Here we present water isotope and noble gas data from the Allan Hills, Antarctica, which provide insight into the local and global climate extending through the Mid Pleistocene Transition and beyond. The Allan Hills blue ice archive provides snapshots of climate that extend well beyond continuous ice core records, but their interpretation has challenges, including complex stratigraphy, potential preservation bias, and highly thinned records.  The water isotope and noble gas data (which come from the same ice samples) suggest a statistically significant correlation between Antarctic temperature and mean ocean temperature, consistent with previous studies. However, we observe subtle differences between these climate reconstructions, including within the mid-Pleistocene transition. We discuss these datasets in the context of broader global changes, and the nuances of the Allan Hills archives.

How to cite: Shackleton, S. and the Allan Hills Blue Ice Coring Team: Early-Mid Pleistocene ice core records of Antarctic and global cooling , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14335, https://doi.org/10.5194/egusphere-egu24-14335, 2024.

Currently, chronologically discontinuous ice cores from the Allan Hills Blue Ice Area (BIA), Antarctica, are our only direct insight into the atmospheric composition of periods beyond the continuous ice core record (800 ka BP). An accurate and precise greenhouse gas history beyond 800 ka would aid understanding of the mechanisms involved in the climatic transitions across the late Pliocene and early Pleistocene. Here we present carbon dioxide (CO₂) and methane (CH₄) results from a new core from the Allan Hills BIA (ALHIC1901). The bottom 25 m of ALHIC1901 contain 52 sampled depths with co-registered ⁴⁰Ar_atm dates (Shackleton et al. in prep), measurements of δD of ice, δ¹⁸O_atm, and concentrations of CO₂ and CH₄ in trapped air. Of these samples, 25 are older than the continuous ice core record, with ages from 821 ± 80 ka to 2700 ± 270 ka. The bottom meter contains ice from the Pliocene with ages from 2700 ± 270 ka to 4000 ± 400 ka. The carbon isotope ratio of CO₂ (δ¹³C-CO₂) was measured on 18 samples to examine the possibility of input of non-atmospheric CO₂ from oxidation of organic matter. Our results indicate that CO₂ and CH₄ levels were similar in the early Pleistocene to those found for the last 800 ka. A small decline of approximately 20 ppm is seen in CO₂ across the Pleistocene, and no secular trend is observed in CH₄. Pliocene-aged samples appear to contain a mixture of atmospheric CO₂ and CO₂ derived from respiration of organic matter at the glacier bed. Using an isotope mixing model we estimate that atmospheric CO₂ was lower than 350 ppm at ~3.1 Ma,

How to cite: Marks Peterson, J., Shackleton, S., Severinghaus, J., Brook, E., Higgins, J., Kurbatov, A., Yan, Y., Buizert, C., Kalk, M., Beaudette, R., Carter, A., Epifanio, J., and Morgan, J.: Late Pliocene and Early Pleistocene CO2 and CH4 from ice cores from the Allan Hills, Antarctica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6781, https://doi.org/10.5194/egusphere-egu24-6781, 2024.

Changes in atmospheric CO₂ and global ice volume as a response to changes in insolation are one of the Earth’s most important feedback mechanisms during glacial-interglacial cycles. During the obliquity paced glacial-interglacial cycles of the 41kyr world prior to 1.2 million years ago (Ma), the response between insolation and the CO₂-ice volume feedback is relatively linear. However, during the Mid-Pleistocene transition (0.6Ma – 1.2Ma), this linear response breaks down leading to a large increase in ice volume with a relatively modest decrease in CO₂ during glacials in late Pleistocene. Here, we present atmospheric CO₂ records derived from boron isotopes measured in the planktic foraminifera G. ruber sensu stricto from 3 ocean sediment cores, each well validated against ice records of CO₂. We find two notable CO₂ features during the MPT, an early de-coupling of CO₂ and ice volume from insolation during MIS 36 (~1.05 Ma), where CO₂ stays relatively constant despite multiple (but muted) orbital cycles. Secondly, during MIS 22 (0.9Ma), CO₂ decreases step-wise, in combination with rising global ice volume, and recovers to “luke-warm style” interglacial levels in the following interglacial MIS 21. The periods of low CO₂ and high ice volume occur in line with saltier Atlantic deep waters enriched in δ¹³C which we interpret as southern origin water masses, and increased ocean carbon storage. We therefore conclude that changes in ocean circulation may have caused an increased uptake of atmospheric carbon during these periods. In contrast, global sea surface temperatures during MIS36 follow insolation and not CO₂ suggesting a de-coupling of the CO₂/ice volume feedback from insolation and temperature. This may have prepositioned the climate system for the significant CO₂ reduction and ice sheet expansion during and after 0.9Ma.

How to cite: Chalk, T., Nuber, S., Stap, L., Scherrenberg, M., Zhang, X., Hain, M., Brown, R., Yu, J., Anderson, M., Barker, S., Rae, J., and Foster, G.: Continuous atmospheric CO2 across the Mid Pleistocene Transition from boron isotopes: decoupling of CO2 from insolation and temperature, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18813, https://doi.org/10.5194/egusphere-egu24-18813, 2024.

Increased export production in the Subantarctic Zone of the Southern Ocean has been proposed as a key mechanism for explaining carbon drawdown during glacial times. Therefore, reconstructions of oceanic particle fluxes from the sedimentary record in this sector are vital. Traditionally, fluxes of various materials to the seafloor have been estimated from stratigraphy-based mass accumulation rates (MARs), which are calculated using a combination of sediment dry bulk density and linear sedimentation rates between dated sediment horizons. We refer to these MARs here as age model-derived bulk MAR (BMARs). However, BMARs and any resulting paleoceanographic interpretations may suffer from substantial errors if lateral redistribution of sediments is not considered. In fact, this is expected to be a common phenomenon in the Southern Ocean due to the strong bottom water circulation of the Antarctic Circumpolar Current. Here, using material from a marine sediment core recovered at the Pacific entrance of the Drake Passage, we evaluate export production and its drivers over the past 1.4 million years by applying several paleoproductivity indicators (biogenic barium, organic carbon, biogenic opal, calcium carbonate, and iron). Crucially, we determine MARs of these various sediment components that are corrected for the lateral movement of sediments that occurred simultaneously to or soon after initial deposition. The results show that the export production indicators varied according to some of the characteristic features of the main climatic events of Earth over the past 1.4 Ma. (The Mid-Pleistocene Transition and Mid-Brunhes Event). Additionally, the productivity response in the area was enhanced (weakened) during globally strong (faint) glacials or interglacials (e.g., MIS 16, MIS 11, MIS 5, and the Holocene for strong and MIS 15-12 for weak responses, respectively).

How to cite: Toyos Simon, M. H., Lamy, F., Lange, C. B., Abell, J. T., Lembke-Jene, L., Arz, H. W., and Winckler, G.: A 1.4 Myr record of export production at the Pacific entrance of the Drake Passage considering syndepositional redistribution of sediments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8444, https://doi.org/10.5194/egusphere-egu24-8444, 2024.

The evolution of Earth’s climate during the Quaternary is characterized by glacial cycles with the periodic waxing and waning of large ice sheets most notably in the Northern Hemisphere. A fundamental transition in Earth climate occurred between 1250 and 650 kyr ago (ka) as the dominant glacial-interglacial variability shifted from ~40 to ~100-kyr cycles with more intense glacial climate. This is known as the Mid-Pleistocene Transition (MPT). The absence of appreciable change in the Milankovitch astronomical climate forcings during the MPT indicates that its occurrence might be a result of self-perpetuating climate feedback processes which would have been stimulated at ~900ka (i.e. Marine Isotope Stage 25-22) when global ice volume (e.g. Elderfield et al., 2012, Ford & Raymo, 2020), glacial ocean circulation (e.g. Pena &Goldstein 2014; Kim et al., 2021), deep ocean carbon reservoir (e.g. Lear et al., 2016; Farmer et al., 2019) and atmospheric CO₂ levels (e.g. Hoenish et al., 2009; Chalk et al., 2017; Yamamoto et al., 2022) coherently experienced stepwise changes to post-MPT like glacial states from MIS22 onwards. Nevertheless, the triggering mechanism remains enigmatic because of the intertwined nature of these internal processes which precludes the disentanglement into their individual roles in the onset of post-MPT glacial ice volume and the pCO₂ level at MIS22.

In this study, applying a combined climate – ice sheet – marine biogeochemical modeling approach, we investigate unidirectional impacts of changes in either atmospheric CO₂ levels or northern hemisphere ice sheet (NHIS) volume on the other in transient simulations spanning two successive obliquity cycles. Our results show that the emergence of a 100-kyr glacial cycle is controlled by the interglacial rather than glacial CO₂ levels. A lower glacial CO₂ levels does increase NHIS volume and hence intensify the glacial climate. But only when the interglacial CO₂ levels are below a threshold (~250ppm in our model) at the first obliquity peak, the developed glacial NHISs can skip the summer insolation maximum and reach a larger volume in the following glaciation, heralding the onset of 100-kyr glacial cycles. Meanwhile, the increased glacial NHISs, as a positive climate feedback process, promote atmospheric CO2 absorption in the subpolar North Atlantic via the strengthened upper cell of Atlantic Meridional Overturning Circulation. This, coupled with the enhanced formation of Antarctic Bottom Water, eventually sequesters the absorbed carbon in the North Pacific, further lowering glacial CO2 levels. Consistent with available proxy records, our results thus reconcile previously competing hypotheses for the occurrence of the MPT, providing a new and coherent dynamic framework accounting for the emergence of 100-kyr glacial cycles.

How to cite: Zhang, X., Stap, L. B., Du, J., and Nuber, S.: Control of subpar interglacial CO2 levels on the emergence of 100-kyr glacial cycles during the Mid-Pleistocene Transition, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17220, https://doi.org/10.5194/egusphere-egu24-17220, 2024.

Around ~800-1200 ka, the transition of glacial-interglacial cycles from earlier ~40-kyr into later ~100-kyr cyclicities without obvious changes in orbital parameters, known as the Middle-Pleistocene Transition (MPT), suggests that Earth’s internal factors, in addition to external astronomical forcing, are also essential for the glacial cycles. However, it is still unclear how internal and external factors interact to lead to the MPT and the ~100-kyr cycle. Here, we statistically analyzed the power spectral relationship between the ~21-kyr, ~41-kyr, and ~100-kyr components within 57 paleoclimate archives and reconstructed the astronomical phase relative to the maximal changing rate of benthic foraminifer oxygen isotopes (δ¹⁸O) over the past 2700 ka to explore the role of astronomical forcings in driving glacial cycles and their relationship with internal factors. The statistical results show that the ~21-kyr power ratio complements the ~100-kyr power ratio. The precession phase covaries with pCO₂-modulated glacial dynamics and exhibits a contrasting correlation with the precession power ratio of benthic δ¹⁸O before and after ~1500 ka. These findings suggest that pCO₂-modulated latitudinal extension of the icesheets determined the glacial response to precession. Around 1500 ka, the response apparently shifted into a nonlinear mode, enabling the gradual extension of glacial cycles into ~100-kyr periodicities at the expense of precession power, which signified the onset of the ~100-kyr glacial cycles. Our study confirms the nonlinear precession origin of ~100-kyr glacial cycles, featuring the possible low- and high-latitude interplay at the precession band.

How to cite: Zhang, Z., Huang, Y., Ma, C., Yin, Q., Yang, H., Lee, E. Y., Cheng, H., Sames, B., Wagreich, M., Liu, Q., Wang, T., and Wang, C.: CO2-forced change in glacial response to precession likely causes the Middle-Pleistocene Transition and ~100-kyr glacial cycles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4211, https://doi.org/10.5194/egusphere-egu24-4211, 2024.

While global changes in temperature during the last 1.5 Ma are well constrained, the terrestrial response to these changes is less understood, particularly in the Southern Hemisphere. Late Pleistocene ice-age climates are routinely characterised as having imposed moisture-stress on low/mid-latitude ecosystems. This idea is largely based on fossil pollen evidence for widespread, low-biomass glacial vegetation, interpreted as indicating climatic dryness. However, woody plant growth is inhibited under low atmospheric CO₂, so understanding glacial environments requires the development of new palaeoclimate indicators that are independent of vegetation. Here, we present two new, well-dated speleothem records from subtropical, southern Australia, both spanning the last three glacial-interglacial cycles. We show that, contrary to expectations, over the past ~350 ka, peaks in southern Australian climatic moisture availability were largely confined to glacial periods, including the last glacial maximum, while warm interglacials were relatively dry. By measuring the timing of speleothem growth in the Southern Hemisphere subtropics, which today has a predominantly negative annual moisture balance, we developed a record of climatic moisture availability that is independent of vegetation and extends through multiple glacial-interglacial cycles. Our results demonstrate that a cool-moist response is consistent across the austral subtropics, and in part may result from reduced evaporation under cool glacial temperatures. Insofar as cold glacial environments in the Southern Hemisphere subtropics have been portrayed as uniformly arid, our findings suggest that their characterisation as evolutionary or physiological obstacles to movement and expansion of animal, plant and, potentially, human populations should be reconsidered.

How to cite: Weij, R., Sniderman, K., Woodhead, J., Hellstrom, J., Brown, J., Drysdale, R., Reed, L., Bourne, S., and Gordon, J.: Precisely dated climate records challenge the Southern Hemisphere glacial aridity paradigm, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-511, https://doi.org/10.5194/egusphere-egu24-511, 2024.

Little is known about the evolution of the Antarctic ice-sheet (AIS) during the Pleistocene and its response to external drivers, such as CO₂, orbital and sea-level forcing. Here, we apply realistic transient climate forcings generated by the 3Ma Community Earth System model (CESM, version 1.2) simulation [1],[2] to the bi-hemispheric Pen State University ice-sheet shelf model (PSUIM). The CESM-simulated surface air temperature, surface solar insolation, precipitation, and sub-surface ocean temperature serve as inputs for PSUIM. This application enables us to simulate a more reliable variability of the AIS over the past 3 million years ago (Ma). Our simulation reveals a more pronounced precessional modulation of early to mid-Pleistocene AIS variability than previously suggested. The results further show the mid-Pleistocene transition (MPT, ~ 1Ma) of AIS, with dominant frequencies changing  from 20-40 kyrs to 80-120 kyrs and a clear regime shift in its surface mass balance. We also find that the pre-MPT precessional variability is significant only in the marine (floating) ice-sheet, not in terrestrial (grounded) ice. This suggests the influence of competing ocean and atmospheric processes in controlling the AIS variability over the past 3Ma. We will further discuss the mechanisms of simulated AIS variability and its climate interactions on orbital timescales and compare our results with paleo reconstructions.

[1] 3Ma-Data: Transient CESM1.2 model simulation data over the 3 million years ago (Ma),   https://climatedata.ibs.re.kr/data/3ma-transient-climate-simulation, doi:10.22741/iccp.20230001.

[2] Yun, K.-S., Timmermann, A., et al. (2023), A transient coupled general circulation model (CGCM) simulation of the past 3 million years, Clim. Past, 19, 1951–1974, https://doi.org/10.5194/cp-19-1951-2023.

How to cite: Yun, K.-S. and Timmermann, A.: Simulating Antarctic ice-sheet variability of the past 3 million years, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3753, https://doi.org/10.5194/egusphere-egu24-3753, 2024.

The Mid-Pleistocene Transition (MPT) is a period marked by significant changes in the LR04 benthic δ18O record. During this interval (ca. 1.25-0.65 Ma), glacial cycles shifted from symmetrical 41-thousand-year (kyr) cyclicity to asymmetrical 100-kyr cycles. This transition was characterized by a number of large-scale ice sheets, including terrestrial-based ice sheets in the northern hemisphere (e.g.,  the Laurentide and Greenland ice sheets) and a marine-based ice sheet in the southern hemisphere (e.g., the West Antarctic Ice Sheet). Since these different ice sheets respond to climate variability in unique ways, the LR04  stack may fail to capture certain aspects of regional cryospheric behavior across the MPT. Specifically, a potential lag in ice-rafted debris (IRD) fluxes, hinting at possible bipolarity in glacial terminations. This study aims to investigate glacial weathering fluxes from West Antarctica across the MPT by constructing an osmium isotope chemostratigraphic record in conjunction with published IRD records from IODP site U1536 in Iceberg Alley, in the Scotia Sea. This record will be compared to a previously published record from the IRD Belt in the North Atlantic. By examining glacial weathering products in regions with high accumulation of glacially-derived debris, these two records will provide a comprehensive comparison of cryospheric behavior in the North and South Atlantic across the MPT. This new dataset will offer a more detailed perspective of global cryospheric behavior during the MPT and may reveal synchronicity between the two hemispheres.

How to cite: Goss, G. and Rooney, A.: Investigating glacial weathering fluxes from West Antarctica across the Mid-Pleistocene: Insights from  Os isotopes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11738, https://doi.org/10.5194/egusphere-egu24-11738, 2024.

Previous research has suggested that different processes in the Southern Ocean contributed to the drawdown and release of atmospheric carbon dioxide (CO₂) during the last glacial cycle (0–130 ka), yet their dynamics and interplay are not fully understood. To unravel the interactions between different processes and to allow for more comprehensive analyses, we aim to compile all previously published proxy data and convert them into the Linked open Paleo Data (LiPD) format, overall increasing interoperability, reusability and impact.

The PAGES C-SIDE working group recently highlighted substantial open-ocean sea-ice extent changes during the mid-glacial period (Marine Isotope Stage 4, ~72 to 60 ka), coinciding with a significant drop in atmospheric CO₂. However, sea-ice changes are notably absent during the early glaciation (Marine Isotope Stage 5d, ~115 to 105 ka), suggesting they cannot account for the early CO₂ decrease (Chadwick et al., 2022).

As an initial step, we present our compilation of sea surface temperature (SST) reconstructions from marine sediment records across the Southern Ocean (Latitude > 30°S) for the last glacial cycle. This compilation assesses SST changes in different zones and basins, evaluates SST gradients, and explores the interplay between SST and sea ice.

Our findings reveal a consistent glacial-interglacial amplitude of 5-10°C across all basins and zones, with uniform timing. SST gradients from the Antarctic to Subantarctic Zone remain unchanged over time, eliminating them as a mechanism for early CO₂ decrease. Additionally, we observe that 50% of the total MIS 5e-to-LGM cooling in Southern Ocean SST occurred from MIS 5e to MIS 5d, with a second drop from MIS 5a to MIS 4, essentially reaching LGM cooling. A distinct decoupling of SST cooling and sea ice expansion over MIS 5 suggests that circulation and/or subsurface temperatures may exert a stronger influence than SST on sea ice extent. This emphasizes the necessity for additional proxy compilations to further disentangle these complex relationships.

How to cite: Thöle, L., Kohfeld, K., Chase, Z., Crosta, X., Bijl, P., and McKay, N.: A first step towards a complete Southern Ocean proxy compilation for the Last Glacial Cycle: glacial-interglacial changes in Sea Surface Temperature, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20306, https://doi.org/10.5194/egusphere-egu24-20306, 2024.

Glacial export productivity in the glacial Southern Ocean may have been enhanced due to iron fertilization from aeolian dust input. Marine sediments indicate such a glacial increase north of the modern Antarctic Polar Front but reduced biogenic activity and reduced nitrogen supply by upwelled deep waters south of it. Due to the sparsity of Southern Ocean sediment data, deriving an overall estimate of marine productivity changes is, however, difficult to achieve. Due to their larger spatial footprint, additional information on basin-wide productivity changes can be obtained from marine biogenic aerosol tracers in Antarctic ice cores.

We use SO₄^2- concentrations and its sulfur isotopic composition as well as other geochemical tracers in the EPICA Dronning Maud Land (EDML) ice core in the Atlantic Sector of the Southern Ocean (AS-SO) to provide the first complete glacial/interglacial source decomposition of total SO₄^2- from the penultimate glacial to the last glacial inception. Our isotopic source decomposition shows that despite other (e.g. terrestrial) sources being significant contributors to total SO₄^2- during glacial times, biogenic SO₄^2- production is always the dominant source at EDML. Using information on recent dimethylsulfide emissions and aerosol forward modeling, we can show that biogenic sulfate recorded in the EDML ice core is derived from the AS-SO south of 35°S but the major source lies south of 50°S, i.e., mainly the seasonal sea ice zone. During the penultimate glacial these sources shifted about 4° northward in parallel to sea ice expansion.

Taking reduced wet deposition of biogenic sulfate aerosol during glacial times into account, we can show that the biogenic sulfate production during the Penultimate Glacial Maximum and the Last Interglacial integrated over the AS-SO may have been only slightly higher in the penultimate glacial and differed by less than 15%. We see millennial biogenic sulfur changes of the same order during the Last Interglacial, which we attribute to temporal changes in the seasonal sea ice zone. An early interglacial productivity minimum in our biogenic sulfate record parallels within age uncertainties features previously reported in the literature, i.e., a minimum in winter, thus seasonal, sea ice extent, a stagnation event in Antarctic Bottom Water and a maximum in summer surface temperature encountered during the early LIG.

How to cite: Fischer, H., Burke, A., Ray, J., Sugden, P., Wolff, E., Pryer, H., Doyle, E., Severi, M., Markle, B., Hörhold, M., Freitag, J., Twarloh, B., and Erhardt, T.: Little glacial/interglacial net change in Southern Ocean bioproductivity over termination II - an integrated sulfate isotope ice core perspective, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5995, https://doi.org/10.5194/egusphere-egu24-5995, 2024.

The Antarctic Circumpolar Current (ACC) represents the world’s largest ocean current system and impacts global ocean circulation, climate, and Antarctic ice sheet stability. Today, ACC dynamics are controlled by atmospheric forcing, Southern Ocean density gradients, and mesoscale eddy activity in the southern high latitudes. Yet, its role in driving the lengthening and intensification of glacial cycles is insufficiently studied. Here, we present a 1.5 Ma-record of changes in ACC strength based on bottom water flow reconstructions and sedimentary opal contents at IODP Sites U1540 and U1541 drilled in the Subantarctic Zone (SAZ) of the Pacific Southern Ocean. Our new data indicate that glacial and interglacial ACC strength gradually increased between ~1.3 and ~ 1 Ma coinciding with the early part of the MPT. This interval culminates in a pronounced ACC maximum during Marine Isotope Stage (MIS) 31 at ~1 Ma reaching ~160 % of the mean Holocene values. The increase in subantarctic ACC strength during the initial part of the MPT is paralleled by the emergence of stronger orbital-scale fluctuations in opal contents at both Sites U1540 and U1541 after MIS 31, suggesting a link between the onset of consistently higher amplitude glacial-interglacial fluctuations of ACC changes and latitudinal shifts of the ‘opal belt’ in the Southern Ocean. Specifically, higher opal contents correlate to decreased ACC strength, suggesting that the opal belt extended northward into the SAZ during glacials. We argue that the early change in ACC dynamics at the beginning of the MPT might be linked with sea surface temperature changes in the eastern subtropical and tropical Pacific, because surface cooling by ~2-3 °C at ODP Site 1237 off Peru between ~1.05 Ma and ~0.8 Ma parallels the reconstructed ACC strengthening at IODP Sites U1540 and U1541. This may result from enhanced advection of subantarctic water masses northward along the Humboldt Current system as a response to the intensification of the ACC starting during MIS 31. Our findings emphasize a contribution of Southern Ocean processes to the climate events causing intensification of glacial-interglacial climate variability during the MPT.

How to cite: Lamy, F., Winckler, G., Arz, H., Farmer, J. R., Lembke-Jene, L., Gottschalk, J., and Toyos, M.: Increase in strength and orbital variability of the South Pacific Antarctic Circumpolar Current across the Mid-Pleistocene Transition, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16932, https://doi.org/10.5194/egusphere-egu24-16932, 2024.

The recovery of a new Antarctic ice core spanning the past ∼ 1.5 million years will advance our understanding of climate system dynamics during the Quaternary. Recently, glaciological field surveys have been conducted to select the most suitable core location near Dome Fuji (DF), Antarctica. Specifically, ground-based radar-echo soundings have been used to acquire highly detailed images of bedrock topography and internal ice layers. In this study, we use a one-dimensional (1-D) ice-flow model to compute the temporal evolutions of age and temperature, in which the ice flow is linked with not only transient climate forcing associated with past glacial–interglacial cycles but also transient basal melting diagnosed along the evolving temperature profile. We investigated the influence of ice thickness, accumulation rate, and geothermal heat flux on the age and temperature profiles. The model was constrained by the observed temperature and age profiles reconstructed from the DF ice-core analysis. The results of sensitivity experiments indicate that ice thickness is the most crucial parameter influencing the computed age of the ice because it is critical to the history of basal temperature and basal melting, which can eliminate old ice. The 1-D model was applied to a 54 km long transect in the vicinity of DF and compared with radargram data. We found that the basal age of the ice is mostly controlled by the local ice thickness, demonstrating the importance of high-spatial-resolution surveys of bedrock topography for selecting ice-core drilling sites.

How to cite: Abe-Ouchi, A., Obase, T., Saito, F., Tsutaki, S., Fujita, S., Kawamura, K., and Motoyama, H.: A one-dimensional temperature and age modeling study for selecting the drill site of the oldest ice core near Dome Fuji, Antarctica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15013, https://doi.org/10.5194/egusphere-egu24-15013, 2024.

The region between Dome A and South Pole has likely preserved ice older than the current 800-thousand-year limit of continuous ice core records; however, until now this region has been largely unexplored. The Center for Oldest Ice Exploration (COLDEX) is currently performing the second of two planned years of airborne geophysical surveys on the Southern flank of Dome A. These surveys are providing new geological and glaciological constraints that we combine with ice-flow models to help target suitable deep ice core sites with the goal of recovering a continuous ice-core record going back at least 1.5 million years.

Using the new airborne ice penetrating radar data from COLDEX, as well as existing data from the AGAP project, we investigate how local variations in surface conditions may affect the ice record over time. First, we trace englacial layers and date them at the intersection with the South Pole ice core to infer the rate and pattern of past accumulation averaged over different time intervals. Second, we assess the impact of local zones of wind scour that occur on the Southern flank of Dome A (Das et al, 2013), which is at the upstream edge of the COLDEX airborne survey. Local zones of wind scour that lead to ablation or no accumulation, create time-transgressive unconformities that can be mapped from ice penetrating radar data. While the unconformity is initiated due to a relatively local change in surface conditions, the unconformity trace is imaged for many tens of kilometers downstream as it is advected by ice flow. Because the airborne survey flight lines are oriented along flowlines, the unconformities act as particle trajectories.

We use an ice-flow model set up along a flowline to evaluate the surface and flow conditions that develop an unconformity similar to a well-imaged unconformity that is observed in the COLDEX data. The unconformity can be well matched with the simple ice-flow model using a fixed position of the scour zone, indicating that the scour zone has been a persistent feature for the past glacial-interglacial cycle (~100 ka). Consistent with previous work (Das et al, 2013), the scour zones are co-located with subglacial ridges that create steeper surface topography. Thus, the positions of the scour zones are likely independent of the climate state and permanent features on long timescales.

By modeling this unconformity trace we can constrain the modern horizontal velocity to ~1.5 m/yr near the scour zone that is located ~400 km from Dome A. The unconformity disrupts the continuity of all of the dated internal layers, which extend to 94 ka. Running the model back 1.5 Ma, we can evaluate where the climate record is disrupted at different positions along the flowline. The farther downstream a potential drill site is, the more problematic the unconformities become for obtaining a continuous climate record because the unconformity disrupts the continuity at deeper depths and older ages.

How to cite: Fudge, T., Koutnik, M., Young, D., Singh, S., Holschuh, N., Yan, S., Blankenship, D., and Kerr, M.: Stability of interior East Antarctic wind scour and ice flow on glacial-interglacial timescales, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14100, https://doi.org/10.5194/egusphere-egu24-14100, 2024.

Ice cores are unique archives capturing records of past temperature (through the ice isotopic composition, e.g. δD) and past atmosphere composition over the last 800 kyr. In particular, their analysis revealed that glacial-interglacial transitions, altering the Earth's climate since the beginning of the Quaternary, are associated with significant variations in the atmospheric levels of CO₂ and CH₄. However, comparison of past temperatures imprinted in ice-phase and atmospheric composition records imprinted in the air-phase is difficult. Indeed, the air is trapped at a depth of 50-100 m, at the bottom of the firn, where snow transforms into ice. Therefore, at a given depth, the air is always younger than the ice and firn densification modeling is needed to estimate the age difference between the air and the ice at each level. Firn densification modeling is associated with large uncertainties when it is applied to low accumulation and low temperature drilling sites of the East Antarctic plateau.

An alternative approach to reconstruct air temperature directly in the air bubbles involves analyzing the isotopic composition of N₂ (δ¹⁵N). Indeed, local temperature and accumulation rate evolutions affect firn thickness and hence modulated the δ¹⁵N in air bubbles trapped at the bottom of the firn via gravitational enrichment of δ¹⁵N over large glacial-interglacial transition on the East Antarctic plateau. The observation of a robust correlation between ice core records of δ¹⁵N and δD (Dreyfus et al., 2010) confirms the strong influence of local climate on the δ¹⁵N. δ¹⁵N measurements have already been applied to determine the phasing between CO₂ and temperature increases over Antarctic temperature increase associated with glacial terminations. However, this strong relationship between δ¹⁵N and δD is not necessarily valid outside of glacial terminations. Here, we address the question to what extent the δ¹⁵N can be used to infer past temperatures and to study the CO₂-temperature relationship, hence circumventing age uncertainties that arise when comparing ice and gas phase measurements.

We first examine the δ¹⁵N record from EPICA Dome C with respect to East Antarctic climate over the last eight glacial-interglacial cycles. We use the good agreement between δD and δ¹⁵N over Termination II as a satisfactory criterion to discern when the δ¹⁵N is a reliable proxy of past temperature. Using this criterion, we assert that the correlation between δ¹⁵N and δD is robust over the past eight terminations. Focusing on the 100-300 ka BP period, we note also three intervals characterized by a weak correlation: the glacial inceptions from MIS 7e to 7d and 7a to 6e, and the MIS 6 glacial period. To explain why δ¹⁵N and δD evolutions contrast over these periods, we connect water stable isotopes with new δ¹⁵N measurements from EDC ice core and explore various snow densification scenarios yielded by a firn model under different climate conditions at the ice sheet surface. Our study permits to identify a criterion to safely use δ¹⁵N as an indicator of the past temperature in the air bubbles of the EDC ice core to study the CO₂-local temperature relationship.

How to cite: Bouchet, M., Landais, A., Parrenin, F., Legrain, E., Capron, E., Grisart, A., Prié, F., Extier, T., Jacob, R., Quiquet, A., Dumas, C., and Klüssendorf, A.: Inferring past temperature from δ15N measurements in air bubbles trapped in Antarctic ice, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5630, https://doi.org/10.5194/egusphere-egu24-5630, 2024.

The European Project for Ice Coring in Antarctica (EPICA) Beyond EPICA – oldest ice aims at retrieving a continuous ice core record of climate feedback and forcing spanning about 1.5 Ma back in time. After determining a suitable drill-site LDC during an extensive pre-site survey 35 km southwest of Concordia station, we are in the second deep drilling season. At the time of submission of this abstract, we penetrated beyond 1604.92 m, roughly spanning one glacial-interglacial cycle. We will report on the drilling and core processing activities and verify the prognostic age scale by comparison with dielectric profiling (DEP) and stable isotope saw dust measurements in the field.

How to cite: Wilhelms, F. and the LDC field parties 2022/2023 & 2023/2024, drill & science workpackage, stable isotope field measurements group: Progress report on Beyond EPICA – oldest ice (BE-OI) Little Dome C (LDC) activity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8041, https://doi.org/10.5194/egusphere-egu24-8041, 2024.

We investigate the effect of Earth’s orbitally governed incoming solar radiation on global ice volume over the past 800,000 years. It is well established that the orbital parameters play some role in the pacing of the glacial-interglacial cycles. However, due to limited data and enigmatic dynamics, the mechanics that could facilitate this relationship remain unresolved. We therefore consider a simple linear model of ice volume that imposes minimal assumptions about its dynamics. This model adequately reproduces the ice volume data for most of the 800,000 year period, with the exception of Marine Isotope Stage 11. This suggests that, aside from a few extrema, the ice volume dynamics primarily result from an approximately linear response to orbital forcing. We substantiate this finding by addressing some of the key criticisms of the orbitally forced hypothesis. In particular, we show that eccentricity can significantly vary the ocean temperature without the need for amplification on Earth. We also present a feasible mechanism to explain the absence of eccentricity’s 400,000 year period in the ice volume data. This requires part of the forcing from eccentricity to be lagged via a slow-reacting mechanism, resulting in a signal that closer approximates the change in eccentricity. A physical interpretation of our model is proposed, using bulk ocean and surface temperatures as intermediate mechanisms through which the orbital parameters affect ice volume. These show reasonable alignment with their relevant proxy data, though we acknowledge that these variables likely represent a combination of mechanisms.

How to cite: Wheen, L., Gernon, T., Hall, C., Wright, J., and Benjamin, O.: The Largely Linear Response of Earth’s Ice Volume to Orbital Forcing, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-64, https://doi.org/10.5194/egusphere-egu24-64, 2024.

The Quaternary period (0–2.58 million years) is an important time in the early evolution of our human ancestors. This period is featured by distinctive glacial-interglacial cycles, primarily caused by variations in orbital parameters (i.e., eccentricity (100 thousand years (kyr)), obliquity (41 kyr), and precession (23/19 kyr)), atmospheric CO₂ concentration (GHG), and their feedbacks. Therefore, it is essential to understand the climate system, mainly focusing on the variability of the Atlantic Meridional Overturning Circulation (AMOC) due to its huge impact.

In this study, we have employed a quasi-continuous simulation to understand the AMOC variability in response to changes in orbital, GHG, and continental ice sheet forcings over the past 2.6 million years. It is found that the AMOC variability is associated with the sea ice coverage and mixed layer depths over the Labrador Sea and Irminger and Iceland basins. The overall mixed layer depth over the Labrador Sea, Irminger, and Iceland basins and the corresponding AMOC variability vary in precession and obliquity periodicity. Meanwhile, the mixed layer depth in the Labrador Sea exhibits a dominant precession, and the Irminger and Iceland basins exhibit a dominant obliquity periodicity. Further, we have divided the entire Quaternary period into three subsets based on the dominant periodicity of the climate state: 0ka–700ka (post-MPT; 100kyr dominant), 700ka–1200ka (MPT; 100–80kyr and 41kyr dominant), and 1200ka–2600ka (pre-MPT; 41kyr dominant). We have found that sea ice coverage and mixed layer depth in the Labrador Sea (Irminger and Iceland basins) are out of (in) phase with a Pearson correlation coefficient of −0.70 (0.42) during post-MPT, −0.78 (0.32) during MPT, and −0.85 (0.38) during pre-MPT periods. These results indicate that during glacial periods, the southward expansion of Labrador sea ice covered the deep convection sites, which impeded deep convection and weakened the AMOC strength. Therefore, the expansion and contraction of Labrador sea ice and its feedback contributed to AMOC variability over glacial‐interglacial cycles for the past 2.6 million years.

How to cite: Mandal, G., An, S.-I., Timmermann, A., Yun, K.-S., and Park, J.-H.: The association of AMOC and Atlantic sea ice in a transient CGCM simulation for the past 2.6 million years., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2423, https://doi.org/10.5194/egusphere-egu24-2423, 2024.

The weakening or shutdown of Atlantic Meridional Overturning Circulation (AMOC) likely played a significant role in the glacial inception during the past million years. Previous modeling studies have shown that orbital forcing could have been triggered multiple AMOC weakening or shutdown, but it is still unclear which orbital parameter is the most essential trigger. In this study, we performed multiple long simulations with a fully coupled atmosphere-ocean general circulation model, CESM1.2.2, to investigate the influence precession on AMOC. It is found that precession is able to trigger a shutdown of AMOC. However, this happens only when the eccentricity is high and the atmospheric CO₂ concentration is relatively low. The growth and expansion of Arctic sea ice is responsible for the shutdown. Therefore, the results may advance our understanding of the mechanisms driving the glacial-interglacial cycles of the Quaternary Period, and may be related to the mid-Pleistocene transition.

How to cite: Liu, Y. and Liu, H.: Shutdown of Atlantic Meridional Overturning Circulation Induced by Precession, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14755, https://doi.org/10.5194/egusphere-egu24-14755, 2024.

Ocean thermohaline circulation (THC) is driven by temperature and salinity in source areas of deep-water formation. THC underwent a fundamental change ~950 to 860 thousand years ago (ka) during the Middle Pleistocene Transition (Pena and Goldstein, 2014). However, the relative contributions of temperature (‘thermo-’) versus salinity ('haline’) change to this fundamental THC reorganization remain unclear. Here we compiled North Atlantic and Pacific Ocean stacks of deep-water temperature (estimated using foraminiferal Mg/Ca) and salinity (estimated from δ¹⁸O_seawater) for the past 1.5-million-years (Myr). The deep North Atlantic became colder and the deep Pacific saltier during glacial periods younger than ~900 ka. Cooling of northern sourced water likely led to increased salinity of southern sourced water by decreasing the melting of land-based ice around Antarctica (Adkins, 2013) and increasing sea ice formation and associated brine rejection. With increased stratification the abyssal ocean became a more effective carbon trap lowering the concentration of atmospheric pCO₂, thereby permitting ice sheets to grow larger and lengthening the glacial cycle. Expansion of Antarctic ice sheets would have also contributed to increasing the salinity of southern source areas as Antarctica shifted from dominantly terrestrial melting to marine-based calving margins (Raymo et al., 2006). Our temperature and salinity reconstructions support a fundamental reorganization of the density structure and stratification of the abyssal glacial ocean across the Middle Pleistocene Transition.

References:

Adkins, J.F. (2013) ‘The role of deep ocean circulation in setting glacial climates’, Paleoceanography, 28(3), pp. 539–561. Available at: https://doi.org/10.1002/palo.20046.

Pena, L.D. and Goldstein, S.L. (2014) ‘Thermohaline circulation crisis and impacts during the mid-Pleistocene transition’, Science, 345(6194), pp. 318–322. Available at: https://doi.org/10.1126/science.1249770.

Raymo, M.E. et al. (2006) ‘Plio-Pleistocene Ice Volume, Antarctic Climate, and the Global’, Nature, 313(July), pp. 492–495. Available at: https://doi.org/10.1126/science.1123296.

How to cite: Thomas, N., Hodell, D., Ford, H., and Greaves, M.: Temperature and salinity changes in the abyssal Atlantic and Pacific Oceans over the Middle Pleistocene Transition, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5365, https://doi.org/10.5194/egusphere-egu24-5365, 2024.

During the early Pleistocene (prior to 1.25 Ma), obliquity dominated the cyclicity of climatic variations, resulting in glacial and interglacial cycles. A significant change occurred between 1.25 Ma and 0.7 Ma, which altered the dominant frequency from 41 kyr to 100 kyr. This transition period is known as the Mid-Pleistocene Transition (MPT). Although several climate models and records have focused on the MPT, our understanding of how climatic variations in the 41 kyr-world affected the planktonic foraminiferal fauna, and their response to the millennial-scale sea surface temperature (SST) oscillations remains limited. Here, we present a sub-millennial scale planktonic foraminiferal assemblage and G. bulloides stable isotope data from southern Iberian margin IODP Site U1387 (36°48.321´N 7°43.1321´W, 559 water depth), influenced by subtropical surface waters from the Azores current. The main goal is to reconstruct temporal trends in SST and to infer ecological changes during the interval from Marine Isotope Stage (MIS) 50 to MIS 40 (1.5-1.28 Ma).

Planktonic foraminifera assemblages show a distinct pattern between glacial and interglacial periods, correlating with changes in the mid-latitudinal North Atlantic's surface circulation. Interglacial periods (MIS 49, MIS 47, MIS 43) exhibit a strong influence of warm, oligotrophic waters. The abundances of subtropical species vary between 40% and 65%, whereas tropical species reach up to 10%. The SSTs were around 23.7°C during summer and 18.5°C during winter. In these periods, insolation appears to influence interglacial intensity, peaking at the onset of MIS 47 and MIS 43. In contrast, during cooler MIS 45, the subtropical species only reached values up to 20% and tropical species up to 2%, with temperatures about 21°C in summer and 16°C in winter. The expansion of the subtropical gyre during the interglacials, but also interstadial periods, could have played a significant role in those species’ assemblages and the SST fluctuations.

In contrast, during glacial periods (MIS 50, MIS 48, MIS 46, MIS 44), extreme cold events of short duration were documented, with MIS 50 and MIS 48 recording distinct terminal stadial events. Those short-term episodes were marked by abrupt abundance increases of polar species N. pachyderma up to 40% to 65%, respectively, and SSTs dropping down to 8°C in summer and 5°C in winter. The coldest temperatures were documented during the MIS 48 stadial terminal event and is consistent with alkenone-derived SST data, indicating colder deglacial conditions compared to MIS 46 and MIS 40. The SSTs, and the faunal data, including the increase in cold water calcareous nannofossil taxa, are consistent with evidence of the southward displacement of subpolar waters and the contraction of the subtropical gyre. In addition to the faunal data, changes in the G. bulloides δ¹⁸O record reveal a gradual increase of values during MIS 48 and abrupt oscillations during MIS 46, MIS 44, MIS 42, and MIS 40. Overall, we confirm the presence of millennial-scale climate variability during the 41 kyr-world with strong impacts on the planktonic foraminifera fauna and implications for the dimension of the subtropical gyre in the North Atlantic.

How to cite: Duque Castaño, M., Voelker, A. H. L., Rodrigues, T., and Trotta, S.: Millennial-scale climate variability in the 41 kyr world of MIS 50 to MIS 40 (1.5-1.28 Ma): Insights from planktonic foraminifera and sea surface temperature data from the southern Iberian margin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-439, https://doi.org/10.5194/egusphere-egu24-439, 2024.

The late Middle Pleistocene Transition (MPT, ~ 800-670 thousand years before present, ka) was characterised by the emergence of large glacial ice-sheets associated with anomalously warm  mid North Atlantic sea surface temperatures (SST) enhancing moisture production. Still, the moisture transport across Eurasia towards high northern latitudes is poorly constrained despite its potential role as feedback mechanisms feeding the ice caps. To reconstruct late MPT moisture production and spreading, we combine records of upper ocean temperature and pollen-based Mediterranean forest cover, a tracer of westerlies and precipitation, from a subtropical drill-core collected off SW Iberia Margin, with records of East Asia summer monsoon (EASM) strength and West Pacific surface temperatures, and compare them with the iLOVECLIM model simulations. We observe that the strongest Mediterranean forest development occurred during Marine Isotope Stage (MIS) 17, centered at 700 ka, reflecting a high amount of regional winter precipitation. In contrast, MIS 19 (~785 ka), under the influence of both similar ice volume and higher atmospheric CO₂ concentration, is marked by limited forest expansion indicating lower winter precipitation in SW Europe compared to MIS 17. More interestingly, the MIS 18 glacial was more forested, reflecting stronger winter rainfall, compared to the preceding MIS 19, despite that the latter interglacial was characterised by higher insolation, sea level, atmospheric CO₂ concentrations and similar warm SST. The long-term increasing trend in winter precipitation in SW Europe parallels the trend of the EASM strength that reached high levels during MIS 18. The model results show high amount of winter rainfall in SW Europe and enhanced EASM (based on the modelled East Asian δ¹⁸O_calcite and summer precipitation) for the three MISs. Similar SW European tree fraction percentages are also modelled during MIS 18 and MIS 19, as inferred from the pollen data. In contrast to the proxy data, the simulated tree fraction is the weakest during MIS 17. The simulated winter rainfall is the highest during MIS 17, but the simulated EASM is the lowest during MIS 18. This mismatch between model and proxy reconstructions could be explained by the difficulty in quantitatively estimating the forest cover from pollen data and/or the result of a feedback process that is not well reproduced in the model such as the poor prediction of the intensity and position of the oceanic moisture source despite a robust SST simulation. Here the data show that SW European winter precipitation and EASM strength reached high levels during the MIS 18 glacial. We explained that this anomalous situation was caused by nearly-continuous moisture supply from both Pacific and Atlantic oceans and its transport to higher latitudes through the westerlies, likely fueling the accelerated expansion of northern hemisphere ice-sheets during the late MPT.

How to cite: Sanchez Goñi, M. F., Extier, T., Polanco-Martinez, J. M., Zorzi, C., Rodrigues, T., and Bahr, A.: Reinforced precipitation in Eurasia as an important feedback mechanism contributing to the Middle Pleistocene Transition ice-sheet expansion, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11250, https://doi.org/10.5194/egusphere-egu24-11250, 2024.

   Nowadays, the hydroclimate of the semi-arid Northeast Brazil is tightly linked with, inter alia, temperature of the adjacent Atlantic Ocean and its interactions with the Atmosphere. The short humid season peaks in April while (i) the intertropical convergence zone (ITCZ) is at its southernmost position, (ii) the southern tropical Atlantic is warm and (iii) the southeast trade winds are weak. Uncertainties remain on past long-term hydroclimate changes and on the drivers controlling these variations in the NE Brazil region. One of the reasons is the lack of available long-term paleoclimate records.

   Very recently, we reconstructed ocean temperature changes in the western tropical Atlantic on glacial-interglacial time scales and highlighted relatively cold (warm) upper ocean waters during glacial (deglacial and interglacial) intervals over the last 300 000 kyr¹. It remains unknown how these changes impacted the NE Brazilian hydroclimate on orbital time scales. This work aims at examining the response of continental vegetation to variations in the western tropical Atlantic heat content over the last two climatic cycles. We used the same sedimentary core (GL-1180) collected off the NE Brazilian margin on which temperature reconstructions were conducted.

   We developed a multi-proxy approach at a 2 kyr temporal resolution to reconstruct the sources and the composition of the sedimentary organic matter (OM) produced on-land and within the water column at both bulk and molecular scales. We first investigated the organic signature of present-day dry (caatinga) and humid (Atlantic tropical forest) vegetation in our study area using modern litter samples. After statistical investigations, we developed new local vegetation proxies based on the relative abundance of long-chain n-alkanes (n-C₃₃/[n-C₂₉+n-C₃₁+n-C₃₃]), n-alkenes (n-C₂₇/[n-C₂₇+n-C₂₈]) and n-alkan-1-ols (n-C₂₈/[n-C₂₈+n-C₃₀]). Secondly, we reconstructed vegetation dynamics and hydroclimate changes using these new proxies together with the bulk elemental (%Corg, %Ntot) and isotopic (δ¹³C_org, δ¹⁵N_tot) composition and the molecular isotopic composition (δ¹³C) of specific C₂₉ and C₃₁ n-alkanes. We found that a caatinga-like dry vegetation expanded during arid glacial periods while humid conditions prevailed over interglacial intervals in agreement with previous regional studies. Comparing our vegetation and upper ocean temperature records, we highlighted that continental humid (arid) conditions occured during intervals of warm (cold) western tropical Atlantic and weak (strong) southeast trade winds.

   In conclusion, our work highlights glacial-interglacial vegetation and hydroclimate changes in NE Brazil. It further shows that the heat content of the tropical Atlantic was a major driver of these changes over the last 300 000 kyr. In addition, we suggest that the three major features (Atlantic heat content – ITCZ – SE trades) were likely controlling together hydroclimate changes and vegetation dynamics over, at least, the last two climatic cycles.

¹Rouyer-Denimal et al., 2023. QSR 321, DOI: 108370. 10.1016/j.quascirev.2023.108370

How to cite: Rouyer-Denimal, L., Govin, A., Bouloubassi, I., Albuquerque, A. L., Nguyen Tu, T. T., Mandeng-Yogo, M., Anquetil, C., and Huguet, A.: Late Pleistocene hydroclimatic and vegetation changes in Northeast Brazil: which role played the western tropical Atlantic?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9485, https://doi.org/10.5194/egusphere-egu24-9485, 2024.

The driving mechanisms of the middle Pleistocene Transition (MPT) are still unclear but the most likely hypotheses are related to ice-sheet dynamic feedbacks, such as ice albedo, precipitation at the ice margins, elevation-temperature and the regoliths. Here we focus on the “precipitation at the ice margins” hypothesis. To test this hypothesis, we have analysed the pollen content of SW Iberian margin sedimentary sequences that document changes in the direction and intensity of the westerlies during the MPT. In my presentation I will show the pollen-based vegetation and winter rainfall changes in the adjacent landmasses of the southwestern Iberian margin during the MPT. Changes in the reconstructed vegetation from IODP Site U1386 (1.2-0.8 Ma), combined with IODP Site U1385 (0.8-0.67 Ma), and their comparison with changes in the North Atlantic Ocean thermal gradient reveal the variability in the intensity and position of the westerlies and in the position of the moisture source, respectively. Preliminary pollen results reveal a long-term decreasing trend in the Mediterranean forest cover during MIS 31-20 (1.1-0.8 Ma), associated with long-term southward migration of the thermocline water source. This trend abruptly shifted northward at 0.8 Ma, and probably was related to progressive northward shift of the westerlies that bring moisture to the margin of the ice sheets feeding the ice caps, and leading to the shift of the dominant ice sheet cyclicity from 41 kyrs to 100 kyrs.

How to cite: Quan, X., Sanchez Goñi, M. F., Moal-Darrigade, P., Yin, Q., and Polanco-Martinez, J.: Variability of the North Atlantic westerlies during MIS 31-16 (1.1- 0.65 Ma) from SW Iberian margin records, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1845, https://doi.org/10.5194/egusphere-egu24-1845, 2024.

Isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGT) and alkenones are widely used tools for reconstructing past sea surface and subsurface temperatures. IsoGDGTs are membrane lipids synthesized by ammonia-oxidizing Nitrososphaerota and contain up to four cyclopentane moieties. Alkenones are unsaturated carbon chains, whose origin is mainly the coccolithophorid algae Emiliania huxleyi. The number of moieties of the isoGDGTs, as well as the degree of unsaturation of the alkenones depends on the ambient water temperature. Both biomarker-synthesizing organisms (Nitrososphaerota and E. huxleyi) are subject to several environmental influences, like nutrient availability, light conditions, salinity changes, changes in the biomarker-producing community, or terrigenous input, that can bias the temperature signal. In this study, we focus on the influence of terrigenous input and changes in salinity on both biomarkers. We use the 17 m-long piston core MR16-069 PC03, which is located at 46° S and ~150 km offshore the Chilean margin. This core covers a full glacial-interglacial cycle (140 ka) and shows recurring high inputs of terrigenous material and freshwater during the glacial period. This extreme contrast between interglacial and glacial is suitable for examining the influence of high terrigenous input on the temperature signal. Due to changes in the depositional setting, our results show a significant change in the expected temperature signal in both proxies during phases of high terrigenous input. We further discuss which temperature calibration is most appropriate for both biomarkers and conclude that for GDGT-based temperatures at this site, a calibration based on the TEX^L₈₆ index is more suitable, while for alkenone-based temperatures the U^K₃₇ index appears to be most accuracy.

How to cite: Hagemann, J. R., Martínez-Garcia, A., Lamy, F., Kaiser, J., Lembke-Jene, L., Arz, H. W., Lange, C. B., and Tiedemann, R.: Environmental parameters affect palaeothermometry of Alkenones and GDGTs: A case study at the Southern Chilean Margin (46° S), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18691, https://doi.org/10.5194/egusphere-egu24-18691, 2024.

The Mid-Pleistocene Transition (MPT) is commonly characterized as a change in both temperature and ice volume from smaller amplitude, 41-kyr variability to higher amplitude, ~100-kyr variability in the absence of any significant change in orbital forcing. Here we reassess these characteristics based on our new reconstructions of changes in global mean surface temperature (DGMST) and global mean sea level over the last 2.5 Myr. Our reconstruction of DGMST includes an initial phase of long-term cooling through the early Pleistocene followed by a second phase of accelerated cooling during the MPT (1.5-0.9 Ma) that was accompanied by a transition from dominant 41-kyr low-amplitude periodicity to dominant ~100-kyr high-amplitude periodicity. Changes in rates of long-term cooling and variability are consistent with changes in the carbon cycle driven initially by geologic processes followed by additional changes during the MPT in the Southern Ocean carbon cycle. The spectrum of our sea-level reconstruction is dominated by 41-kyr variance until ~1.2 Ma with subsequent emergence of a ~100-kyr signal that, unlike global temperature, has nearly the same concentration of variance as the 41-kyr signal during this time. Moreover, our sea-level reconstruction is significantly different than all other reconstructions in showing fluctuations of large ice sheets throughout the Pleistocene as compared to a change from fluctuations in smaller to larger ice sheets during the MPT. We attribute their longer period variations after the MPT to modulation of obliquity forcing by the newly established low-frequency CO₂ variability. Specifically, prior to reaching their maximum size at the end of each ~100-kyr interval, ice-sheet response to periods of lower CO₂ was modulated by higher obliquity, and vice versa, with the times of maximum ice-sheet growth only occurring when low CO₂ combined with the next obliquity low. Ice sheets then began to melt in response to the next increase in obliquity, with the subsequent sequence of events and feedbacks leading to a termination. High-resolution ice-core CO₂ records that extend beyond 0.8 Ma are needed to test this hypothesis. Otherwise, large ice sheets shared a common size threshold throughout the Pleistocene equivalent to sea level below -80 m that, when exceeded, resulted in a termination that was paced by the next increase in obliquity.

How to cite: Clark, P. U., Shakun, J., Rosenthal, Y., Pollard, D., Köhler, P., Hostetler, S., Bartlein, P., Liu, Z., Zhu, C., Schrag, D., and Pisias, N.: A Revisionist View of the Mid-Pleistocene Transition, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3722, https://doi.org/10.5194/egusphere-egu24-3722, 2024.