Abstract
The southeastern United States endures environmental change from human population increase, climate change, and land-use alterations creating the need to understand baseline conditions and environmental patterns prior to human impacts. While paleoenvironmental data can be reconstructed from a variety of archives (e.g., lake sediments, tree rings, speleothems), some geographic areas contain fewer such records. One archive capable of recording moisture regimes and other paleoenvironmental changes over millennia but has received little attention relative to other climate proxies is bat guano deposits in cave systems. Bat guano deposits are found in many cave environments in the southeastern United States and can be used as an archive of paleoenvironmental data including precipitation, vegetation, and aspects associated with the ecology of bats. Here, we present a 12,000-year record of paleoenvironmental change based on δ2H stable isotopes in a guano core collected from Cave Springs Cave in Alabama, USA. Results suggest distinct shifts in moisture with dryer conditions during the early Holocene/late Pleistocene (12,200–9500 cal year BP) (δ2H values − 86.82 to −-77.70), wetter conditions during the middle Holocene (9300–3900 cal year BP) (δ2H values − 125.74 to − 80.63), roughly coinciding with the Holocene Climatic Optimum event time interval (9000–5000 cal year BP). During the last 4000 years, conditions in the region shifted in the southeastern United States region becoming dryer once again. Climate inferences based on guano δ2H are consistent with the role of atmospheric moisture on regional vegetation changes suggested by previous pollen records obtained from lake sediment cores. This study suggests bat guano δ2H may be a reliable method to provide a long-term paleoclimate record.
Highlights
In recent decades, interannual variability in summer precipitation has intensified significantly over the southeastern United States, with an increased frequency of both abnormally wet and dry summers (Wang et al 2010)
The Cave Springs Cave guano core consisted of chitinous insect pieces, bat hair, and fecal material
This section of the core was inferred to be the mineral brushite, due to the lack of organic matter and high percentages of P and Ca which are characteristic of brushite found in other guano cores (Giurgiu and Tamas 2013; Onac et al 2015)
Summary
Interannual variability in summer precipitation has intensified significantly over the southeastern United States, with an increased frequency of both abnormally wet and dry summers (Wang et al 2010). It remains unclear the extent to which natural climate varies and climate changes due to the increasing atmospheric concentration of greenhouse gases which controls precipitation fluctuation in the southeastern region of the United States. Regional and global climate models suggest that increasing greenhouse gases can influence summer rainfall patterns in the southeast the extent to which natural processes versus anthropogenic drivers control rainfall variability remains uncertain (Li et al 2013). The subtropical southeastern United States is home to environments typically useful for paleoreconstructions (e.g., natural lakes, tree rings), but most records contain short chronologies beginning in the mid Holocene or later (Larson and Schaetzl 2001). Current instrumental data of air surface temperatures and rainfall amount, in addition to tree ring chronologies provide accurate and high-resolution environmental information but do not extend beyond the time interval when the region experienced the first European settlements ~1600s-1700s (Stahle et al 1998; Cleaveland et al 2003; Therrell et al 2006)
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