Making the Invisible Stratigraphy Visible: A Grid-Based, Multi-Proxy Geoarchaeological Study of Umhlatuzana Rockshelter, South Africa

Abstract

Umhlatuzana rockshelter is an archaeological site with an occupational record covering the Middle Stone Age, Later Stone Age, and Iron Age. The presence of both Middle and Later Stone Age assemblages makes Umhlatuzana the ideal location for the study of the MSA–LSA transition (20–40 ka) in southern Africa. This transitional period is characterized by important modifications in stone tool technology, from prepared core technology to a toolkit based on microlith production. These changes are argued to have occurred in response to changes in climate and environment leading up to the Last Glacial Maximum. The deposits bearing the transitional assemblages at Umhlatuzana rockshelter appear homogeneous with no visible stratigraphic boundaries. This study integrates geoarchaeological techniques in order to explore fine-resolution geochemical differentiations of the sediments that are macroscopically invisible, and that will provide insight into (post-)depositional processes over time. Samples were systematically retrieved from the western profile of the site following a grid-based sampling strategy and analyzed for pH, elemental composition (XRF), and Magnetic Susceptibility. Additionally, the results were integrated with preliminary micromorphological observations. Our study reveals a steady, gradual change in the geochemistry of the deposits throughout the Pleistocene, related to a combination of environmental change and occupation intensity. We suggest that the part of the sequence reported to bear Middle to Later Stone Age transitional industries is characterized by wetter environmental conditions compared to the underlying deposits. Additionally, we support results from previous studies that excluded large scale post-depositional movement of the sedimentary sequence. Our study offers a successful multi-proxy approach to systematically sample and study archaeological deposits at the macro and micro scale, integrating a variety of geoarchaeological techniques. The approach provides insight into the depositional and post-depositional history of the site, and allows for questions of stratigraphic integrity, anthropogenic input, preservation, and environmental change to be addressed.

Laboratory-based experimental research into the effect of diagenesis on heated bone: implications and improved tools for the characterisation of ancient fire

“…This study shows that it is essential to take the effect of diagenesis into account when reconstructing heating conditions. This involves using pH-specific reference data as well as measuring the pH and other geochemical signatures of the sediments (e.g., see 20 , 51 ). The latter can also provide valuable insight into the preservation conditions of unheated materials and help explain their distribution or absence.…”

Sand, hearths, lithics and a bit of bioturbation: Site formation processes at Umhlatuzana rockshelter, South Africa

“…The upper stratigraphic units (Group H) show better charcoal and bone preservation conditions than the underlying Group P deposits. This can be the result of progressive decomposition with depth and/or the increase of sediment alkalinity due to more intensive combustion activities (Reidsma et al, 2021). The progressive decomposition is apparent in the micromorphological study with the uppermost layers (H1)…”

“…The newly excavated western profile was documented in detail and sampled for various analyses. Geoarchaeological and paleoenvironmental results were presented by Sifogeorgaki et al (2020), Reidsma et al (2021) and Sifogeorgaki and Dusseldorp (2022), while a study of raw material categories was published by Sifogeorgaki et al (2023).…”

“…We analysed a total of six samples using XRD. Samples H1, D2, A14, A11 and A13 are sediment samples, originally collected for the study of Reidsma et al (2021). We selected these samples to illuminate the mineralogical composition of layers associated with specific micromorphological samples.…”

“…Geochemically, the sediments of Group H demonstrate an abrupt increase at this boundary in elements that connect to combustion activities such as CaO, MgO and Sr (Reidsma et al, 2021).…”