The Ocean's Role in the Early Pleistocene Aridification of East Africa

Much of East Africa is characterized by vast expanses of open savannah grasslands. Savannah (mostly C4) grasses are functionally adapted to warm, semiarid regions where rainfall is highly seasonal.  These grasslands first expanded (globally) during the late Miocene, but the East African grasslands only reached their modern extent relatively recently, during the early Pleistocene between 2.0-1.6 Ma. This is documented by a number of independent proxies including stable isotopic analyses of soil carbonates, increases in the proportion of grazing bovids in fossil mammalian fauna, and increases in grassland-derived plant wax biomolecules preserved in deep-sea sediments of the Gulf of Aden.

A puzzling aspect of this East African climate transition has been its timing. Grassland expansion after 2 Ma occurred fully one million years after the onset of glacial cycles near 3 Ma. An emerging view is that changes in tropical ocean temperature gradients, particularly across the Indian Ocean, shaped the climate history of East Africa. Atmospheric convergence, convection, and rainfall occur where ocean waters are warmest (such as the IndoPacific warm pool region), whereas divergence, descent, and aridity occurs where tropical oceans are coolest (such as upwelling regions along the NE African-Arabian margins).  Current studies note changes in the zonal Indian Ocean SST gradient are the dominant cause of East African rainfall anomalies today. Existing paleoceanographic data from the tropical Pacific and Indian Oceans document large and fundamental changes in tropical SST gradients during the early Pleistocene.

Here, we explore the possible role of the tropical ocean SST changes in shaping East African paleoclimate history by developing records of Indian Ocean zonal SST gradient  over the Pliocene and Pleistocene. We developed a Mg/Ca record of SST variability at Site 762 near the Exmouth Plateau off NW Australia and compare this to two published SST records from the Arabian Sea. These records demonstrate that the modern zonal Indian Ocean SST gradient of 3-4°C was first established near 2.0 Ma. Prior to 2 Ma, zonal SSTs were nearly uniform at 27-28°C. After 2 Ma, Arabian Sea Site 722 SSTs cooled to 25°C, and Site 762 SSTs synchronously warmed to 28-29°C.

The absence of the zonal Indian SST gradient prior to 2 Ma is consistent with more humid conditions in East Africa as the warmer waters off the continent would promote greater convection and rainfall. We conducted a series of climate model experiments using the NCAR model with zonally uniform SST gradients imposed over the Indian, Pacific, and Indo-Pacific basins to determine the ocean’s role in shaping East African climate. The experiments clearly document the leading influence of Indian Ocean SST on East African climate: Annual East African rainfall increases by 30% is permanent zonally-uniform SSTs are imposed. 

We suggest that the leading cause for the earty Pleisotcene expansion of East African grasslands was the emergence of the modern Indian Ocean SST gradient near 2 Ma.