TRACE ELEMENT VARIATIONS IN SPELEOTHEM CALCITE: INFLUENCE OF NON-ENVIRONMENTAL FACTORS

Trace elements in cave calcite deposits (speleothems) vary on annual to millennial time scales and have been used as paleoenvironmental proxies for temporal changes in temperature, effective precipitation, and groundwater flow paths. Partitioning of trace elements may also be affected by site-specific factors inherent to speleothem growth. In order to assess the controls on trace element partitioning in speleothems, we have grown calcite on frosted glass plates placed beneath drips on actively growing stalagmites in caves in Barbados and central Texas. We sampled calcite spatially across the plates for Ca, Mg, Sr, and Ba concentrations and drip waters before and after plate deployment. The median effective distribution coefficients (Kd's) for Mg and Sr are 0.023 and 0.12, respectively, similar to those observed in previous experimental studies. The trace element ratios are not constant across a given plate; Mg/Ca ratios increase radially away from the center of calcite growth on most plates from both Texas and Barbados. Whereas most of the plates from one site in the Texas cave exhibit radial decreases in Sr/Ca and Ba/Ca (a negative correlation between Mg and both Sr and Ba), one plate from the Barbados cave site exhibits radial increases in Ba/Ca and Sr/Ca (i.e., positive correlation among Mg, Sr and Ba). The radial increase in trace element ratios may be due to the chemical evolution of the drip water as Ca is incorporated into, and trace elements are preferentially excluded from, the calcite lattice as the water flows across the plate. This is consistent with higher trace element ratios in water sampled from the edge of one plate relative to the bulk drip water. The radial decrease in Ba/Ca and Sr/Ca may result from calcite growth being most rapid at the plate center, coupled with the known increase of Sr and Ba Kd values with increasing growth rate. This is supported by the plates with the most rapid growth having the most systematic radial increases in Ba and Sr, and exhibiting higher effective Sr Kd values. These results indicate that trace element variations in stalagmites may reflect the position of samples along a growth layer, growth rate, and/or the extent of mineral-solution reaction, in addition to independent environmental parameters such as temperature, rainfall, or groundwater residence time.