Speleothem reconstructions of Holocene interannual climate variability in Botswana

Abstract

The natural variability of rainfall in the Southern African region remains, as yet, poorly understood due to scarce availability of long instrumental and pre-instrumental records and the sparse distribution of weather stations in remote areas. It is known that regional atmospheric circulation features, particularly the Botswana High, control the moisture distribution across the region on seasonal and interannual timescales. The El Niño-Southern Oscillation (ENSO) further influences moisture transport, resulting in alternating wet and dry periods. Understanding the interplay between these forcing in modulating natural rainfall variability is crucial for effective water resource management, agricultural planning, and climate adaptation in a region heavily reliant on seasonal rainfall.Speleothems (secondary mineral cave deposits) are known to record local hydrology and rainfall over thousands of years and can provide valuable knowledge about natural rainfall variability in Southern Africa. Here, we present two speleothems from Gcwihaba Cave, located in northwestern Botswana, that span the late Holocene period between 200 and 2500 years before present (yrs BP). Robust age models for speleothems were constructed using a combination of U-Th and 14C dating techniques, despite signs of biocorrosion from bat guano in the cave. The two well-laminated speleothems exhibit alternating bands of calcite and aragonite, likely identifying annual to multi-annual timescales. High-resolution stable-isotope (δ18O and δ13C) and trace-element data from these speleothems reveal pronounced interannual variability, suggesting large fluctuations in rainfall amounts in the area, which can be linked to ENSO, as suggested by water tagging experiments with an isotope-enabled climate model. Analyzing multidecadal changes in interannual isotope and trace-element variability provides further insights into low-frequency ENSO dynamics during the late Holocene, which can then be compared with other paleo-ENSO reconstructions.