Environmental controls on the petrology of a late Holocene speleothem from Botswana with annual layers of aragonite and calcite

A~cr: A carbonate stalagmite from Drotsky's Cave in northwestern Botswana consists of alternating layers of calcite and aragonite. Layer counts and radiocarbon ages indicate that the calcite-aragonite pairs are annual layers representing about 1500 years of deposition. The annual layering probably resulted from highly seasonal rainfall. Comparison of the uppermost layers of the speleothem with meteoro-logical records shows that precipitation of CaCO3 in Drotsky's Cave was controlled by climate. Thickness of calcite layers correlates with rainfall, suggesting that calcite precipitation was largely dependent on the quantity of water supplied to the speleothem. By contrast, thickness of aragonite layers correlates with temperature, although variation in temperature can-not explain greater aragonite abundance on the sides of the speleothem compared to its center. Mg/Ca ratios in calcite layers increase upward to the bases of overlying aragouite layers, and analyses of cave waters suggest that fluid Mg/Ca ratios reach levels sufficient to cause aragonite precipi