Positive precipitation-evaporation budget from AD 460 to 1090 in the Saloum Delta (Senegal) indicated by mollusk oxygen isotopes

摘要

There is a critical need to document the long-term variability of the West African Monsoon (WAM) in the Sahel region. We present here a multidecadal proxy record of the past hydrology from AD 460 to 1090 in the Saloum Delta, Senegal. The Saloum Delta is a hypersaline estuary where the salinity and the water isoto-pic composition are highly sensitive to rainfall variations. The past hydrology was studied using the oxygen isotopic ratio of Anadara senilis fossil shells, since mollusk shell isotopic composition (δ~(18)O) in this environment is primarily determined by the precipitation-evaporation budget. Successive samples of shells were taken along the stratigraphy of the massive Dioron Boumak fossil shell middens for new insights into the past WAM multi-decadal to centennial variability. The averaged δ~(18)O value of fossil shells was more negative by 1.4‰ compared to modern shells' isotopic signature. This result indicates substantially fresher mean conditions in the Saloum Delta, that was likely not hypersaline as it is today. The precipitation-evaporation budget was thus more positive in response to a more intense and/or longer monsoon season during the studied period. Our record suggests that strong multidecadal droughts as observed in the Sahel in the late 20th century did likely not occur in Senegal during this ～600-yr time period.