Search

At present, a strong latitudinal sea surface temperature (SST) gradient of ~16 °C exists across the Southern Ocean, maintained by the Antarctic Circumpolar Current (ACC) and a set of complex frontal systems. Together with the Antarctic ice masses, this system has formed one of the most important global climate regulators. The timing of the onset of the ACC-system, its development towards modern-day strength, and the consequences for e.g., the latitudinal SST gradient around the southern Atlantic Ocean, are still uncertain. Here we present new TEX86-biomarker records, calibrated to SST, from two sites located east of Drake Passage (southern South Atlantic) to assist in better understanding two critical time intervals of prominent climate transitions during the Cenozoic: The Late Eocene–Early Oligocene (ODP Site 696) and Middle–Late Miocene (IODP Site U1536) transitions. Our results overall show rather temperate conditions (20–11 °C) during the Late Eocene to Early Oligocene interval, with a weaker latitudinal SST gradient (~8 °C) across the Atlantic sector of the Southern Ocean compared to present day (~16 °C). We ascribe the regional similarity in SSTs across the Late Eocene–Early Oligocene South Atlantic to a persistent, strong Subpolar Gyre circulation, connecting all sites, which can only exist in absence of a strong throughflow across the Drake Passage. Surprisingly, the southern South Atlantic records show comparable SSTs (~12–14 °C) during both the Earliest Oligocene Oxygen Isotope Step (EOIS, ~33.65 Ma) and the Miocene Climate Optimum (MCO, ~16.5 Ma). Apparently, maximum Oligocene Antarctic ice volume could coexist with warm ice-proximal surface ocean conditions, while at similar ocean temperatures, the Middle Miocene Antarctic ice sheet was strongly reduced. Southern South Atlantic SSTs cooled to ~5 °C at the onset of the Middle Miocene Climate Transition (MMCT, 14 Ma), making it the coldest oceanic region recorded around Antarctica and the likely main location for deep water formation. The already cold southern South Atlantic conditions at MMCT meant it experienced little cooling during the latter part of the Miocene, which contrasts the profound cooling due to northward expansion of the Southern Ocean frontal systems in the lower latitudes and other sectors of the Southern Ocean.

Antarctic sea-ice extent, primary productivity and ocean circulation represent interconnected systems that form important components of the global carbon cycle. Subdecadal to centennial-scale variability can influence the characteristics and interactions of these systems, but observational records are too short to evaluate the impacts of this variability over longer timescales. Here, we use a 170-m-long sediment core collected from Integrated Ocean Drilling Program Site U1357B, offshore Adélie Land, East Antarctica to disentangle the impacts of sea ice and subdecadal climate variability on phytoplankton bloom frequency over the last ~11,400 years. We apply X-ray computed tomography, Ice Proxy for the Southern Ocean with 25 carbon atoms, diatom, physical property and geochemical analyses to the core, which contains an annually resolved, continuously laminated archive of phytoplankton bloom events. Bloom events occurred annually to biennially through most of the Holocene, but became less frequent (~2–7 years) at ~4.5 ka when coastal sea ice intensified. We propose that coastal sea-ice intensification subdued annual sea-ice break-out, causing an increased sensitivity of sea-ice dynamics to subdecadal climate modes, leading to a subdecadal frequency of bloom events. Our data suggest that projected loss of coastal sea ice will impact the influence of subdecadal variability on Antarctic margin primary productivity, altering food webs and carbon-cycling processes at seasonal timescales., This research used samples and data provided by IODP expedition 318, sponsored by the US National Science Foundation (NSF) and participating countries under the management of the Consortium for Ocean Leadership, including the Australian and New Zealand International Ocean Discovery Program Consortium. Funding was provided by Royal Society Te Apārangi Marsden Fund (18-VUW-089 to R.M.M. and 15-VUW-131 to N.A.N.B.) and the New Zealand Ministry of Business, Innovation and Employment through the Antarctic Science Platform (ANTA1801). Funding was also provided by the New Zealand Ministry of Business, Innovation and Employment Strategic Science Investment Fund (SSIF) through GNS Science (grant 540GCT32). We acknowledge funding from the Dumont d’Urville NZ-France Science and Technology Programme, MARICE project (Marine and Ice core reconstruction of East Antarctic sea ice variability over the past 2,000 years) (project nos. 45455NF and 19-VUW-047-DDU Catalyst Fund, RSNZ). J.E. and X.C. acknowledge funding by the ERC StG ICEPROXY (203441), the ANR CLIMICE and the FP7 Past4Future (243908) projects. F.J.J.-E. was funded by project 201830I092 (Spanish Research Council). C.E. and F.J.J.-E acknowledge funding by the Spanish Ministry of Science and Innovation (grant CTM2017-89711-C2-1-P), co-funded by the European Union through FEDER funds. C.R.R. was funded by a University of Otago research grant and a L’Oréal-UNESCO For Women in Science Australia and New Zealand Fellowship. The Natural Environment Research Council funded K.E.A. (CENTA PhD; NE/L002493/1) and J.B. (standard grant Ne/I00646X/1). Y.Y. was funded by the Japan Society for Promotion of Science (JSPS) grant no. JP20H00193. S.F.P. was supported by National Science Foundation grant OPP-0732796. R.B.D. was supported by National Science Foundation grants PLR-1644118 and OCE-1129101. The authors acknowledge the Norwegian Polar Institute’s Quantarctica package, and the use of imagery from the NASA Worldview application (https://worldview.earthdata.nasa.gov/), part of the NASA Earth Observing System Data and Information System.

The reconstruction of past sea-ice distribution in the Southern Ocean is crucial for an improved understanding of ice–ocean–atmosphere feedbacks and the evaluation of Earth system and Antarctic ice sheet models. The Antarctic Peninsula (AP) has been experiencing a warming since the start of regular monitoring of the atmospheric temperature in the 1950s. The associated decrease in sea-ice cover contrasts the trend of growing sea-ice extent in East Antarctica. To reveal the long-term sea-ice history at the northern Antarctic Peninsula (NAP) under changing climate conditions, we examined a marine sediment core from the eastern basin of the Bransfield Strait covering the last Deglacial and the Holocene. For sea-ice reconstructions, we focused on the specific sea-ice biomarker lipid IPSO25, a highly branched isoprenoid (HBI), and sea-ice diatoms, whereas a phytoplankton-derived HBI triene (C25:3) and warmer open-ocean diatom assemblages reflect predominantly ice-free conditions. We further reconstruct ocean temperatures using glycerol dialkyl glycerol tetraethers (GDGTs) and diatom assemblages and compare our sea-ice and temperature records with published marine sediment and ice core data. A maximum ice cover is observed during the Antarctic Cold Reversal 13 800–13 000 years before present (13.8–13 ka), while seasonally ice-free conditions permitting (summer) phytoplankton productivity are reconstructed for the late Deglacial and the Early Holocene from 13 to 8.3 ka. An overall decreasing sea-ice trend throughout the Middle Holocene coincides with summer ocean warming and increasing phytoplankton productivity. The Late Holocene is characterized by highly variable winter sea-ice concentrations and a sustained decline in the duration and/or concentration of spring sea ice. Overall diverging trends in GDGT-based TEX86L and RI-OH' subsurface ocean temperatures (SOTs) are found to be linked to opposing spring and summer insolation trends, respectively.

Reconstructions of ocean primary productivity (PP) help to explain past and present biogeochemical cycles and climate changes in the oceans. We document PP variations over the last 50 kyr in a currently oligotrophic subtropical region, the Gulf of Cadiz. Data combine refined results from previous investigations on dinocyst assemblages, alkenones, and stable isotopes ( 18O, 13C) in planktonic (Globigerina bulloides) and endobenthic (Uvigerina mediterranea) foraminifera from cores MD04-2805 CQ and MD99-2339, with new isotopic measurements on epibenthic (Cibicides pachyderma–Cibicidoides wuellerstorfi) foraminifera and dinocyst-based estimates of PP using the new n = 1,968 modern database. We constrain PP variations and export production by integrating qualitative information from bio-indicators with dinocyst-based quantitative reconstructions such as PP and seasonal sea-surface temperature and information about remineralization from the benthic  13C (difference between epi- and endo-benthic foraminiferal  13C signatures). This study also includes new information on alkenone-based SST and total organic carbon which provides insights into the relationship between past regional hydrological activity and PP regime change. We show that PP, carbon export, and remineralization were generally high in the NE subtropical Atlantic Ocean during the last glacial period and that the Last Glacial Maximum (LGM) had lower  13C than the Heinrich Stadials with sustained high PP, likely allowing enhanced carbon sequestration. We link these PP periods to the dynamics of upwelling, active almost year-round during stadials, but restricted to spring-summer during interstadials and LGM, like today. During interstadials, nutrient advection through freshwater inputs during autumn–winter needs also to be considered to fully understand PP regimes. info:eu-repo/semantics/publishedVersion

Antarctic sea ice plays a critical role in the Earth system, influencing energy, heat and freshwater fluxes, air–sea gas exchange, ice shelf dynamics, ocean circulation, nutrient cycling, marine productivity and global carbon cycling. However, accurate simulation of recent sea-ice changes remains challenging and, therefore, projecting future sea-ice changes and their influence on the global climate system is uncertain. Reconstructing past changes in sea-ice cover can provide additional insights into climate feedbacks within the Earth system at different timescales. This paper is the first of two review papers from the Cycles of Sea Ice Dynamics in the Earth system (C-SIDE) working group. In this first paper, we review marine- and ice core-based sea-ice proxies and reconstructions of sea-ice changes throughout the last glacial–interglacial cycle. Antarctic sea-ice reconstructions rely mainly on diatom fossil assemblages and highly branched isoprenoid (HBI) alkenes in marine sediments, supported by chemical proxies in Antarctic ice cores. Most reconstructions for the Last Glacial Maximum (LGM) suggest that winter sea ice expanded all around Antarctica and covered almost twice its modern surface extent. In contrast, LGM summer sea ice expanded mainly in the regions off the Weddell and Ross seas. The difference between winter and summer sea ice during the LGM led to a larger seasonal cycle than today. More recent efforts have focused on reconstructing Antarctic sea ice during warm periods, such as the Holocene and the Last Interglacial (LIG), which may serve as an analogue for the future. Notwithstanding regional heterogeneities, existing reconstructions suggest that sea-ice cover increased from the warm mid-Holocene to the colder Late Holocene with pervasive decadal- to millennial-scale variability throughout the Holocene. Studies, supported by proxy modelling experiments, suggest that sea-ice cover was halved during the warmer LIG when global average temperatures were ∼2 ∘C above the pre-industrial (PI). There are limited marine (14) and ice core (4) sea-ice proxy records covering the complete 130 000 year (130 ka) last glacial cycle. The glacial–interglacial pattern of sea-ice advance and retreat appears relatively similar in each basin of the Southern Ocean. Rapid retreat of sea ice occurred during Terminations II and I while the expansion of sea ice during the last glaciation appears more gradual especially in ice core data sets. Marine records suggest that the first prominent expansion occurred during Marine Isotope Stage (MIS) 4 and that sea ice reached maximum extent during MIS 2. We, however, note that additional sea-ice records and transient model simulations are required to better identify the underlying drivers and feedbacks of Antarctic sea-ice changes over the last 130 ka. This understanding is critical to improve future predictions.

The Antarctic coastal zone is an area of high primary productivity, particularly within coastal polynyas, where large phytoplankton blooms and drawdown of CO2 occur. Reconstruction of historical primary productivity changes and the associated driving factors could provide baseline insights on the role of these areas as sinks for atmospheric CO2, especially in the context of projected changes in coastal Antarctic sea ice. Here we investigate the potential for using carbon isotopes (δ13C) of fatty acids in marine sediments as a proxy for primary productivity. We use a highly resolved sediment core from off the coast of Adélie Land spanning the last ∼ 400 years and monitor changes in the concentrations and δ13C of fatty acids along with other proxy data from the same core. We discuss the different possible drivers of their variability and argue that C24 fatty acid δ13C predominantly reflects phytoplankton productivity in open-water environments, while C18 fatty acid δ13C reflects productivity in the marginal ice zone. These new proxies have implications for better understanding carbon cycle dynamics in the Antarctica coastal zone in future palaeoclimate studies.

The scarcity of paleo-records from the Antarctic Peninsular region of the Southern Ocean hinders our understanding of the timing of the opening of Drake Passage, specifically in the region of the South Orkney Microcontinent (SOM) and Powell Basin, between the Scotia and Antarctic plates. At Ocean Drilling Program (ODP) Hole 696B, SOM sediments recovered from the upper Eocene-lower Oligocene (~37.6–32.2 Ma) enable us to gain insight into paleoceanographic and paleoclimatic changes during gateway opening across the major Cenozoic climate shift —the Eocene-Oligocene transition— when the Antarctic ice sheet first reached sea-level. We propose the following sequence of events, based on a multi-proxy analysis of sediment facies, mineralogy, and organic matter geochemistry (TOC, TN, C/N ratio, δC, δN, and n-alkanes). During the late Eocene (~37.6–35.5 Ma) the SOM was attached to the Antarctic Peninsula, and terrigenous sediments of likely local origin were deposited in shallow waters under conditions of reduced‑oxygen/low-salinity, and temperate climate. In the latest Eocene (~35.5–34.1 Ma), terrigenous input was reduced due to the separation of the SOM from the Antarctic Peninsula by proto-Powell Basin opening. Decreased sediment supply during continuous deepening of the SOM led to deposition of a condensed section with significant glauconitization, recurrent winnowing by bottom currents, and suboxic conditions near the sediment-water interface. At the time of the Eocene-Oligocene transition (EOT; ~34.1–33.6 Ma) two upward-coarsening sediment sections were deposited within an overall upward-fining section, which we interpret as records of regressive phases due to ice sheet expansion. During the early Oligocene (~33.6–33.2 Ma) the SOM deepened further because of continued opening of Powell Basin, and organic-rich sediments were deposited as a result of enhanced biological production, partially driven by enhanced upwelling. Major cooling caused a change from forests indicating relatively humid temperate conditions in the late Eocene, to forests indicative of dry and cool conditions in the Oligocene, as shown by biomarker records, which also indicate weakening contributions of terrestrial organic matter to the marine sedimentary record. We thus conclude that a shallow gateway, the proto-Powell Basin, formed at ~35.5 Ma, as seen in the decreased delivery of proximal, coarse terrigenous sediments to the SOM margin; this was followed by a long-term deepening trend, interrupted by EOT regression due to continent-wide ice sheet build-up. Opening of the proto-Powell Basin could have provided a shallow-water pathway for water flowing from the Drake Passage-Scotia Sea towards the northern Weddell Sea, enhancing upwelling at the southern SOM shelf margin. Hence, increased marine productivity across the EOT might have resulted from the combined effects of the opening of Powell Basin and climate cooling., This research relied on samples provided by the International Ocean Discovery Program (IODP). We thank the staff at the Gulf Coast core repository (GCR) for assistance in ODP Leg 113 core handling and shipping. We also acknowledge the help of Dr. Rocío Marquez ´ Crespo (Scientific Instrumentation Center, University of Granada) for her assistance using the ESEM. Thanks are given to Dr. Claus-Dieter Hillenbrand (British Antarctic Survey) for critical reading of an earlier version of this paper, and to Carlos Exposito-Ceballos ´ (University of Granada) for helpful discussions about benthic foraminifera preservation at Hole 696B. We also appreciate the constructive comments provided by Dr. Ellen Thomas and Dr. Sandra Passchier that greatly improved the manuscript. We thank Jean Sanders for correction of the English text. Funding for this research was provided by the Spanish Ministry of Science and Innovation (grants CTM2014-60451-C2-1/2-P and CTM2017-89711-C2-1/2-P) cofounded by the European Union through FEDER funds. RT acknowledges funding provided by Project PID2019-104625RB-100 (Secretaría de Estado de I + D + I, Spain), BRNM-072-UGR18 (FEDER Andalucía), and P18-RT-4074 (Junta de Andalucía). NT received funding from the Natural Environment Research Council (NERC)-funded Doctoral Training Partnership ONE Planet [NE/S007512/1]. The European Research Council under the European Community's Seventh Framework Program provided funding for this work through ERC Starting Grant #802835 (OceaNice) to PKB. This paper is a contribution to the SCAR PAIS Programme.

This study presents new high resolution sedimentary δN records from piston cores collected within and outside the present-day eastern south Pacific oxygen minimum zone along a latitudinal transect from 3.5°S to 15°S. Radiocarbon dating of foraminifera and organic matter show that the cores cover the Holocene and the last deglaciation with high sedimentation rate allowing interpretations at millennial to centennial timescale. High δN values, reaching 10‰ and large amplitude changes, with a magnitude of ~4‰ are observed in the southern part of the studied area during the last 18 ka BP. In contrast, the northern Peruvian cores located on the edge of the OMZ show low δN values varying from 4 to 6‰ with amplitude of only 1‰ during the same time period. δN values decrease in all the studied cores from the last deglaciation to the early Holocene (17 to 8.5 ka BP) and reach a minimum value during the mid-Holocene. The δN variations are attributed to microbial N-loss to N, e.g. denitrification and/or anammox, and the characteristic N-enriched signal that is recorded in the underlying sediments under suboxic to anoxic conditions where denitrifiers thrive. Surprisingly, δN values from cores located within the OMZ show similar values as the more northern cores located outside the OMZ between 5 and 8.5 ka BP. This minimum is not related to local changes in export production, reconstructed from sedimentary organic carbon, total nitrogen and bromine, but appears to be controlled by changes in the ventilation of the area. The low δN values recorded between 8.5 and 5 ka BP are well correlated with more arid conditions developed along the Peruvian margin and an increase of the sea surface temperature gradient along the Peruvian margin and between the West and East Pacific along the equator, implying an intensification of the Hadley circulation and climatic conditions similar to La Niña-like state. Consequently, these mid-Holocene conditions led to greater ventilation of subsurface waters that deepened the Peruvian oxycline then revealing similar conditions as observed today in the northern part of the study area., This work is a contribution of Sonderforschungsbereich 754 “Climate-Biogeochemistry interactions in the tropical ocean” (www.sfb754.de), which is supported by the Deutsche Forschungsgemeinschaft. Financial support was partly provided by French research program LEFE-EVE (INSU, CNRS) to P.M. We thank the anonymous reviewers for comments and suggestions on earlier version of this manuscript.

The Antarctic coastal zone is an area of high primary productivity, particularly within coastal polynyas where large phytoplankton blooms and drawdown of CO2 occur. Reconstruction of historical primary productivity changes, and the associated driving factors, could provide baseline insights on the role of these areas as sinks for atmospheric CO2, especially in the context of projected changes in coastal Antarctic sea ice. Here we investigate the potential for using carbon isotopes (δ13C) of fatty acids in marine sediments as a proxy for primary productivity. We use a highly resolved sediment core from off the coast of Adélie Land spanning the last ~ 400 years and monitor changes in the concentrations and δ13C of fatty acids along with other proxy data from the same core. We discuss the different possible drivers of their variability and argue that C24 fatty acid δ13C predominantly reflects phytoplankton productivity in open water environments, while C18 fatty acid δ13C reflects productivity in the marginal ice zone. These new proxies have implications for better understanding carbon cycle dynamics in the Antarctica coastal zone in future paleoclimate studies.

International audience; Dramatic changes in sea ice have been observed in both poles in recent decades. However, the observational period for sea ice is short, and the climate models tasked with predicting future change in sea ice struggle to capture the current Antarctic trends. Paleoclimate archives, from marine sedimentary records and coastal Antarctic ice cores, provide a means of understanding sea ice variability and its drivers over decadal to centennial timescales. In this study, we collate published records of Antarctic sea ice over the past 2000 years (2 ka). We evaluate the current proxies and explore the potential of combining marine and ice core records to produce multi-archive reconstructions. Despite identifying 92 sea ice reconstructions, the spatial and temporal resolution is only sufficient to reconstruct circum-Antarctic sea ice during the 20th century, not the full 2 ka. Our synthesis reveals a 90 year trend of increasing sea ice in the Ross Sea and declining sea ice in the Bellingshausen, comparable with observed trends since 1979. Reconstructions in the Weddell Sea, the Western Pacific and the Indian Ocean reveal small negative trends in sea ice during the 20th century (1900-1990), in contrast to the observed sea ice expansion in these regions since 1979.

Recent paleoclimatic studies suggest that changes in the tropical rainbelt across the Atlantic Ocean during the past two millennia are linked to a latitudinal shift of the Intertropical Convergence Zone (ITCZ) driven by the Northern Hemisphere (NH) climate. However, little is known regarding other potential drivers that can affect tropical Atlantic rainfall, mainly due to the scarcity of adequate and high-resolution records. In this study, we fill this gap by reconstructing precipitation changes in Northeastern Brazil during the last 2,300 years from a high-resolution lake record of hydrogen isotope compositions of plant waxes. We find that regional precipitation along the coastal area of South America was not solely governed by north-south displacements of the ITCZ due to changes in NH climate, but also by the contraction and expansion of the tropical rainbelt due to variations in sea surface temperature and southeast trade winds in the tropical South Atlantic Basin., We thank Fanny Kaczmar (LOCEAN-UPMC) and Ralph Kreutz (MARUM) for their support during the analyses, Igor M. Venancio (INPE) for comments about this paper. The lipid extractions were supported by LOCEAN/IRD. The isotope analyses were supported by the DFG-Research Center/Cluster of Excellence, “The Ocean in the Earth System” at MARUM - Center for Marine Environmental Sciences. This work was supported by the Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP), Brazil through NSF Dimensions-Biota FAPESP grant 2013/50297 and PIRE NSF-FAPESP Grants 2017/50085-3 to F.W.C as well as the post-doc fellowship to V.F.N (2016/15807-5) and the US National Science Foundation NSF grants AGS-1303828 and OISE-1743738 to MV and 1103403 to R.L.E and H.C. We thank Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Brazil (CAPES) fellowship to G.U. Grant 99999.012754/2013-07 as well as the post-doc fellowship through the Postgraduate Program on Geochemistry and Geotectonics, Geoscience Institute of the University of São Paulo.

The emergence of high‐amplitude, low‐frequency glacial‐interglacial cycles during the mid‐Pleistocene climate transition (MPT; 800–1,200 ka) is associated with global cooling. In the eastern equatorial Pacific, sea surface temperatures cooled, and the upwelling‐induced cold tongue expanded significantlyduringtheMPT.Hereweusesedimentaryrecordsofiron,biogenicsilica,andnutrient‐nitrogen consumption to evaluate biogeochemical changes hypothesized to accompany the cold tongue expansion. Our results suggest that the eastern equatorial Pacific of the MPT hosted surface waters with higher nitrate contents and biogenic silica production relative to the last 600 ka. Increased production occurred despite low iron supply. We attribute this to enhanced upwelling and nutrient enrichment of thermocline waters, both likely related to the northward migration of Southern Ocean fronts. The return of these fronts to their southward positions after the MPT may be associated with stronger drawdown of nutrients and, potentially, atmospheric CO2 in the Southern Ocean., This work was supported in part by NSF‐MGG (1060779) and USSSAC postcruisesupportforLeg202toR.S.R. We thank P. Rumford and the IODP Core repository for providing samples and assistance with XRF core scanning; K. Kelley for analytical assistance; and Z. Kerrigan, J. Freedman, and A. Stahl for the help with sample preparation. New data will be archived at the NCEI Paleoceanography Data Repository., National Science Foundation (US)

In this study, we present a unique high-resolution Holocene record of oceanographic and climatic change based on analyses of diatom assemblages combined with biomarker data from a sediment core collected from the Vega Drift, eastern Antarctic Peninsula (EAP). These data add to the climate framework already established by high-resolution marine sedimentary records from the Palmer Deep, western Antarctic Peninsula (WAP). Heavy sea ice conditions and reduced primary productivity were observed prior to 7.4 ka B.P. in relation with the proximity of the glacial ice melt and calving. Subsequent Holocene oceanographic conditions were controlled by the interactions between the Westerlies-Antarctic Circumpolar Current (ACC)-Weddell Gyre dynamics. A warm period characterized by short seasonal sea ice duration associated with a southern shift of both ACC and Westerlies field persisted until 5 ka B.P. This warm episode was then followed by climate deterioration during the middle-to-late Holocene (5 to 1.9 ka B.P.) with a gradual increase in annual sea ice duration triggered by the expansion of the Weddell Gyre and a strong oceanic connection from the EAP to the WAP. Increase of benthic diatom species during this period was indicative of more summer/autumn storms, which was consistent with changes in synoptic atmospheric circulation and the establishment of low- to high-latitude teleconnections. Finally, the multicentennial scale variability of the Weddell Gyre intensity and storm frequency during the late Holocene appeared to be associated with the increased El Nino–Southern Oscillation frequency.

International audience; Ocean-driven basal melting has been shown to be the main ablation process responsible for the recession of many Antarctic ice shelves and marine-terminating glaciers over the last decades. However, much less is known about the drivers of ice shelf melt prior to the short instrumental era. Based on diatom oxygen isotope (δ18Odiatom; a proxy for glacial ice discharge in solid or liquid form) records from western Antarctic Peninsula (West Antarctica) and Adélie Land (East Antarctica), higher ocean temperatures were suggested to have been the main driver of enhanced ice melt during the Early-to-Mid Holocene while atmosphere temperatures were proposed to have been the main driver during the Late Holocene. Here, we present a new Holocene δ18Odiatom record from Prydz Bay, East Antarctica, also suggesting an increase in glacial ice discharge since ~4500 years before present (~4.5 kyr BP) as previously observed in Antarctic Peninsula and Adélie Land. Similar results from three different regions around Antarctica thus suggest common driving mechanisms. Combining marine and ice core records along with new transient accelerated simulations from the IPSL-CM5A-LR climate model, we rule out changes in air temperatures during the last ~4.5 kyr as the main driver of enhanced glacial ice discharge. Conversely, our simulations evidence the potential for significant warmer subsurface waters in the Southern Ocean during the last 6 kyr in response to enhanced summer insolation south of 60°S and enhanced upwelling of Circumpolar Deep Water towards the Antarctic shelf. We conclude that ice front and basal melting may have played a dominant role in glacial discharge during the Late Holocene.

As compared to the late Pleistocene, Alkenone-based sea surface temperature (SST) in the Benguela region revealed relatively warm and stable SST recorded between approximate to 3.5 and 2.0 Ma, and coincide with a period of increasing biological productivity as revealed by increasing deposition of biogenic opal. We assess how the hydrological patterns recorded in SST proxies are embedded in the geological record by performing a proxy-proxy comparison. We used Laser-Ablation Inductively Coupled Plasma-Mass Spectrometry to measure the Mg/Ca on the planktonic foraminifera species Globigerina bulloides, allowing in situ measurements of Mg/Ca on individual foraminiferal tests. Mg/Ca-derived temperatures provide much colder temperatures than alkenone-derived SST by up to 10 degrees C. We build a scenario involving contrasting sensitivities of paleothermometers upon the annual cycle, namely alkenones preferentially capturing SST when the surface ocean is warmer than the mean-annual average SST, and G. bulloides capturing SST when upwelling intensifies. Multichamber analysis also suggests that G. bulloides migrates below the sea surface while calcifying its last chambers prior to gametogenesis, allowing the extraction of both surface and subsurface temperature from Mg/Ca measured on different chambers. The range of temperatures recorded between our multiple SST proxies is supported by the range of temperatures simulated with a general circulation model when different seasons, different water depth and different orbital configurations occurring during the late Pliocene are considered. A greater seasonal cycle in SST during the Pliocene can account for alkenone and Mg/Ca-derived temperature contrast, pointing to a radically different mode of upwelling activity in the Benguela region compared to today.

In this study we present combined high-resolution records of sea surface temperature (SST), phytoplankton productivity, and nutrient cycling in the Benguela Upwelling System (BUS) for the past 3.5 Ma. The SST record provided evidence that upwelling activity off Namibia mainly intensified ca.2.4-2.0 Ma ago in response to the cooling of the Southern Ocean and the resultant strengthening of trade winds. As revealed by productivity-related proxies, BUS intensification led to a major transition in regional biological productivity when considering the termination of the Matuyama Diatom Maximum (a diatom high-production event). Major oceanic reorganization in the Benguela was accompanied by nutrient source changes, as indicated by a new nitrogen isotopic (delta N-15) record that revealed a stepwise increase at ca. 2.4 and ca. 1.5 Ma ago. The change in source region likely resulted from significant changes in intermediate water formation tied to the reorganization of oceanic conditions in the Southern Ocean, which may have in turn mainly controlled the global ocean N cycle, and therefore the N isotopic composition of nutrients since 3.5 Ma ago.

International audience; The largest increase in export production in the eastern Pacific of the last 5.3 Myr (million years) occurred between 2.2 and 1.6 Myr, a time of major climatic and oceanographic reorganization in the region. Here, we investigate the causes of this event using reconstructions of export production, nutrient supply and oceanic conditions across the Pliocene-Pleistocene in the eastern equatorial Pacific (EEP) for the last 3.2 Myr. Our results indicate that the export production peak corresponds to a cold interval marked by high nutrient supply relative to consumption, as revealed by the low bulk sedimentary 15N/14N (δ15N) and alkenone-derived sea surface temperature (SST) values. This ∼0.6 million year long episode of enhanced delivery of nutrients to the surface of the EEP was predominantly initiated through the upwelling of nutrient-enriched water sourced in high latitudes. In addition, this phenomenon was likely promoted by the regional intensification of upwelling in response to the development of intense Walker and Hadley atmospheric circulations. Increased nutrient consumption in the polar oceans and enhanced denitrification in the equatorial regions restrained nutrient supply and availability and terminated the high export production event.

The West Antarctic ice sheet is particularly sensitive to global warming and its evolution and impact on global climate over the next few decades remains difficult to predict. In this context, investigating past sea ice conditions around Antarctica is of primary importance. Here, we document changes in sea ice presence, upper water column temperatures (0–200 m) and primary productivity over the last 9000 yr BP (before present) in the western Antarctic Peninsula (WAP) margin from a sedimentary core collected in the Palmer Deep basin. Employing a multi-proxy approach, we derived new Holocene records of sea ice conditions and upper water column temperatures, based on the combination of two biomarkers proxies (highly branched isoprenoid (HBI) alkenes for sea ice and TEXL86 for temperature) and micropaleontological data (diatom assemblages). The early Holocene (9000–7000 yr BP) was characterized by a cooling phase with a short sea ice season. During the mid-Holocene (~ 7000–3000 yr BP), local climate evolved towards slightly colder conditions and a prominent extension of the sea ice season occurred, promoting a favorable environment for intensive diatom growth. The late Holocene (the last ~ 3000 yr) was characterized by more variable temperatures and increased sea ice presence, accompanied by reduced local primary productivity likely in response to a shorter growing season compared to the early or mid-Holocene. The stepwise increase in annual sea ice duration over the last 7000 yr might have been influenced by decreasing mean annual and spring insolation despite an increasing summer insolation. We postulate that in addition to precessional changes in insolation, seasonal variability, via changes in the strength of the circumpolar Westerlies and upwelling activity, was further amplified by the increasing frequency/amplitude of El Niño-Southern Oscillation (ENSO). However, between 4000 and 2100 yr BP, the lack of correlation between ENSO and climate variability in the WAP suggests that other climatic factors might have been more important in controlling WAP climate at this time.

The West Antarctic ice sheet is particularly sensitive to global warming and its evolution and impact on global climate over the next few decades remains difficult to predict. In this context, investigating past sea ice conditions around Antarctica is of primary importance. Here, we document changes in sea ice presence, upper water column temperatures (0–200 m) and primary productivity over the last 9000 yr BP (before present) in the western Antarctic Peninsula (WAP) margin from a sedimentary core collected in the Palmer Deep Basin. Employing a multi-proxy approach, based on the combination of two biomarkers proxies (highly branched isoprenoid (HBI) alkenes for sea ice and TEX86L for temperature) and micropaleontological data (diatom assemblages), we derived new Holocene records of sea ice conditions and upper water column temperatures. The early Holocene (9000–7000 yr BP) was characterized by a cooling phase with a short sea ice season. During the mid-Holocene (~7000–3800 yr BP), local climate evolved towards slightly colder conditions and a prominent extension of the sea ice season occurred, promoting a favorable environment for intensive diatom growth. The late Holocene (the last ~2100 yr) was characterized by warmer temperatures and increased sea ice presence, accompanied by reduced local primary productivity, likely in response to a shorter growing season compared to the early or mid-Holocene. The gradual increase in annual sea ice duration over the last 7000 yr might have been influenced by decreasing mean annual and spring insolation, despite increasing summer insolation. We postulate that, in addition to precessional changes in insolation, seasonal variability, via changes in the strength of the circumpolar Westerlies and upwelling activity, was further amplified by the increasing frequency/amplitude of the El Niño–Southern Oscillation (ENSO). However, between 3800 and 2100 yr BP, the lack of correlation between ENSO and climate variability in the WAP suggests that other climatic factors might have been more important in controlling WAP climate at this time.

The global Late Pliocene/Early Pleistocene cooling (~3.0–2.0 million years ago – Ma) concurred with extremely high diatom and biogenic opal production in most of the major coastal upwelling regions. This phenomenon was particularly pronounced in the Benguela upwelling system (BUS), off Namibia, where it is known as the Matuyama Diatom Maximum (MDM). Our study focuses on a new diatom silicon isotope (δ30Si) record covering the MDM in the BUS. Unexpectedly, the variations in δ30Si signal follow biogenic opal content, whereby the highest δ30Si values correspond to the highest biogenic opal content. We interpret the higher δ30Si values during the MDM as a result of a stronger degree of silicate utilisation in the surface waters caused by high productivity of mat-forming diatom species. This was most likely promoted by weak upwelling intensity dominating the BUS during the Late Pliocene/Early Pleistocene cooling combined with a large silicate supply derived from a strong Southern Ocean nutrient leakage responding to the expansion of Antarctic ice cover and the resulting stratification of the polar ocean 3.0–2.7 Ma ago. A similar scenario is hypothesized for other major coastal upwelling systems (e.g. off California) during this time interval, suggesting that the efficiency of the biological carbon pump was probably sufficiently enhanced in these regions during the MDM to have significantly increased the transport of atmospheric CO2 to the deep ocean. In addition, the coeval extension of the area of surface water stratification in both the Southern Ocean and the North Pacific, which decreased CO2 release to the atmosphere, led to further enhanced atmospheric CO2 drawn-down and thus contributed significantly to Late Pliocene/Early Pleistocene cooling.

Most high-southern latitude records of carbon and nitrogen isotopic ratios and elemental ratios are from the Antarctic and Polar Front zones, thus hindering a comprehensive view of nutrient cycling in the Southern Ocean. We present, for the first time, two records from the Subantarctic Zone and the southern Subtropical Zone of the Indian Ocean. These records provide a latitudinal transect covering the main oceanographic systems of the Southern Ocean. Carbon and nitrogen content of the diatom-bound organic matter increases during the last glacial in the Antarctic and Subantarctic zones but does not show a clear climate-related signal in the Subtropical Zone. Comparison of these records with sea-surface temperatures reconstructed at the core locations and record of dust deposition over Antarctica suggests that eolian iron input possibly switched diatom physiology toward higher carbon to silica and nitrogen to silica uptake and storage south of the Subantarctic Front due to the dependency of photosynthesis on iron concentration levels. Conversely, the northernmost core was too remote from any iron source over the last 50,000 years. Antarctic diatoms therefore have the sole potential to change the nutrient content of the waters escaping from the Southern Ocean, hence providing partial support to the silicic leakage hypothesis as a potential cause of lower glacial atmospheric CO 2 .

Reconstructions of ocean primary productivity (PP) help to explain past and present biogeochemical cycles and climate changes in the oceans. We document PP variations over the last 50 kyr in a currently oligotrophic subtropical region, the Gulf of Cadiz. Data combine refined results from previousinvestigations on dinocyst assemblages, alkenones, and stable isotopes (18O, 13C) in planktonic (Globigerina bulloides) and endobenthic (Uvigerina mediterranea) foraminifera from cores MD04-2805 CQ and MD99-2339, with new isotopic measurements on epibenthic (Cibicidoides species) foraminifera and dinocyst-based estimates of PP using the new n = 1,968 modern database. We constrain PP variations and export production by integrating qualitative information from micropaleontological bio-indicators with quantitative reconstructions of parameters such as dinocyst-based PP and seasonal sea-surface temperature and information about remineralization from the benthic Dd13C (i.e. difference between epi- and endo-benthic foraminiferal d13C signatures). We show that PP, carbon export, and remineralization were generally high in the NE subtropical Atlantic Ocean during the last glacial period and that the Last Glacial Maximum (LGM) had lower Dd13C than the Heinrich Stadials with sustained high PP, likely allowing enhanced carbon sequestration. This study also provides information on the dynamics of PP regime changes, as the dataset includes alkenone-based SST and total organic carbon. We link these PP periods to the dynamics of the GoC upwelling, active almost year-round during stadials, but restricted to spring–summer during interstadials and LGM, like today. During interstadials, nutrient advection through freshwater inputs during autumn–winter rains needs also to be considered to fully understand PP regimes.