Past dynamic of continental atmospheric humidity and vegetation: new insights from the triple isotopic composition of oxygen – HUMI-17

Continental atmospheric relative humidity is a key-climate parameter poorly captured by global climate models. A model-data comparison approach, applicable beyond the instrumental period, is essential to progress on this issue. However, there is a lack of past humidity proxies. The first objective of the HUMI-17 project is to combine expertise from LSCE and CEREGE in triple oxygen isotopic measurements to develop a new proxy of past relative humidity in continental areas, namely the triple oxygen isotopic composition of plant silica (phytoliths). Calibration of the proxy requires determination of the triple oxygen isotopic fractionations in play in the water cycle at the soil/plant/atmosphere interface. For that purpose, we will benefit from the unparalleled capacity of environmental conditioning and online measurements in growth chambers offered by the CNRS Ecotron facility. Relative humidity, temperature, pCO2 and day/night alternation will vary independently. Then, special attention will be paid to scale up the proxy and the water cycle monitoring from the growth chamber to the natural environment using the equipped outdoor sites of the O3HP and of the AMMA-CATCH national observation service. Additionally, impact of vegetation sources and of taphonomy on the 17O-excess of phytoliths will be investigated using plants, soils and surface sediment samples collected along current gradients of climate and vegetation. The second objective of HUMI-17 is to foster our knowledge of the interactions between climate (including humidity) and vegetation during pre-anthropic periods. For that purpose, we will produce three local records of past changes in atmospheric humidity and vegetation that will be compared to multi-proxy reconstructions already available and to downscaled outputs of coupled climate-vegetation models. Additionally, we will use the triple isotopic composition of atmospheric oxygen (O2) to reconstruct past changes in biosphere productivity at the global scale. The O2 cycle is directly linked to the water cycle and evolution of global productivity is a key parameter when studying past changes in the global water cycle. We will take advantage of the Ecotron growth chamber experiment to quantify, at the soil/plant/atmosphere interface, the successive fractionations leading to the ?17O of atmospheric O2. Then a coupled model-data approach will be used to produce an update estimation of the biosphere productivity during the last climatic cycle. Records of ?17O of O2 measured in ice cores are available for that purpose. The HUMI-17 project is based on an already existing collaboration between CEREGE/IMBE, LSCE and Ecotron to which will be added HSM (for AMMA-CATCH), the Department of Earth Sciences (Western University, Canada) and The Rosenstiel School of Marine and Atmospheric Science (University of Miami, USA). Our research will be structured around the following tasks: 1) growth chamber calibrations, 2) field calibrations, and 3) past climate and vegetation changes reconstructions. The amount of aid claimed to the ANR is 531k€ (for 4 years).