Investigation of Drought Intensity and Periodicity in South Texas using Chemical and Biological Records in Bat Guano Cores

Studies of bat guano deposits have indicated that chemical and isotopic variations recorded in the guano record can be used to examine historical variations in climate. 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 suggest that guano deposits as thick as 108 ft (33 m) may be present in some areas of the cave. This study examined 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 Bracken Bat Cave during three sampling campaigns in early 2016. 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. Chemical, isotopic, and mineralogical analyses were conducted on sample isolates. Radiocarbon (14C) and stable isotopic data (2H/1H, 13C/12C, 15N/14N), along with other supporting chemical data, have been used to establish age and chemical signatures associated with core sample depths and stratigraphic layers. Contrary to initial expectations, the sampled areas appear to represent guano deposition over decades rather than centuries according to isotope data correlated with layer sequences, which are thought to represent annual cycles. The time frame of deposition can be correlated with historical local weather data. This allows evaluation of the utility of guano core in evaluating short-term and relatively recent weather patterns. The success of the correlation of chemical and isotopic variations to historical weather data of known periods of drought dictates whether longer guano core records may be used to ascertain frequency and duration of historical – and possible prehistorical – periods of drought that predate other conventional climate records