2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 3
Presentation Time: 8:00 AM-4:45 PM

Carbon Isotopes In Fossil Sequences as Aridity Proxies

KOHN, Matthew J., Geosciences, Boise State University, 1910 University Dr, Boise, ID 83725 and ZANAZZI, Alessandro, Geological Sciences, University of South Carolina, EWS 617, 701 Sumter St, Columbia, SC 29208, mattkohn@boisestate.edu

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.