Carbon Isotopes In Fossil Sequences as Aridity Proxies

Carbon isotope compositions of C3 plants correlate negatively with precipitation. Literature data including over 100 δ¹³C values from a variety of C3 plants and corresponding mean annual precipitation (MAP) show a generally sigmoidal pattern. At low MAP (<300 mm/yr) and high MAP (>1100 mm/yr), modern δ¹³C values are relatively constant at ~-26‰ and ~-30‰ respectively. At intermediate MAP, δ¹³C values increase quasi-linearly with decreasing MAP, i.e., with a slope of ~0.5‰/100 mm/yr MAP. Although δ¹³C values from fossil plants, soil organic matter, paleosol carbonate, and bone carbonate are commonly scattered (±1-2‰), large datasets can reduce standard errors for specific intervals to ~±0.25‰, implying MAP resolution of ±50 mm/yr.

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.