Margaux Brandon
email: margaux.brandon@locean.ipsl.fr

Stéphanie Duchamp-Alphonse

Elisabeth Michel

Amaelle Landais

Gulay Isguder

Patricia Richard

Nicolas Pige

Franck Bassinot

Samuel L Jaccard

Annachiara Bartolini

Brandon Margaux

Duchamp-Alphonse Stéphanie

Michel Elisabeth

Landais Amaëlle

Isguder Gulay

Richard Patricia

Pige Nicolas

Bassinot Franck

Jaccard Samuel

Bartolini Annachiara

Enhanced Carbonate Counter Pump and upwelling strengths in the Indian sector of the Southern Ocean during MIS 11

Keywords: Southern Ocean, Marine Isotope Stage 11, Coccolith, Planktonic foraminifera, Geochemistry, Carbonate Counter Pump, Upwelling, Atmospheric CO2 concentration

établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The Earth's climate system of the past 800 000 years (800 ka) has been characterised by glacial-interglacial cycles that are tightly coupled to pCO2 changes [START_REF] Lüthi | High-resolution carbon dioxide concentration record 650,000-800,000 years before present[END_REF][START_REF] Bereiter | Revision of the EPICA Dome C CO2 record from 800 to 600 kyr before present[END_REF]. Causes underlying these changes, especially in conjunction with deglaciations (glacial Terminations) that registered pCO2 rises of about 50-100 ppm, are still under debate. It is recognised that complex, synergistic interactions involving changes in sea-ice cover, surface water stratification and marine productivity in the SO together with the dynamics of the westerly winds may have played a major role in modulating the efficiency of the physical and biological carbon pumps [START_REF] François | Contribution of Southern Ocean surface-water stratification to low atmospheric CO2 concentrations during the last glacial period[END_REF][START_REF] Stephens | The influence of Antarctic sea ice on glacial-interglacial CO2 variations[END_REF][START_REF] Sigman | Glacial/interglacial variations in atmospheric carbon dioxide[END_REF][START_REF] Hodell | Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean[END_REF][START_REF] Toggweiler | Midlatitude westerlies, atmospheric CO2 , and climate change during the ice ages[END_REF][START_REF] Jaccard | Two Modes of Change in Southern Ocean Productivity Over the Past Million Years[END_REF][START_REF] Ferrari | Antarctic sea ice control on ocean circulation in present and glacial climates[END_REF][START_REF] Abelmann | The seasonal sea-ice zone in the glacial Southern Ocean as a carbon sink[END_REF][START_REF] Galbraith | Deglacial weakening of the oceanic soft tissue pump: global constraints from sedimentary nitrogen isotopes and oxygenation proxies[END_REF][START_REF] Menviel | Southern Hemisphere westerlies as a driver of the early deglacial atmospheric CO2 rise[END_REF][START_REF] Gottschalk | Mechanisms of millennial-scale atmospheric CO2 change in numerical model simulations[END_REF][START_REF] Stein | Timing and magnitude of Southern Ocean sea ice/carbon cycle feedbacks[END_REF][START_REF] Sigman | The Southern Ocean during the ice ages: A review of the Antarctic surface isolation hypothesis, with comparison to the North Pacific[END_REF]. The reinvigoration of SO upwelling has been held responsible for a substantial part of the abrupt pCO2 rise observed during Terminations [START_REF] Hodell | Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean[END_REF][START_REF] Anderson | Wind-Driven Upwelling in the Southern Ocean and the Deglacial Rise in Atmospheric CO2[END_REF][START_REF] Skinner | Ventilation of the Deep Southern Ocean and Deglacial CO2 Rise[END_REF][START_REF] Siani | Carbon isotope records reveal precise timing of enhanced Southern Ocean upwelling during the last deglaciation[END_REF][START_REF] Jaccard | Covariation of deep Southern Ocean oxygenation and atmospheric CO2 through the last ice age[END_REF][START_REF] Rae | CO2 storage and release in the deep Southern Ocean on millennial to centennial timescales[END_REF][START_REF] Gottschalk | Glacial heterogeneity in Southern Ocean carbon storage abated by fast South Indian deglacial carbon release[END_REF][START_REF] Sigman | The Southern Ocean during the ice ages: A review of the Antarctic surface isolation hypothesis, with comparison to the North Pacific[END_REF] while an increase in the efficiency of the Biological Carbon Pump (BCP) might have accounted for up to 40-50 ppmv CO2 drawdown during glacial maxima [START_REF] Hain | Carbon dioxide effects of Antarctic stratification, North Atlantic Intermediate Water formation, and subantarctic nutrient drawdown during the last ice age: Diagnosis and synthesis in a geochemical box model[END_REF]Martinez-Garcia et al., 2009;[START_REF] Jaccard | Two Modes of Change in Southern Ocean Productivity Over the Past Million Years[END_REF]. However, due to a lack of high-resolution primary productivity reconstructions and, more generally, to the scarcity of multiproxy approaches [START_REF] Duncan | Interglacial/glacial changes in coccolith-rich deposition in the SW Pacific Ocean: An analogue for a warmer world?[END_REF][START_REF] Duchamp-Alphonse | Enhanced ocean-atmosphere carbon partitioning via the carbonate counter pump during the last deglacial[END_REF], it remains difficult to apprehend the spatio-temporal patterns underpinning changes in the BCP and compare them to the SO upwelling dynamics to understand their impact on pCO2. Furthermore, most of the studies aiming to reconstruct changes in the strength and efficiency of the BCP focus on the export of phytoplankton-derived organic carbon to the deep ocean (i.e., the Soft Tissue Pump (STP)) that directly lowers atmospheric CO2 [START_REF] Martin | Glacial-interglacial CO2 change: The iron hypothesis[END_REF]Martinez-Garcia et al., 2009;[START_REF] Martinez-Garcia | Iron Fertilization of the Subantarctic Ocean During the Last Ice Age[END_REF][START_REF] Thöle | Glacial-interglacial dust and export production records from the Southern Indian Ocean[END_REF]. They typically overlook the export of phytoplankton-and zooplankton-derived calcium carbonate (CaCO3), which raises surface water CO2 (i.e., the CCP) [START_REF] Baumann | Coccolith contribution to South Atlantic carbonate sedimentation[END_REF][START_REF] Barker | Globally increased pelagic carbonate production during the Mid-Brunhes dissolution interval and the CO2 paradox of MIS 11[END_REF][START_REF] Broecker | Ratio of coccolith CaCO3 to foraminifera CaCO3 in late Holocene deep sea sediments[END_REF][START_REF] Salter | Carbonate counter pump stimulated by natural iron fertilization in the Polar Frontal Zone[END_REF][START_REF] Rembauville | Planktic foraminifer and coccolith contribution to carbonate export fluxes over the central Kerguelen Plateau[END_REF], and contributes to the CaCO3/organic carbon ratio in the biogenic particle flux to the sea-floor (namely the rain ratio), especially in the SO [START_REF] Duchamp-Alphonse | Enhanced ocean-atmosphere carbon partitioning via the carbonate counter pump during the last deglacial[END_REF][START_REF] Krumhardt | Southern Ocean Calcification Controls the Global Distribution of Alkalinity[END_REF]. This omission may relate to the fact that calcareous phytoplankton is in part represented by coccoliths, which detailed analysis requires sophisticated tools such as SYRACO (SYstème de Reconnaissance Automatique des Coccolithes), a software using automated optical microscope that allows the robust determination of coccolith mass and thus, coccolithophore calcification rates, based on the optical properties (birefringence) of calcite under cross-polarized light [START_REF] Beaufort | Weight estimates of coccoliths using the optical properties (birefringence) of calcite[END_REF]2014). It is also probably linked to the lack of well-preserved carbonate fraction in sediment cores of the SO, making the reconstruction of biogenic carbonate export in the past challenging [START_REF] Barker | Globally increased pelagic carbonate production during the Mid-Brunhes dissolution interval and the CO2 paradox of MIS 11[END_REF][START_REF] Jaccard | Two Modes of Change in Southern Ocean Productivity Over the Past Million Years[END_REF][START_REF] Thöle | Glacial-interglacial dust and export production records from the Southern Indian Ocean[END_REF].

Amongst the interglacials of the past 800 ka, MIS 11 is the longest (~30 ka) as well as one of the warmest (Tzedakis et al., 2022 and references therein). Following the cold glacial MIS 12, MIS 11 begins at the end of Termination V (TV), the first Termination recording high amplitude increases in both pCO2 (∼ 90 ppmv) [START_REF] Bereiter | Revision of the EPICA Dome C CO2 record from 800 to 600 kyr before present[END_REF][START_REF] Nehrbass-Ahles | Abrupt CO2 release to the atmosphere under glacial and early interglacial climate conditions[END_REF] and Antarctic air temperatures (> 12°C) [START_REF] Jouzel | Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years[END_REF]. MIS 11 occurs during an eccentricity minimum [START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF] and coincides with strong global biosphere productivity [START_REF] Brandon | Exceptionally high biosphere productivity at the beginning of Marine Isotopic Stage 11[END_REF], a major phase in coral reef expansion [START_REF] Husson | Reef Carbonate Productivity During Quaternary Sea Level Oscillations[END_REF], and a climax in coccolithophore (Gephyrocapsa sp.) production [START_REF] Barker | Globally increased pelagic carbonate production during the Mid-Brunhes dissolution interval and the CO2 paradox of MIS 11[END_REF][START_REF] Beaufort | Cyclic evolution of phytoplankton forced by changes in tropical seasonality[END_REF]. Recent studies document the increase in coccolith abundances over this time interval in the Pacific and Atlantic sectors of the SO [START_REF] Flores | Calcareous plankton dissolution pattern and coccolithophore assemblages during the last 600 kyr at ODP Site 1089 (Cape Basin, South Atlantic): paleoceanographic implications[END_REF]Saavedra-Pellitero et al., 2017a;2017b). However, no studies exist in the Indian sector of the SO yet, and none focus on the CCP and its impact on atmospheric pCO2 based on combined coccolith and planktonic foraminifera carbonate export data.

Here, we provide coccolith and planktonic foraminifera abundance and mass from core MD04-2718 retrieved in the Indian sector of the SO. Using these data, we demonstrate the biogenic origin of the sedimentary CaCO3 at this site and calibrate the carbonate signal to reconstruct high resolution changes in CCP strength across the time interval covering MIS 12 -MIS 10 (440-360 ka), including TV and MIS 11. Combined with reconstructed summer SST and δ 13 CN. pachyderma data obtained from the same core, as well as previously published micropaleontological and geochemical data from the Pacific and Atlantic sectors of the SO as well as geochemical data from EPICA Dome C (EDC), our study further highlights the synergistic contribution of changing global productivity and terrestrial carbon storage, SO ventilation and CCP strength on atmospheric CO2 during this key period of the past 800 ka.

Regional oceanic setting

The SO is delimited by the Antarctic continent to the South and by the Subtropical front (STF) to the North (Fig. 1a). The eastward-flowing Antarctic Circumpolar Current (ACC) connects the Atlantic, Pacific and Indian sectors of the SO. This current, driven by Coriolis and the Southern Hemisphere westerlies, is characterised by the largest mass transport of all ocean currents, averaging 150 Sv (10 6 m 3 s -1 ) in the Indian Ocean [START_REF] Park | Direct observations of the ACC transport across the Kerguelen Plateau[END_REF]. Its Northern and Southern Boundaries (NB and SB) are often delimited around 40°S and 60°S respectively, but their exact location depends on bottom topography (Sokolov andRintoul, 2009, Park et al., 2019) and varies latitudinally [START_REF] Freeman | Temporal variability in the Antarctic Polar Front (2002-2014)[END_REF]. Three hydrological fronts generally characterised by stronger flows, are defined in the ACC: the Subantarctic Front (SAF), the Polar Front (PF) and the Southern Antarctic Circumpolar Front (SACCF) (Fig. 1). They help defining the Subantarctic Zone (SAZ) between the STF and the SAF, the Polar Front Zone (PFZ) between the SAF and the PF, and the Antarctic Zone (AAZ) delimited by the PF to the North, and the SB to the South [START_REF] Orsi | On the meridional extent and fronts of the Antarctic Circumpolar Current[END_REF][START_REF] Park | Observations of the Antarctic Circumpolar Current Over the Udintsev Fracture Zone, the Narrowest Choke Point in the Southern Ocean[END_REF]. Within the Indian sector of the SO, the Kerguelen Plateau that extends from ∼ 46°S to 63°S and between ∼ 62°E and 85°E, is the second largest submarine plateau in the world. Due to its great meridional extent and topography, it constitutes a major bathymetric constrain to the ACC flow and thus exerts a significant impact on both ACC transport and front patterns [START_REF] Park | Direct observations of the ACC transport across the Kerguelen Plateau[END_REF][START_REF] Sokolov | Circumpolar structure and distribution of the Antarctic Circumpolar Current fronts: 1. Mean circumpolar paths[END_REF]. In this context, the PF is located alongside the southern coast of the Kerguelen archipelago [START_REF] Park | Polar Front around the Kerguelen Islands: An up-to-date determination and associated circulation of surface/subsurface waters[END_REF]. The circulation in the Southern Ocean is also regulated by the upwelling of CO2-and nutrient-rich Circumpolar Deep Waters (CDW) that resurface at the Antarctic divergence in the AAZ (Fig. 1c). These waters represent the source for the Subantarctic Mode Water (SAMW) and the Antarctic Intermediate Water (AAIW) that flow northward. Today, the change in the strength of upwelling affects the partitioning of CO2 between the ocean subsurface and the atmosphere on decadal timescales [START_REF] Lovenduski | Toward a mechanistic understanding of the decadal trends in the Southern Ocean carbon sink[END_REF][START_REF] Landschützer | The reinvigoration of the Southern Ocean carbon sink[END_REF][START_REF] Gruber | The oceanic sink for anthropogenic CO2 from 1994 to 2007[END_REF].

In the SO, primary productivity is primarily steered by bottom-up control involving both physical and biogeochemical processes, which are spatially and temporally heterogeneous. Today, the presence/absence of sea ice together with the progressive temperature decrease polewards subdivide the SO into distinct provinces characterised by specific functional assemblages [START_REF] Boyd | Environmental factors controlling phytoplankton processes in the Southern Ocean[END_REF][START_REF] Constable | Climate change and Southern Ocean ecosystems I: how changes in physical habitats directly affect marine biota[END_REF]. In ecosystems that remain ice-free throughout the year, i.e., within the Permanently Open Ocean Zone (POOZ), primary productivity is commonly co-limited by light and micronutrient availability [START_REF] Boyd | Environmental factors controlling phytoplankton processes in the Southern Ocean[END_REF][START_REF] Moore | Processes and patterns of oceanic nutrient limitation[END_REF]. While the residual surface ocean macronutrient concentrations in the SO are among the highest globally [START_REF] Pollard | Quantifying nutrient supply to the Southern Ocean[END_REF] largely due to the large-scale upwelling of CDW at the Antarctic divergence, primary productivity is relatively low, owing to the scarcity of bioavailable iron (Fe) imposes on phytoplankton growth [START_REF] Martin | Glacial-interglacial CO2 change: The iron hypothesis[END_REF][START_REF] Pollard | Southern Ocean deep-water carbon export enhanced by natural iron fertilization[END_REF]. In all cases, phytoplankton that flourishes south of the PF is dominated by diatoms, with blooms generally occurring during austral summer (Rigual-Hernandez et al., 2015).

Phytoplankton that thrives north of the PF is more heterogeneous and represented by nano-and picoplankton of different origins. One of the most striking patterns associated with phytoplankton in this area, is the significant coccolithophore blooms that occur during spring and summer between ∼40 and ∼60°S, and that forms the so-called Great Calcite Belt that enriches surface waters in CaCO3 [START_REF] Balch | The contribution of coccolithophores to the optical and inorganic carbon budgets during the Southern Ocean Gas Exchange Experiment: New evidence in support of the "Great Calcite Belt" hypothesis[END_REF]2016;[START_REF] Rembauville | Planktic foraminifer and coccolith contribution to carbonate export fluxes over the central Kerguelen Plateau[END_REF]. Within the Indian sector of the SO, near the Kerguelen plateau, this Great Calcite Belt is observed in the PFZ [START_REF] Smith | The influence of environmental variability on the biogeography of coccolithophores and diatoms in the Great Calcite Belt[END_REF] and the northern part of the AAZ (between Kerguelen and Heard and McDonald Islands) and may contribute ∼ 60 to 85 % of the CCP [START_REF] Blain | Effect of natural iron fertilization on carbon sequestration in the Southern Ocean[END_REF][START_REF] Rembauville | Planktic foraminifer and coccolith contribution to carbonate export fluxes over the central Kerguelen Plateau[END_REF][START_REF] Smith | The influence of environmental variability on the biogeography of coccolithophores and diatoms in the Great Calcite Belt[END_REF]Rigual-Hernandez et al., 2020). Relatively high primary productivity may also be observed locally, near the PF and the Crozet and Kerguelen plateaus, where oceanic current and/or bottom topography enriches surface waters with iron [START_REF] Blain | Effect of natural iron fertilization on carbon sequestration in the Southern Ocean[END_REF][START_REF] Sokolov | On the relationship between fronts of the Antarctic Circumpolar Current and surface chlorophyll concentrations in the Southern Ocean[END_REF][START_REF] Pollard | Southern Ocean deep-water carbon export enhanced by natural iron fertilization[END_REF][START_REF] Quéguiner | Iron fertilization and the structure of planktonic communities in high nutrient regions of the Southern Ocean[END_REF][START_REF] Tagliabue | Surface-water iron supplies in the Southern Ocean sustained by deep winter mixing[END_REF].

Material and methods

Material core

The 50-meter-long marine sediment core MD04-2718 (48°53,31S; 65°57,42E, 1428 m water depth) was retrieved from the NW flank of the Kerguelen Plateau in 2004, during the MD136/VIGO expedition aboard the R.V. Marion Dufresne (Fig. 1, [START_REF] Michel | MD 136/VIGO à bord du R/V Marion Dufresne[END_REF] within the PFZ.

Surface water properties are largely controlled by CO2-and nutrient-rich waters upwelling in the Antarctic Zone, that are advected northward by the Ekman transport. Sediment recovered at this site is primarily siliciclastic with alternations of nannofossil (calcareous) and diatom (opal) oozes. MIS 11 is clearly identifiable and represented by calcareous-rich ooze, between 11.5 and 13.1 m core depth. Southern Boundary of the ACC .c Salinity (psu) section at 66°E using WOA18 (Zweng et al., 2018) on ODV software [START_REF] Schlitzer | Ocean Data View[END_REF]. SAMW: Subantarctic Mode Water, AAIW: Antarctic Intermediate Water, CDW: Circumpolar Deep Water, AABW: Antarctic Bottom Water.

Geochemical analyses

Planktonic foraminifera δ 18 O and δ 13 C compositions (expressed in ‰) were measured at the LSCE (Laboratoire des Sciences du Climat et de l'Environnement), based on 67 Neogloboquadrina pachyderma senestral (200-250 µm fraction, Supplementary Fig. 1) samples, subsampled every 3-4 cm between 11.03 and 13.3 m depth (except for 13.07, 13.10 and 13.13 m, which are barren of N. pachyderma s). For each sample, 30 specimens were hand-picked, cleaned using methanol, ultrasonicated, weighted, and analysed using an IRMS GV ISOPRIME mass spectrometer coupled with an extraction line. The measurements are reported versus the Vienna Pee Dee Belemnite standard (VPDB) with respect to NBS-19 standard, with a mean external reproducibility (1σ) for carbonate standards of ± 0.04 ‰ and 0.06 ‰ for δ 13 C and δ 18 O, respectively. The reproducibility of isotopic measurements on N. pachyderma s. are 0.06 and 0.1 ‰, for δ 13 C and δ 18 O, respectively.

High-resolution, relative sedimentary calcium (Ca) concentrations were measured every 1 cm using an Aavatech profiling X-ray Fluorescence (XRF) core scanner at ETH Zurich, Switzerland with a 10s integration at 10 kV and 500 µA. In parallel, sedimentary CaCO3 concentrations were determined on 26 samples at GEOPS Laboratory with a ± 2 % precision, using the vacuum-gasometric technique described in [START_REF] Duchamp-Alphonse | Enhanced ocean-atmosphere carbon partitioning via the carbonate counter pump during the last deglacial[END_REF] where the amount of CO2 degassed during the reaction of 100 mg ± 2 mg of dried sediments with 6N HCl is converted into CaCO3 percentages. Comparison between Ca counts and discrete CaCO3 contents show a highly significant statistical correlation (r 2 =0.97), and CaXRF signal is therefore directly converted into CaCO3 content [START_REF] Jaccard | Subarctic Pacific evidence for a glacial deepening of the oceanic respired carbon pool[END_REF]2010;[START_REF] Duchamp-Alphonse | Enhanced ocean-atmosphere carbon partitioning via the carbonate counter pump during the last deglacial[END_REF]; Supplementary Fig. 2).

Micropaleontological analyses

Coccoliths

Coccolith abundance and mass were determined at the GEOPS (GEOscience Paris Saclay) Laboratory. Smear slides were prepared for 68 samples, based on the protocol detailed in [START_REF] Duchamp-Alphonse | Enhanced ocean-atmosphere carbon partitioning via the carbonate counter pump during the last deglacial[END_REF], after [START_REF] Beaufort | Optical measurements to determine the thickness of calcite crystals and the mass of thin carbonate particles such as coccoliths[END_REF]. For each sample, the abundance and mass of individual coccoliths (with an average of 6,172 coccoliths per sample (176 to 13,233)) were automatically determined with the help of the SYRACO software using a Leica DM6000B microscope.

SYRACO detects and classifies most of the coccoliths (Gephyrocapsa caribbeanica, small

Gephyrocapsa, G. oceanica, G. muellerae, Calcidiscus leptoporus and Coccolitus pelagicus) present on the slides. Coccolith mass was also directly obtained using SYRACO based on a quasi-linear relationship between their brightness (birefringence in grey scale) and their thickness under crosspolarized light. Because this method applies to coccoliths thinner than 1.55 µm, we interpret only thickness and mass data for Gephyrocapsa specimens, as C. leptoporus and C. pelagicus display thirdorder interferences. The presence of relatively large detrital particles may lead to an underestimation of small specimens including Gephyrocapsa sp. and an overestimation of larger ones including C. pelagicus. To correct for these potential biases, the abundance of each morphotype was verified manually on 18 samples covering the full spectrum of coccolith abundance observed from MIS 12 to MIS 10. Visual counts were performed on the same view fields analysed by SYRACO and were compared to the results obtained by the software. Gephyrocapsa sp. abundances from SYRACO counts were calibrated using the correlation coefficients between the two datasets, while C. pelagicus abundances have been obtained visually for the entire sample set because of the confusion made by SYRACO between detrital particles and this species. C. leptoporus abundances did not differ significantly between SYRACO and visual counts were not corrected.

The number of coccoliths per gram of sediment (absolute coccolith abundance -ACA) was determined for each morphotype/species using the equation of [START_REF] Grelaud | Glacial to interglacial primary production and El Niño-Southern Oscillation dynamics inferred from coccolithophores of the Santa Barbara Basin[END_REF] as follows:

= * * *
where Nc represents the total coccolith number per sample, Sf, the flat beaker area (3117 mm²), No, the number of view fields (165), So, the view field area (0.01 mm²) and Ws, the dry sediment weight.

The coccolith calcite mass per gram of sediment (per g. sed) was determined using the same equation, where Nc is replaced by Mc, that represents the coccolith calcite mass per view fields.

Gephyrocapsa calcite mass (CMgephy) was obtained based on the estimation of specimen abundance and mean mass as follows: = *

Planktonic foraminifera

The abundance and planktonic foraminifera species composition were determined at the LSCE with a binocular microscope, using the > 150 µm fraction of 57 samples ranging between 11.30 m and 13.3 m depth. For each sample, an average of 450 planktonic foraminifera were counted. The absolute planktonic foraminifera abundance (AFA) per gram of sediment was determined as follows:

= * 2
where Nf represents the number of planktonic foraminifera counted per sample (> 300), split is the number of splits performed on the sediment before counting (0 ≤ split ≤ 6), and Wdry, the dry sediment sample weight.

The planktonic foraminifera calcite mass was determined at the LSCE using mean weights of the major planktonic foraminifera species from the >150 µm fraction. The foraminifera calcite mass is thus underestimated as it does not consider juveniles planktonic foraminifera or smaller species such as G. uvula. Yet the weight variations of this fraction are considered representative of the weight variations of the total planktic foraminifera fraction. The calcite mass produced by planktonic foraminifera, CaCO3plank.foram, is (in mg/g):

!"#. !% = (∑ ( ∑ ( * )* ( ) * , ) * ( 10 /
Where mjj represents the mean weight in size range j of specie I (in µg), sizeji the percentage of population in size range j for specie i, and Xi the percentage of the specie i in the sample. The error on the mass will be: Neogloboquadrina pachyderma sinistral and dextral, Globigerinita glutinata, Turborotalia quinqueloba, Globigerina bulloides, Globorotalia inflata, Globoratalia crassaformis and Globorotalia truncatilunoides. We determine mean weights of : 3.55 ± 0.32 µg for N. pachyderma, 9.42 ± 2.15 µg for G. bulloides, 3.48 ± 0.42 for G. glutinata, 13.9 ± 2.37 µg for G. crassaformis and 22.4 ± 5.39 µg for G. inflata. We assumed a similar weight to G. glutinata for T. quinqueloba. For G. truncatilunoides, we assumed a mean weight similar to G. crassaformis. For N. pachyderma, 90 to 97 % of the population ranges in the minimum measured size range (150-200 µm), and between 71 to 100 % for G. glutinata and T. quinqueloba. For the other species the size range distribution varies with depth with the main size range varying from 150-200 µm to 250-315 µm for G. bulloides, G. crassaformis and G. truncatulinoides and from 150-200 µm to 315-355 µm for G. inflata. The error on the calcite mass of the planktonic foraminifera depends thus mainly on the error on the population fraction in the different size range for the larger species. The error on the calcite mass of the planktonic foraminifera also considers the variations in planktonic foraminifera weight and size with depth observed in the core, that can be linked with the changes in atmospheric CO2 concentration [START_REF] Barker | Foraminiferal Calcification Response to Glacial-Interglacial Changes in Atmospheric CO2[END_REF][START_REF] Moy | Reduced calcification in modern Southern Ocean planktonic foraminifera[END_REF][START_REF] Henehan | Size-dependent response of foraminiferal calcification to seawater carbonate chemistry[END_REF]. For the 57 samples, the calcite mass of the planktonic foraminifera and its error have been calculated from the counted abundance of the different species and their mean size and mean weight calculated from the six depths. The error on this calcite mass varies from 26 to 57 % and is linearly linked with the abundance of G. bulloides (r 2 = 0.91). Although large, the error remains negligible compared to the observed variations that are almost a factor of a thousand.

Summer SST estimates were determined on the same 57 samples as those used for planktonic foraminifera assemblages based on two independent approaches: the Modern Analog Technique and the percentage of N. pachyderma s. The former one relies on the use of a database of modern foraminifera assemblage [START_REF] Haddam | Improving past sea surface temperature reconstructions from the Southern Hemisphere oceans using planktonic foraminiferal census data[END_REF] and the PaleoAnalog software [START_REF] Therón | Rapid reconstruction of paleoenvironmental features using a new multiplatform program[END_REF] to find the closest modern analogues for the fossil assemblage [START_REF] Prell | Stability of low-latitude sea-surface temperatures: an evaluation of the CLIMAP reconstruction with emphasis on the positive SST anomalies[END_REF][START_REF] Haddam | Improving past sea surface temperature reconstructions from the Southern Hemisphere oceans using planktonic foraminiferal census data[END_REF]. The latter one is based on the linear relation between SST and the percentage of N. pachyderma s. in the total planktonic foraminifera assemblage following the equation of [START_REF] Govin | Evidence for northward expansion of Antarctic Bottom Water mass in the Southern Ocean during the last glacial inception[END_REF]:

T°C (% pachy s.) = -0.087 (% pachy s.) + 11.339

No SST could be determined between 13.13 and 13.07 m depth as the number of planktonic foraminifera was insufficient (<300) for a robust temperature estimate. During glacial periods the number of foraminifera per gram of sediment decreases drastically and, in these three levels, the >150 µm fraction contains almost exclusively ice rafted detritus and radiolarians.

Results

Geochemical signals

Planktonic foraminifera δ 18 O values range between 2.2 ‰ and 3.8 ‰ (Fig. 2a). After a rapid decrease from 3.7 to 2.2 ‰ between 13.00 and 12.33 m, values gradually increase to reach around 3.6 ‰ at 11.33 m. The δ 13 C signal ranges from 0.75 ‰ to 1.67 ‰ (Fig. 2b). δ 13 C values are characterised by a slightly increasing plateau between 12.80 and 12.23 m and two sharp decreases to the minimum value of 0.8 ‰ from 13.23 to 13.03 m and from 11.57 to 11.33 m.

Sedimentary CaCO3 concentrations range between 5 % and 99 % (Fig. 2c). Values never exceed 20 % between 13.30 and 13.10 m and between 11.

Micropaleontological signals

Coccolith assemblages are predominantly composed of Gephyrocapsa sp. i.e., specimens from the Noelaerhabdaceae family whose proportions exceed 98.5 %, reaching maximum values (99.9 %) between 12.87 and 11.33 m depths (Supplementary Fig. 3). C. pelagicus and C. leptoporus are present as well, but in much lower proportions. They are mainly represented by the subspecies C. pelagicus ssp. braarudii (10-14 µm) and C. leptoporus (5-8 µm) and range between 0 and 0.9 %, and 0 and 0.6 % of the assemblages, respectively. The number of total coccoliths per gram of sediments (per g. sed.) ranges from 0.017*10 9 (at 13.10 m) to 40*10 9 (at 12.10 m) and is mainly represented by the number of Gephyrocapsa sp. per g. sed. that fluctuates between 0.016*10 9 and 40*10 9 (Fig. 3b and 3c). show the error on SYRACO abundance for each specie and for Gephyrocapsa mean mass.

After an increasing trend from 12.90 to 12.67 m, values show a plateau around 22.5*10 9 specimens per g. sed. between 12.67 and 11.80 m and decreases to reach values as low as 0.35*10 9 specimens per g. sed. at 11.17 m. C. leptoporus abundance fluctuates between 0.06*10 6 and 21.6*10 6 specimens per g. sed. and its distribution generally follows that of Gephyrocapsa sp. (and thus of total coccoliths), with an increasing trend between 13.07 and 12.67 cm followed by a longterm decreasing trend up to 11.47 m, interrupted by a transient a peak at 12.10 m (Fig. 3d). C. pelagicus abundance varies between 0 and 1.6*10 6 coccoliths per g. sed (Fig. 3e). It is totally absent from the sediment between 12.73 and 11.54 m and is thus only present between 13.30 and 12.77 m and between 11.53 and 11.03 m, where it reaches values as high as 1.6 and 0.77*10 6 specimens per g. sed, respectively. Gephyrocapsa sp. mean mass varies between 1.03 pg and 2.24 pg (Fig. 3f). After reaching maximum values (from 2.07 to 2.24 pg) between 13.23 and 13.13 m, data are consistent with a long-term decrease to reach minimum values around 11.40 m depth.

The planktonic foraminifera assemblages are mainly composed of N. pachyderma sinistral and G. bulloides with proportions ranging between 0 % and 99.8 % and between 0 and 75.1 %, respectively. The other species, represented by G. glutinata, G. inflata, G. crassaformis, N.

pachyderma dextral and T. quinqueloba never exceed 6.8 %. (Supplementary Fig. 4). The total number of planktonic foraminifera fluctuates between 0 and 18.5*10 3 specimens per g. sed (Fig. 4b).

No specimen is found between 13.13 and 13.07 m, and the total abundance of planktonic foraminifera never exceeds 5.7*10 3 between 13.30 and 12.9 m and between 11.5 and 11.3 m.

Between 12.9 and 11.5 depth the number of N. pachyderma s. varies between 0.3 and 12.4*10 3 specimens per g. sed (Fig. 4c), and significantly influences the total number of planktonic foraminifera which generally shows the same trends. Its abundance shows an overall plateau around 3*10 3 specimens per g. sed, and 2 maxima (12.4*10 3 and 10.9*10 3 specimens per g. sed) at 12.70 and 11.73 m, respectively. The number of G. bulloides fluctuates between 2.10 and 4.7*10 3 specimens per g. sed (Fig. 4d). After reaching maximum value at 12.70 m, it depicts a long-term decreasing trend to the minimum abundance of 2.1 specimens per g. sed. at 11.3 m. The number of N. pachyderma d. varies between 0.14 and 548.6 specimens per g. sed. and follows the general same trends as those of total planktonic foraminifera and N. pachyderma s. (Fig. 4e). G. glutinata abundance ranges between 0.2 and 551 specimens per g. sed (Fig. 4f). It depicts rather scattered values with two peaks (512 and 551 specimens per g. sed.) at 12.70 and 12.33 m. G. crassaformis abundance fluctuates between 0 and 259.5 specimens per g. sed, with values averaging 150 specimens per g. sed between 12.70 and 12.13 m (Fig. 4g). T. quinqueloba abundance fluctuates between 0 and 219.4 specimens per g. sed, and no specific trend may be highlighted (Fig. 4h). Finally, G. inflata abundance ranges between 0 and 106 specimens per g. sed (Fig. 4i). Similarly, to G. crassaformis, no or very few specimens are found between 12.23 and 11.60 m.

The reconstructed summer SSTs fluctuate between 1.7 and 9.9 °C (Fig. 4j). The maximum temperature is reached after a drastic warming amounting ~8 °C between 13.27 and 13.00 m. It is followed by temperatures fluctuating between 4.6 and 8.3°C from 12.93 to 12.70 m, and a plateau around 8°C between 12.67 and 12.27 m. Then, summer SSTs gradually decrease and reach down to 3 °C between 12.03 and 11.30 m.

Age model

The age model of core MD04-2718 is based on the planktonic foraminifera δ 18 O record and sedimentary CaCO3 signal obtained on the uppermost 30 m of the core (Supplementary Fig. 5). These records helped identifying glacial-interglacial cycles and determining that the uppermost 20 m of the core covered the last 800 ka. It also allowed us to define the depth interval 13.00 to 11.00 m as representing the MIS 12 -MIS 10 time-interval and confirm our assumption that the nannofossil ooze between 11.5 and 13.1m was deposited during MIS 11 (see section 3.1). Numerous studies showed that the correlation between reconstructed SSTs in the Southern Ocean and air temperature reconstructed from Antarctic ice cores was helpful to better constrain sediment core age model (e.g., [START_REF] Pichon | Surface water temperature changes in the high latitudes of the southern hemisphere over the Last Glacial-Interglacial Cycle[END_REF][START_REF] Sowers | A 135,000-year Vostok-SPECMAP common temporal framework[END_REF][START_REF] Waelbroeck | A comparison of the Vostok ice deuterium record and series from Southern Ocean core MD 88-770 over the last two glacial-interglacial cycles[END_REF][START_REF] Thöle | Glacial-interglacial dust and export production records from the Southern Indian Ocean[END_REF][START_REF] Ai | Southern Ocean upwelling, Earth's obliquity, and glacial-interglacial atmospheric CO2 change[END_REF].

Here, we used the record of δD of the EPICA Dome C ice core as a proxy for Antarctic surface temperature [START_REF] Jouzel | Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years[END_REF]. As such, for the interval MIS 12 -10, the age model of core MD04-2718 was refined based on the graphical alignment of our reconstructed summer SST with record ofthe δD record on the AICC2012 timescale [START_REF] Bazin | An optimized multi-proxy, multi-site Antarctic ice and gas orbital chronology[END_REF], assuming an in-phase relationship (Fig. 5a). Using Analyseries software [START_REF] Orsi | On the meridional extent and fronts of the Antarctic Circumpolar Current[END_REF], we set 5 tie points which helped us identifying the 11.03-13.3 m core section as representing the time interval between 438.4 and 360.2 ka i.e., between the end of MIS 12 and the beginning of MIS 10. Over this period, the sedimentation rate averages 3.1 cm.ka -1 with lowest values (1 cm.ka -1 ) being recorded during MIS 12, and highest ones (4.9 cm.ka -1 ) being observed at the beginning of MIS 11 (Fig. 5b). Given these sedimentation rates, the temporal resolution of our record ranges from ∼ 0.6 to 3.8 ka for micropaleontological and stable isotope data and of 0.2 to 2 ka for the CaCO3 signal. core [START_REF] Jouzel | Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years[END_REF] on the AICC2012 timescale [START_REF] Bazin | An optimized multi-proxy, multi-site Antarctic ice and gas orbital chronology[END_REF].

Black dots are the tie points between SST and δD b

Estimated sedimentation rate of MD04-2718 core. Black curve represents the depth versus the calibrated age (ka BP on AICC2012) and the grey shaded area represent the variation of sedimentation rate with time.

Discussion

CaCO3 as a record of CCP strength

In pelagic environments of the SO, sedimentary carbonate concentration and/or coccolith abundances have been assumed to reflect biogenic carbonate export production -and by inference the strength of the CCP -based on the paradigm that in modern environments, and more particularly in the Great Calcite Belt, coccolithophores represent an important -if not dominant -part of exported mineralized organisms [START_REF] Salter | Carbonate counter pump stimulated by natural iron fertilization in the Polar Frontal Zone[END_REF][START_REF] Rembauville | Planktic foraminifer and coccolith contribution to carbonate export fluxes over the central Kerguelen Plateau[END_REF]. However, the bulk sedimentary carbonate signal may include detrital or diagenetic phases and within the biogenic fraction, contributions of planktonic foraminifera and pteropods, the latter often considered negligible [START_REF] Salter | Carbonate counter pump stimulated by natural iron fertilization in the Polar Frontal Zone[END_REF][START_REF] Rembauville | Planktic foraminifer and coccolith contribution to carbonate export fluxes over the central Kerguelen Plateau[END_REF]. Therefore, special attention must be devoted to assessing the significance of bulk sedimentary carbonate signals before making interpretations in terms of planktonic carbonate productivity and, by inference, of the CCP strength. It requires to carefully select the sedimentary cores that robustly reflect biogenic carbonate accumulation and to consider the potentially obfuscating impact of diagenesis.

Core MD07-2418 was retrieved at 1,428 m water depth i.e., well above the modern lysocline located at ∼4,300 m [START_REF] Kolla | Calcium carbonate distribution in the surface sediments of the Indian Ocean[END_REF][START_REF] Howard | Late Quaternary CaCO3 production and preservation in the Southern Ocean: Implications for oceanic and atmospheric carbon cycling[END_REF]. The area only receives carbonatefree terrigenous material sourced from volcanic rocks of the Crozet and Kerguelen plateau [START_REF] Nougier | Contribution à l'étude géologique et géomorphologique des îles Kerguelen (Terres Australes et Antarctiques Françaises)[END_REF]. Thus, the sedimented carbonate material directly relates to local biogenic carbonate production, for which the diagenetic overprint and potential detrital contribution are minimal. These assumptions are corroborated by Scanning Electron Microscopy (SEM) observations highlighting that the sedimentary carbonate fraction is predominantly constituted by very well-preserved foraminifera and coccoliths, sometimes represented as intact coccospheres (Fig. 6). In addition, we measured the relative proportion of fragmented planktonic foraminifera shells versus intact foraminifers, a wellcharacterised preservation index (Vazquez-Riveiros et al., 2010;[START_REF] Flores | Calcareous plankton dissolution pattern and coccolithophore assemblages during the last 600 kyr at ODP Site 1089 (Cape Basin, South Atlantic): paleoceanographic implications[END_REF] (Supplementary Fig. 6). This foraminifera fragmentation index varies between 0 and 15.2 % with values higher than 3 % observed only between 426 and 417 ka. These results clearly indicate that planktonic foraminiferal preservation is generally good [START_REF] Le | Carbonate Dissolution Fluctuations in the Western Equatorial Pacific During the Late Quaternary[END_REF].

Dissolution processes could result in differential sedimentary preservation in the coccolith assemblages as well, preferentially preserving resistant specimens. However, the assemblage is mainly constituted by well-preserved Gephyrocapsa sp., as seen on SEM observations. Furthermore, within this genus, small Gephyrocapsa sp. (<3 µm) follow the exact same trends as the larger ones, which suggests that differential dissolution does not significantly affect coccolith assemblages in sediments from core MD04-2718 (Supplementary Fig. 6). ka to 372 ka to reach low values of 0.43 mg/g during glacial MIS 10. The Gephyrocapsa calcite mass record closely resembles the one of Gephyrocapsa sp. abundance, whereas no relationship may be observed with Gephyrocapsa mean mass (Fig. 7 and Supplementary Fig. 7). Moreover, while the variability in Gephyrocapsa sp. mean mass along the records are of a factor of two, the observed variations in abundance are more than a factor of two thousand. We therefore posit that changes in Gephyrocapsa calcite mass are mainly the consequence of changes in Gephyrocapsa abundance rather than in shell mean mass linked with shift in calcification or morphotypes. The planktonic foraminifera (>150 µm) calcite mass varies between 0 and 93.9 mg/g. Briefly, it is characterised by null or low values during MIS 12 (< 7.2 mg/g) and MIS 10 (< 1.08 mg/g) and generally higher values during MIS 11 and the beginning of MIS 10 (> 20 mg/g). Maximum values are observed around 412.7 ka i.e., at the beginning of MIS 11. According to the significant linear correlation that exists between the Gephyrocapsa sp. + planktonic foraminifera calcite mass and sedimentary CaCO3 content (r² = 0.93), it appears that both plankton groups constitute the major component of the biogenic carbonate signal. Gephyrocapsa sp. seems to have a relatively larger contribution on the sedimentary CaCO3 content as shown by its greater correlation to CaCO3 (r 2 = 0.89) than planktonic foraminifera calcite mass itself (r 2 = 0.83) (Supplementary Fig. 2). Such results are consistent with the major control of coccoliths and more particularly morphotypes of the Noelaerhabdaceae family exert on PIC export today [START_REF] Rembauville | Planktic foraminifer and coccolith contribution to carbonate export fluxes over the central Kerguelen Plateau[END_REF][START_REF] Patil | Interannual changes of austral summer coccolithophore assemblages and southward expanse in the Southern Indian Ocean[END_REF].

In the following, changes in the downcore CaCO3 signal at site MD04-2718 will be interpreted in terms of changes in planktonic carbonate export, and by inference changes in CCP strength. As represented by the variation in sedimentary CaCO3 content, it appears that the CCP was strengthened during MIS 11 compared to MIS 12 and MIS 10 (Fig. 8). By analogy to the modern SO [START_REF] Baumann | Coccolithophores as Indicators of Ocean Water Masses, Surface-Water Temperature, and Paleoproductivity -Examples from the South Atlantic[END_REF][START_REF] Zondervan | The effects of light, macronutrients, trace metals and CO2 on the production of calcium carbonate and organic carbon in coccolithophores-A review[END_REF] and the Pacific sector of the SAZ during MIS 12-MIS 10 [START_REF] Schaefer | Planktic foraminiferal and sea surface temperature record during the last 1 Myr across the Subtropical Front, Southwest Pacific[END_REF][START_REF] Ullermann | Glacial-interglacial oceanic changes in the central Pacific sector of the Southern Ocean during the past 500 ka[END_REF]Saavedra-Pellitero et al., 2017a;2017b), it is probable that such recorded pattern is related to substantial changes in SST and nutrient conditions. In addition to reconstructed SSTs, the abundances of C. pelagicus and C. leptoporus can be used to indirectly assess past changes in sea surface nutrient and temperature conditions. Indeed, while C. pelagicus seems to proliferate under colder conditions than C. leptoporus [START_REF] Cobianchi | Pleistocene biogeochemical record in the south-west Pacific Ocean (images site MD97-2114, Chatham Rise)[END_REF], both these species appear to have affinities with relatively nutrient-rich waters [START_REF] Andruleit | Coccolithophore fluxes in the Norwegian-Greenland Sea: Seasonality and assemblage alterations[END_REF][START_REF] Balestra | Coccolithophorids from the Southeast Greenland Margin (Northern North Atlantic): production, ecology and the surface sediment record[END_REF][START_REF] Amore | A Middle Pleistocene Northeast Atlantic coccolithophore record: Paleoclimatology and paleoproductivity aspects[END_REF]. This behaviour has been well documented on glacial/interglacial timescales in the SO [START_REF] Flores | Pleistocene fluctuations in the Agulhas Current Retroflection based on the calcareous plankton record[END_REF][START_REF] Findlay | Subtropical Front fluctuations south of Australia (45°09'S, 146°17'E) for the last 130 ka years based on calcareous nannoplankton[END_REF][START_REF] Marino | Response of calcareous nannofossil assemblages to paleoenvironmental changes through the mid-Pleistocene revolution at Site 1090 (Southern Ocean)[END_REF]Saavedra-Pellitero et al., 2017a;2017b) to characterizse frontal zone dynamics [START_REF] Flores | Pleistocene fluctuations in the Agulhas Current Retroflection based on the calcareous plankton record[END_REF][START_REF] Findlay | Extant calcareous nannoplankton in the Australian Sector of the Southern Ocean (austral summers 1994 and 1995)[END_REF]2000;[START_REF] Flores | Calcareous plankton dissolution pattern and coccolithophore assemblages during the last 600 kyr at ODP Site 1089 (Cape Basin, South Atlantic): paleoceanographic implications[END_REF][START_REF] Baumann | Coccolith contribution to South Atlantic carbonate sedimentation[END_REF][START_REF] Malinverno | Coccolithophore assemblage distribution along a temperate to polar gradient in the West Pacific sector of the Southern Ocean (January 2005)[END_REF][START_REF] Marino | Changes in calcareous nannofossil assemblages during the Mid-Pleistocene Revolution[END_REF]Saavedra-Pellitero et al., 2017a;2017b). At core site MD04-2718, reconstructed SSTs range between ∼2-4°C during glacial MIS 12 and MIS 10, and ∼8°C during MIS 11 (Fig. 8). In parallel, relatively fertile conditions are inferred during TV and MIS 11 by the coccolith species C. pelagicus and C. leptoporus that peak between 426 and 415 ka and between 412 and 392 ka, respectively. The CCP was thus stronger during the relatively warm and nutrient-rich MIS 11 interglacial compared to the relatively cold and nutrient-depleted MIS 12 and MIS 10. The CCP trend shows however some temporal lag with the SST and nutrient records, highlighting other mechanisms in potentially regulating the variability of CCP strength. This is particularly obvious during TV (430-424 ka), when the CCP remains relatively weak despite drastic increases in SSTs and nutrient conditions. At that time, decreasing δ 13 CN.pachyderma values and increasing mean mass of Gephyrocapsa are recorded (Fig. 8). The rapid δ 13 C decrease during the deglaciation is also documented in the South Pacific (Saavedra-Pellitero et al., 2017b) and South Atlantic [START_REF] Hodell | Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean[END_REF] (Fig. 8). This isotopic signal is consistent with the resumption of SO upwelling, which supplies abundant nutrients, previously sequestered in the ocean interior owing to greater water-column stratification, to the productive ocean surface [START_REF] François | Contribution of Southern Ocean surface-water stratification to low atmospheric CO2 concentrations during the last glacial period[END_REF][START_REF] Spero | The Cause of Carbon Isotope Minimum Events on Glacial Terminations[END_REF][START_REF] Adkins | The role of deep ocean circulation in setting glacial climates[END_REF][START_REF] Siani | Carbon isotope records reveal precise timing of enhanced Southern Ocean upwelling during the last deglaciation[END_REF]. Coeval increase in Gephyrocapsa mean mass has also been associated with increase in CO2-rich upwelled waters in the SO [START_REF] Duchamp-Alphonse | Enhanced ocean-atmosphere carbon partitioning via the carbonate counter pump during the last deglacial[END_REF]. The reason behind these staggered and seemingly disconnected patterns could relate to the ecological competition between diatoms and coccolithophores in the SO. During glacial periods, favoured by higher iron (Fe) availability, diatom productivity outcompetes coccolithophore growth [START_REF] Martin | Glacial-interglacial CO2 change: The iron hypothesis[END_REF][START_REF] Kohfeld | Role of Marine Biology in Glacial-Interglacial CO2 Cycles[END_REF]Martinez-Garcia et al., 2009;[START_REF] Thöle | Glacial-interglacial dust and export production records from the Southern Indian Ocean[END_REF], and call for a weaker CCP. While the availability of Fe dwindled over deglacial periods, reinvigorated upwelling of nutrient-rich waters would have promoted diatom growth during Termination V. In such a scenario, we hypothesize that it is only during peak MIS 11 that silicic acid concentrations became too low, thereby limiting diatom growth [START_REF] Sarmiento | High-latitude controls of thermocline nutrients and low latitude biological productivity[END_REF], and thus had the potential to favour coccolith production, enhancing the CCP. Overall, since the upwelled waters that (re)surface in the AAZ act under the forcing of the prevailing westerlies [START_REF] Anderson | Wind-Driven Upwelling in the Southern Ocean and the Deglacial Rise in Atmospheric CO2[END_REF][START_REF] Lauderdale | Wind-driven changes in Southern Ocean residual circulation, ocean carbon reservoirs and atmospheric CO2[END_REF][START_REF] Menviel | Southern Hemisphere westerlies as a driver of the early deglacial atmospheric CO2 rise[END_REF], it is probable that winds migrated poleward between MIS 12 and MIS 11, concomitantly forcing the poleward migration of the entire frontal system [START_REF] Flores | Pronounced mid-Pleistocene southward shift of the Polar Front in the Atlantic sector of the Southern Ocean[END_REF][START_REF] Kemp | Migration of the Antarctic Polar Front through the mid-Pleistocene transition: evidence and climatic implications[END_REF]Saavedra-Pellitero et al., 2017a;2017b).

According to the reconstructed SST (< 8°C in mean) as well as the relatively low percentages of C. leptoporus, while close to the SAF, it is likely that site MD04-2718 remained in the PFZ. That is only around 390 ka, when SSTs decreased, that westerlies (along with the frontal system) might have shifted back to the North, leading to a decrease in the CCP. [START_REF] Hodell | Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean[END_REF], g Gephyrocapsa mean mass (pg), h C. pelagicus abundance (*10 6 /g sed), i C. leptoporus abundance (*10 6 /g sed), j CaCO3 (%) derived from CaXRF, k PS75/059-2

Coccoliths abundance (10 9 coccoliths/g of sediment, Saavedra-Pellitero et al., 2017b), showed on AICC2012 chronology.

Increased CCP during TV and MIS 11 and its impact on atmospheric CO2

The onset of deep-water ventilation following MIS 12 is recorded at ∼432 ka, while the CCP starts to increase a few thousand years later, around 426 ka (Fig. 8). This distinct timing suggests that the rapid increase in atmospheric pCO2 during Termination V was triggered, in part, by the reinvigoration of the SO upwelling early in the deglacial progression. Concomitant with the reinvigoration of SO upwelling, the increase in global biosphere productivity [START_REF] Brandon | Exceptionally high biosphere productivity at the beginning of Marine Isotopic Stage 11[END_REF] that reflects in part, an increase in vegetation cover and terrestrial carbon storage [START_REF] Tzedakis | The last 1.35 million years at Tenaghi Philippon: revised chronostratigraphy and long-term vegetation trends[END_REF][START_REF] De Vernal | Natural Variability of Greenland Climate, Vegetation, and Ice Volume During the Past Million Years[END_REF]Prokopenko et al., 2009;[START_REF] Melles | 2.8 Million Years of Arctic Climate Change from Lake El'gygytgyn, NE Russia[END_REF][START_REF] Kousis | Centennial-scale vegetation dynamics and climate variability in SE Europe during Marine Isotope Stage 11 based on a pollen record from Lake Ohrid[END_REF] has probably acted as a sink of atmospheric CO2, thereby transiently buffering the impact of oceanic CO2 degassing (Fig. 8). pCO2 reached maximum concentration (up to ∼ 282 ppm) during the second part of MIS 11, as the CCP functioned at maximum strength. Enhanced CCP in the SO contributed to reduce the efficiency of the BCP. Overall, enhanced Gephyrocapsa sp. export and burial both in the SO (Saavedra-Pellitero et al., 2017a;2017b) and at low latitudes [START_REF] Barker | Globally increased pelagic carbonate production during the Mid-Brunhes dissolution interval and the CO2 paradox of MIS 11[END_REF][START_REF] Rickaby | Coccolith chemistry reveals secular variations in the global ocean carbon cycle?[END_REF]Alvarez et al., 2010;[START_REF] Amore | A Middle Pleistocene Northeast Atlantic coccolithophore record: Paleoclimatology and paleoproductivity aspects[END_REF][START_REF] Saavedra-Pellitero | Paleoceanographic evolution of the Japan Sea over the last 460 kyr -A coccolithophore perspective[END_REF], would have led to a global increase in carbonate dissolution at the seafloor [START_REF] Barker | Globally increased pelagic carbonate production during the Mid-Brunhes dissolution interval and the CO2 paradox of MIS 11[END_REF][START_REF] Qin | Deep-Water Carbonate Ion Concentrations in the Western Tropical Pacific Since the Mid-Pleistocene: A Major Perturbation During the Mid-Brunhes[END_REF] and to a significant change in the global rain ratio, eventually impacting atmospheric pCO2 [START_REF] Sigman | The calcite lysocline as a constraint on glacial/interglacial low-latitude production changes[END_REF][START_REF] Sigman | Glacial/interglacial variations in atmospheric carbon dioxide[END_REF][START_REF] Antia | Basin-wide particulate carbon flux in the Atlantic Ocean: Regional export patterns and potential for atmospheric CO2 sequestration[END_REF][START_REF] Barker | Globally increased pelagic carbonate production during the Mid-Brunhes dissolution interval and the CO2 paradox of MIS 11[END_REF]. This hypothesis warrants further investigations to fully quantify changes in the rain ratio and their impact on atmospheric pCO2. From 398 ka to 381 ka, the gradual, ~35 ppm decrease in atmospheric pCO2 coincided well with the rapid increase in benthic foraminifera Δδ 13 C in the South Atlantic (Hodell et al., 2003), consistent with enhanced stratification of the SO and increased carbon storage in the ocean interior. The second part of atmospheric CO2 drawdown during MIS 10 would be supported in part by the reduction in the strength of the CCP combined with a globally more efficient marine BCP.

At a global scale, the most striking pattern of MIS 11 relates to the low eccentricity context that is documented around 400 ka [START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF]. Low eccentricity, through the lengthening of the growing season, might have triggered an acme of primary producers such as Gephyrocapsa sp., at both high and low latitudes [START_REF] Rickaby | Coccolith chemistry reveals secular variations in the global ocean carbon cycle?[END_REF]. One cannot exclude either that our record reflects the long-term cyclic Noelaerhabdaceae evolutionary pattern recently highlighted for the tropical area over the Pleistocene [START_REF] Beaufort | Cyclic evolution of phytoplankton forced by changes in tropical seasonality[END_REF], with Gephyrocapsa blooms during MIS 11 being the consequence of reduced seasonality and phytoplankton niches. In all cases, we propose that, under this particularly low eccentricity context, enhanced terrestrial productivity and carbon storage on land, together with enhanced biological carbonate productivity and CCP strength in the SO may have contributed to shape the 30 ka-long CO2 plateau characteristic of MIS 11.

Conclusion

Our study documents temporal variations in the strength of the CCP based on Southern Ocean sediments and their impact on atmospheric CO2 concentrations over the period ranging between MIS 12 and MIS 10. First, using micropaleontological and geochemical data, we demonstrate that the sedimentary CaCO3 signal robustly reflects the production and export of Gephyrocapsa sp. and (to a lesser degree) planktonic foraminifera and can therefore be used as a high-resolution proxy of the CCP. Second, using planktic foraminifera, C. leptoporus and C. pelagicus abundances combined with δ 13 C of N. pachyderma s. from the same site, we reconstruct changes in sea surface temperature and nutrient conditions in conjunction with the regional upwelling dynamics, which are closely linked to the latitudinal migration of the westerly winds. Third, the combination of the results obtained in this study together with previous micropaleontological and geochemical data from literature allow us to highlight the sequence of events leading to the deglaciation and to understand the specific atmospheric pCO2 pattern that characterises this key interval of the past. We show that the abrupt atmospheric CO2 rise observed during TV is led by the reinvigoration of SO upwelling probably due to a poleward migration of the westerlies and ACC fronts, while the sustained CO2 plateau observed during MIS 11 is the result of enhanced global primary productivity and increased CCP due to the exceptionally low eccentricity context.

Figure 1

 1 Figure 1 Core location and study area. a, b Location of the studied core (MD04-2718, red dot) as well as cores from previous studies (ODP sites 1088 and 1090 (Hodell et al., 2003) and PS75/059-2 (Saavedra-Pellitero et al., 2017a; 2017b) (black dots). Oceanic fronts are from Park et al. (2019). NB: Northern Boundary of the ACC, SAZ: Subantarctic Zone, SAF: Subantarctic Front, PFZ: Polar Front Zone, PF: Polar Front, AAZ: Antarctic Zone, SACCF: Southern Antarctic Circumpolar Front, SB:

  the error on each of the variables.The size distribution, considering eight intervals(150-200 µm, 200-250 µm, 250-315 µm, 315-355 µm, 355-400 µm, 400-450 µm, 450-500 µm and >500 µm), and the mean weight for each size has been determined at six depths for the eight species representing > 99 % of the assemblage. The mean weight has been determined from 15 weighings of 1 to 4 individuals (to reach weight between 10 to 40 µg), to decrease the weight error to 19 % [(1/(2(n-1)) 0.5 ]. The eight species include

  30 and 11.03 m, while they average 85 % between 12.67 and 11.90 m. The most striking trends are observed between 13.10 and 12.66 m and 12.03 and 11.41 m, where CaCO3 concentrations undergo a drastic increase to maximum values of 99 % at 12.65 m and a two-fold decreasing trend to reach minimum values of 7 % at 11.38.

Figure 2

 2 Figure 2 Bulk carbonate signals and stable isotope records of planktonic foraminifera N. pachyderma s. from sediment core MD04-2718. a δ 18 ON. pachy s (‰), b δ 13 CN. pachy s (‰), c CaCO3 (%) derived from CaXRF signal, the envelope representing the error on the calculation of CaCO3.

Figure 3

 3 Figure 3 Coccolith and δ 18 ON. pachy s signals between 1103 and 1330 cm of the sediment core MD04-2718. a. δ 18 ON. pachy s (‰), abundances of : b coccoliths (number of coccoliths*10 9 per g sed.), c Gephyrocapsa (number*10 9 per g sed.), d C. leptoporus (number*10 6 per g sed.),. e C. pelagicus (number*10 6 per g sed.), f Gephyrocapsa mean mass (pg). Envelopes in b, c, d and f

Figure 4

 4 Figure 4 Planktonic foraminifera and δ 18 ON. pachy s signals between 1103 and 1330 cm of the sediment core MD04-2718. a Signal of δ 18 ON. pachy s (‰), b Number of total planktonic foraminifera (per g sed.), Number (per g. of sed.) of: c N. pachyderma s., d G. bulloides, e N. pachyderma d., f G. glutinata, g G. crassaformis, h T. quinqueloba, i G. inflata, j Reconstructed summer SST (°C) using N. pachyderma s. percentages (blue curve) and based on Modern Analog Technique method (green curve).

Figure 5 a

 5 Figure 5 a Age model of MD04-2718 based on the correlation between MD04-2718 summer SST reconstructions and EDC δD signal on AICC2012 timescale. Blue curve is the reconstructed summer SST using N. pachyderma s. percentages. Green curve is the reconstructed summer SST based on Modern Analog Technique method. Black curve is the δD measured in EDC ice

Figure 6

 6 Figure 6 Visual estimation of coccolith preservation of sediment core MD04-2718 based on Tescan VEGA II SEM observation (Museum National d'Histoire Naturelle, MNHN, Paris) showing good preservation a at 389.1 ka and b at 405.8 ka.

Figure 7

 7 Figure 7 Origin of the carbonate fraction. 1 Correlation between CaCO3 and Gephyrocapsa + planktonic foraminifera calcite mass, 2 a planktonic foraminifera calcite mass (mg/g), b Gephyrocapsa mean mass (pg), c Gephyrocapsa calcite mass (mg/g), d CaCO3 (%). Smoothed curves use a three-point average using IGOR software. The shaded areas represent the error on the curve. The dotted blue lines represent the beginning and end of MIS 11 from right to left respectively, and the dotted red line represent the middle of the rising slope of CO2 from glacial to interglacial.

  Figure 8 Records of CCP and upwelling strengths in the SO and relation with variation of atmospheric CO2 concentration.a CO2 (ppm)[START_REF] Nehrbass-Ahles | Abrupt CO2 release to the atmosphere under glacial and early interglacial climate conditions[END_REF], b Biopshere productivity (Fbio,t/Fbio,PST)[START_REF] Brandon | Exceptionally high biosphere productivity at the beginning of Marine Isotopic Stage 11[END_REF], c δ 18 O of N.

	pachyderma s. from MD04-2718, d summer SST reconstructed using N. pachyderma s percentages (blue curve) and using
	Modern Analog Technique method (green curve), e δ 13 C of N. pachyderma s. from MD04-2718, f Δδ 13 C of benthic
	Cibicidoides wuellerstorfi from ODP Site 1088-ODP Site 1090

Acknowledgments

This work benefited from founding from the INSU-LEFE-IMAGO program BIOCOD, from the European Research Council under the European Union H2020 Programme (H2020/20192024)/ ERC grant agreement no. 817493 ERC ICORDA as well as from the Institut Pierre Simon Laplace (IPSL). M.B. doctoral scholarship is supported by a public grant overseen by the French National research Agency (ANR) as part of the « Investissement d'Avenir » program, through the "IDI 2017" project funded by the IDEX Paris-Saclay (grant ANR-11-IDEX-0003-02). S.L.J. acknowledges support from the Swiss

National Science Foundation (grants PP00P2_144811 and PP00P2_163003). We thank Sylvain Pont and the Plateau technique de Microscopie Electronique (PtME, MNHN, Paris, France) for their assistance in providing SEM images, which helped for the estimation of the preservation of the carbonate fraction. We also thank Yvonne Hamann for instructions for the XRF core scanner at ETH, Zurich.

Data availability

Data will be made available in the Pangaea database prior to publication.