Past biological productivity inferred from the isotopic composition of O2 and CO2

Biological activity is key to control the CO₂ atmospheric concentration and strong retroactions involve climate, carbon cycle and biosphere productivity. Still, determining the contribution of the biological The BIOPROXY project aims to estimate the past productivity of the biosphere based on the isotopic composition of oxygen and carbon dioxide.fluxes on the CO₂ atmospheric concentration over the Quaternary remains difficult. The isotopic composition of CO₂ is a useful tool but disentangling the different drivers of d¹³C of CO₂ is still a challenge : some studies suggested that the changes in d¹³C of CO₂ and CO₂ during the last deglaciation should be explained by increase in sea surface temperature and weakened biological pump, other studies invoked upwelling of carbon-enriched waters in the Southern Ocean, growth of the terrestrial biosphere or the role of thawing permafrost. In parallel, the elemental and isotopic composition of O₂ has also been proposed as a tool to provide information on total biospheric productivity or relative proportion of terrestrial vs marine biological productivity. Here, we want to combine the information carried by both tracers to better constrain the evolution of the biological carbon and oxygen cycles with respect to climate change. To reach this goal, we aim at (1) identifying the elemental and isotopic signature of CO₂ and O₂ changes during biological processes, (2) complete a 1.5 Myear record of emental and isotopic composition of O₂ and CO₂ using the newly recovered Beyond EPICA ice core and (3) interpret the ice core series using a simple box model equiped with CO₂ and O₂ isotopes and adapted for sensitivity tests. The approach followed in this project combines instrumental development (laser spectroscopy), laboratory experiments (controlled and multiplexed biological chambers) using expertises in ecophysiology and isotopic geochemistry, analytical measurements (mass spectrometry and optical spectroscopy), ice core analyses as well as simple modeling including explicit representation of the isotopes in the carbon and oxygen cycles over glacial – interglacial cycles.

Participants : LSCE (PI A. Landais), ECOTRON Sud de France (PI C. Piel), ECOTRON Ile de France (PI S. Fiorini), IGE (PI E. Capron), ENS Géologie (PI S. Abiven)