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.
BIOCOD is the rational consequence of ongoing collaborations between GEOPS (partner 1), LSCE (partner 2), EPOC (partner 3) and IGE (partner 4) via inter-laboratory projects focusing on past changes in the global cycle C. It also makes part of well-established international collaborations focusing on the Southern Ocean and its impacts on the global C cycle in the past as well as model intercomparison projects within PMIP4.
The work program is divided into four tasks related to specific scientific objectives and methods. Task 1 is devoted to the marine productivity and the Biological C Pump efficiency based on the geochemical (TOC, CaCO3, XRF, C/N, alkenones, sterols, n-alkanes, Cd-Sr-U-Ba/Cabenth-foram,d13Corg, d13Cforam d18Oforam), micropaleontological (coccoliths, foraminifera, diatoms) and sedimentological (clay mineralogy and grain size) analyses of sediment cores MD04-3728 and MD97-2115, retrieved in the Indian and Pacific sectors of the Subantarctic Zone respectively. Task 2 focuses on the terrestrial productivity and C stocks and relies on compilations of published pollen, TOC and biomarker data as well as new pollen data from IODP sites U1385 (Iberian margin) and 1446 (Bay of Bengal) during the targeted terminations. Task 3 aims at quantifying global biological productivity fluxes and controls on CO2 changes based on measurements of isotopic composition of CO2 and O2 (D17O of O2, CO2 contents and d13C of CO2) in the air trapped in Epica Dome C and Beyond EPICA Oldest ice cores, collected in Antarctica. In such a task, BIOCOD also aims to finalize the innovative optical spectroscopy instrumental development run at the IGE since 2021 that will allow for the first rapid acquisition of joint d13CCO2 and CO2 analyses on 20 g samples with respective analytical uncertainties of 0.04 ‰ and 1 ppm. Task 4 is dedicated to local/regional vs global productivity patterns and their impacts on C stocks and C fluxes, over the targeted terminations using the intermediate complexity model iLOVECLIM. Recent implementations of d13C and d18O of O2 as well as the undergoing implementation of D17O of O2 into the model are particularly helpful to establish the relationship between terrestrial, marine and global biological productivity on the C-cycle. Overall, selected sediment and ice cores, together with the iLOVECLIM model that is particularly well suited for transient simulations of several thousand years, allow us to work at resolutions better than 2-5ka over the past 800 ka and 0.3-0.5 ka across TVII, TV and TIII.
Quaternary Glacial-Interglacial transitions are key to answer significant challenges pose by anthropogenic climate changes, as they witness important atmospheric concentration CO2 (pCO2) rises (50-100 ppm), in a few thousand years. However, quantitative interpretations of these pCO2 changes are not yet possible as carbon stocks and carbon fluxes related to the major reservoirs of the climate system, remain unknown. Particularly, those involving the biosphere productivity, at the interface between the atmosphere, the hydrosphere and the lithosphere, have a major role in the atmospheric pCO2 changes. In this context, BIOCOD aims to quantify biological productivity changes and their impacts on atmospheric pCO2 variations during the glacial-interglacial cycles of the last 800,000 years, through the multiproxy study of sedimentary and glacial cores, and the use of climate models.
Madame Stéphanie DUCHAMP-ALPHONSE (Géosciences Paris-Saclay)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
EPOC Ecole Pratique des Hautes Etudes Paris
IGE Institut des Géosciences de l'Environnement
GEOPS Géosciences Paris-Saclay
LSCE Centre national de la recherche scientifique
Help of the ANR 569,359 euros
Beginning and duration of the scientific project: December 2022 - 48 Months