A monitoring and 20th century stalagmite study from a shallow cave in New Mexico : elucidating climate controls on geochemical variability with insight into stalagmite suitability for paleoclimate reconstructions




Sekhon, Natasha

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Confronting climate change with a delayed onset of the Monsoon season and an increase in anthropogenic aerosol concentrations is the new normal. In water-scarce regions such as New Mexico, that rely on rainfall during the summer season, there is a strong concern on how water resources will be affected by future climate change enhanced due to increase in greenhouse gases. Paleoclimate studies offer a lens through which to better prepare for future climate scenarios by investigating trends in pre-instrumental paleoclimate data. However, a thorough understanding of how climate variables are reflected in the geochemistry of paleoclimate data is required. Further, sub-annually resolved paleoclimate data are essential to delineate seasonal variability. The geochemical variability of stalagmites harvested from near cave entrances preserve sub-annual climate variability. Further, the geochemical variability from 20th century sub-annually stalagmite records can serve as important links between cave-monitoring and longer stalagmite records. Through monitoring a shallow cave in New Mexico (Sitting Bull Falls Cave), we document seasonal variability in Mg concertation of modern cave calcite to reflect variability in surface-air temperature. We next investigate the controls on the geochemical variability of two 20th century growing stalagmites retrieved from the same cave. To retrieve an age model, we develop a depth-to-age peak counting algorithm using the temperature-dependent variation of calcite Mg concentrations. We next qualitatively compare the sub-annually resolved time series against instrumental data of rainfall, wind speed, and aerosol deposition to ascertain seasonal trends. We find that Fe concentrations increase during the Monsoon Season (July-September). Zn and Cu concentrations increase during the pre-Monsoon season, which we infer to record aerosol deposition. Finally, we use XRCT scans to reconstruct the internal growth morphology of a new core in Sitting Bull Falls cave with the aim to retrieve a record that pre-dates the instrumental record. The result from this research demonstrates that increase in transition metal concentrations reflect season of recharge (Fe) and aerosol deposition (Zn). This relationship between proxy and climate variable should be consistent in stalagmites from other regions. And stalagmites that pre-date the instrumental can potentially reconstruct the variability in groundwater recharge and delineate between natural and anthropogenic deposition


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