# Carbonate counter pump strength and its impact on atmospheric pCO2 over the past 800 ka: evidence from Southern Ocean micropaleontological and geochemical data

Nicolas Pige, Yu Wang, Stéphanie Duchamp-Alphonse, Sophie Sépulcre, Vishnu Thuruttath Unnikrishnan, Margaux Brandon, Amaëlle Landais, Elisabeth Michel

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Nicolas Pige, Yu Wang, Stéphanie Duchamp-Alphonse, Sophie Sépulcre, Vishnu Thuruttath Unnikrishnan, et al.. Carbonate counter pump strength and its impact on atmospheric pCO2 over the past 800 ka: evidence from Southern Ocean micropaleontological and geochemical data. 2025. hal-05236257

HAL Id: hal-05236257

https://hal.science/hal-05236257v1

Preprint submitted on 11 Sep 2025

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# Carbonate counter pump strength and its impact on atmospheric pCO2 over the past 800 ka: evidence from Southern Ocean micropaleontological and geochemical data

Nicolas Pige<fnref n="1" />, Yu Wang<fnref n="1" />, Stéphanie Duchamp-Alphonse<fnref n="1" />, Sophie Sépulcre<fnref n="1" />, Vishnu Thuruttath Unnikrishnan<fnref n="1" />,<fnref n="2" />, Margaux Brandon<fnref n="1" />,<fnref n="3" />,<fnref n="4" />, Amaëlle Landais<fnref n="4" />, and Elisabeth Michel<fnref n="4" />

During the glacial-interglacial transitions of the past 800 000 years (800 ka), commonly referred to as "glacial terminations", atmospheric CO₂ concentrations (pCO₂) rose by 50-100 ppm. Biological productivity from the Southern Ocean (SO) significantly impacted these variations through changes in the Biological Carbon Pump strength, which includes the Soft Tissue Pump (STP) i.e., the net downward flux of phytoplanktonic organic carbon, and the Carbonate Counter Pump (CCP) i.e., the export of planktonic calcium carbonates increasing the surface-to-depth alkalinity gradient. Both modulate ocean-atmosphere exchanges as they respectively decrease and increase CO₂ concentrations in the surface ocean and hence the atmosphere. Paleoclimate studies focusing on the SO highlight decreasing STP from the Subantarctic as a potential driver of increasing pCO₂ during glacial terminations. A few studies have demonstrated the probable impact of CCP on pCO₂ over specific glacial terminations (Duchamp-Alphonse et al., 2018; Brandon et al., 2022; Anderson et al., 2024) but very little is known about CCP patterns over the past 800 ka.

This study aims to reconstruct changes in CCP strength over the past 800 ka and assess their impacts on pCO₂. Following the exact same strategy as the one developed by Brandon et al., (2022), we performed micropaleontological (coccolith and foraminifera abundances and morphometrics) and geochemical analyses (CaCO₃, CaₓRFi, d¹³Cₙ. pachyderma, d¹⁸Oₙ. pachyderma) on sediment core MD97-2115 (43°10,84S, 171°48,55W), retrieved in the Pacific sector of the Subantarctic Zone. Preliminary results show that the carbonate fine fraction (< 20µm) of the sediment is mainly composed of coccoliths (*Emiliania huxleyi* and *Gephyrocapsa* morphotypes; *Coccolithus pelagicus*; *Calcidiscus leptoporus*) and might be used as a CCP signal.

<footnote n="1">¹Université Paris-Saclay, CNRS, GEOPS, 91405 Orsay, France</footnote>

<footnote n="2">²Univ. Lille, CNRS, UMR 8198, Evo-Eco-Paleo, F-59000 Lille, France</footnote>

<footnote n="3">³Department of Ocean Systems, Royal Netherlands Institute for Sea Research (NIOZ), Den Burg, Texel, the Netherlands</footnote>

<footnote n="4">⁴Université Paris-Saclay, CNRS, CEA, UVSQ, Laboratoire des sciences du climat et de l'environnement, 91191 Gif-sur-Yvette, France</footnote>
(2024).

Brandon, M. *et al*. Enhanced Carbonate Counter Pump and upwelling strengths in the Indian sector of the Southern Ocean during MIS 11. *Quaternary Science Reviews* 287, 107556 (2022).

Duchamp-Alphonse, S. *et al*. Enhanced ocean-atmosphere carbon partitioning via the carbonate counter pump during the last deglacial. *Nature Communications* 9, 1–10 (2018).