Carbon Isotopes In Fossil Sequences as Aridity Proxies
Carbon and oxygen isotope data from the CO3 component of 400 ka fossil bone, South Carolina, strongly suggest that bone CO3 oxygen and carbon isotopes equilibrate to soil conditions after burial. Enamel CO3 from fossil horse, camel and deer preserves original biogenic compositions, and shows a wide range of δ13C values (-14 to -2‰, VPDB), reflecting dietary selection within a mixed C3 and C4 landscape (Kohn et al., 2005; Geology, 33, 649-652); δ18O values range from 27 to 32‰ (V-SMOW). In contrast, δ13C values of fossil bone clusters significantly at -9 to -14‰, and is at best weakly dependent on taxon. Oxygen isotope values are significantly decreased at 22 to 26‰. These data generally suggest that fossil bone CO3 can serve as an aridity proxy, at least at intermediate MAP, supporting inferences that aridity changed little across the Eocene-Oligocene transition in the northern Great Plains, US (Zanazzi et al., 2007; Nature, 445, 639-642). In contrast, a stepped increase in aridity is suggested by an increase in fossil enamel δ13C, and constant bone compositions in view of decreasing marine δ13C values.