced Glacial Antarctic Bottom Water at the onset of the deglaciation, followed by a net release of CO2 from the Atlantic sector of the Southern Ocean. However, our results indicate a different ocean circulation pattern during Termination III, when atmospheric CO2 increases by 85 ppm[2]. Unlike Termination III, the north-sourced water seems to take a large proportion of the deep Atlantic Ocean during this period.

References:

[1] Nehrbass-Ahles, C. et al. (2020), Science vol. 369 1000–1005.

[2] Bereiter, B. et al. (2015), Geophys. Res. Lett. 42, 542–549.

How to cite: Ji, X. and Yu, J.: The mechanism controlling air-sea CO2 exchange under different ocean circulation conditions, a case study from Iberian Margin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1157, https://doi.org/10.5194/egusphere-egu24-1157, 2024.

X5.133
|
EGU24-19515
Eva Calvo, Lucía Quirós-Collazos, Marta Rodrigo, Stefan Schouten, Jaap Sinninghe-Damsté, Leopoldo Pena, Isabel Cacho, and Carles Pelejero

The Pacific Ocean equatorial upwelling region is of great interest to understand climate dynamics within the context of current global change. It plays a key role in global biogeochemical cycles, especially in the carbon cycle, as it stands for being one of the areas with largest CO2 fluxes from the ocean to the atmosphere. Moreover, tropical regions play a key role in regulating global climate, since they control the transfer of thermal energy from low to high latitudes. In this context, and with the aim of reconstructing paleoclimate conditions at glacial-interglacial time scales in this region, we analysed selected molecular biomarkers in the marine sediment core ODP 1240, at the easternmost region of the Eastern Equatorial Pacific (EEP), covering the last 160 kyr. We focused on long-chain alkenones, glycerol dialkyl glycerol tetraethers (GDGTs) and long-chain alkyl diols (LCDs). Upon quantification of these lipids, we calculated the UK'37, TEXH86 and LDI indices, and discussed their suitability as paleotemperature proxies to reconstruct sea surface conditions in the study region. We found that UK'37 and TEXH86 derived-temperatures track the warming and cooling trends typical of glacial-interglacial variations. However, while they provide similar temperatures during the last two interglacial maxima, they disagree during glacial periods, when the TEXH86-based estimations display significantly cooler temperatures. The LDI-derived record also shows similar temperatures to those from the UK'37 and TEXH86during the more recent interglacial but, for the last glacial-interglacial period, LDI-derived temperatures remain colder than those of the UK’37 and even colder than those of the TEXH86 at some periods. Multiple factors could be behind this variability and disagreement between the three paleothermometers: depth dwelling, production or exportation of the different biological producers of each lipid, seasonality, diagenetic processes and changes in biogeochemistry conditions of the studied marine region, amongst others. In this presentation, the factors that we believe are most important in the study region will be presented and discussed, to improve our understanding of the biological dynamics of the precursors of each proxy and of their reconstructed marine temperatures in the EEP.

How to cite: Calvo, E., Quirós-Collazos, L., Rodrigo, M., Schouten, S., Sinninghe-Damsté, J., Pena, L., Cacho, I., and Pelejero, C.: Sea surface temperature variations in the Eastern Equatorial Pacific (ODP Site 1240) over the last 160 kyr from three lipid paleothermometers (UK'37, TEXH86 and LDI), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19515, https://doi.org/10.5194/egusphere-egu24-19515, 2024.

X5.134
|
EGU24-20563
A millennial-scale tephra event-stratigraphic record of the South China Sea since the penultimate interglacial
(withdrawn after no-show)
Deming Kong
X5.135
|
EGU24-4936
Coupled Impacts of Ventilation of the North Pacific Intermediate Water and Biological Productivity on Late Quaternary sedimentary oxygenation in the Subtropical North Pacific
(withdrawn after no-show)
Yanguang Dou

Posters virtual: Thu, 18 Apr, 14:00–15:45 | vHall X5

Display time: Thu, 18 Apr 08:30–Thu, 18 Apr 18:00
Chairpersons: Svetlana Radionovskaya, Himadri Saini, Madison Shankle
vX5.22
|
EGU24-6599
|
ECS
Anastasia Zhuravleva, Kirsten Fahl, and Henning A. Bauch

Paleo-data and models show that reductions in the strength of the Atlantic meridional overturning circulation (AMOC) lead to significant subsurface warming in the western tropical North Atlantic. The thermal response at the sea surface is less constrained due to the competing nature of the atmospheric and oceanic processes that produce opposite signs of temperature change. Here, we used alkenone unsaturation in sediments to reconstruct sea surface temperature (SST) evolution in the southeastern Caribbean (core MD99-2198, 1330 m water depth) during the last glacial-interglacial cycle, including Heinrich Stadial 11, which was a period of intense AMOC weakening. Our data show a 1 °C SST warming associated with the onset of Heinrich Stadial 11, and a 1 °C cooling during the late Heinrich, followed by a gradual 1 °C warming during the early last interglacial. Although stadial events are generally associated with wind-induced surface cooling in the tropical North Atlantic, the positive Caribbean SST anomaly during Heinrich Stadial 11 is consistent with previous findings. It likely originates from the upwelling of subsurface water that warmed in response to the initial AMOC weakening. Reduction in the Caribbean SST during the late Heinrich, associated with a particularly weak AMOC strength as suggested by our benthic d13C values, can indicate that the subsurface warming has diminished in the tropical North Atlantic possibly due to a general cooling in the source region (i.e., the subtropical gyre). A two-phased Heinrich is supported by the planktic foraminifera assemblage data, indicating that cooling occurred in the late Heinrich. In addition, this late phase is characterized by coarser sediments, which can be due to a strongly reduced outflow of the Orinoco and a particularly southern position of the intertropical convergence zone. For the last interglacial, our alkenone-derived SST record suggests stable conditions. However, the obtained interglacial values are characterized by very high alkenone unsaturation indexes that can incorporate large measurement and calibration errors due to the lack of Caribbean sediment traps and core-top data. These results, therefore, emphasize the need to better quantify the effectiveness of alkenones in reconstructing interglacial SST history in the Caribbean.

How to cite: Zhuravleva, A., Fahl, K., and Bauch, H. A.: A two-phased Heinrich Stadial 11 as revealed by alkenone-based temperature record from the western tropical North Atlantic , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6599, https://doi.org/10.5194/egusphere-egu24-6599, 2024.