Orbital-scale climate and environmental responses of the Western Sahel to shifts in Cenozoic boundary conditions

Nearly 100 million people live in and depend on the Sahel for agriculture and natural resources. The region is sensitive to natural climate and environment variations caused by the seasonal movement of the tropical rainbelt. In the paleoclimate record, insolation plays a clear role on West African Monsoon strength, but responses to other forcings like temperature, greenhouse gases, ice volume, and land surface cover are unclear due to the lack of highly resolved, terrestrial records that span major global and regional shifts through time. Here we present leaf wax precipitation and vegetation records from five targeted study windows throughout the last 25 million years, derived from long-chain n-alkane hydrogen (δD_wax) and carbon (δ¹³C_wax) isotopes, respectively, in a sediment core from ODP Site 959 in the Gulf of Guinea, where westerly winds and major river systems transport Western Sahel-sourced material. Analyses of trend and variability document a range of rainfall and vegetation responses to orbital forcings in different boundary conditions in the Oligocene, Miocene, Pliocene, and Pleistocene. We find that both the climate and environment was more variable in times of higher CO₂ and global temperatures, suggesting an increase in ecosystem instability moving forward into the future. Because of the high resolution and temporal coverage of these new biomarker isotope records, we can examine relationships between precipitation and vegetation fluctuations, even prior to C₄-expansion when there was a strong correlation despite minimal variation in δ¹³C_wax in a C₃ world. Further, we find a drying trend throughout the record, demonstrating that vegetation on long timescales was decoupled from hydroclimate and was like driven by global CO₂, advancing our understanding of climate and ecosystem relationships across the Cenozoic.