INVESTIGATION OF DROUGHT INTENSITY AND PERIODICITY IN SOUTH TEXAS USING CHEMICAL AND BIOLOGICAL RECORDS IN BAT GUANO CORES

Isotopic and chemical variations of bat guano deposits have been used to examine historical variations in climate. The Bracken Bat Cave in south central Texas hosts a large bat colony that is thought to have inhabited the cave for many hundreds of years. Recent geophysical surveys have indicated that guano deposits as thick as 108 ft (33 m) may be present in some areas of the cave. This study examines the potential use of the Bracken Bat Cave guano deposits to develop a robust proxy for climate change and drought history in south Texas.

Approximately 24 ft (7 m) of guano core samples were collected from the Bracken Bat Cave during three sampling events in early 2016. The individual 12-inch core sample tubes were packaged and stored under refrigeration to minimize potential changes due to microbial or other biologic activity until they were processed for detailed characterization and subsampling. The guano cores revealed distinct physical and compositional layering. For example, guano pellets along with insect and bone detritus enabled identification of more than twenty distinct layer sequences within a 9-ft (2.7 m) vertical section.

Contrary to initial expectations, the sampled areas appear to represent guano deposition over decades rather than centuries according to a count of the number of layer sequences, which are thought to represent yearly cycles. However, chemical, isotopic, and mineralogical analyses are being conducted on sample isolates. Radiocarbon (¹⁴C) and stable isotopic data (²H/¹H, ¹³C/¹²C, ¹⁵N/¹⁴N, and ¹⁸O/¹⁶O), along with other supporting chemical data, will be used to establish age and chemical signatures associated with core sample depths and stratigraphic layers. Should sampled guano deposits exhibit chemical and isotopic variations that can be correlated with periods of known drought, this record could be used to model regional changes in recharge to groundwater systems in the region. While the relatively recent and short time frame of the guano record will likely preclude determination of longer-term climate trends, it may be extremely useful for calibrating the guano chemical and isotopic markers with known climate and regional precipitation data.