Speleothem trace element signatures: A hydrologic geochemical study of modern cave dripwaters and farmed calcite

摘要

Trace element variations in ancient cave speleothems are often interpreted as indicators of changes in paleo-rainfall and hydrologic conditions. However, these records are difficult to interpret without an understanding of the physicochemical controls on stalagmite chemistry plus site-specific calibration of changes in net rainfall to variations in dripwater and speleothem hemistry. In this study we examine geochemical relationships between net rainfall (Precipitation minus Evapotranspiration; P-ET), drip rates, drip water chemistry, and contemporaneous calcite chemistry to test the hypothesis that speleothem Mg/Ca and Sr/Ca records are proxies for rainfall amount. HRC is contained within four low-magnesium limestone units capped sporadically by a remnant dolomitic limestone. Aqueous concentrations of magnesium (post evapotranspiration) decrease with increasing vertical travel distance between the soil zone and the point of in-cave drip emergence (Drip Path Length – DPL) as dissolved high-Mg solutions sourced from the dolomitic caprock are diluted with dissolved low-Mg limestone waters sourced from the host limestone. Dripwater Mg/Ca and Sr/Ca ratios covary and provide diagnostic indicators of the two dominant mechanisms controlling dripwater chemistry: (1) mixing of post-evaporative solutions derived from two geochemical endmembers (dissolution of dolomite and limestone); and (2) evolution of hydrochemistry away from dissolved bedrock compositions due to Prior Calcite Precipitation (PCP) above the drip sites. By resolving the linear mixing relationships for drip water Mg/Ca and Sr/Ca sources and the distribution coefficients for trace element transfer in the PCP dripwater-to-calcite precipitation reactions and applying these principles to our time series, we find that the extent of PCP production within the karst is directly controlled by the balance between Precipitation (P) and Evapotranspiration (ET): higher net rainfall (P-ET > 1: wet conditions) reduces PCP, and lower net rainfall with increased evapotranspiration (P-ET < 1) increases PCP. Farmed calcite X/Ca ratios faithfully track hydrologically-influenced seasonal variations in dripwater chemistry for X = Mg, Sr, and Na. However, the relationship between changes in net rainfall and changes in Mg/Ca and Sr/Ca ratios in modern calcite is unique at each site and differs significantly at closely-spaced drip/stalagmite locations. This suggests that in situ modern hydrochemical calibrations should be performed atop individual speleothems prior to harvesting for paleoclimate investigations, and that such calibrations may not reflect past conditions as drip paths change. We apply this understanding to published dripwater data and speleothem time series from other caves. A major implication is that in order to interpret stalagmite Sr/Ca and Mg/Ca ratios as ‘wet vs. dry’ proxies, speleothem Sr/Ca and Mg/Ca variations must be coherent and in-phase over all time periods (i.e., Sr/Mg ratios must be constant). These criteria will help to distinguish ‘rainfall amount’ versus ‘rainfall source’ in speleothem δ~(18)O records.