Uranium concentrations from an aragonite speleothem as a proxy for Mesoamerican Monsoon Variability over the last 2,250 years

Abstract:

Trace element concentrations (Mg and U) were measured in an aragonite stalagmite (JX-6) from Juxtlahuaca Cave (“JX Cave”), in southwestern Mexico, 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 correlate well with δ¹³C and δ¹⁸O values, and we interpret them to be a 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 the consistency of wet season (May – November) rainfall and temperature between 240 BCE to 1800 CE. Comparison to previous studies suggests that speleothem U concentrations are controlled by local cave conditions and are best used 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 increased U mobilization in overlying soils related to multiple deforestation events in association with the clearing of land above Juxtlahuaca Cave for agricultural use.