Application of bat guano deposits in paleoenvironment research

Bat guano is fecal excrement from bats. Guano accumulates beneath bat roost sites and is commonly found in attics, beneath bridges, and in caves where the large volumes can accumulate over time. Most bats forage for food near their roost sites and thick deposits of guano can record changes in climate that affect food sources. Guano contains a rich information source of paleoenvironment changes and the sedimentary sequence, which can be used to reconstruct the paleoenvironment. This paper reviews the current status of bat guano research worldwide and discusses various proxies and their significance for use in reconstructing paleoenvironments. Since the 1980s, scientists noticed the potential of bat guano in paleoenvironment studies. So far, the studying regions of bat guano mainly located in southwestern Romania, southwestern United States, and southeast Asia. The currently applied geochemical proxies of bat guano in the paleoenvironment are isotopes, such as carbon, nitrogen, and hydrogen isotopes, and elements such as copper, lead, and cadmium. The organic carbon in bat guano is from phytophagous insects. As a result, carbon isotope could reflect regional vegetation changes, which are controlled by climate. The main source of nitrogen in bat guano is the inorganic nitrogen storage in soil. The opening and closing of the nitrogen cycle in soil could affect the δ¹⁵N values of bat guano, so it can indicate the regional nitrogen circulation process. Deuterium in bat guano inherits from the natural water body, which comes into bat guano through the food chain. So, it can reflect the metabolism processes of the insect bat eaten, the evaporation and other isotopic fractionation processes during precipitation. The heavy metals in bat guano come from cave bedrock and the overlying soil, and some also resulted from human activities. As a result, they can record the changes in the guano deposition rate and local environmental pollution. For example, Wurster et al. (2010) measured the element ratios, δ¹³C, δ¹⁵N and δD of the bat guano deposits in Bat Cave from the Grand Canyon of the United States. They suggested the ratios of C/N and N/H, as well as δ¹³C and δD values consistently reveal a cooler and wetter climate in the Late Pleistocene than that during the Holocene. O'shea et al. (2001) found organic pollutants and mercury deposits in the bat guano deposits, which was close to the Rocky mountain, recorded the pollution from the local military factory. Because of the richness of organic materials, accurate age can usually be obtained by using AMS14C dating of the total organic carbon from bat guano. However, the acidic materials in the excrement might corrode the bedrock, resulting in the dead carbon effect on bat guano, especially for those experienced diageneses. To solve this problem, it is better to extract lipid extracts, saturated hydrocarbon components, and insect chitin in bat guano for AMS14C dating. This article also briefly introduces our bat guano study at Devil's Sinkhole in Texas, USA. Texas is located in the subtropical climate zone, with mild to warm winters and hot summers. Precipitation in the region is not evenly distributed and is mainly concentrated in the early spring and early autumn. Devil's Sinkhole (30.062°N, 100.103°W) is about 107 m deep, 15 m wide at the top and 97 m wide at the bottom. The top (1~9 cm) samples collected at the site were loose and grayish brown. The middle section (10~15 cm) was light brown. The lower section (15.5~23.0 cm) was relatively compact, and the areas in contact with the bedrock were dark tan. We selected the top and bottom samples and preformed AMS14C dating of the total organic carbon from the samples at the Xi'an accelerator mass spectrometry center. Results show that the age of the bottom samples ranges from 695~739 cal.a B.P. and the top samples are modern deposits. Further proxy analysis and chronological work are in progress. Finally, the future direction of guano studies is discussed. More reliably geochemical and biological proxies are suggested to develop in guano studies. It is also important to separate the anthropogenic and natural influences on different proxies of guano. Applying the recently developed DNA technology to study the changes in bat species and cave biological communities could be another important direction.