la Composition Isotopique TRiple de l'OxygèNe : de Nouveaux Indicateurs de l'Evolution de la biosphEre et du cycle hydRologique – CITRONNIER

About 3.10E16 moles of oxygen are exchanged annually through the respiration and photosynthesis processes. This makes the oxygen cycle the most important biogeochemical cycle on Earth with a strong control by biospheric productivity. Long records of d18O of atmospheric air on the recent Quaternary show a strong influence of orbital parameters (precession), ice sheets volume and abrupt climate changes (Dansgaard-Oeschger events) on the oxygen cycle. Our aim is to quantify the different fluxes associated with the oxygen cycle: how did biospheric productivity vary in the past with respect to climate change? What was the role of orbital parameters? of ice sheet volume? of the tropical regions (shift of ITCZ, of the monsoon system and thus of the water sources for the plant)? Questions on the interactions between climate and water cycle are connected to those on the oxygen fluxes since the terrestrial biosphere productivity is closely linked to the water cycle organization through the photosynthesis process. What are the location and intensity of water recycling? How does the repartition of evaporative regions vary when climate change (at the orbital and millennial timescales)? In order to better constrain oxygen cycle and water cycle, we propose here a new method based on the measurements of the triple isotopic composition of oxygen (16O, 17O, 18O) in air and in water. The triple isotopic composition of oxygen of air is a tracer integrating different parameters (global biospheric productivity, ice sheet volume, vegetation repartition, relative humidity during evapotranspiration…). Our project will thus integrate a data-model approach that will enable the precise and quantitative extraction of the past variations of biosphere productivity. This comes as a primordial complement to the numerous studies performed during the last decades on (1) d18O and dO2/N2 of fossil air from ice cores and (2) the hydrologic cycle through dD and d18O of water and ice. This project includes the original development at LSCE of an original line enabling high precision measurements of the triple isotopic composition of oxygen in the air rapped in ice cores. Combined to the recently developed line for the measurement of the 3 isotopes of oxygen in water, this experimental set-up will enable a complete description of spatial and seasonal variations of the 3 isotopes of oxygen in air and in water. This experimental part will be associated with a modeling part through the development of the oxygen cycle and its isotopes in the 'system Earth' model CLIMBER-2 enabling transient simulations on major climatic changes. Moreover, the triple isotopic composition of water has been recently added in the 'water cycle' part of several atmospheric general circulation model (AGCM) so that we project comparison between models of low and high resolution to better constrain and parameterize the main processes (recycling, convection, advection, …). The data-model intercomparisons are central in our project and will enable a quantitative interpretation of the triple isotopic composition of oxygen. After the first studies on the spatial and seasonal variability (especially role of the tropical and polar regions on the triple isotopic composition of oxygen in water), our project aim at studying the interactions between climate, biosphere, water cycle on selected time periods highlighting the role of orbital parameters, ice sheet volume, millennial climatic variability. We will thus study the last glacial –interglacial transition through isotopic measurements in several ice cores (Antarctica, Greenland) and transient simulations with the CLIMBER-2 model. With the same tool, we plan a complete study of the last interglacial period and glacial inception. Finally, we aim at documenting the variability of the glacial period with the study of Dansgaard-Oeschger events during periods of high and low sea level as well as a study of marine isotopic stage 6.5 (~ -175 kyrs) characterized by a minimum in precession.