Disconnected Drivers of Late Pleistocene Northwest African Hydroclimate and Vegetation

Abstract:

African savanna ecosystems were the landscapes for human evolution, migration and food production, yet the fundamental drivers of rainfall and vegetation dynamics in these ecosystems remain unclear. Many proxy records suggest that Northwest African monsoon strength and extent is forced by changes in Northern Hemisphere ice volume or by local insolation, however neither can fully explain the observations. Here we use plant-wax δD and constant flux-normalized (³He_ET) dust fluxes to explore the orbital-scale pacing and drivers of the Northwest African monsoon, and plant-wax δ¹³C to reconstruct ecosystem variability. We show that late-Pleistocene rainfall in this region is controlled by low-latitude insolation and principally tracks changes in moisture delivery associated with the summer inter-hemispheric insolation gradient. While increases in precipitation are associated with expansion of savanna grasslands into the desert landscapes, changes in pCO₂ predominantly drive the C₃/C₄ composition of savanna ecosystems. These findings suggest that, despite widely varying forecasts of Northwest African precipitation change, future predictions of woody plant cover within existing African savannas may in fact be better informed by pCO₂.