ENVIRONMENTAL CONTROLS ON THE PETROLOGY OF A LATE HOLOCENE SPELEOTHEMFROM BOTSWANA WITH ANNUAL LAYERS OF ARAGONITE AND CALCITE

A carbonate stalagmite from Drotsky's Cave in northwestern Botswana co nsists of alternating layers of calcite and aragonite. Layer counts an d radiocarbon ages indicate that the calcite-aragonite pairs are annua l layers representing about 1500 years of deposition. The annual layer ing probably resulted from highly seasonal rainfall. Comparison of the uppermost layers of the speleothem with meteorological records shows that precipitation of CaCO3 in Drotsky's Cave was controlled by climat e. Thickness of calcite layers correlates with rainfall, suggesting th at calcite precipitation was largely dependent on the quantity of wate r supplied to the speleothem. By contrast, thickness of aragonite laye rs correlates with temperature, although variation in temperature cann ot 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 aragonite layers, and analyses of cave wate rs suggest that fluid Mg/Ca ratios reach levels sufficient to cause ar agonite precipitation. Increasing evaporation, which caused greater io nic strength and supersaturation, resultant increasing Mg/Ca ratios in the fluid, and perhaps increasing temperature probably combined to ca use aragonite precipitation. Detailed petrographic analysis suggests t hat each annual cycle of CaCO3 precipitation began with relatively int ense fluid flow, sometimes sufficient to dissolve some of the underlyi ng aragonite before precipitation of calcite. Calcite precipitation un der a thick fluid layer allowed euhedral crystals to form at first but thinning of the fluid to a film allowed only flatly terminated calcit e crystals by season's end. As fluid flow diminished, increasing evapo ration, increasing Mg/Ca ratios in the fluid, and perhaps increasing t emperature combined to cause aragonite precipitation to begin, particu larly on the sides of the speleothem. In some years, fluid flow dimini shed to the point that dust accumulated on aragonite surfaces before t he onset of the next year's precipitation.