Influence of fluid Mg/Ca ratios on speleothem petrography – Insights from cave analogue experiments

Speleothems are recognized as reliable archives of past continental climate dynamics. Depending on the research focus, both geochemical and petrographic proxies are employed. While previous studies have explored the petrographic features of speleothems - particularly mineralogy and crystal fabric development - the relationship between drip water geochemistry and the petrographic attributes of speleothems remains underexplored. Research indicates that several physicochemical parameters, such as drip rate, pH, supersaturation, growth rate, fluid Mg/Ca ratio, and organic matter content, influence the mineralogy and crystal morphology of cave carbonates. Among these, the Mg/Ca ratio of drip water is the most influential, directly affecting crystal morphology and serving as a proxy for prior calcite precipitation (PCP). Cave environments are subject to various factors that can alter drip water Mg/Ca ratios. To disentangle these effects, a series of cave analogue experiments were conducted in a climate chamber set to 15 °C and 70 % humidity under atmospheric CO₂ conditions. Each solution was purified of organic material and maintained at a constant pH of 7.9 with a steady drip rate of 98 µL/min. Roughened watch glasses provided a crystallization surface for the carbonate precipitates. The fluid Mg/Ca ratio was the only variable, adjusted between experiments (0.5, 0.375, 0.25, 0.125). Each Mg/Ca ratio was tested both with and without the influence of PCP, with experiments lasting 90 days. Throughout this period, temperature, humidity, CO₂ level, drip rate, conductivity, pH, and outflow element concentrations were continuously monitored. Carbonate precipitates were analyzed using SEM, EBSD, and EMPA. Initial results suggest that calcite crystal morphology varies with changes in fluid Mg/Ca ratio, and aragonite precipitates only form in experiments influenced by PCP at the same initial Mg/Ca ratio as non-PCP experiments.