Environmental comparisons of the Awash Valley, Turkana Basin and lower Omo Valley from upper Miocene to Holocene as assessed from stable carbon and oxygen isotopes of mammalian enamel

摘要

The Awash Valley, Turkana Basin and lower Omo Valley of East Africa are three regions that have been particularly important for documenting the environment from the late Miocene to the Holocene, but these basins have never been compared throughout that large temporal sequence. In this context, we compare changes in the diet of herbivores with mixed diets (hippopotamids, elephantids, suids and bovids), the total large-vertebrate diet in the ecosystem, as well as water deficit from these three basins between 7.4 Ma and 10 ka to determine how they were different. Our sample consists of a compilation of more than 3000 published mammalian stable isotopic values. Our results show that the Awash valley becomes more arid through time, corresponding broadly with an increase in C-4 plants, but that relationship is not clear in the other two basins. The Awash and Turkana are broadly similar in overall aridity while the lower Omo Valley is clearly more mesic between 4 and 2.5 Ma. However, the Turkana and Omo are similar in ecosystem values, while it is the Awash that presents a landscape with more C-4 plants. When comparing the diets, the three basins are similar, with an increase in C-4 plants after similar to 4 Ma (after 3.0-3.4 Ma in the lower OmoValley), with all taxa converging by 1.9-2.4 Ma on a similar diet with mostly C-4 plants. Elephants vary little throughout the sequence when compared to the other taxa. Our data suggest that, except for elephantids, all taxa studied track the expansion of C-4 plants in East Africa and that this expansion may be, in part, related to a general increase in aridity.