Guano stable isotopic evidence of Anthropocene climate change influence on aridity and vegetation dynamics

Abstract

A 2.8 m-long core of cave bat guano was obtained from an excavated pit within Bracken Cave (Texas, USA) and analyzed for δ13C and δ15N. Radiocarbon dating of the core indicated that guano deposition occurred between 1974–1989 and 1998–2016 CE. The hiatus between 1989 and 1998 CE was interpreted to be related to documented mining activities during this interval. The average Suess-corrected δ13C values for the entire series (–19.7 ± 1.0 ‰) suggest that insects ingested by the bats had a diet incorporating both C3 and C4 plants. The proportion of each appeared to be dependent on the sensitivity of vegetation to prolonged and frequent droughts. Two documented La Niña events centered around 2008 and 2011 CE occurred contemporaneously with intervals of higher δ13C values. The drought recorded in 2006 may have corresponded with foraging of insects located over C3 vegetation, which resulted in lower δ13C (–22.0 ‰). δ15N values agree well with records of drought and precipitation in Texas, suggesting an aridity control on the local nitrogen cycle. The highest δ15N values occurred during periods of a weaker North American Monsoon (1977 and 1982 CE). Higher δ15N values also occur at 2006, 2009, and 2012 CE, corresponding with years of reduced precipitation locally and throughout the region. This study further demonstrates that guano δ13C and δ15N records can provide paleo-information with respect to climatic influence on the environment in semi-arid regions.