Precipitation variability in the Sahel during the last 2000 years revealed by archaeological shell middens in Senegal

Abstract

In the second half of the 20th century, a strong multidecadal drought occurred in the Sahel that had catastrophic social and economic consequences. This event made it urgent to better understand the forcing driving precipitation changes in the Sahel and potential impact of anthropogenic global warming. Here, we reconstruct past changes of the precipitation regime in the Saloum Delta, Senegal, from stable isotope analyses in fossil shells of Senilia senilis. The shells were collected in large radiocarbon dated shell middens built by ancient fishermen during the last two millennia. Oxygen isotopic composition in shells is related to salinity conditions in this hypersaline estuary that is very sensitive to changes in precipitation and evaporation. Our dataset shows that salinity is today at its highest level in the past 2000 years. These values were reached abruptly in the 20th century and lie outside the range of natural climatic variability in this site. Monthly resolved isotopic profiles in fossil shells combined with analyses of shell growth lines allowed us to explore the role of rainfall seasonality in the variability of mean conditions.