The mid-Pleistocene transition [MPT; between ~1.25 and 0.7 million years ago (Ma)] marks when Earth's glacial pacing switched from 41,000- to 100,000-year cyclicity. Here, we present a South Pacific hydrogenous neodymium isotope record tracking changes in deep ocean circulation spanning 1.5 to 0.5 Ma. This record indicates that the MPT was not triggered by Northern Hemisphere (NH) processes. Instead, the growth of Antarctic ice sheets to a critical size (~1.8 Ma) primed the climate system for the MPT, with iceberg release and melting transferring buoyancy out of the Southern Ocean. This iceberg-associated buoyancy redistribution ultimately reduced deep water formation in the NH, an observation supported by our data. Once the MPT was initiated in the Southern Hemisphere, its subsequent climatic changes were dominated by NH processes.

Abstract. The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.

The Mid-Pleistocene Transition (MPT), from ~ 1.2 to 0.6 million years is characterized by glacial interglacial cycles periodicity shifting from 41 kyr to ~100 kyr with larger amplitude variations (Pisias et Moore, 81; Imbrie et al., 93). This transition cannot be explained as a direct consequence of the astronomical forcing. Among the different mechanisms that has been suggested to explained this change in periodicity, a number involve a long-term decrease in atmospheric carbon, linked to greater carbon sequestration in the ocean. Various processes may have caused this carbon sequestration in the ocean (Paillard, 207; Willeit et al., 2019), including ocean temperature decrease, increased productivity in the Southern Ocean (Martinez-Garcia et al., 2011), and changes in deep ocean circulation (Raymo et al., 1997, Peña et Goldstein, 2012, Hasenfratz et al., 2019). For some processes, reconstructions indicate different timings, with mean surface temperature decrease occurring before 1.2 Myr (Snyder 2016) while ocean circulation changes started after 0.95 Myr (Raymo et al., 1997, Peña et Goldstein, 2012, Hasenfratz et al., 2019). Here we present the evolution of CO32- concentration along the MPT, in the Pacific sector of the Southern Ocean, that is linked to carbon accumulation. This record has been reconstructed from benthic foraminifera B/Ca ratio (Yu and Elderfield, 2007) from 1.25 to 0.45 Myr. While the two records from the Atlantic Ocean, North (Sosdian et al. 2018) and South (Farmer et al., 2019), indicate a reduction in CO32-, thus an increase in carbon accumulation, starting with the deep Atlantic Ocean circulation change at ~0.95 Myr (Raymo et al., 1997, Peña et Goldstein, 2012), these new South Pacific data indicate a delayed decrease by ~100kyr in agreement with a Pacific tropical record (Qin et al., 2022) obtained from changes in normalized weights of planktic foraminifera. This different timing, even within the Southern Ocean, questions the role of the balance between the dissolution/accumulation of carbonates between the Atlantic and Pacific basins. Hasenfratz et al.,2019, Science, 363, 1080–1084.Imbrie J. et al., 1993, Paleoceanography, https://doi.org/10.1029/93PA02751Paillard, D., 2017, Clim. Past, 13, 1259-1267.Peña L.D & Goldstein S.L., 2014, Science 345 (6194):318-22.Pisias, N.G. and T.C. Moore, 1981, EPSL, 52, 450-458, https://doi.org/10.1016/0012-821X(81)90197-7Qin, B. et al. (2022). Geophysical Research Letters, 49, e2021GL097121. https://doi. org/10.1029/2021GL097121Raymo, M.E. et al., 1997, Paleoceanography 12, 546–559. 15. 16.Snyder C.W., 2016, Nature, 538,226-228Sosdian, S. et al., (2018), Paleoceanography and Paleoclimatology, 33, 546–562., https://doi.org/10.1029/2017PA003312Willeit et al., 2019, Sci. Adv. 5 : eaav7337.Yu, J., & Elderfield, H. (2007), Earth and Planetary Science Letters, 258(1–2), 73–86. https://doi.org/10.1016/j.epsl.2007.03.025

Antarctic ice core evidence indicates that atmospheric CO2 levels increased during Heinrich Stadial (HS) 1 and the Younger Dryas (YD) during the last deglaciation. A substantial fraction of this carbon is believed to have stemmed from the ocean interior, released, in part, through enhanced wind-driven upwelling and air-sea CO2 exchange in the Southern Ocean. This was highlighted by two deglacial opal flux peaks identified in sediment core TN057-13-PC4 (53.17 °S, 5.13 °E, 2818 m water depth) from the Atlantic Southern Ocean south of the Polar Front, proximal to the Antarctic Divergence Zone (Anderson et al., 2009). However, there is limited information on changes in deep-ocean 14C ventilation and surface ocean hydrography in the Atlantic Antarctic Divergence, and their role in atmospheric CO2 variations during these two periods of deglacial CO2 rise. Here, we provide a new set of 12 mixed-benthic and 63 planktonic foraminiferal (i.e., Neogloboquadrina pachyderma) 14C ages obtained with a MIni-CArbon-DAting-System (MICADAS) in sediment core TN057-13-PC4, along with high-resolution multi-proxy (sub-)sea surface temperature reconstructions for the same site (N. pachyderma Mg/Ca ratios, TEX86, diatom assemblages). Our data help better constrain the nature, timing, and impacts of deep-ocean upwelling on surface ocean hydrography and on atmospheric CO2 exchange near the Antarctic Divergence of the Southern Ocean. Our data show strong (sub-)surface warming in the Antarctic Divergence during HS1 and YD that is accompanied by a rapid decline in benthic-minus-planktic 14C ages towards mean Holocene values at the onset of the deglaciation. We also observe millennial-scale increases in seawater d18O (paired N. pachyderma Mg/Ca-d18O analyses), hence local surface salinity and marked variations in 14C surface ocean reservoir ages that parallel changes in Antarctic sea ice extent. This corroborates previous evidence indicating increased upwelling of Circumpolar Deep Water in the Atlantic Antarctic Divergence during HS1 and YD, yet suggests an onset of strong Southern Ocean ventilation earlier than what is expected from increases in opal fluxes alone. Our data support a fundamental role of upwelling and CO2 outgassing in the Antarctic Divergence of the Southern Ocean in the two-step atmospheric CO2 rise during the last deglaciation, and further suggest that possible variations in CO2 solubility and sea-ice retreat amplified the effects of physical circulation changes on Southern Ocean air-sea CO2 exchange.References: Anderson, R.F., Ali, S., Bradtmiller, L.I., Nielsen, S.H.H., Fleisher, M.Q., Anderson, B., Burckle, L.H., 2009. Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO2. Science 323, 1443–1448. doi: 10.1126/science.116744

This study investigates surface sediments from the seafloor of the Tayrona Basin, an understudied forearc basin in the Colombian Caribbean, offshore La Guajira. We analyzed grain size, mineralogical composition, major and minor elemental concentrations, organic carbon content, C/N ratios, and benthic foraminiferal (BF) assemblages to assess sedimentation processes and environmental conditions. Sediment and bottom-water samples were collected using a box corer and CTD during the Maria S. Merian MSM112 cruise in 2022. Multivariate statistical methods were applied to explore the relationships between environmental and ecological parameters. Despite the relative uniformity of bottom-water conditions, multivariate analyses of BF and sedimentological parameters revealed four distinct zones, each characterized by specific BF assemblages and sediment compositions. The Tayrona Basin exhibits signs of high surface productivity, yet, high oxygen levels in bottom waters promote active organic matter degradation, leading to lower total organic carbon (TOC) content in the sediments. Sedimentation is influenced by a combination of contour currents, terrigenous input from the Magdalena and Rancheria river systems, and autochthonous biogenic deposition. Benthic foraminiferal assemblages are dominated by agglutinated and epifaunal species, indicative of deep-marine environments influenced by bottom currents and organic matter flux. The study highlights the role of bottom-water oxygenation, terrigenous input, and deep-sea currents in shaping the sedimentary record of this forearc basin. This baseline dataset contributes to the understanding of sedimentary and ecological processes in deep-marine forearc settings.

Alongside shifts in Pacific and Southern Ocean carbon cycling, reductions in the extent and formation of North Atlantic Deep Water (NADW) and the expansion of southern sourced waters in the Atlantic Ocean were linked to enhanced marine carbon storage during glacial periods and are considered key mechanisms explaining late Pleistocene atmospheric CO2 variations on glacial–interglacial timescales. However, changes in the formation of deep waters in the Nordic Seas, an important source of NADW, and their influence on the geometry and intensity of Atlantic overturning remain poorly understood, especially beyond the last glacial maximum, leaving possible impacts on atmospheric CO2 changes elusive. Here, we present high-resolution Cibicidoides wuellerstorfi B / Ca-based bottom water [CO32-] reconstructions, alongside complementary C. wuellerstorfi stable oxygen and carbon isotopes and abundance estimates of aragonitic pteropods in marine sediment core PS1243 from the deep Norwegian Sea to investigate past deep-water dynamics in the Nordic Seas and potential impacts on Atlantic overturning and carbon cycling. Our data suggest continuous formation of dense and well-ventilated (high [CO32-]) deep waters throughout Marine Isotope Stages (MIS) 5 and 4, alongside a deepening of the aragonite compensation depth by at least 700 m between MIS 5b and MIS 4 (91–57 ka before present), consistent with sustained Nordic Seas convection. In addition, slightly higher, yet statistically significant, mean bottom water [CO32-] during MIS 5e (sensu stricto, 126–116 ka before present) compared to the Holocene (last 10 ka) highlights the resilience of Nordic Seas overturning towards a warmer North Atlantic, decreased Arctic sea ice extent and meltwater supply from surrounding ice sheets, although centennial-scale perturbations cannot be excluded. A compilation of bottom water [CO32-] records from the Atlantic Ocean indicates that dense waters from the Nordic Seas may have continuously expanded into the intermediate and/or deep (western) North Atlantic via supply of dense-water overflows across the Greenland–Scotland Ridge, diminishing the capacity of the North Atlantic to store carbon during MIS 4. Our study emphasises differences in the sensitivity of North Atlantic and Nordic Seas overturning dynamics to climate boundary conditions of the last glacial cycle that have implications for the carbon storage capacity of the Atlantic Ocean and its role in atmospheric CO2 variations.

The mid-Pleistocene transition (MPT) is arguably the most enigmatic long-term climate shift of the Quaternary and is characterized by increasingly severe glacial conditions about 1.2 to 0.6 million years ago. Although the MPT was suggested to be linked with a continuous lowering of glacial atmospheric CO2 (CO2,atm) levels, the processes underlying this CO2,atm decline are incompletely understood. Here we compare two new benthic foraminiferal (Cibicidoides/Cibicides sp.) δ13C records reflecting Circumpolar Deep Water (CDW), from central South Pacific International Ocean Discovery Program Site U1541 (54.2°S, 125.4°W, 3606 m water depth) and Southeast Atlantic Ocean Drilling Program Site 1094 (53.2°S, 05.1°E, 2807 m water depth), with similar records from the global ocean to identify possible reorganizations in the oceanic respired carbon pool over the past 2 million years that may explain CO2,atm changes across the MPT. We show a good agreement between lower CDW δ13C signatures in the central South Pacific and in the Southeast Atlantic, and a wide-spread glacial decline in CDW δ13C signatures across five Southern Ocean sites during the MPT. This points at a contribution from reduced glacial CDW ventilation and increased glacial respired carbon storage in the Southern Ocean to the glacial CO2,atm decline across the MPT. We also highlight an Atlantic-Pacific Southern Ocean-wide increase in the magnitude of deglacial CDW δ13C shifts during the MPT, which coincides with an amplitude increase in glacial-interglacial Antarctic Circumpolar Current flow strength variations (Lamy et al., 2024). This highlights that not only an increased Southern Ocean respired carbon storage might have driven CO2,atm variations across the MPT but also more efficient outgassing of that carbon during deglacial phases post-MPT. We will address potential linkages of glacial respired carbon storage and deglacial outgassing to changes in Antarctic ice sheet dynamics and southern hemisphere westerlies across the MPT.References:Lamy, F., Winckler, G., Arz, H., Farmer, J., Gottschalk, J., Lembke-Jene, L., Middleton, J.L., et al., 2024. Five million years of Antarctic Circumpolar Current strength variability. Nature 627, 789–796. doi: 10.1038/s41586-024-07143-3

Changes in the formation of North Atlantic Deep Water (NADW) and the expansion of southern-sourced waters in the Atlantic Ocean are linked to enhanced marine carbon storage during glacial and stadial periods, explaining late Pleistocene atmospheric CO2 variations. However, the role of deep-water formation in the Nordic Seas, a key NADW source, and its influence on Atlantic overturning remains unclear, especially after the last glacial maximum. In this study, we present high-resolution reconstructions of bottom water [CO32-] from Cibicidoides wuellerstorfi, along with stable isotopes and aragonitic pteropod abundances in marine sediment core PS1243 from the deep Norwegian Sea, to explore past deep-water dynamics and their impact on carbon cycling. Our data suggest continuous formation of dense, well-ventilated deep waters during Marine Isotope Stages 5 and 4, with a deepening of the aragonite compensation depth during the MIS 5b-to-4 transition. MIS 5e indicates resilience of Nordic Seas overturning in spite of a warmer North Atlantic and suggested summer Arctic sea ice reduction. A compilation of Atlantic [CO32-] records suggests that dense waters from the Nordic Seas expanded into the western North Atlantic, reducing its carbon storage capacity during MIS 4 and stadial MIS 5. Our study highlights differences in the sensitivity of Atlantic and Nordic Seas overturning to past climate conditions, with implications for the Atlantic's role in atmospheric CO2 variations.

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.

In the recent past, the Antarctic ice sheet has experienced significant ice mass loss, which is suggested to be driven primarily by the intrusion of relatively warm deep waters on continental shelves. Given its vast ice shelves and bedrock below sea level, the West Antarctic Ice Sheet has been considered to be strongly sensitive to oceanic forcing and associated heat supply to its margins. Recently, however, also marine-based portions of the East Antarctic Ice Sheet (EAIS) were identified of reacting sensitively to oceanic changes with a direct consequence for rising sea levels as large subglacial areas such as the Aurora or Wilkes Basin hold a sea level equivalent of around 20 meters. So far, past EAIS dynamics and their interaction with ocean dynamics remain poorly understood. Here, we reconstruct past EAIS dynamics from ice-rafted detritus (IRD) counts and estimates of marine productivity in the Indian Southern Ocean. Our opal and carbonate percentages derive from sediment core PS141_49-3 (64° 55.795' S, 106° 51.606' E, 2454 m) retrieved during RV Polarstern Expedition PS141 on the upper East Antarctic continental slope offshore Vanderford Glacier, reaching back to marine isotope stage (MIS) 8, i.e., ~300 ka before present. During glacials, the dominant input of terrigenous sediments suggests a decrease of marine productivity, possibly due to enhanced sea-ice cover extending over the continental slope region. Deglacial phases coincide with high IRD input indicating enhanced iceberg discharge during periods of increased ice mass loss. In contrast, high interglacial opal contents suggest enhanced surface ocean productivity likely associated with a reduced seasonal sea-ice cover. Comparison of our findings with other marine records from offshore Sabrina Coast, Prydz Bay and Wilkes Land reveals consistency of this glacial-interglacial pattern to slope and abyssal sediments around the East Antarctic margin. Our data therefore contributes to an Indian Southern Ocean-wide perspective on terrigenous sediment mobilisation on the slope and EAIS-proximal marine productivity, likely controlled by the grounding line migration across the shelf, sea-ice extent, and oceanic heat supply towards the EAIS margin.

A growing body of observations has revealed rapid changes in both the total inventory and the distribution of marine oxygen over the latter half of the 20th century, leading to increased interest in extending oxygenation records into the past. The use of paleo-oxygen proxies has the potential to extend the spatial and temporal range of current records, constrain pre-anthropogenic baselines, provide datasets necessary to test climate models under different boundary conditions, and ultimately understand how ocean oxygenation responds beyond decadal-scale changes. This review seeks to summarize the current state of knowledge about proxies for reconstructing Cenozoic marine oxygen: sedimentary features, sedimentary redox-sensitive trace elements and isotopes, biomarkers, nitrogen isotopes, foraminiferal trace elements, foraminiferal assemblages, foraminiferal morphometrics, and benthic foraminiferal carbon isotope gradients. Taking stock of each proxy reveals some common limitations as the majority of proxies functions best at low-oxygen concentrations, and many reflect multiple environmental drivers. We also highlight recent breakthroughs in geochemistry and proxy approaches to constraining pelagic (in addition to benthic) oxygenation that are rapidly advancing the field. In light of both the emergence of new proxies and the persistent multiple driver problem, the need for multi-proxy approaches and data storage and sharing that adhere to the principles of findability, accessibility, interoperability, and reusability (FAIR) is emphasized. Continued refinements of proxy approaches and both proxy–proxy and proxy–model comparisons are likely to support the growing needs of both oceanographers and paleoceanographers interested in paleo-oxygenation records.

Understanding the evolution of deep ocean circulation and chemistry over the last glacial cycle is key to elucidating the ocean’s role in modulating atmospheric CO2 changes on millennial and orbital timescales. MIS 4 is a key paleoclimatic interval of the last glacial inception for assessing the role of the deep-ocean carbon storage in driving atmospheric CO2 levels, because it is characterized by a large decrease of air temperature and a rapid atmospheric CO2 drop of ~40 ppmv, and includes several millennial climatic events, for example Heinrich Stadial 6. Although various paleo proxy records suggest a weakened Atlantic overturning during MIS 4, and particularly HS 6, changes in AMOC strength and the geometric extent of NADW shoaling remain poorly understood. Here, we present deep-water temperature reconstructions based on infaunal benthic foraminiferal Mg/Ca ratios and bottom water oxygen concentration reconstructions using redox-sensitive foraminiferal U/Ca, from the deep North (~2.65km) and South (~3.8km) Atlantic to assess the changes in deep water hydrography and by extension circulation.Our reconstructed deep-water temperature changes from the Iberian Margin (~2.65 km water depth) suggest a stronger influence of colder southern sourced waters during MIS 4 and particularly during HS 6; and a clear subsurface warming during MIS 5a stadials. Meanwhile, changes in deep-water temperatures in the Atlantic Sector of the Southern Ocean (SO) closely follow variations in Antarctic temperature, atmospheric CO2 and the mean ocean temperature, likely mediated by buoyancy forcing in the SO, which is in turn likely linked to sea-ice expansion at the MIS 5a/4 transition. Together with (arguably smaller) contributions from reduced air-sea gas exchange efficiency in the SO, these combined changes would have lowered atmospheric CO2through more efficient carbon sequestration in an expanded deep Atlantic reservoir during MIS 4, through their impact on the solubility- and soft tissue “pumps” (i.e. the ocean’s disequilibrium and respired carbon budgets). Indeed, bottom water oxygenation reconstructions from the South Atlantic support the conclusion that the Southern Ocean appears to have represented a significant reservoir for sequestering CO2 away from the atmosphere during MIS 4.

Understanding the evolution of deep ocean circulation and chemistry over the last glacial cycle is key to elucidating the ocean’s role in modulating atmospheric CO2 changes on millennial and orbital timescales. MIS 4 is a key paleoclimatic interval of the last glacial inception for assessing the role of the deep-ocean carbon storage in driving atmospheric CO2 levels, because it is characterized by a large decrease of air temperature and a rapid atmospheric CO2 drop of ~40 ppmv, and includes several millennial climatic events, for example Heinrich Stadial 6. Although various paleo proxy records suggest a weakened Atlantic overturning during MIS 4, and particularly HS 6, changes in AMOC strength and the geometric extent of NADW shoaling remain poorly understood. Here, we present deep-water temperature reconstructions based on infaunal benthic foraminiferal Mg/Ca ratios and bottom water oxygen concentration reconstructions using redox-sensitive foraminiferal U/Ca, from the deep North (~2.65km) and South (~3.8km) Atlantic to assess the changes in deep water hydrography and by extension circulation. Our reconstructed deep-water temperature changes from the Iberian Margin (~2.65 km water depth) suggest a stronger influence of colder southern sourced waters during MIS 4 and particularly during HS 6; and a clear subsurface warming during MIS 5a stadials. Meanwhile, changes in deep-water temperatures in the Atlantic Sector of the Southern Ocean (SO) closely follow variations in Antarctic temperature, atmospheric CO2 and the mean ocean temperature, likely mediated by buoyancy forcing in the SO, which is in turn likely linked to sea-ice expansion at the MIS 5a/4 transition. Together with (arguably smaller) contributions from reduced air-sea gas exchange efficiency in the SO, these combined changes would have lowered atmospheric CO2through more efficient carbon sequestration in an expanded deep Atlantic reservoir during MIS 4, through their impact on the solubility- and soft tissue “pumps” (i.e. the ocean’s disequilibrium and respired carbon budgets). Indeed, bottom water oxygenation reconstructions from the South Atlantic support the conclusion that the Southern Ocean appears to have represented a significant reservoir for sequestering CO2 away from the atmosphere during MIS 4.

Paleoceanographic interpretations of Plio-Pleistocene climate variability over the past 5 million years rely on the evaluation of event timing of proxy changes in sparse records across multiple ocean basins. In turn, orbital-scale chronostratigraphic controls for these records are often built from stratigraphic alignment of benthic foraminiferal stable oxygen isotope (δ18O) records to a preferred dated target stack or composite. This chronostratigraphic age model approach yields age model uncertainties associated with alignment method, target selection, the assumption that the undated record and target experienced synchronous changes in benthic foraminiferal δ18O values, and the assumption that any possible stratigraphic discontinuities within the undated record have been appropriately identified. However, these age model uncertainties and their impact on paleoceanographic interpretations are seldom reported or discussed. Here, we investigate and discuss these uncertainties for conventional manual and automated tuning techniques based on benthic foraminiferal δ18O records and evaluate their impact on sedimentary age models over the past 3.5 Myr using three sedimentary benthic foraminiferal δ18O records as case studies. In one case study, we present a new benthic foraminiferal δ18O record for International Ocean Discovery Program (IODP) Site U1541 (54°13′ S, 125°25′ W), recently recovered from the South Pacific on IODP Expedition 383. The other two case studies examine published benthic foraminiferal δ18O records of Ocean Drilling Program (ODP) Site 1090 and the ODP Site 980/981 composite. Our analysis suggests average age uncertainties of 3 to 5 kyr associated with manually derived versus automated alignment, 1 to 3 kyr associated with automated probabilistic alignment itself, and 2 to 6 kyr associated with the choice of tuning target. Age uncertainties are higher near stratigraphic segment ends and where local benthic foraminiferal δ18O stratigraphy differs from the tuning target. We conclude with recommendations for community best practices for the development and characterization of age uncertainty of sediment core chronostratigraphies based on benthic foraminiferal δ18O records.

The Antarctic Circumpolar Current (ACC) represents the world’s largest ocean-current system and affects global ocean circulation, climate and Antarctic ice-sheet stability 1 – 3 . Today, ACC dynamics are controlled by atmospheric forcing, oceanic density gradients and eddy activity 4 . Whereas palaeoceanographic reconstructions exhibit regional heterogeneity in ACC position and strength over Pleistocene glacial–interglacial cycles 5 – 8 , the long-term evolution of the ACC is poorly known. Here we document changes in ACC strength from sediment cores in the Pacific Southern Ocean. We find no linear long-term trend in ACC flow since 5.3 million years ago (Ma), in contrast to global cooling 9 and increasing global ice volume 10 . Instead, we observe a reversal on a million-year timescale, from increasing ACC strength during Pliocene global cooling to a subsequent decrease with further Early Pleistocene cooling. This shift in the ACC regime coincided with a Southern Ocean reconfiguration that altered the sensitivity of the ACC to atmospheric and oceanic forcings 11 – 13 . We find ACC strength changes to be closely linked to 400,000-year eccentricity cycles, probably originating from modulation of precessional changes in the South Pacific jet stream linked to tropical Pacific temperature variability 14 . A persistent link between weaker ACC flow, equatorward-shifted opal deposition and reduced atmospheric CO 2 during glacial periods first emerged during the Mid-Pleistocene Transition (MPT). The strongest ACC flow occurred during warmer-than-present intervals of the Plio-Pleistocene, providing evidence of potentially increasing ACC flow with future climate warming.

The bottom water conditions in the Central South Pacific (CSP) and associated changes in the Lower Circumpolar Deep Water (LCDW) and Antarctic Bottom Water (AABW) under warmer-than-present conditions need to be better understood. These water masses transfer their properties to the major ocean basins. We analyzed Late Miocene to Early Pliocene (5.6 -3.6 Ma) marine sediment core sections from the CSP for benthic foraminifera, ice rafted debris (IRD), Ostracoda, planktic foraminifera Orbulina universa abundance, and organic geochemical proxies to assess the bottom water characteristics under warmer -than -present day conditions. A significant increase in IRD abundance between 5.3 and 4.9 Ma marks the Early Pliocene warm phase. The benthic foraminiferal assemblages indicate shifts in bottom water conditions over time in the CSP region. Between 5.6 and 5.3 Ma, predominantly oxygenated bottom water with moderate organic matter flux prevailed. This shifted to suboxic conditions with increased organic matter flux from 5.3 to 4.9 Ma. Subsequently, between 4.9 and 4.4 Ma, bottom water conditions alternated frequently between oxic and suboxic states. Enhanced bottom water formation and inflow of LCDW and AABW in the CSP during 4.4-4.0 Ma promoted oxygenated conditions, accompanied by low organic export flux. However, sluggish bottom water circulation from 4.0 to 3.6 Ma reverted to suboxic conditions, associated with increased carbon burial. Notably, productivity peaked intermittently between 5.3 and 3.6 Ma, as indicated by the occurrence of suboxic species assemblages and increase in the abundance of Orbulina universa , benthic microfauna (ostracods), and other paleoproductivity indicators.

The last glacial period was punctuated by cold intervals in the North Atlantic region that culminated in extensive iceberg discharge events. These cold intervals, known as Heinrich Stadials, are associated with abrupt climate shifts worldwide. Here, we present CO 2 measurements from the West Antarctic Ice Sheet Divide ice core across Heinrich Stadials 2 to 5 at decadal-scale resolution. Our results reveal multi-decadal-scale jumps in atmospheric CO 2 concentrations within each Heinrich Stadial. The largest magnitude of change (14.0 ± 0.8 ppm within 55 ± 10 y) occurred during Heinrich Stadial 4. Abrupt rises in atmospheric CO 2 are concurrent with jumps in atmospheric CH 4 and abrupt changes in the water isotopologs in multiple Antarctic ice cores, the latter of which suggest rapid warming of both Antarctica and Southern Ocean vapor source regions. The synchroneity of these rapid shifts points to wind-driven upwelling of relatively warm, carbon-rich waters in the Southern Ocean, likely linked to a poleward intensification of the Southern Hemisphere westerly winds. Using an isotope-enabled atmospheric circulation model, we show that observed changes in Antarctic water isotopologs can be explained by abrupt and widespread Southern Ocean warming. Our work presents evidence for a multi-decadal- to century-scale response of the Southern Ocean to changes in atmospheric circulation, demonstrating the potential for dynamic changes in Southern Ocean biogeochemistry and circulation on human timescales. Furthermore, it suggests that anthropogenic CO 2 uptake in the Southern Ocean may weaken with poleward strengthening westerlies today and into the future.

Abstract. Reductions in the extent and formation of North Atlantic Deep Water (NADW) and the expansion of southern-sourced waters in the Atlantic Ocean were linked to enhanced marine carbon storage during glacial and stadial periods and are considered a key mechanism explaining late Pleistocene atmospheric CO2 variations on glacial-interglacial and millennial timescales. However, changes in the formation of deep waters in the Nordic Seas, an important source of NADW, and their influence on the geometry and intensity of Atlantic overturning remain poorly understood, especially beyond the last glacial maximum, leaving possible impacts on atmospheric CO2 changes elusive. Here, we present high-resolution Cibicidoides wuellerstorfi B/Ca-based bottom water [CO32-] reconstructions, alongside with complementary C. wuellerstorfi stable oxygen and carbon isotopes and abundance estimates of aragonitic pteropods in marine sediment core PS1243 from the deep Norwegian Sea to investigate past deep-water dynamics in the Nordic Seas and potential impacts on Atlantic overturning and carbon cycling. Our data suggest continuous formation of dense and well-ventilated (high-[CO32-]) deep waters throughout Marine Isotope Stages (MIS) 5 and 4, alongside a deepening of the aragonite compensation depth by at least 700 m during the MIS 5b-to-4 transition, consistent with sustained Nordic Seas convection. In addition, higher-than-Holocene bottom water [CO32-] during MIS 5e highlight the resilience of Nordic Seas overturning towards a warmer North Atlantic, decreased Arctic sea ice extent and meltwater supply from surrounding ice sheets. A compilation of bottom water [CO32-] records from the Atlantic Ocean indicates that dense waters from the Nordic Seas may have continuously expanded into the intermediate and/or deep (western) North Atlantic via supply of dense water overflows across the Greenland-Scotland Ridge, diminishing the capacity of the North Atlantic to store carbon during MIS 4 and stadial conditions of MIS 5. Our study emphasises differences in the sensitivity of North Atlantic and Nordic Seas overturning dynamics to climate boundary conditions of the last glacial cycle that have implications for the carbon storage capacity of the Atlantic Ocean and its role in atmospheric CO2 variations.

The high-latitude regions are known for a diverse array of benthic meiofauna, yet our understanding of these communities remains limited, particularly in the deep ocean. This study aims to assess the variability and adaptation of nematodes and agglutinated foraminifera in the modern sediments of the Southern Pacific Ocean at >3500 m water depth. Seawater and sediment slurry from the first piston cores (i.e., mudline samples) from International Ocean Discovery Program Sites U1539, U1540, U1541, and U1543 were analyzed for Rose Bengal stained nematode and agglutinated benthic foraminifera. During the microscopic study seven nematode specimens belonging to the genus Desmoscolex, with 16-17, 36 or 38 main rings were found. Scanning electron microscopy study suggest that one morphotype with 17 main rings used only coccoliths of the species Calcidiscus leptoporus while the other specimens used fine-grained siliciclastic material on their concretion ring. Besides nematodes, a few benthic agglutinated foraminifera specimens exclusively used a single species of planktic foraminifera ( Globoconella inflata) and/or robust coccoliths, in addition to other fine-grained siliciclastic material, for their test construction. These patterns appear to be highly selective. Specimens of the same benthic nematode Desmoscolex genus and agglutinated foraminifera that have diverse grain types show that these specimens can adapt to their environments, choose specific grains as per their preference, and have no relationship with the grain/particle abundance. This study of Desmoscolex and agglutinated foraminifera species suggests low to moderate organic matter flux and increased ventilation in the abyssal depth of the Southern Pacific Ocean.