A speleothem mineralogy perspective on interannual wet-dry cycles in Botswana during the Late Holocene

Abstract

Interannual rainfall variability in the Kalahari Desert is strongly controlled by the El Niño-Southern Oscillation (ENSO). Paleo-reconstructions of hydroclimate (wet-dry) cycles in this area may therefore provide insights into the past behaviour of ENSO. Here, we present new petrographic and geochemical data of Late Holocene speleothem samples from Gcwihaba Cave, Botswana. The cave system, which is home (colony) to large numbers of bats, formed in highly karstified metamorphic dolomite. The studied speleothems consist of calcite and aragonite laminae at micrometer to millimetre-scales. High-resolution mineralogical, stable carbon (δ13C) and oxygen (δ18O) isotope ratios, and trace elemental concentrations, combined with chronological constraints (14C and U-Th data) and layer counting under the optical microscope, suggest that calcite/aragonite duplets record annual to interannual fluctuations in hydroclimate. Wet conditions favor calcite formation, whereas aragonite forms preferentially during the dry period. Speleothem lamina thickness is closely linked to the annual infiltration, which is controlled by seasonal aquifer recharge cycles. High-resolution laser-ablation trace-element (TE) analysis and isotope data support the petrographic observations. Calcite carbonate farming experiments in the cave revealed that aragonite and calcite are in distinct layers and well-preserved. There is no evidence in modern precipitates of dissolution-reprecipitation processes that lead to the transformation of aragonite to calcite. Of all TE, Y and La appear to be the best rainfall proxies, reflecting their transport pathway from the soil horizon at the top of the cave to the speleothem via drip water. Synchronous occurrences of higher Y and La with calcite phases suggest wet conditions, i.e., more rainfall. In contrast, aragonite layers exhibit higher concentrations of Sr, Ba, and U, and increased fluorescence due to the presence of organic matter, which possibly originates from bat guano deposits. However, this proposition requires further investigation. Aragonite formation can be linked to drier conditions in the cave, which are accompanied by an increase in the drip water Mg/Ca ratio. Drier conditions also increase the likelihood of preserving air-borne dust (guano particle) deposition rich in phosphorus from the cave interior within speleothem layers. Our results highlight that mixed calcite-aragonite speleothems provide a robust archive of high-frequency (annual to interannual) hydroclimate variability in southern Africa.