Multi-proxy speleothem-based reconstruction of mid-MIS 3 climate in South Africa

Abstract

Abstract. The southern coast of South Africa displays a highly dynamical climate as it is at the convergence of both the Atlantic and Indian Ocean, and it is located near the subtropical/temperate zone boundary with seasonal influence of easterlies/westerlies. The region hosts some key archeological sites with records of significant cognitive, technological and social developments. Reconstructions of the state and variability of past climate and environmental conditions around sites of archeological significance can provide crucial context for understanding the evolution of early humans. Here we present a short but high-resolution record of hydroclimate and temperature in South Africa. Our reconstructions are based on trace elements, calcite and fluid inclusion stable isotopes, and fluid inclusion microthermometry from a speleothem collected in Bloukrantz Cave, in the De Hoop Nature Reserve in the Western Cape region of South Africa. Our record covers the time period from 48.3 to 45.2 ka during Marine Isotope Stage 3. Both 𝛿18Oc and 𝛿 13Cc show strong variability and covary with Sr/Ca. This correlation suggests that the control on these proxies originates from internal cave processes such as Prior Calcite Precipitation, which we infer to be related to precipitation amount. The hydroclimate indicators furthermore suggest a shift towards overall drier conditions after 46 ka, coincident with a cooling in Antarctica and drier conditions in the eastern part of South Africa corresponding to the Summer Rainfall Zone. Fluid inclusion-based temperature reconstructions show good agreement between the oxygen isotope and microthermometry methods, and results from the latter display little variation throughout the record, with reconstructed temperatures close to the present-day cave temperature of 17.5 °C. Overall, the BL3 record thus suggests stable temperature from 48.3 to 45.2 ka whereas precipitation was variable with marked drier episodes on sub-millennial timescales.

A new MIS 5 to MIS 2 speleothem record from Sandkraal Cave on the South African Cape south coast

“…Lower deposition rates for the speleothems in SK1 during MIS 3 than in the previous intervals and the hiatus in deposition may also support drier conditions. A similar interpretation in terms of changes of PCP has also been proposed for a nearby speleothem from Bloukrantz Cave (Maccali et al, 2023).…”

“…8), indicating that evaporation biases between rainwater and cave water are likely to affect speleothem δ 18 O at this time (Baker et al, 2019). An effect of hydrology, mainly through PCP, on speleothem δ 18 O has recently also been inferred for a speleothem record from Bloukrantz Cave (∼170 km west of SK1) on the South African south coast during MIS 3 (Maccali et al, 2023). The shift of semblance relationships observed at ∼70 ka indicates that the main influence on δ 18 O switches from infiltration bias in MIS 5 to evaporation bias in MIS 4-3.…”

“…Speleothem δ 13 C records from the region have been generally interpreted as reflecting changes in the proportions of C3 and C4 plants in the vegetation (Talma and Vogel, 1992;Bar-Matthews et al, 2010;Braun et al, 2019Braun et al, , 2020Chase et al, 2021). A short Marine Isotope Stage (MIS) 3 record from Maccali et al (2023) was the first from this region to attribute changes of speleothem δ 18 O and δ 13 C to prior carbonate precipitation (PCP) and by extension changes in precipitation amounts based on a comparison with trace element records.…”