Exploring high-resolution carbon isotopes in archaeological charcoal as a rainfall seasonality proxy
Rainfall in present-day South Africa is distinctly seasonal, with a Summer Rainfall Zone (SRZ) in the east, a Winter Rainfall Zone (WRZ) in the west, and a Year-Round Rainfall Zone (YRZ) along the south coast and interior between them. Understanding shifts in these zones during Late Pleistocene glacial- interglacial cycles is crucial for both regional climate reconstructions and interpreting behavioural variability in the South African Later Stone Age (LSA) archaeological record. However, several key palaeoarchives employed to reconstruct rainfall seasonality during the Late Pleistocene are hindered by low temporal resolution, reliance on inference and untenable ecological relationships, and their spatial and temporal distance from archaeological evidence. This research tests whether a proven seasonal precipitation proxy (SPP) based on high-resolution carbon isotope profiles — successfully applied to evergreen wood in the Northern Hemisphere — can be used to reconstruct rainfall seasonality from South African archaeological Proteaceae (angiosperm) and Podocarpaceae (gymnosperm) charcoal. Modern Protea and Podocarpus wood and charcoal samples from all three rainfall zones are tested to assess the SPP's applicability in South Africa and its ability to extract seasonal climate data from different materials. Results show that seasonal amplitudes (Δδ13Cmeas), changes in precipitation (ΔP), and ratios of summer to winter rainfall (PS/PW) align with local rainfall data. Slight discrepancies between wood and charcoal seasonal amplitudes and therefore values for ΔP and PS/PW are attributed to carbonisation effects, including 13C depletion and volume shrinkage leading to reduced sampling resolution, which lead to a repeated but quantifiable overestimation of summer rainfall in charcoal. In the WRZ, seasonal amplitudes are dulled relative to local rainfall data due to intermittent growth cessation during seasonal dry periods, but these effects could in some cases be corrected for. Analysis of Podocarpaceae charcoals from Waterfall Bluff showed an increase in summer rainfall during the Last Glacial-Interglacial Transition (LGIT), while Proteaceae charcoals from Boomplaas Cave showed an increase in winter rainfall during the Last Glacial Maximum (LGM). These results support existing models of Southern Hemisphere (SH) westerly shifts. The SPP offers a quantitative, high-resolution, on-site proxy for seasonality in archaeological contexts, with potential applicability to global archaeological sites from which evergreen charcoals have been retrieved.
carbon isotopes, archaeological charcoal, rainfall seasonality, dendroclimatology, Protea, Podocarpus
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