The response of central Texas cave drip sites to extreme events : implications for paleoclimatology
Creators: name: Bunnell, Kendra Elizabeth
name: 0000-0002-7345-1385
Publisher: name: The University of Texas at Austin
Year: 2019
Subjects: Paleoclimatology
Speleothems
Oxygen isotopes
Growth rate
Drip Rate

description: As global climate changes, extreme weather events such as droughts and floods are hypothesized to become more frequent and extreme. Therefore, understanding the geographic range, intensity, and frequency of extreme events in the past is important to provide context for future changes. Speleothems have the potential to record these events. This study investigates the response of central Texas deep-cave drip sites to extreme weather events. Deep caves are defined in this study as those caves with high winter speleothem calcite growth rates and lower summer speleothem calcite growth rates. From the resulting observations, it was hypothesized how such events may be preserved in a speleothem record, if at all. To investigate this, the drip rates and oxygen isotope (δ¹⁸O) responses of cave drip water sites, as well as calcite growth rates for calcite precipitated on artificial substrates below the drip sites, of eleven sites in four central Texas caves over eight years (2010-2018) were analyzed. The study period includes three extreme events: a historic drought (2011), a hurricane (2017), and a historically wet fall season (2018). Using a maximum drip rate versus coefficient of variability curve the sites used in this study were defined as diffuse (long water residence-time in the epikarst), conduit (short residence time), or intermediate (intermediate residence time). A weighted running mean model using the unique isotopic signature of Hurricane Harvey (2017) was used to assign residence times. Diffuse sites were shown to have a 34.5-month residence time on average, conduit sites were shown to have a residence time of 18 months on average, and intermediate sites have a 27-month average residence time. This study shows that these short-term events are only likely to be preserved in conduit to intermediate sites. Events that occur in the summer, a regional period of slow to no calcite deposition or speleothem growth, are less likely to be preserved. As would be expected, those events that last longer, or have a longer recovery time, are more likely to be preserved. The most likely preservation styles of the studied events, based on a combination of the magnitude and duration of the drip water response, as well as the statistical significance and commonality of the response, are as follows: 1) droughts show statistically significant longer-than-average periods of low to no growth, with no isotopic response , at 2 of the 11 sites; 2) tropical storms show an abrupt and historic isotopic low (drip water depleted in δ¹⁸O) followed by a multi-month return to baseline isotopic values with no growth rate response, seen at 2 of the 11 sites; and 3) abnormally wet periods show a statistically significant isotopic low, observed at 5 of the 11 sites, which can be coupled with a growth rate increase, this combination was observed at 2 of the 5 sites showing a significant isotopic low. These results indicate that in paleoclimatology studies investigating extreme climatic events, deep-cave temperate speleothems chosen should be those that are supplied by conduit drip sites and have higher growth rates. Monitoring studies of cave drip sites can be used to identify sites that fit these criteria and thus have application to paleoclimate studies.