HR: 0800h
AN: PP31B-1535    [Abstracts]
TI: Coupled C- and O-Isotopes in Mammal Fossils as Monitors of Continental Climate Change and Paleoseasonality
AU: * Kohn, M J
EM: mjk@geol.sc.edu
AF: University of South Carolina, Dept. Geol. Sci. 701 Sumter St; EWS 617, Columbia, SC 29208 United States
AU: Zanazzi, A
EM: mjk@geol.sc.edu
AF: University of South Carolina, Dept. Geol. Sci. 701 Sumter St; EWS 617, Columbia, SC 29208 United States
AB: CO₃substitutes for PO₄ in biogenic phosphates, and provides a combined archive of oxygen and carbon isotope compositions that serve to track climate change on the continents. Mean δ¹⁸O from fossils yields information about water composition, which is related to MAT and atmospheric circulation patterns; mean δ¹³C yields information about plant (food) ecology, e.g., open vs. closed vegetation, which is related to dietary selection and aridity. Both fossil bone and fossil teeth are useful for paleoclimate studies. Fossil bone compositions are analogous to paleosol carbonate; to a first order C-isotopes correlate with aridity, and allow correction of O-isotopes for evaporative enrichment, and accurate inference of paleowater compositions. For example, in central Oregon, fossil bone compositions from 30 Ma to <600 Ka clearly resolve a major trend towards increased aridity and a 3-4‰ decrease in the δ¹⁸O of precipitation after 7 Ma, caused by doubling of the height of the Cascade Range. In contrast to bone, tooth enamel preserves in vivo rather than diagenetic compositions, but mean compositions can be used analogously. In central Oregon, mean tooth δ¹⁸O preserves the same dramatic 3-4‰ decrease after 7 Ma (Kohn et al., 2002; EPSL, 204, 151-165). In Argentina and the northern Great Plains, mean tooth compositions across the Eocene-Oligocene transition reveal a statistically significant ~1‰ increase in δ¹³C as is also observed in marine records, and related to changes in the global carbon cycle. A special advantage of teeth from large mammals is that enamel is precipitated progressively for as much as an entire year, so down-tooth zoning measurements can additionally provide records of seasonal changes in water compositions, and ecosystem/dietary seasonality. In southern Argentina, δ¹⁸O and δ¹³C values from middle Eocene (38.5 Ma) teeth show variable correlations, but by the late Eocene (36.5 Ma) and earliest Oligocene (33.4 Ma), they consistently and positively correlate, perhaps reflecting establishment of regular summer/dry - winter/wet seasons by the late Eocene. However, the variances in δ¹⁸O and δ¹³C in all three time slices are comparable (±0.8 to 0.9‰, 1σ), suggesting minimal changes to climate seasonality, e.g., MART. Interestingly, there remains little evidence in southern Argentina for significant climate change across the Eocene-Oligocene transition, in terms of mean δ¹⁸O and MAT (Kohn et al., 2004; Geology, 32, 621-624), mean δ¹³C and aridity, precipitation seasonality, and MART.
DE: 0330 Geochemical cycles (1030)
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0419 Biomineralization
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 4914 Continental climate records
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005