Using trace elements to characterize subaqueous calcite deposits in Devils Hole (Nevada)

Subaqueous calcite deposits from Devils Hole (Nevada, USA) have been used to date groundwater oscillations on orbital and multi-millennial timescales [1]. The record is based on two types of calcite: 1) slow growing (1 mm/ka), dense mammillary calcite which forms under water and can be reliably dated with U-series techniques; 2) porous and undatable folia which forms directly at the water surface. In order to date short-term (millennia and shorter) oscillations in the last glacial period, layers less than few mm thin need to be clearly identified prior to analysis. In general, the boundary between mammillary calcite and folia is not sharp, making it difficult to assess whether an individual layer can be reliably dated. In an effort to identify suitable layers, we characterized the two types of calcite by optical petrography, stable isotopes (δ13C and δ18O) and trace elements (Mg, Sr, Ba, U). Preliminary results show a significant variability in trace-element concentrations in folia, while mammillary calcite has a characteristically uniform concentration of most elements. At transitions from folia to mammillary calcite, elongated crystals which are distinct from mammillary calcite (compact fabric, comprising composite columnar crystals, each being a “bundle” of multiple rodshaped crystallites) and folia (small, slightly elongated to mosaic-like crystals) were found as an additional, third type of calcite. The Mg/Ca ratio in mammillary calcite and the transition zone is higher by an order of magnitude relative to folia. Ba/Ca and Sr/Ca are elevated in the transition zone relative to mammillary calcite and folia by an order of magnitude. Where thin calcite layers are similar to these transition zones, they can be clearly distinguished from folia and may inform us about the timing of short-term climate oscillations.