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 fractionation at 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 taphonomy on the 17O-excess of phytoliths and applicability of the humidity proxy to record regional climate inside and outside the calibration area will be checked 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 fractionation leading to the D17O of atmospheric O2. Then a coupled model-data approach will be used to produce an updated estimation of the biosphere productivity during the last climatic cycle. Records of D17O of O2 measured in ice cores are available for that purpose.
The HUMI-17 ANR project (2017-2021) is based on an ongoing collaboration between CEREGE/IMBE, LSCE and Ecotron (initiated from 2016 to 2017 in the frame of an INSU-LEFE-IMAGO project), 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).