Quantitative reconstruction of past hydroclimates : the triple oxygen isotope composition (17O-excess) of biogenic silica perspective – PAST-17

In a changing climate, RH is a key-parameter to consider when assessing interactions between land (vegetation, water surface) and atmosphere. The PAST-17 project aims at capitalizing on the recent development in the triple oxygen isotope (17O-excess) analysis of biogenic silica to quantify the evolution of past RH and resolve key-issues regarding past hydroclimates: 1) In a combined data/hydrological-isotope model approach, the 17O-excess of diatoms will be used to reconstruct changes in the hydrological balance of Sahelian and Saharan lakes during the African Humid Period (between 9-5kyrs BP), in a much more precise way than with the d-excess and ?18O alone. This period of Green Sahara has seen large-scale migration of human settlements in northern Africa and provides a possible reference for the near future climate in part of the Sahara and Sahelian regions. 2) Within a multi-proxy approach, the 17O-excess of fossil phytoliths from lake and peat sediments will be used to specify the effects of climate and human activities on past vegetation, as well as possible feedback loops between vegetation and climate in West, Central and East Africa between 3 and 1.5 kyrs BP which corresponds to the Iron Age and the development of agricultural innovations. 3) The 17O-excess of fossil phytoliths from buried soils will be used to produce relative humidity estimates and reassess the isotope lapse rates used for reconstructing the Cenozoic topography of the Tibetan-Himalayan orogen, a major component of the earth’s climate. The quantitative relative humidity estimates produced in the PAST-17 project framework will be available for the enhancement of the Earth System Models outputs that have difficulties to properly capture continental relative humidity, especially in the inter-tropical area. The project relies on a consortium of specialists in triple oxygen isotope composition of water and minerals, climate, vegetation and lake water proxies, climate and vegetation models from CEREGE, LSCE, LGL-TPE, ISEM, AMMA-CATCH and EspaceDEV.