A comparison of paleosols from Oldowan and Acheulian sites at Gona, Ethiopia

Abstract

Previous work has shown that early Oldowan archaeological sites ≥2.0 Ma are associated with restricted areas on the landscape, often close to water and raw material; whereas Acheulian archaeological sites ≤1.7 Ma are found in a wider variety of settings, coinciding with the emergence and proliferation of Homo erectus. We provide more context to this understanding of hominin-landscape interaction using paleopedology and analyzing a sample (n=9) of paleosols from Gona, Ethiopia at artifact and hominin sites that are either ~2.6 Ma (Oldowan) or <1.7 Ma (mainly, but not exclusively, Acheulian). Paleosols at ~2.6 Ma sites are thin paleo-Entisols with A-C horizons; whereas paleosols at <1.7 Ma sites are thicker paleo-Inceptisols and –Vertisols with overprinted Ak-Bk or ABkss-Bkss horizons. The ~2.6 Ma paleosols often had many prominent very fine channel pores commonly created by grasses; whereas the <1.7 Ma paleosols occasionally had medium to coarse rhizoliths indicative of tree roots. Using bulk geochemistry, the mean integrated Na loss is ~9 times higher for <1.7 Ma paleosols (-6.0*10-3 mol cm-2) than for ~2.6 Ma paleosols (-6.8*10-4 mol cm-2). The paleosol data suggest that ~2.6 Ma artifact sites occurred on relatively unstable landscapes that allowed little time for mature paleosol development. Conversely, sites <1.7 Ma occurred on or near paleosols that achieved relative stability that was long enough to develop calcareous subsoil. We quantified duration of weathering by calculating a Residence Time (RT) for each paleosol, where the integrated Na loss over the profile (mol cm-2) was divided by the inferred mean Na outflux for the basin (mol cm-2 y-1). The solution yields a rough estimate of how long, in years, Na was weathered out of the profile. The RTs for ~2.6 Ma sites leached Na on average for 540±290 years prior to burial, ranging from 349 to 970 years; whereas, RTs for <1.7 Ma sites leached Na on average for 3000±4300 years, ranging from 102 to 10,431 years. An F test for equality of variances shows that for the two RT groups, ~2.6 Ma and <1.7 Ma, the variances are not equal (p=0.0005). Our observation of longer and overall more variable residence times associated with <1.7 Ma sites is consistent with the hypothesis that Acheulian technology was found, or used, on a greater variety of landscapes than Oldowan sites.