Magnesium isotope time-series analyses of dolostone cave dripwater and speleothems: Proxy calibration and application
Mineralogy, Petrology and Geochemistry
Final Report Abstract
The time-series analyses of Mg isotopes of drip waters from dolostone dominated caves from Germany and Morocco revealed for most drip water sites significant 26Mg variations. These variations are induced by changes in the silicate versus carbonate weathering ratio. Silicate weathering is enhanced under dry and warm conditions, whereas carbonate weathering is induced by cold and wet climate conditions. Due to the significant differences of the Mg isotope composition of these minerals, changes in the weathering regime are detectable in drip water Mg isotopes. In case of German caves, where changes in temperature are more pronounced than in rainfall amount, the weathering ratio is driven by temperature variations. In Morocco, however, both temperature and rainfall amount complement each other and drive the changes in the silicate versus carbonate weathering ratio. Furthermore, the different transfer times for each specific drip site could be observed varying between a few months up to a year. Some drip water sites do not show variations in their Mg isotopic composition. In these cases, the signal of the weathering ratio is strongly buffered due to longer water transfer times / residence times and mixing of waters in the aquifer. Although possible, no dependency of the Mg isotope variations of the drip waters on prior calcite precipitation could be observed. The Mg isotopic composition of several speleothems recorded the changes in the silicate versus carbonate weathering ratio and the Mg isotope fractionation factor of aragonite dominated samples is smaller than for calcite speleothem samples. The Mg isotope proxy was successfully applied in dolostone cave systems. Nevertheless, the application of this proxy is only at its beginning and it is recommended to include the Mg isotope proxy in speleothem research in future projects.
