Magnesium isotope time-series analyses of dolostone cave dripwater and speleothems: Proxy calibration and application

Speleothems are an important archive for reconstructing past climate variability. The Magnesium isotope proxy tested so far in limestone-hosted caves provides the possibility of reconstructing climate conditions from changes in the silicate-to-carbonate weathering ratio. Other caves, however, are situated in dolostone host rock. Consequently, the Mg content of the host rock is much higher than that of limestone. Dripwater monitoring in a set of dolostone-dominated caves in Germany and Morocco, as well as the collection of soil (silicate minerals), host rock (carbonate), and speleothem samples, aims to apply the Mg isotope proxy in dolostone-hosted caves. The time-series analyses of the Mg isotope composition of dripwaters revealed, for most dripwater sites, significant variations in δ²⁶Mg values, which can be related to changes in the silicate-to-carbonate weathering ratio. Silicate weathering is enhanced under dry, warm conditions, whereas cold, wet conditions favour carbonate weathering. Due to significant differences in the Mg isotope composition of silicate (soil) and carbonate (host rock) minerals, changes in the weathering regime are detectable in drip-water Mg isotope ratios in both climate regions. In German caves, where changes in temperature are more pronounced than changes in rainfall amount, the weathering ratio is driven by temperature variations. In Morocco, however, both temperature and rainfall amount complement each other and drive changes in the silicate-to-carbonate weathering ratio. Furthermore, the different transfer times at each drip site ranged from a few months to at least a year. Some drip water sites show no variation in Mg isotope composition. In these cases, the signal of the weathering ratio is strongly buffered by longer water transfer times/residence times and mixing of waters in the aquifer. Although possible, no dependence of Mg isotope variations in the dripwaters on prior calcite precipitation was observed. Corresponding speleothems from the monitored dripwater sites exhibit varying Mg isotope compositions of calcite and aragonite. There is no overprint of other factors during carbonate precipitation; thus, these variations are solely due to changes in the silicate-to-carbonate weathering ratio and, consequently, changes in temperature and rainfall amount. Furthermore, observations on the Mg isotope fractionation factor of aragonite-dominated samples revealed a smaller Δ²⁶Mg than for calcite speleothem samples. Generally, the Mg isotope proxy is a valuable tool for reconstructing past climate conditions in both limestone- and dolostone-dominated caves.