SPATIAL VARIATION IN PALEO CRITICAL ZONE ECOLOGY FROM BIOMARKERS AND COMPOUND SPECIFIC δ13C EVIDENCE, OLDUVAI GORGE, TANZANIA

The 1959 discovery of hominin fossil Zinjanthropus boisei brought the world’s attention to the rich paleontological records at Olduvai Gorge, Tanzania. Excavations of the Zinj archaeological level 22, a 20 cm thick silty-clay paleosol from the Critical Zone, uncovered remains of Homo habilis and a high density collection of fossils (with some cut-marked bones) and Oldowan stone tools. The site is ~ 1.84 Ma. The origin of this dense concentration of material within a 300 m² area has been controversial for decades. We present plant biomarker and compound-specific carbon-isotopic data (n=71) for organic matter preserved in the stratigraphically well-constrained Critical Zone sediments from FLK Zinj, as well as surrounding sites at the same stratigraphic level. Associated plant biomarker (leaf-waxes) δ¹³C values vary between -19.4‰ and -33.1‰, revealing distinct differences in plant community composition within just 100s of meters of each other.

Central sites (including FLK Zinj) cover about 250 m² and contain leaf-waxes with low δ¹³C values, which we interpret as evidence of a localized forest habitat. About 200 m to the north, plant biomarkers and their carbon-isotope signatures are consistent with the presence of aquatic plants and occur in conjunction with mound-like tufa deposits, suggesting a spring and wetland habitat. The tufa has freshwater isotopic signatures (δ¹⁸O values ranging from -5.0‰ to -1.0‰) which indicate a meteoric water source for the spring. At southern sites, plant biomarkers and their carbon-isotope signatures suggest an open grassland habitat.

The time equivalency and close physical proximity of the two environments (woodland and the spring-fed wetland) indicate the two are related. Freshwater was likely attractive to hominins, a site selected for plant resources, as well as for accessing carcasses that were then transported to the woodland for consumption in safety. These data have important implications for the interpretation of hominin behavior in meat acquisition and the ongoing debate on scavenging versus hunting.