African grasslands became widespread over five million years earlier than previously known
New study in PNAS: "The origin and development of the Miocene northwest African savanna"
image: Map of study area showing the major ecosystem divisions in the modern day summarized from Sayre et al. The dark red circles mark the locations of the core sites (ODP 659 and DSDP 368) investigated in this study.
Credit: Credit: Schartman, et. al.
The iconic grassland savannas of Africa are geographically extensive and important for both local fauna and how they affect global climate. But where did they come from, and how will they change in the future? Ongoing warming, rising CO2 levels, and shifting rainfall patterns are likely to profoundly change these ecosystems. These effects are difficult to predict because they can alter these ecosystems in competing ways.
This study investigates the transformation of Northwest African ecosystems over the past 24 million years using molecular fossils and their isotopic chemistry to understand their history and allow projections for the future. The molecular fossils are blown from the continent to the ocean where they can be preserved in ocean sediments. The research team sampled drillcores from the International Ocean Discovery Program and extracted and measured these fossils to understand when grasslands developed, and the possible causes for their rise to prominence over time.
They found that the first open (grassland) ecosystems occurred approximately 15 million years ago, or about 5 million years earlier than previously thought. This ecosystem was different from today’s in the types of grasses present, and it was not until millions of years later that grasslands more similar to our modern savanna became dominant.
The emergence of African grasslands is a hallmark event in global climate and ecosystem change. The development of grassland ecosystems at 15 million years also set the stage for faunal evolution in Africa that includes our human ancestors. The establishment at 15 million years ago and subsequent transformation occurred during periods of global cooling and aridification, but involved complex interactions between climate, wildfire, and herbivores that still operate today. The geologic record indicates that the response of savannas and other subtropical open ecosystems to future climate perturbations will depend on the nature and strength of these interactions, in addition to global climate changes.
This research was led by Anna Schartman during her Ph.D. at the University of California, Santa Cruz, in collaboration with her advisor, Ocean Sciences Professor Pratigya Polissar, and Professor Caroline Strömberg at the University of Washington.