Three Millennia of Drought in the Southwest: New Results from Trace Element Analysis of Speleothems

Abstract:

Understanding the nature of drought on timescales ranging from seasons to millennia is critical for water resource management, particularly in areas such as the southwest U.S. where water is scarce. Previous work in semiarid regions indicates that trace elements in speleothems can record hydrologic changes over time. Here we present a ~3500-year, high-resolution record of drought based on trace elements in a stalagmite from Fort Huachuca Cave, southern Arizona. Trace element concentrations (normalized to Ca) were measured via LA-ICP-MS at 10 μm resolution, with 10 measurements corresponding to approximately one year of stalagmite growth. Annual cycles are apparent in Sr, Ba, P, and Y, verified with ²³⁰Th dates taken at ~50-year increments. Ba and Sr covary strongly, and P and Y covary inversely to Ba and Sr. Based on published studies, and the independence of the processes governing these elements, we infer that Ba and Sr increase during dry intervals and P and Y during wet. The combination of these elements allows us to reconstruct past variations in drought and moisture. We compare our record of drought to the instrumental record of local precipitation as well as to tree-ring paleoclimate reconstructions in the Southwest. We propose that multi-elemental analysis of speleothems can be used as a measure of relative aridity that characterizes the multidecadal to multicentury variability of drought and helps to resolve ambiguities in the more commonly used oxygen isotopes. This new indicator for drought will contribute to a deeper understanding of the risk of prolonged and/or severe drought in the southwest U.S.