Reconstructing the Sahelian atmospheric relative humidity associated with vegetation changes using the 17O-excess of phytoliths from Lake Guiers sediments.

Atmospheric relative humidity (RH) and vapor pressure deficit (VPD, a combination of RH and temperature) are primary drivers of vegetation productivity and dynamics, especially in the tropics where air temperature changes are limited. Local RH and VPD conditions are also important factors used to predict the probability of fire occurrence and behavior because they control fuel moisture. RH is therefore a crucial climate variable that drives vegetation distribution and modifies disturbance regimes. Since the end of the last century, continental RH has been decreasing (and VPD increasing) resulting in increasing mortality in tropical trees. However, RH predictions from Earth System Models (ESMs) are poorly constrained and vary on a regional scale from one model to another, probably due to our poor knowledge of feedbacks between vegetation and the atmosphere. Reconstructing independently past vegetation and RH changes beyond the instrumental period is of intense interest in order to better understand interactions between vegetation and the RH and produce quantitative data available for comparison with ESMs outputs for past climates. A set of calibrations recently showed that the triple isotope composition of oxygen in phytoliths, which are biogenic silica particles formed in plants, expressed by the 17O-excess (d’17O – 0.528*d’18O), is a proxy of RH conditions prevailing during the plant water evaporation, independently of vegetation type. A quantitative relationship between 17O-excess of phytoliths and RH of the growing season was therefore established. Combining the study of phytoliths morphology, a commonly used indicator of past vegetation, and their 17O-excess signature, makes it possible to produce independent reconstructions of past vegetation and RH. Here, the triple oxygen isotope composition of sedimentary phytoliths from Lake Guiers (16°15′00”N, 15°50′00”W), located in the Sahelian area of Senegal, was analyzed by IR-laser-fluorination-IRMS. The morphological assemblages of phytoliths were also defined using optical microscopy and phytolith indices. The sediments were dated using 210Pb and radiocarbon, and record the last 4000 years. Modern soil and plant samples were also analyzed for phytoliths content and the associated RH was estimated using their 17O-excess signature. We provide reconstructions of regional changes in RH and vegetation over the last 4000 years, with a focus on the last 400 years due to a higher temporal resolution. We also studied the potential biases in the regional RH reconstructions due to the contribution of phytolith from local lakeshore vegetation (growing under higher RH conditions) and to the contribution of diatoms (produced in isotopic equilibrium with the lake water). This study is part of the HUMI-17, PAST-17 and Thresholds projects funded by the French National Research Agency  (ANR) and the European Union’s Horizon 2020 research and innovation program. The coring was made possible thanks to the contribution of the OLAC (Office des LAcs et Cours d’eau du Sénégal).