VARIABLE CALCITE DEPOSITION RATES AS PROXY FOR PALEO-PRECIPITATION DETERMINATION AS DERIVED FROM SPELEOTHEMS IN CENTRAL FLORIDA, U.S.A.

Abstract

Deposition rates derived from speleothems have been shown to be a useful paleoclimatic proxy. Past studies have shown that the most common climatic parameter measured by variable deposition rates is precipitation, where increased precipitation leads to increased calcite deposition. This was the premise of our study, where three Floridian stalagmites’ deposition rates were measured and compared to paleohydrologic indicators taken from the sample or from other regional records. Deposition rates were measured by determining the volume of calcite precipitated between TIMS U-series dates (mm3 yr21 ), thereby accounting for morphological changes on the stalagmite over its depositional history. Most prior research relied on a simple linear interpolation between known ages to calculate rate (mm yr21 ). Results show three distinct periods of increased deposition for our stalagmites centered on 2.0, 1.25 and 0.5 ka BP. A comparison with Mg/Ca and Sr/Ca ratios and calcite deposition tentatively shows elevated elemental ratios during the three aforementioned periods. Elevated trace element ratios have been shown to be correlated with increased residence time of percolation waters in the overlying bedrock above caves and consequently decreased rainfall. To corroborate this finding, paleo-precipitation records from Little Salt Spring, Florida and Lake Miragoane, Haiti, were examined for coeval arid periods with our stalagmites. Both records do possess similar dry periods and provide added support that the region experienced periods of abrupt aridity over the last two millennia. The combined effect of a change in the mean position of the Intertropical Convergence Zone and the easterly winds associated with the North Atlantic High appear to be the major causes for these times of aridity.