BIOlogical productivity changes and their leverage on the CarbOn cycle during past Deglaciations – BIOCOD

BIOCOD: BIOlogical productivity changes and their leverage on the CarbOn cycle during past Deglaciations

The study of biological productivity in the both, ocean and continent, during glacial-interglacial transitions, will allow us to test the main hypotheses linking past variations in atmospheric carbon dioxide concentration and biological productivity. The underlying mechanisms will also be explored through the use of climate and carbon cycle models.

Toward a better understanding of C stocks and C fluxes associated with biological productivity and their impact on atmospheric CO2 concentration changes during deglaciations of the past 800 000 years

During each of the nine glacial-interglacial transitions of the past 800 000 years (800 ka), often referred to as “glacial terminations”, atmospheric carbon dioxide concentrations (pCO2) rose by 50-100 ppm within a few thousand years, yielding important feedbacks on deglacial warming. Despite the central role played by CO2 forcing in climate changes, including recent ones (IPCC, 2022), the synergistic mechanisms leading to these deglacial transitions remain elusive. While the ocean and permafrost reservoirs are suspected to have transferred vast amounts of CO2 to the atmosphere during glacial terminations through the release of previously sequestered remineralised carbon (C) from the ocean interior and thawing permafrost respectively, quantitative estimates of the global and regional C stocks and C fluxes associated with biological productivity are missing.<br /><br />Very little is known about past changes in marine and terrestrial biological productivity, despite their link to the global C cycle, as photosynthetic organisms continuously convert CO2 from the ocean-atmosphere system into organic matter before promoting its sequestration in the ocean interior and the lithosphere. Model simulations and paleoclimatic reconstructions suggest that decreasing marine productivity in the sub-Antarctic zone (SAZ) owing to the southward migration of the polar frontal system and dwindling iron fertilization, may have contributed to release previously sequestered CO2 to the atmosphere by decreasing the strength of the Soft Tissue Pump i.e., the organic C uptake and export to depth and by inference, the deep ocean C storage. However, these studies typically overlooked the strength of the Carbonate Counter Pump i.e., the export of plankton-derived calcium carbonate which raises surface water CO2 and contributes together with the Soft Tissue Pump, to the ocean’s Biological C Pump. In parallel, evidence is growing that the terrestrial biosphere may have also played a key role in regulating pCO2. Increasing terrestrial productivity and C storage in high and sub-tropical latitudes during terminations might have led to decrease pCO2, thus partially counteracting the release of C from the ocean interior. <br /><br />In this project, we aim to quantify multi-centennial to multi-millennial changes in marine and terrestrial biological productivity and unravel their impacts on glacial-interglacial pCO2 patterns over the past 800 ka combining direct and indirect productivity proxies inferred from natural climate archives together with global climate modelling experiments. To achieve our objectives, we propose to join our efforts on the multi-centennial patterns of terminations TVII (620 ka), TV (430 ka) and TIII (250 ka) that encompass the end of the Mid-Pleistocene Transition (1 200-600 ka) and the Mid-Brunhes Event (430 ka) corresponding to the transitions from a world dominated by 41 ka Glacial-Interglacial cycles to a 100-ka global climate periodicity, exhibiting large pCO2 changes.