A Plio-Pleistocene molecular isotopic record of Turkana Basin vegetation

Abstract:

Paleovegetation and paleontological data from East Africa suggest that over the last five million years, the evolution of mammals has been significantly influenced by the emergence and expansion of C₄ grasslands. Isotopic data from soil carbonate and marine core biomarkers indicate increased C₄ grasses on the landscape beginning in the late Miocene. However, most Plio-Pleistocene sediments at vertebrate fossil localities lack sufficient soil carbonates that could allow us to directly link organisms to the particular environment where they lived. Here we take advantage of the organic biomarker record of vegetation in East Africa to understand landscape variability, its long-term evolution, and the relationship to the fossil record. Biomarker isotope records from Omo Group sediments, a fluvial-lacustrine sequence in the Turkana Basin, differ from their marine core counterparts in that they provide a local vegetation signal that can be directly linked to fossil material from the same sediments; they differ from soil carbonate records in that they are not limited to the specific environmental conditions necessary for carbonate precipitation.

We present carbon isotope data from n-alkane and n-alkanoic acids from Omo Group sediments that span 3.6 to 1.1 Ma. The δ¹³C values from nC31 alkanes and nC30 alkanoic acids indicate high landscape variability ranging from C₃-dominated to pure C₄ environments. In both the Shungura and Nachukui Formations, biomarker data record significantly more C₄ vegetation on the landscape than existing soil carbonate data. Biomarker data from the Shungura Formation indicate a more dynamic landscape than what is shown in the soil carbonate record. Biomarker samples come from sediments initially collected for pollen or paleomagnetic analyses that generally do not contain soil carbonates. They represent a wide variety of sediment types and therefore capture a broad range of depositional environments and environmental variability. Combined δ¹³C records from biomarkers and soil carbonates indicate a more dynamic landscape than what is recorded by a single proxy alone. The combined paleovegetation record from these two proxies suggests that East African fauna evolved in a highly variable landscape during the Plio-Pleistocene.