Petrology and geochemistry of annually laminated stalagmites from an Alpine cave (Obir, Austria) : seasonal cave physiology.

摘要

Calcite stalagmites from Obir, an Alpine (1100 m altitude), perennially wet cave, were studied using optical and electron backscatter diffraction petrology, bulk ICP-MS analysis, and microanalysis by ion microprobe and micro-X-ray fluorescence using synchrotron radiation. Drip water penetrates 70 m through Triassic limestones (with some Pb-Zn mineralization) to the chamber Säulenhalle where the stalagmites were collected and with which we compared drip water compositions. However, the cave is also characterized by strong seasonal ventilation, leading to low PCO2 in winter. All samples display autumnal event lamination defined by a narrow, optically visible zone with trace element enrichments, within which synchrotron studies have resolved micron-scale enrichments of Pb and Zn. Crystallite-scale (10 μm) lateral trace element variations reflect alternate smooth faces and rough crystal edges, induced by high Zn content. Seasonal falls in sulphate S and increase in δ13C can be associated with the winter ventilation. The elemental covariations support the transport of Pb, Zn, P, F, Br and I by organic colloids, but final depositions from aerosols rather than dripwater should be considered. The chemical variations are powerful expressions of seasonal cave physiology, but the study points to important gaps in our understanding of colloidal element transport in ventilated caves.