West African hydroclimate trends and variability during the Cenozoic

Abstract:

Hydroclimate in the western Sahel is controlled by the West African Monsoon (WAM), bringing heavy rains to the region during the summer months. Modest changes in this system over the historical record have had outsized impacts on ecosystems and resources. Prior studies have documented the remarkable sensitivity of this system to low-latitude orbital insolation changes as well as high-latitude temperature changes throughout the Neogene. Here we explore the Cenozoic history of WAM hydroclimate sensitivity to orbital forcing as well as long-term, secular trends and transitions. Land surface changes, particularly the expansion of C₄ grasslands, which has been dated in this region to ~10 Ma, could have a dramatic effect on tropical hydroclimate by way of various feedbacks. However, our understanding of the potential impacts on tropical monsoonal hydroclimate from major boundary regime changes is poor because we lack quantitative, high resolution records from the tropics that span the Cenozoic. We present new orbitally resolved hydrogen isotope records from leaf waxes (δD_wax) preserved in ODP Site 959 marine sediment core, located in the Gulf of Guinea. This sediment core archives ~25 myr of fairly constant sedimentation, and contains distinct intervals of color and chemical oscillations that we have targeted for variability analyses. We compare these results with Plio-Pleistocene δD_wax records from the same region to quantify how the monsoon responded to orbital forcings in differing global and regional boundary conditions throughout the Cenozoic. These are the first orbitally resolved δD_wax records from Africa prior to the Pliocene, and hence provide a new deeper-time perspective on hydroclimate variability in a region with uncertainty surrounding its response to future global warming.