Displays

Perhaps the most important feedback to orbital climate change is CO₂ storage in the deep ocean.  By regulating atmospheric CO₂, ocean carbon storage synchronizes glacial climate in both hemispheres, and drives the full magnitude of glacial-interglacial climate change.  However few data exist that directly track the deep ocean’s carbon chemistry over a glacial cycle.  Here, we present geochemical reconstructions of deep ocean circulation, redox, and carbon chemistry from sediment cores making up a detailed depth profile in the South Atlantic, alongside a record of Southern Ocean surface water CO₂, spanning the last glacial cycle.  These data indicate that initial glacial CO₂ drawdown is associated with a major increase in surface ocean pH in the Antarctic Zone of the Southern Ocean, cooling at depth, enhanced deep ocean stratification, and carbon storage.  Deep ocean carbon storage and deep stratification are further enhanced when CO₂ falls at the onset of Marine Isotope Stage 4, and are also pronounced during the LGM, illustrating a link between orbital scale climate stages and deep ocean carbon.  However our data also illustrate non-linear feedbacks to orbital forcing during glacial terminations, which show abrupt decreases in pH in Southern Ocean surface and subsurface waters, as CO₂ is rapidly expelled from the deep ocean at the end of the last ice age.

How to cite: Rae, J., Foreman, A., Crumpton-Banks, J., Burke, A., Charles, C., and Adkins, J.: Ocean carbon storage and release over a glacial cycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17480, https://doi.org/10.5194/egusphere-egu2020-17480, 2020.

The Antarctic Circumpolar Current (ACC) is the largest current system in the world, linking the Pacific, Atlantic and Indian Ocean basins. However, the variability of the ACC, which plays a fundamental role on global ocean circulation and climate variability, is still poorly constrained. This information is crucial for understanding the role of the ACC on global ocean circulation in response to global warming. Here, we reconstruct changes in the ACC over the past 155,000 years based on sediment grain size variations recorded in a highly-resolved marine sedimentary record from the central Drake Passage near the Polar Front. Our results show significant changes in the ACC during the last glacial cycle and a remarkable boundary between the glacial and interglacial periods. Substantial decreases (~33% to ~47%) in the ACC flow speed from interglacial to glacial period, which corroborates and extends results of previous studies along the subantarctic northern limit of the ACC into the central Drake Passage. This strong variation of ACC likely plays a significant role in regulating Pacific-Atlantic water mass exchange via the “cold water route” and could significantly affect the Atlantic Meridional Overturning Circulation. Superimposed on these glacial-interglacial changes, we found strong millennial-scale variations in ACC current speed, increasing in amplitude close to full glacial conditions. We hypothesise that the central ACC increases its sensitivity to Southern Hemisphere millennial-scale climates oscillations, likely associated with westerlies’ wind stress and Antarctic sea ice extent once glacial conditions fully formed.

How to cite: Wu, S., Lamy, F., Kuhn, G., Lembke-Jene, L., Zhang, X., Haas, C., Nowaczyk, N., W. Arz, H., and Tiedemann, R.: Millennial-scale variability in Antarctic Circumpolar Current and its impacts during the last glacial cycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6778, https://doi.org/10.5194/egusphere-egu2020-6778, 2020.

Indian Ocean surface salinity dynamics are thought to play an important role in shaping glacial-interglacial climate through controlling Agulhas leakage efficiency. It is proposed that a strong Agulhas leakage supplies warm and salty Indian ocean surface waters to Atlantic surface currents influencing convective potential at North Atlantic deep-water formation sites. Here, we present new planktonic foraminiferal Mg/Ca and stable isotope-derived salinity reconstructions for the last 1.2Ma from the northern Mozambique channel. We find salinity increases well before terminations, followed by early decrease before glacial inception. We present a possible link between the hydrography in the northern Mozambique channel and whole ocean salinity changes due to unique surface circulation in the Indian ocean. Despite being a mostly tropical and subtropical ocean, salinity in the modern tropical Indian Ocean is fresher than at comparable latitudes in the Atlantic or Pacific. This is due to the inflow of freshwater from the Indonesian throughflow and recycling via an active Agulhas leakage. We show that salinity in the glacial western Indian Ocean was significantly higher due to a reduced ITF and a weaker Agulhas leakage. We hypothesise that opening and closing of these two gateways influences the development/diminishment of a strong subtropical Indian Ocean gyre which controls sea surface salinity and temperature of tropical Indian Ocean water masses and subsequently the efficiency of the Agulhas Leakage.

How to cite: Nuber, S., Rae, J. W. B., Andersen, M. B., de Boer, B., Zhang, X., Hall, I. R., and Barker, S.: The Role of Changing Indian Ocean Salinity in shaping Pleistocene Climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9828, https://doi.org/10.5194/egusphere-egu2020-9828, 2020.

The geometry of large-scale deep ocean circulation is closely linked to processes occurring in the Southern Ocean (SO). The SO is the ‘window’ through which much of the world’s ocean interior interacts with the atmosphere, and understanding the complex relationships coupling SO dynamics to deep circulation can provide valuable insights into biogeochemical and physical processes important to global climate. Of particular interest is how these processes interacted with, and behaved under different climate states, such as the glacial-interglacial cycles of the Pleistocene (0-2.8 Ma), and the intensification of Northern Hemisphere glaciation during the transition from the warm Mid-Pliocene (3.3-3.1 Ma) to the early Pleistocene. Here, we utilise new composite sediment core records (41^oS, 25^oE, 2700-2900 m water depth) to reconstruct deep chemical and physical ventilation at the Agulhas Plateau, as well as the competing presence of warm Subtropical waters vs cold Subantarctic waters in the surface, over the past ~3 Ma. We present records of the ‘sortable silt’ flow speed proxy, the stable isotope (δ¹⁸O, δ¹³C) composition of benthic foraminifera, bulk sediment element concentrations, and the accumulation of ice-rafted debris (IRD). The sortable silt proxy demonstrates that deep physical ventilation is largely decoupled from deep chemical ventilation as indicated by benthic δ¹³C, with higher flow speeds coincident with more depleted δ¹³C. Furthermore, deep ventilation is related to changes in the terrigenous sediment composition: deep flow speeds and δ¹³C vary concurrently with bulk sediment geochemistry (K/Al, Ti). At the Agulhas Plateau, we interpret deep chemical ventilation and near-bottom flow speeds to reflect changes in the advection of northern-sourced deep waters (e.g. North Atlantic Deep Water and its glacial equivalent) and meridional variability in the location of the deep-reaching Antarctic Circumpolar Current (ACC) and its associated fronts. The presence of IRD at the Agulhas Plateau is controlled primarily by the equatorward survivability far-travelling Antarctic icebergs, and therefore represents the relative presence of cold, iceberg-bearing Subantarctic Zone (SAZ) surface waters. Generally, at times of high near-bottom flow speed and more ‘southern’ terrigenous sediment composition, IRD is higher, implying a meridional expansion of the SAZ. Together, these proxy records provide a continuous and long-term insight into the evolution of coupled surface-deep conditions at the Agulhas Plateau. We postulate that these conditions may reflect the wider geometry of ocean circulation in the SO, documenting the interactions between the ACC and circum-Antarctic fronts with the upwelling, conversion, and export of deep water masses. Our records represent the first multi-proxy reconstruction of this system across climate transitions of the past ~3 Ma, allowing us to explore its evolution across a range of timescales, from million-year to orbital-scale. Furthermore, by measuring multiple proxies on the same samples, we are able to determine the relative phasing between different processes independent of chronostratigraphic uncertainties, for example the timing of SAZ changes vs perturbations in deep ocean circulation at the site.   

How to cite: Starr, A., Hall, I. R., Barker, S., van der Lubbe, J., Hemming, S. R., Jimenez-Espejo, F. J., and Lathika, N.: The Evolution of Subantarctic Fronts, Deep Ocean Ventilation and Flow Vigour at the Agulhas Plateau: Surface-Deep Coupling Across Climate Transitions of the past 3 Ma, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10495, https://doi.org/10.5194/egusphere-egu2020-10495, 2020.

The formation of North Atlantic Deep Water (NADW) in the North Atlantic is an important modulator of the climate system, as it drives the global termohaline circulation, responsible for the distribution of heat, salts and nutrients across the oceans. ODP Site 1063 (4584 m), on the deep Bermuda Rise, is located in the mixing zone between NADW and Antarctic Bottom Water (AABW) and appears to be a good location to study how ocean circulation and climate interconnect. Here we present a new record based on Nd isotope ratios that covers ~1 Ma at that Site. Our data shows Nd isotope ratios during parts of interglacials that are much lower than present day NADW. These results are coherent with recent published studies on the last interglacial–glacial cycle that show that the deep North Atlantic Nd isotope ratios are also lower than NADW during the early interglacial. However, Nd isotope values from the shallower DSDP Site 607 (3427 m), within the core of NADW, have remained similar to modern NADW during interglacials over the same time interval. Site 607 is thought to represent the deep North Atlantic, as shown by an Atlantic meriodional transect that displays Nd isotopes ratios for glacial and interglacial maxima over the last ~1 Ma. We suggest that Nd isotope ratios at Site 1063 do not fully represent the North Atlantic endmember of the AMOC during interglacials, but regional or local processes. However, glacial values at Site 1063 fitting those of Site 607 suggest that Nd isotope ratios represent, indeed, water mass mixing during glacial periods. The low Nd-isotope ratios in the deep Bermuda Rise during interglacials would be the result of particle-seawater exchange derived from the arrival of freshly ground, poorly weathered bedrock from the Canadian shield to the North Atlantic during major ice sheet retreats, such as deglaciations as well as stadial-to-interstadial transitions. Consequently, a deep, regionally constrained layer of seawater is tagged with this extreme Nd isotope signature that is not representative of the AMOC. We suggest that a benthic nepheloid layer, whose development is driven by a deep-recirculating gyre system regulated by the interaction between the northward flowing Gulf Stream and the southward flowing deep western boundary current, facilitates the periodical masking of the deep Atlantic Nd isotope signature at Site 1063. The intermittence of the masking allows for a speculation on how the deep-recirculating gyre system might have changed over the last ~1 Ma glacial-to-interglacial cycles.

How to cite: Jaume-Seguí, M., Kim, J., Knudson, K. P., Yehudai, M., Goldstein, S. L., Bolge, L., Ferretti, P., and Pena, L. D.: Glacial-to-interglacial variations in the deep water at the Bermuda Rise inferred from a Nd isotope record covering the last million years , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18340, https://doi.org/10.5194/egusphere-egu2020-18340, 2020.

The causes for major climate transitions in the Upper Pleistocene are based on the assumption that orbital forcing, i.e. the increase in northern hemisphere summer insolation (NHSI), initiates glacial ice sheets to melt away leading to the formation of warm interglacials. Good examples are plentiful available, e.g. major glacial terminations (T) such as T1, 2, or 5. Besides these major climate transitions there are also other glacial terminations across marine substage boundaries that, although seemingly of minor scale, had nevertheless massive climate impacts either globally (MIS4/3, MIS7d/7c) or regionally (MIS5b/5a). While an interglacial decrease in NHSI seems to run in parallel with early glacial inception - as can be noted for the later Holocene and MIS5e - the onset of T2 vs. T1 has long been controversially discussed with respect to its orbitally forced timing. This study therefore explores the involvement of other mechanisms. Primarily, these have to do not so much with internally produced feedback processes but are the consequence of temperature changes to be found in the low-latitudes. Transferred northward through the atmosphere and ocean these changes then feed ice sheet growth and determine its geographical configuration of different magnitudes, also eventually leading to a glacial maximum. During the past, climate transitions from a glacial into an interglacial world therefore did not start with the end of a glacial maximum. It is the time just prior to that particular maximum when the major change occurred.

How to cite: Bauch, H.: Exploring the nature and timing of glacial climate transitions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5427, https://doi.org/10.5194/egusphere-egu2020-5427, 2020.

From the combination of orbital theory with benthic δ¹⁸O it has been suggested which obliquity cycles led to interglacials during the Quaternary (e.g. Tzedakis et al., 2017). Here, we define interglacials, as deduced for the last 800 kyr (Past Interglacials Working Group of PAGES, 2016), by the absence of substantial northern hemispheric land ice outside of Greenland. When applied to land-ice distribution derived from a 3D-ice-sheet model-based deconvolution of the LR04-benthic δ¹⁸O stack into its temperature and sea-level components (de Boer et al., 2014) we find an irregular pattern of interglacials not only, as suggested so far, in the late Pleistocene but across most of the last 2.6 Myr. In the early Pleistocene eight obliquity cycles miss the onset of new interglacials, therefore increasing the average interglacial periodicity to 60 kyr. Both prolonged glacials (due to skipped terminations) and prolonged interglacials (so-called continued interglacials) are the reasons for these new irregularities. This finding adds new irregularities to the already known glacial/interglacial pattern during the last 1 Myr that include eleven obliquity cycles without new interglacials. Only in the Mid-Pleistocene in-between interglacials reappear regularly once in each obliquity cycle (every 41 kyr) with an exception around 1.1 Myr BP in which the onset of two successing interglacials is more than 100 kyr apart. This finding suggests that the notation of the Quaternary as an obliquity driven period with a growing influence of ice volume on the timing of deglaciations is too simple, or that our definition of interglacials, that seems to be suitable for the last 1.6 Myr, is not applicable to the whole Quaternary.

References:

How to cite: Köhler, P. and van de Wal, R.: Land ice distribution suggests an irregular pattern of interglacials across most of the Quaternary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1340, https://doi.org/10.5194/egusphere-egu2020-1340, 2020.

In a recent paper Tzedakis et al (2017) described a simple rule that predicts, using only caloric summer half-year insolation as input, which insolation cycles lead to the onset of an interglacial. The rule is based on an energy threshold, one of whose characteristics is that it reduces with time since the last interglacial onset, reflecting increased fragility of glacial climate as ice sheets get larger. The rule correctly predicts every complete deglaciation of the past million years, a period in which interglacial onset skips both precession and obliquity cycle maxima. This then raises the question to what extent the approximate 100 ka period observed in the last million years is due simply to internal dynamics rather than to the period of eccentricity present in the insolation record. Here we will test this by creating synthetic insolation curves from which eccentricity (or other orbital components) have been removed.  We will then use the proposed rule to test to what extent eccentricity influences the spacing of interglacials. We will also assess the impact of other orbital components and the impact earlier in the Quaternary when the energy threshold was lower.

How to cite: Wolff, E., Crucifix, M., and Tzedakis, C.: The role of eccentricity in determining the spacing between interglacials, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9574, https://doi.org/10.5194/egusphere-egu2020-9574, 2020.

Within the Late Pleistocene, a ‘termination’ is the name given to the rapid (~10kyr) deglacial transition marking the end of a (~100kyr) glacial cycle. These massive events involve all the critical elements of Earth’s climate system: global temperatures, precipitation patterns, ice sheet extent, ocean and atmospheric circulation systems, atmospheric composition and biological activity. Investigations into the mechanisms of glacial termination have been many and it is now thought that abrupt shifts in the ocean/atmosphere system play a ubiquitous and critical role in deglaciation. However, significant uncertainties remain concerning the timing and magnitude of deglacial changes and the likelihood that they will be interrupted by ‘terminal oscillations’ such as the Bølling-Allerød / Younger Dryas oscillation during Termination 1. In this presentation we will address these uncertainties in the light of recent developments in the understanding of glacial terminations as the ultimate expression of the interaction between millennial and orbital timescale variations in Earth’s climate. Innovations in numerical climate simulation and new geologic records that enable us to test these simulations allow us to highlight new avenues of research as well as to emphasise the importance of lingering uncertainties in key climatic parameters such as sea level variability through time.

How to cite: Knorr, G. and Barker, S.: Glacial Termination: Going, Going, Gone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4227, https://doi.org/10.5194/egusphere-egu2020-4227, 2020.

We use a global array of ~120 sea-surface temperature (SST) records based on Mg/Ca, alkenone, and faunal proxies to reconstruct global and regional temperature change over the last 5 Myr. All records are placed on the LR04 age model. Here we report the reconstructions and discuss their implications for characterizing global climate evolution (frequency, variance, transitions) over this interval and its relationship to changes in CO2, orbital forcing, and mean ocean temperature. Average global temperature has cooled by ~6.5oC since 5 Ma, with significant breakpoints tentatively identified at ~3.38 Ma, 1.34 Ma, and 0.88 Ma. We also invert the global reconstruction to reconstruct global sea level for the last 5 Myr.

How to cite: Clark, P. U., Shakun, J., Rosenthal, Y., Bartlein, P., Koehler, P., and Mix, H.: Reconstructions of Global and Regional Temperature Change for the Last 5 Myr, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12928, https://doi.org/10.5194/egusphere-egu2020-12928, 2020.

Understanding the evolution of, and the interactions between, ice sheets and the global climate over geological time is important for being able to constrain earth system sensitivity. However, direct observational evidence of past CO₂ concentrations only exists for the past 800,000 years. Records of benthic d¹⁸O date back millions of years, but contain signals from both land ice volume and ocean temperature. In recent years, inverse forward modelling has been developed as a method to disentangle these two signals, resulting in mutually consistent reconstructions of ice volume, temperature and CO₂. We use this approach to force a hybrid ice-sheet – climate model with a benthic d¹⁸O stack, reconstructing the evolution of the ice sheets, global mean sea-level and atmospheric CO₂ during the late Pliocene and the Pleistocene, from 3.6 Myr ago to the present day. The resulting reconstructions of CO₂ and sea level agree well with the ice core record and different sea-level proxies, indicating that this model set-up yields useful information for colder-than-present climates. For the warmer-than-present climates of the Late Pliocene, different proxies for both CO₂ and sea level are contradictory, making model validation difficult. During the early Pleistocene, 2.6 – 1.2 Myr ago, we simulate 40 kyr glacial cycles with CO₂ ranging between 270 – 280 ppmv during interglacials and 210 – 240 ppmv during glacial maxima. After the Mid-Pleistocene Transition (MPT), when the glacial cycles change from 40 kyr to 80/120 kyr cyclicity, these values change to 260 to 280 ppmv during interglacials, and 180 – 200 ppmv during glacial maxima.

How to cite: Berends, T., de Boer, B., and van de Wal, R.: Reconstructing the evolution of ice sheets, sea level and atmospheric CO2 during the past 3.6 million years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8762, https://doi.org/10.5194/egusphere-egu2020-8762, 2020.

The late Pliocene corresponds to a large cooling over Northern Hemisphere associated with sporadic occurrences of glaciations. The most important event occurred during the marine isotope stage M2 (MIS M2, 3.312–3.264 Ma) when a large glaciation took place with a sea level drop from 20 to 60 m, but its duration is short and the summer insolation forcing change at 65°N is weak. De Schepper et al (2013) invoked to explain the onset and termination of this glaciation with the opening and closing of the Central American Seaway (shallow CAS). Based on their hypothesis, we have intensively studied the onset mechanism of  MIS M2 through a series of sensitivity experiments using the IPSL AOGCM and the asynchronous coupling with an Ice sheet model (GRISLI). Our results demonstrate that the shallow CAS helps to precondition the low-latitude oceanic circulation and affects the related northward energy transport, but cannot alone explain the onset of the M2 glaciation, the most important contribution on MIS M2 are from the large change of pCO₂ as well as the internal feedbacks of vegetation and ice sheet. Moreover, we have also investigated the period from the late Pliocene to the early Pleistocene (3-2.5 Ma) through a transient-like simulation using the same AOGCM and ISM. This enables to simulate the Greenland Ice Sheet (GRIS) onset and development using the pCO₂ reconstructions from different proxies. All these simulations were analyzed with emphasis on cryosphere and focused on the Northern Hemisphere (mid-to-high latitudes). Here we used the same modeling simulations but with a focus over the tropical Africa. We first depict the large changes of temperatures and hydrological cycle produced over this area during these two periods and compare our data to reconstructions. Moreover, by prescribing our climate results as inputs for the vegetation model (Biome4), we compare more directly the simulated plant functional types (PFTs) with that constructed by the pollen data. In addition, we further quantify the respective impact of various driving factors on these PFTs variations.

How to cite: Tan, N., Yule, E., Ramstein, G., Barboni, D., Raj, R., and Dumas, C.: Simulations of large climate transition occurring at high and low latitudes during the late Pliocene (3.3 Ma) and the Plio/Pleistocene (3-2.5 Ma) boundary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20914, https://doi.org/10.5194/egusphere-egu2020-20914, 2020.

The intensification of Northern Hemisphere glaciation (iNHG) at 3.4-2.5 million years ago (Ma) represents the last great transition in Cenozoic climate state with the development of large scale ice sheets in the Northern Hemisphere that waxed and waned with changes in insolation. Declining atmospheric CO₂ levels are widely suggested to have been the main cause of iNHG but the CO₂ proxy record is too poorly resolved to provide an adequate test of this hypothesis. The boron isotope-pH proxy, in particular, has shown promise when it comes to accurately estimating past CO₂ concentrations and is very good at reconstructing relative changes in CO₂ on orbital timescales. Here we present a new orbitally resolved record of atmospheric CO₂ (1 sample per 3 kyr) change from Integrated Ocean Drilling Program Site 999 (12.74˚N, -78.74 ˚E) spanning ~2.6–2.4 Ma based on the boron isotope (δ¹¹B) composition of planktic foraminiferal calcite, Globingerinoides ruber (senso stricto, white).  We find that δ¹¹B values of G. ruber show clear glacial-interglacial cycles with a magnitude that is similar to those of the Mid-Pleistocene at the same site and elsewhere.  This new high-resolution view of CO₂ during the first large glacial events of the Pleistocene confirms the importance of CO₂ in amplifying orbital forcing of climate and offers new insights into the mechanistic drivers of natural CO₂ change. 

How to cite: Brown, R., Chalk, T., Wilson, P., Rohling, E., and Foster, G.: High resolution CO2 record of the great Plio-Pleistocene glaciations using boron isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10925, https://doi.org/10.5194/egusphere-egu2020-10925, 2020.

The Pliocene to early Pleistocene yields a close analogy to near-future climate, with atmospheric pCO₂ between pre-industrial and anthropogenically perturbed levels as they may be reached in few decades. A sedimentary archive that is well suited to study Plio-Pleistocene climate dynamics in the terrestrial realm has recently become available through the ICDP-sponsored HOTSPOT project on the evolution of the Snake River Plain (Idaho, USA). At the Mountain Home site, HOTSPOT drilling has yielded the MHAFB11 core that comprises 635 m of fine-grained lacustrine sediments (Shervais et al. 2013). Based on the yet available paleomagnetic age control, these sediments span from the late Pliocene to the early Pleistocene, which makes them the first archive in continental North America that covers this time interval at one site. Based on their geographic position, the sediments from paleo-Lake Idaho can contribute to a better understanding of climate variability across the Plio-Pleistocene transition in western North America, notably with respect to the hypothesis that enhanced moisture transport into the higher latitudes of North America from ~2.7 Ma onwards allowed the initiation of Northern Hemisphere glaciation (Haug et al., 2005).

To gain insight into the paleoclimatic evolution of northwestern North America during the late Pliocene to early Pleistocene, we have palynologically analyzed 131 samples from the 732–439 m depth interval (corresponding to an age of ~2.8 to ~2 Ma) of the MHAFB11 core. The obtained palynological dataset, which has a mean temporal resolution of ~7 ka, documents that a Pinus-dominated coniferous forest biome prevailed in the catchment area of paleo-Lake Idaho throughout the study interval. However, percentages of pollen from conifer taxa decrease in the latest Pliocene before reaching consistently lower values in the early Pleistocene at ~2.4 Ma. In contrast, pollen taxa representing an open vegetation (e.g., Artemisia, Asteraceae) and deciduous trees (e.g., Quercus, Betula and Alnus) become increasingly abundant in the early Pleistocene (at ~2.4 Ma). We interpret this vegetation shift to an open mixed conifer/deciduous forest to be caused by wetter climate conditions. This interpretation is supported by quantitative climate estimates, which show a gradual increase in mean annual precipitation in the early Pleistocene. This trend towards wetter conditions supports the notion that enhanced moisture transport to northern North America from the subarctic Pacific Ocean contributed to the onset of Northern Hemisphere glaciation at ~2.7 Ma (Haug et al., 2005).

References:

Haug, G.H., Ganopolski, A., Sigman, D.M., Rosell-Mele, A., Swann, G.E., Tiedemann, R., Jaccard, S.L., Bollmann, J., Maslin, M.A., Leng, M.J. and Eglinton, G., 2005. North Pacific seasonality and the glaciation of North America 2.7 million years ago. Nature, 433, 821-825.

Shervais, J.W., Schmitt, D.R., Nielson, D., Evans, J.P., Christiansen, E.H., Morgan, L.A., Shanks, P. W.C., Prokopenko, A.A., Lachmar, T., Liberty, L.M., Blackwell, D.D., Glen, J.M., Champion, D., Potter, K.E., Kessler, J., 2013. First Results from HOTSPOT: The Snake River Plain Scientific Drilling Project, Idaho, U.S.A. Scientific Drilling, 3, 36-45.

How to cite: Allstädt, F., Koutsodendris, A., Appel, E., Rösler, W., Prokopenko, A., Reichgelt, T., and Pross, J.: Reconstruction of environmental and climatic change during the late Pliocene and early Pleistocene in northwestern North America based on a new drill core from paleo-Lake Idaho , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14645, https://doi.org/10.5194/egusphere-egu2020-14645, 2020.

We propose a simple physically-based model of the coupled evolution of Northern Hemisphere (NH) landmass ice-volume, atmospheric CO₂ concentration and global mean temperature. The model only external forcings are the orbital forcing (maximum solar insolation at 65°N) and anthropogenic CO₂ emissions. The model consist of a system of 3 coupled non-linear differential equations, representing physical mechanisms relevant for the evolution of the climate system in time-scales longer than thousands of years.

When forced by the orbital forcing only, the model is successful in reproducing the natural glacial-interglacial cycles of the last 800kyr, in agreement with paleorecords and simulations performed with the CLIMBER-2 Earth System Model of intermediate complexity. The model is successful in reproducing both the timing and amplitude of the glacial-interglacial variations, producing a correlation with paleodata of 0.75 in terms of NH ice-volume.

For the next million years, we analyse the model results under different scenarios: the natural scenario (in which only orbital forcing is applied) and scenarios in which various magnitudes of fossil fuel CO₂ emissions are considered (in addition to the orbital forcing).

When anthropogenic emissions are included the model shows that even fairly low CO₂ anthropogenic emissions (100 Pg or larger) are capable of affecting the next glacial inception, expected to occur in 120kyr from now, delaying large NH ice formation by 50kyr. Considering total carbon releases ranging between 1000 and 5000 Pg (a reasonable expectation of fossil fuel CO₂ emissions to occur in the next few hundred years) the temporal evolution of the climate system could be significantly different from the natural progression. Emissions larger than 3000 Pg could have long-lasting effects, being natural conditions not resumed even after 1 Million years have passed. In addition, emissions larger than 4000 Pg prevent glacial cycles in the next half million years.

How to cite: Talento, S. and Ganopolski, A.: A Simple Model for Glacial Cycles and Impact of fossil fuel CO2 emissions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9940, https://doi.org/10.5194/egusphere-egu2020-9940, 2020.

Over the past 1.5 million years, Earth’s climate has shifted from a predominantly 41 thousand year (kyr) dominated climate cycle to one dominated by longer and larger glacial-interglacial cycles, known as the Mid-Pleistocene Transition (MPT). The MPT occurs over a period of several hundreds of thousands of years, with little change to Earth’s external orbital forcing, thus implicating internal climate feedbacks. Here we interrogate the current capacity, and future potential, of boron isotope records to provide high quality carbon cycle information for the Pleistocene. We also present a compilation of boron isotope-derived pH-CO₂ records from low-latitude ocean drill cores which closely follow the evolution of atmospheric CO₂ over the ice core interval but extend it to 1.5 million years ago with a resolution of up to ~1 sample per 3 kyr. This new, near continuous δ¹¹B-derived CO₂ record is compared against other independent CO₂ data from blue-ice cores and records of ocean and climate change., This confirms there is a decline in mean CO₂ across the MPT which manifests as a lengthening and deepening of glacial CO₂, and highlights the distinct difference in the nature of CO₂ cycles in the 41-kyr world.

How to cite: Chalk, T., Hain, M., Foster, G., Nuber, S., Rohling, E., Barker, S., Cherry, S., and Wilson, P.: Orbital CO2 cycles and the Mid-Pleistocene Transition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11682, https://doi.org/10.5194/egusphere-egu2020-11682, 2020.

The Mid-Pleistocene Transition (MPT, ~1.3-0.7 Ma) is one of the most drastic climatic transition in the recent climatic history of our planet. During this transition, glacial-interglacial variability shifted from 41- to 100-ka cycles, without notable changes in the orbital forcing. Internal forcing mechanisms in Earth’s climate likely shifted the system towards particularly more extreme glacial periods. A decrease in the atmospheric CO₂ contemporary to a severe weakening of the Atlantic deep-ocean circulation around 900 ky suggests that weakened deep-ocean circulation facilitated the capture of CO₂ into the deep ocean and thus contributed to the switch towards more intense and longer glacial periods.

ODP Site 668B, in the deep eastern equatorial Atlantic, has been previously used to reconstruct the atmospheric CO₂ evolution across the MPT using boron isotopes in surface dwelling foraminifera. Here we present new high resolution proxies from the same site covering the last 2 Ma. In particular, benthic foraminifera stable isotopes and trace elements (B/Ca, Mg/Ca, Cd/Ca), as well as Nd isotope data (εNd) from Fe-Mn encrusted foraminifera shells. Using the newly improved chronology based on benthic foraminifera stable isotopes we show that our new εNd data covaries substantially with the atmospheric pCO₂ data and shows a glacial-interglacial variability through the entire record, with εNd values matching typical glacial-interglacial range values in the North-Atlantic basin (~-11 to ~-14). Between ~1 to 2 Ma, when the 41-ka-cycles were dominant, εNd data also covaries with carbonate ion saturation index (ΔCO₃²-) as derived from the new B/Ca data, Bottom Water Temperatures (BWT, Mg/Ca) and, with deep ocean nutrient content (phosphate derived from Cd/Ca). Results indicate a higher fraction of warmer, less corrosive and nutrient-poor northern-sourced waters (higher BWT, higher ΔCO₃^2-, lower Cd/Ca, lower εNd) reaching the deep-equatorial Atlantic during interglacial periods compared to glacial periods. Interestingly, this covariation does not stand after ~0.9Ma. Even though εNd and BWT data suggest an increased contribution of southern-sourced waters to the site during glacial periods after 0.9Ma, as shown by a gradual decrease in glacial BWT (>1°C) and increasing glacial εNd values (~1ε units), both B/Ca and Cd/Ca show a distinctive low frequency variability superimposed to the glacial-interglacial variability. These oscillations can be interpreted as infiltrations and/or overflows of southern-sourced waters across the mid-ocean ridge into the SE Atlantic basin that do not completely follow glacial-interglacial periodicity. We propose that bathymetrical constrains exert a control on the chemistry of the deep waters in the deep eastern equatorial Atlantic with potential impacts on global climate. Partially isolated sub-basins such as the SE Atlantic could have effectively acted as carbon reservoirs over longer time scales than glacial-interglacial changes.

How to cite: Pena, L. D. and Jaume-Seguí, M.: Deep water mass geometry in the south east Atlantic across the Mid-Pleistocene Transition: bathimetric vs oceanographic controls , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13196, https://doi.org/10.5194/egusphere-egu2020-13196, 2020.

Abrupt climate events are generally believed to be characteristic of glacial (intermediate-to-large cryosphere) climate states, requiring either sizeable ice-sheets or large freshwater pulses to act as triggers for abrupt climate changes to occur. Amplification occurs when these triggers bear upon the Atlantic Meridional Overturning Circulation (AMOC). However, the focus on glacial climate states in abrupt climate change research has led to an underrepresentation of research into interglacial periods. It thus remains unclear whether high-magnitude climate variability requires large cryosphere-driven feedbacks or whether it can also occur under low ice conditions. Here we present a high resolution analysis of surface and deep water components of the AMOC spanning the transition from Marine Isotope Stage (MIS) 19c to 19a to test if orbital boundary conditions similar to our current Holocene can accommodate abrupt climate events. Sediment core DSDP 610B (53°13.297N, 18°53.213W), located approximately 700-km west of Ireland, was specifically chosen due to its high sedimentation rate during interglacial periods, excellent core recovery over the Quaternary and its unique geographical location. Above the core site, the dominant oceanographic feature is the North Atlantic Current and at 2417-m water depth, 610B is influenced by Wyville Thomson Overflow Water flowing southwards. A multiproxy approach including paired grain size analysis, planktic foraminifer assemblage counts, and ice-rafted debris counts within the same samples allows us to resolve the timing between both surface and bottom components of the AMOC and their response to abrupt climate events during MIS-19 in the eastern subpolar gyre. This study is societally relevant as future freshwater inputs from a melting Greenland ice sheet may impact ocean circulation, potentially causing shifts in climate for many European countries.

How to cite: Holmes, D. and Morley, A.: Are Cryosphere-Driven Feedbacks a Requisite for Abrupt Climate Events?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19225, https://doi.org/10.5194/egusphere-egu2020-19225, 2020.

Anthropogenic CO₂ release into the atmosphere leads to temperature projections for 2100 only experienced on Earth since many million years. However, those periods are poorly known due to low temporal and spatial data and ill-defined climate forcings. However past warm periods (interglacials), occurring during the Quaternary, under variable boundary conditions (e.g. greenhouse gases concentration, sea level and ice sheets size, insolation and orbital forcing), can provide invaluable information on the dynamics and processes behind natural warm climates. Here we present records for the sea surface temperature based in Uk’37-SST at orbital and millennial-scale over the last 1.25 Ma, with special focus on the past interglacials of two SW Iberian margin sedimentary sequences recovered during IODP Expedition 339,  Sites U1385 (37°34.285′N, 10°7.562′W;  2589m) and U1391 (37°21.5322′N, 9°24.6558′W; 991m). We also performed a data-model comparison to explore the dynamics related with the role of obliquity on the Atlantic Meridional Overturning Circulation (AMOC) changes. Our data  show that Interglacials are characterized by an interval of maximum warmth followed by a temperature decline punctuated by millennial-scale SST oscillations. In most cases the first stadial marks the beginning of a glacial inception that is characterized by an abrupt SST decrease, followed by high frequency SST oscillations, and large amounts of freshwater input. This suggests a climatic change from interglacial to glacial conditions linked to the start of ice sheets growth (enrichment of d18O) and the AMOC slowdown resulting in an enhanced cooling of the mid-latitudes.

How to cite: Rodrigues, T., Zeng, X., Padilha, M., Oliveira, D., O. Grimalt, J., and Abrantes, F.: The role of obliquity forcing on the interglacial climate instabilities in the mid-latitudes of the North Atlantic , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19780, https://doi.org/10.5194/egusphere-egu2020-19780, 2020.

The Last Glacial Maximum (LGM) is marked by significant cooling of the global ocean, which was recently estimated to 2.6°C using noble gases trapped in ice cores (1). This cooling is not equally distributed throughout the world oceans, since global ocean circulation models predict regional temperature anomalies during the LGM of up to 7°C (annually and zonally averaged) when compared to modern interior ocean temperature (2). The oceans deep interior thus became haline stratified (3) due to the drop in temperature to near freezing and the global increase in salinity from ice sheet growth. In contrast to a deepening of the modern thermocline as a result of anthropogenic global warming, cooling causes the thermocline to rise in the sub-tropics as more polar waters enter the mid-depth ocean.

Here we present glacial thermocline temperature reconstructions since the LGM based on the Li/Mg ratio in aragonite skeletons of precisely dated cold-water corals. Corals have been collected from 300-1000m water depths from sites in the northern and southern Atlantic (62°N to 25°S) and demonstrate synchronous 5 - 7°C glacial cooling, and a dramatic shoaling of the thermocline. Through the deglaciation the warming of the upper thermocline ocean occurs early in the southern hemisphere followed by fluctuating warming and thermocline deepening in the northern Hemisphere, which supports the oceanic climate seesaw proposed by Stocker and Johnson in 2003 (4). We thus propose dramatic changes in export of polar waters towards the Equator and augmented subsurface ocean stratification leading to a mostly polar Atlantic with a shallow permanent thermocline. This shoaling possibly increased the rate of nutrient recycling causing higher biological surface ocean activity and the cooling promoted carbon storage. During the glacial, we assume an atmospheric forcing, such as equatorward displacement of the Hadley circulation, to steer the glacial polar water advance as mid-depth boundary currents in the northern and southern hemisphere to effectively spread the cold water through the entire mid-depth Atlantic.

References:

How to cite: Lausecker, M., Hemsing, F., Krengel, T., Förstel, J., Schröder-Ritzrau, A., Border, E., Orejas, C., Titschack, J., Wienberg, C., Hebbeln, D., Wefing, A.-M., Montagna, P., Douville, E., Matos, L., Raddatz, J., and Frank, N.: Was the Atlantic a predominantly Polar Ocean during the last glacial?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21063, https://doi.org/10.5194/egusphere-egu2020-21063, 2020.

Small air inclusions in ice cores represent a direct archive of past atmospheric compositions, allowing us to measure the concentration of the three most potent non-condensable Greenhouse Gases (GHG) CO₂, CH₄ and N₂O as far back as 800,000 years before present (kyr BP). These records demonstrate that transitions from glacial to interglacial conditions are accompanied by a substantial net increase of CO₂, CH₄ and N₂O in the atmosphere (Lüthi et al. 2008, Loulergue et al. 2008, Schilt et al. 2010). A sound understanding of the interplay between the reorganization of the climate system and the perturbation of GHG inventories during glacial terminations is partly limited by the temporal resolution of the records derived from ice cores. In fact, with the exception of the last deglaciation (23-9 kyr BP) centennial-scale GHG variability remained uncaptured for precedings glacial terminations.

In this work, we exploit the exceptionally long temporal coverage of the EPICA Dome C (EDC) ice core to reconstruct, for the first time, centennial-scale fluctuations of CH₄ mole fractions from 145 to 125 kyr BP, encompassing the entire penultimate deglaciation (138-128 kyr BP). With a temporal resolution of ~100 years, our new record is now unveiling all climate-driven signals enclosed into the EDC ice core, exploiting the maximum resolution possible at Dome C (). This offers us the opportunity to study the timing and rates of change of CH₄ in unprecedented details.

Preliminary analysis reveals that the deglacial CH₄ rise is a superimposition of gradual millennial-scale increases (~0.01-0.02 ppb/year) and abrupt and partly intermittent centennial-scale events (~80-200 ppb in less than a millennium). We will investigate processes modulating the observed changes in the CH₄ cycle, compare the structure of our record with the CH₄ profile of the last deglaciation (Marcott, 2014) and contrast it with the EDC CO₂ and N₂O records over the penultimate glacial termination now available in similar resolution.

How to cite: Schmidely, L., Silva, L., Nehrbass-Ahles, C., Han, J., Shin, J., Schmitt, J., Fischer, H., and Stocker, T.: Centennial-scale evolution of methane during the penultimate deglaciation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2844, https://doi.org/10.5194/egusphere-egu2020-2844, 2020.

Paleo-records of primary productivity (PP) changes from the Arabian Sea (AS) have revealed the major influence of monsoon-wind intensity in controlling productivity variations at different timescales, through mixed-layer dynamics and upwelling activity. Much less is known, however, about past changes in paleo-PP in the Bay of Bengal (BoB).

       In the present study, we have reconstructed PP over the last 26,000 years from a sediment core located on the northeastern (NE-) BoB. Paleo-PP was estimated by a PP empirical equation using the relative abundance of Florisphaera profunda, a deep dwelling coccolithophore that develops in the lower euphotic zone. Our record does not reveal any obvious difference of PP between the Last Glacial Maximum (LGM) and the late Holocene, but strong oscillations characterize the deglaciation. Our NE-BoB record is anti-phased to PP records in the AS, and positively correlated to surface seawater salinity (SSS) changes reconstructed from the same core since the LGM. We propose that the strong correlation to salinity variations reflects the role of salinity-stratification related to monsoon precipitation on PP at both orbital- and millennial-scales. Outputs of a climatic transient simulation (TraCE-21) and runs obtained with the Earth System Model IPSL-CM5 support the above interpretation of a strong control of past PP variations by local hydrological changes in the NE-BoB. Our data also highlight the potential teleconnection of the Atlantic Meridional Overturning Circulation strength and Indian Monsoon intensity during the deglaciation.

How to cite: Zhou, X., Duchamp-Alphonse, S., Kageyama, M., Bassinot, F., Beaufort, L., and Colin, C.: Primary productivity dynamics in the northeastern Bay of Bengal over the last 26,000 years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10253, https://doi.org/10.5194/egusphere-egu2020-10253, 2020.

During the last glacial period atmospheric CO₂ and temperature in Antarctica varied together on millennial timescales, with CO₂ abruptly increasing by 10-20 ppm in <1000 years in some cases. The exact causes of these rapid CO₂ changes during a cold glacial climate remain unclear. Here we examine the role of ocean carbon storage and atmospheric exchange by applying the boron isotope-pH (CO₂) proxy to Globigerina bulloides from core site TAN110628 located in the Pacific Sector of the Southern Ocean.  By reconstructing the surface carbonate system at TAN110628 at high temporal resolution (1 sample every 1 kyr) from 30 to 64 kyr we are able to fully constrain the nature of carbon leakage from the Sub Antarctic Zone of the Southern Pacific Ocean associated with these millennial-scale changes in atmospheric CO₂.  This provides unique insights into the causes of abrupt changes in atmospheric CO₂ during Marine Isotope Stage 3 and the last termination. 

How to cite: Shuttleworth, R., Bostock, H., and Foster, G.: Millennial-scale oceanic CO2 release during marine isotope stage 3, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17440, https://doi.org/10.5194/egusphere-egu2020-17440, 2020.

Glacial Termination V is one of the most extreme glacial-interglacial transitions of the past 800 ka [1]. However, the changes in orbital forcing from Marine Isotope Stage (MIS) 12 to 11 are comparatively weak. In addition, MIS 11c is exceptionally distinct compared to other interglacials with for example a longer duration [2] and a higher-than-present sea level [3] despite a relative low incoming insolation. Therefore, the term “MIS 11 paradox” was coined [4]. However, only little is known about the Atlantic overturning circulation during this time interval [e.g. 5,6].

Here, we present Atlantic-wide deep water circulation patterns spanning the glacial maximum of MIS 12, Termination V, and MIS 11. Therefore, sediment cores throughout the Atlantic were analyzed regarding their Nd isotopic composition of authigenic coatings to reconstruct the provenance of the prevailing bottom water masses.

During the glacial maximum of MIS 12, the deep Atlantic Ocean was bathed with a higher amount of southern sourced water compared to the following interglacial. Termination V is represented by a sharp transition in the high-accumulating sites from the North Atlantic with a switch to northern sourced water masses. MIS 11 is characterized through an active deep water formation in the North Atlantic with active overflows from the Nordic Seas, only disrupted by a short deterioration. A strong export of northern sourced water masses to the South Atlantic points to an overall strong overturning circulation.

[1] Lang and Wolff 2011, Climate of the Past 7: 361-380.

[2] Candy et al. 2014, Earth-Science Reviews 128: 18-51.

[3] Dutton et al. 2015, Science 349: aaa4019.

[4] Berger and Wefer 2003, Geophysical Monograph 137: 41-60.

[5] Dickson et al. 2009, Nature Geoscience 2: 428-433.

[6] Vázquez Riveiros et al. 2013, EPSL 371-372: 258-268.

How to cite: Link, J. M. and Frank, N.: Deep water circulation patterns in the Atlantic during MISs 12-11, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20750, https://doi.org/10.5194/egusphere-egu2020-20750, 2020.

Antarctic ice core and deep ocean sediment core records imply that the interglacial climate during Marine Isotope Stage 13 (MIS 13) was relatively cold, and ice sheets were likely larger than today. We model the MIS 13 climate with a coupled climate-ice sheet model AWI-ESM1.2-LR under different orbital configurations at 495, 506 and 517 kyr BP. Summer insolation at 65 °N at 495 kyr BP is similar to the preindustrial, but the lower greenhouse gas values lead to an ice sheet buildup relative to today. Boreal summer at perihelion at 506 kyr BP causes a warmer summer over Northern Hemisphere continents, inhibiting the development of Northern Hemisphere ice sheets. Lower obliquity induces cooling over the polar regions and is favorable for the ice sheet buildup. Aside from the polar regions, mountains with high elevation also have favorable conditions for ice sheet buildup. The Cordilleran Ice Sheet is more sensitive and has a faster response to boreal summer insolation change than the other large scale Northern Hemisphere ice sheets. This indicates that different ice sheets might have different development processes. In addition, ice sheets do not build up over northeastern North America and Eurasia in our simulations. In our final set of simulations, we address the multi-stability of the ice sheets which could be a reason for causing this phenomenon.

How to cite: Niu, L., Gierz, P., J. Gowan, E., and Lohmann, G.: The influence of orbital configurations on Northern Hemisphere ice sheet evolution during MIS 13 with a coupled climate-ice sheet model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9156, https://doi.org/10.5194/egusphere-egu2020-9156, 2020.