Southern Ocean biological pump over the last glacial cycle from new diatom transfer functions

Abstract. We present new transfer functions to reconstruct deep ocean (~1000 m) particulate organic carbon (POC) flux and particulate inorganic to organic carbon export ratio (PIC : POC) from diatom assemblage in the Southern Ocean. The transfer functions were calibrated with modern sediment trap data covering the three ocean sectors of the Southern Ocean. They were then applied to ten sediment cores located in the Antarctic Zone (AZ) in the three Southern Ocean basins. The diatom community appears to catch efficiently the ecosystem structure that sets the magnitude and stoichiometry of the export fluxes with root mean square errors of the prediction ranging 17–19.6 % depending on the transfer function. A consistent climatic signal is observed in all sediment cores: the reconstructed deep-ocean POC export is higher during glacial than interglacial periods. The PIC : POC ratio is low during glacial periods and increases quickly after glacial maxima. These two signals suggest that both the increase in the biological carbon pump and the decrease in the carbonate counter-pump in the AZ during glacial periods could have contributed to the decrease in atmospheric pCO₂. The reconstructed POC export is consistent with previously published diatom-bound δ¹⁵N and total organic carbon content but differs from elemental Ba/Fe ratio, hinting Ba potential preservation issues in Southern Ocean sediments. At the global Southern Ocean scale, the deep-ocean POC export flux decreases by 50 % and the PIC : POC export ratio increases by 17 % during the last deglaciation. While the glacial/interglacial POC flux change is comparable in the three SO sectors, the PIC : POC change is weaker in the Pacific, suggesting a distinctive response of the calcifying plankton community to glacial conditions in this sector. We suggest two mechanisms likely to increase the biological pump efficiency during glacial periods: 1) iron fertilization increasing primary production combined with diatom spore formation that increases export efficiency, and 2) a northward extension of sea ice edge supporting a greater zooplankton-mediated export that increases transfer efficiency. These new transfer functions quantitatively support a glacial iron fertilization effect in the AZ, contrasting with the view of a fertilization effect restricted to the Subantarctic Zone.