Trace Element Variations in a Stalagmite from Southwest Mexico: Implications for Paleoclimate

The combination of stable isotope and trace element concentrations in speleothems has been shown to be a valuable indicator for interpreting paleoclimatic conditions. Anomalous spikes in trace element concentrations have also been shown to indicate anthropogenic environmental changes to the surface, such as deforestation events. In this study, trace element concentrations (24Mg, 25Mg, 84Sr, 88Sr, 137Ba, and 238U) of a stalagmite (JX-6) from Juxtlahuaca Cave, in southwestern Mexico, were measured using an inductively coupled plasma mass spectrometer (ICP-MS). These trace element concentrations were compared to previously analyzed δ18O and δ13C values from JX-6, and to the results of previous studies comparing U concentrations in speleothems to paleoclimate. U concentrations of JX-6 were shown to correlate well with δ13C and δ18O values and may be a suitable proxy for soil moisture above Juxtlahuaca Cave. This study concludes that U concentrations in JX-6 may be controlled by changes in the pCO2 of overlying soils in relation to plant respiration possibly linked to variations in wet season (May – November) rainfall and temperature between 240 BCE to 1800 CE. Comparison to previous studies suggests U concentrations in speleothems are controlled by local cave conditions and that U is best used as a proxy for paleoclimatic conditions with the support of additional trace element and stable isotope data. Anomalous spikes in trace element concentrations were also observed in JX-6 at ~1862, 1871, 1904, and 1933 CE. These spikes were interpreted to be caused by uranium mobilization from soils overlying the cave, possibly relating to multiple deforestation events in association with the clearing of land above Juxtlahuaca Cave for agricultural use.