Recent climate records from a near-entrance stalagmite




Carlson, Peter Eric

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Sub-annually resolved environmental proxies can be valuable archives of climate change, but they are rare in terrestrial settings, and it can be difficult to verify their annual nature. We suggest that speleothems that grow in well-ventilated (near-entrance) zones of caves may preserve such high-resolution records. Previous monitoring studies of a well-ventilated, temperate-latitude cave (Westcave Preserve, central Texas) have documented seasonal variations in the oxygen isotope composition of calcite grown on glass substrates. We extend this work to provide a high-resolution (weekly to monthly) 52-year record of δ¹⁸O, Mg, Sr, and Ba in Westcave stalagmite WC-3, using the temperature-dependent variation of calcite δ¹⁸O and Mg concentrations as a precise seasonal chronometer. We confirm this chronology using measurements of radiocarbon in the stalagmite, which show an attenuated, lagged, and spread record of the atmospheric “bomb pulse” from nuclear weapons testing. We develop and test a new inverse modeling framework using these results, and show that stalagmite WC-3 carbon is sourced primarily from decomposition of subsurface organic matter. Finally, we correlate the WC-3 geochemical records to global and local climate parameters, finding that the ability of the records to respond to local annual-scale climate signals (particularly of aridity and precipitation) is highly dependent on the multi-decade phase of the Pacific Decadal Oscillation (PDO). We find that water residence times are low during cold-phase PDO, and high during warm-phase PDO. Additionally, we find that summer Mg concentrations increase with drought conditions (Palmer Drought Severity), especially during warm-phase PDO, and summer δ¹³C values decrease with increasing precipitation, during both cold- and warm-phase PDO. As PDO phases can last for decades, proxy calibration using cave monitoring studies may only be valid for a portion of the climate record. Throughout the entirety of this work, we show comprehensively and for the first time that high-resolution near-entrance speleothems can serve as an important intermediary between short-term monitoring and longer speleothem records.


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