Reconstructing past climates using carbon isotopes from fulvic acids in cave sediments

Little is known about the potential of organic substances found within cave sediments to provide useful reconstructions of past climatic changes. Here, we present a study of the carbon isotopes of fulvic acids extracted from sediments from a cave in west-central Florida. Fulvic acids are the most contemporaneous representation of soil humic substances derived from vegetation growing above the cave. Radiocarbon dating of these sediments constrains their depositional period to be late-Holocene. The range of the fulvic acid δ13C record shows the dominance of C3 vegetation during this period, but with changes in abundance and the shifting presence of C4 plants. The δ13C values record both the Little Ice Age and Medieval Climate Anomaly which produce drier and wetter conditions for Florida, respectively. Our conclusion that the stable carbon isotopic values of the cave sediment record as a proxy for vegetation change as driven by variability in precipitation is strongly supported by a comparison with the δ13C and δ18O values of two uranium-series dated speleothems from this part of Florida, with both being proxies for precipitation change.