STABLE ISOTOPE-BASED PLIO-PLEISTOCENE CLIMATE AND VEGETATION RECONSTRUCTIONS OF SOME OF THE EARLIEST HOMINID FOSSIL SITES IN THE EAST AFRICAN RIFT SYSTEM (CHIWONDO BEDS, N MALAW)

The isotope geochemistry of pedogenic carbonate and fossil herbivore enamel is a powerful tool to reconstruct terrestrial paleoenvironments. Here we present the first pedogenic Plio-Pleistocene long-term carbon (δ¹³C) and clumped isotope (Δ₄₇) records from the Chiwondo Beds, one of the earliest hominid fossil sites in the East African Rift System (EARS).

The studied 5.0 to 0.6 Ma deposits in the Karonga-Chilumba area (NE shore of Lake Malawi) comprise abundant pedogenic carbonates and fossil remains of a diverse fauna, including two hominid fossil finds: a maxillary fragment of Paranthropus boisei and a mandible of Homo rudolfensis, both dated around 2.4 Ma.

We contrast δ¹³C values from pedogenic carbonate with data from fossil enamel, of different suid, bovid, and equid species and complement the former by Δ₄₇thermometry data as a proxy for soil temperature. Our data represent a southern hemisphere record in the EARS, a region particularly interesting for reconstructing vegetation patterns and correlating these across the ITCZ with data on the evolution and migration of early hominids and the proposed boundary shift between different savanna types. As our study site is situated between the well-known hominid-bearing sites of eastern and southern Africa it fills an important geographical gap for early hominid research.

Results of over 600 pedogenic carbonates from over 25 sections show δ¹³C values that consistently average around -8.5 ‰ over the past 5 Ma with no significant short-term excursions or longer-term trends. The data from molar tooth enamel omnivores (suids) compliment these findings with average δ¹³C = -10.0 ‰. The absence of long-term trends towards more positive δ¹³C values contrasts the increasing role of C₄-grasslands in the northern EARS. By analyzing enamel of specialized grazers such as equids and bovids, we show that the environment in the southern part of the rift was not homogeneous with a high woody fraction but the δ¹³C values around 0 ‰ indicate instead a presence of open grassland savannas with a large portion of C₄ biomass. Our data hence point to regional differences in climate and vegetation dynamics during the Plio-Pleistocene in the EARS. It therefore documents persistence of paleoenvironmental dynamics in the southern branch of the EARS at times of early hominid evolution.