Terrestrial West African climate and environmental responses to orbital forcing across Neogene boundary condition changes

The Sahel is highly sensitive to both floods and droughts, risking food and other resources on which nearly 100 million people depend. Understanding how natural variations of precipitation and vegetation fluctuate in response to orbital forcing across major shifts in boundary conditions, like temperature, ice volume, and land surface, can help constrain the regional sensitivity to a wide range of external forcings. However, these interactions between climate and ecosystem changes remain uncertain for sub-Saharan Africa due to the lack of long, highly resolved, quantitative, terrestrial records that span major global and regional shifts in deep time. Here we present leaf wax precipitation and vegetation records from 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 leaf waxes. Analyses of trend, amplitude of variability, and periodicity document a range of rainfall and vegetation responses to orbital forcings, depending on the specific boundary conditions of the study window. We find that the Western Sahel got wetter, yet more C₄-rich, over the Neogene. Orbital-scale precipitation was highly variable throughout the study periods, but particularly strong during the warm Miocene. While unlike many East African leaf wax isotope records that are precessionally driven, obliquity appears to play a role in the late Pleistocene, suggesting that climate-driving orbital parameters may vary regionally. Further, because of the high resolution and temporal coverage of these new biomarker isotope records, we can examine nonlinear relationships between precipitation and vegetation fluctuations, including prior to C₄-expansion when we find strong correlation despite minimal variation in δ¹³C_wax, advancing our understanding of climate and ecosystem feedbacks millions of years ago.