HR: 08:30h
AN: T31E-03 INVITED     [Abstracts]
TI: A New Approach to Paleoaltimetry Based on Abundances of 13C-18O Bonds in Soil Carbonates
AU: * Eiler, J M
EM: eiler@gps.caltech.edu
AF: Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Ghosh, P
EM: pghosh@gps.caltech.edu
AF: Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Garzione, C
EM: garzione@earth.rochester.edu
AF: Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627 United States
AB: The elevation of the earth's surface is among the most difficult environmental variables to reconstruct from the geological record. We describe a new approach to paleoaltimetry based on independent and simultaneous determinations of soil temperatures and the oxygen isotope compositions of soil waters, constrained by measurements of abundances of ¹³C-¹⁸O bonds in paleosol carbonates. The concentration of ¹³C¹⁸O¹⁶O in CO₂ produced by phosphoric acid digestion of carbonates is proportional to abundances of ¹³C-¹⁸O bonds in reactant carbonates, and abundances of those bonds increase with decreasing carbonate growth temperature due to the isotope exchange equilibrium: ¹³C¹⁶O₃ + ¹²C¹⁸O¹⁶O₂ = ¹³C¹⁸O¹⁶O₂ + ¹²C¹⁶O₃. The paleothermometer based on this reaction constrains carbonate growth temperature independent of the d¹⁸O of waters from which they grew because the reaction is a homogeneous equilibrium involving only isotopologues of the carbonate ion. We refer to this method as the `clumped isotope thermometer' because it measures the temperature-dependent `clumping' of ¹³C and ¹⁸O into bonds with each other in the carbonate mineral lattice. This thermometer can aid paleoaltimetry by: (1) constraining the growth temperatures of soil carbonate, which can be compared to a known altitudinal gradient in surface temperature; (2) by rigorously constraining the d¹⁸O of water from which carbonate grew, which can be compared to the altitude dependence of the d¹⁸O of meteoric water; and, in a new approach to this problem, (3) by constraining the correlation between soil temperature and the d¹⁸O of soil water. Such correlations can discriminate between the effects of altitude, climate, latitude and seasonality in driving changes in T and d¹⁸O of water, and thus provide an opportunity for quantifying the contributions from altitude changes alone. We use our approach to show that 3700 ± 500 m of surface uplift occurred in the Bolivian Altiplano over 3.6 Ma between 10.3 Ma and 6.7 Ma, at an average rate of 1.03 ± 0.14 mm/yr. This rate is most consistent with removal of dense lower lithosphere as the cause of elevation gain. Surface uplift of the Altiplano coincides with a decrease in the rate of contractional deformation in the Andean plateau, the eastward propagation of deformation into the Subandean zone, and a decrease in the convergence rate between the Nazca and South American plates.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1865 Soils (0486)
DE: 8110 Continental tectonics: general (0905)
DE: 8175 Tectonics and landscape evolution
DE: 8194 Instruments and techniques
SC: Tectonophysics [T]
MN: Fall Meeting 2005