PALEO-ALTIMETRY BASED ON ABUNDANCES OF 13C-18O BONDS IN SOIL CARBONATES

The elevation of the earth's surface is among the most difficult environmental variables to reconstruct from the geological record. Existing methods based on stable isotope geochemistry, cosmogenic nuclides, paleobotany and vesicle sizes in subaerial lavas suffer from various limitations, and the applicability of any method to a specific region depends both on the availability of appropriate samples and the ability to account for confounding factors. A new approach to paleoaltimetry based on independent and simultaneous determinations of soil temperatures and the oxygen isotope ratio of soil waters is described in Ghosh et al., (2006), which introduced a new paleo-thermometer based on abundances of the doubly-substituted CO2 isotopologue in carbonates. Fundamental to this new approach to paleoaltimetry is the determination of changes in temperature of carbonate growth from the abundances of 13C18O16O2 in the soil carbonate. This allows reconstruction of oxygen isotope ratio of soil water through time by rigorously constraining the d18O of water from which carbonate grew (i.e., because both the growth temperature and d18O of carbonate are known), which can be compared to the altitude dependence of the d18O of meteoric water. Further, this method can aid paleoaltimetry by constraining the growth temperatures of soil carbonate independently. We use this approach to show that 3000±500 m of surface uplift occurred in the Bolivian Altiplano over ~3.5 Ma between 10.3 Ma and 6.8 Ma, at an average rate of 1.03±0.12 mm/yr. Subsequent to this study we have analyzed paleosol carbonates deposited between 25 Ma and 23 Ma from the Eastern Cordillera. Based on recent results, insignificant surface uplift was deciphered between ~25 Ma and 10.3 Ma. The drop in the rate of uplift with older ages is inconsistent with evidence of upper crustal shortening in the region. Surface uplift of the Altiplano between 10.3 and 6.8 Ma 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.