Controls on Mg/Ca Ratios in Recent Stromatolites: Insights from Fluvial Systems in the Iberian Range (Spain)

The utility of the Mg/Ca elemental ratio of calcite ((Mg/Ca)(calcite)) as a temperature indicator in continental carbonate deposits is a matter of debate due to the different results obtained by diverse authors. In this study, we aimed to test the reliability of the (Mg/Ca)(calcite) in fluvial carbonates. We selected the recent tufa stromatolite records of four rivers on the Iberian Peninsula for the trace element analysis based on six-monthly sampling. Previous sedimentary and hydrological studies on these fluvial basins provided the information for this work. The water temperature estimates for the stromatolite (Mg/Ca)(calcite) substantially differed from the measured water temperatures in most of the studied cases. We thus assessed other factors that participate in the control of the Mg partitioning between water and calcite. The correction of the detrital Mg content yielded water temperatures that matched the measured ones in one of the rivers. The (Mg/Ca)(water), water discharge and calcite precipitation rates may also occasionally influence the (Mg/Ca)(calcite). The six-month behaviour of some of these parameters could interfere with the relationship between the (Mg/Ca)(calcite) and water temperature. According to these results, and their comparison with other non-marine carbonates, the wide variety of parameters that are involved in the (Mg/Ca)(calcite) limit it as a geochemical thermometer in continental sedimentary environments.