δ¹³C records from fish fossils as paleo-indicators of ecosystem response to lake levels in the Plio-Pleistocene lakes of Tugen Hills, Kenya

The carbon isotopic ratios of organic matter in fish fossils from diatomites and other lake beds in the HSPDP drill core from Tugen Hills, Kenya (2.56–3.29 Ma) reflect trophic resource uses, and can indicate the dietary habitats of fish in the paleolake. This information offers insight into how fish communities responded to lake-level fluctuations during the Plio-Pleistocene in the East African Rift Valley. We have compared this record with fish fossil isotopes from both a previously published study of a Lake Malawi drill core (139 ka–present) and core top (modern ca 1978) samples collected at the water/sediment boundary from Lake Turkana (Kenya) of known environmental provenance. Both the Lake Malawi drill core fossils (−7.2‰ to −27.5‰ VPDB) and modern Lake Turkana samples (−16‰ to −24.6‰ VPDB) have δ¹³C values indicating a mix of near-shore and deep-water pelagic species. In contrast, the δ¹³C values for the Tugen Hills core fossils vary only between −20‰ and −27‰ VPDB. The absence of δ¹³C values greater than −19‰, suggests none of these fossils are derived from near-shore benthic habitats. The lack of shallow water, benthic lacustrine fish fossils through the Tugen Hills lake cycles may indicate that the rate of change from low-lake stands to deeper lake phases was very rapid, and shallow water communities were not established for long enough to leave a fish fossil record at the core site. These results strongly suggest that lake level responses to climate variability in the Baringo basin of the East African Rift were very abrupt during the Plio-Pleistocene transition.