NEAR ENTRANCE SPELEOTHEMS: ARCHIVES OF SEASONALITY?

Reconstructing and understanding past climate variability is important to better inform present and future climate variability. Paleoclimate and geochemistry researchers have developed proxies from various archives (eg., lake cores, tree-ring cores, and speleothems) to discern past climate variability on different time scales. Stable isotope (δ¹⁸O, δ¹³C, δD) variations have been analyzed in tree-rings to tease out sub-annual resolution variability, albeit, the proxies are sensitive to various environmental factors (temperature and precipitation). Terrestrial paleoclimate records reconstructed from speleothems display the potential to solely investigate seasonality in temperature.

Traditionally, speleothems analyzed to reconstruct past terrestrial climate (i.e., rainfall amount, moisture source, vegetation) on annual or centennial (low-resolution) time-scales, are from the deeper parts of caves. The well-mixed vadose zone and consequently, well-mixed cave drip-water hinders the development of sub-annual (high-resolution) records from deeper parts of caves with some exceptions that show potential for seasonal resolution in such records (e.g., Mattey et al., 2010).

In contrast, speleothems from the near-entrance zones of caves may provide the much-needed seasonal (high-resolution) terrestrial paleoclimate records. Speleothem scientists have traditionally overlooked these sites because of concerns regarding evaporation, kinetic isotope effects, biological activity. However, recent cave monitoring and speleothem calcite studies that generated stable isotope (δ¹⁸O; Feng et al., 2014) and trace element (Mg/Ca and Sr/Ca; Casteel and Banner, 2015) records from a small, well-ventilated, near-entrance cave zone (Westcave) in central Texas (30°26’ N, 98°13’W) show the sensitivity of calcite growth to temperature on a seasonal scale. These studies have implications for reconstructing high-resolution terrestrial paleoclimate records to study seasonality in temperature in drought-prone regions such as the South-central USA. Westcave in central Texas is not a unique site, as similar settings in other small caves and the typically overlooked near-entrance zones of larger caves from the region have the similar potential of recording seasonality in temperature.