Paleoelevation Reconstruction using Pedogenic Carbonates

Paleoelevation reconstruction using stable isotopes, although a relatively new science, is making a signifi cant contribution to our understanding of the recent growth of the world’s major orogens. In this review we examine the use of both light stable isotopes of oxygen and the new “clumped-isotope” (Δ47) carbonate thermometer in carbonates from soils. Globally, the oxygen isotopic composition (δ18O) of rainfall decreases on average by about 2.8 ‰/km of elevation gain. This effect of elevation will in turn be archived in the δ18O value of soil carbonates, and paleoelevation can be reconstructed, provided (1) temperature of formation can be estimated, (2) the effects of evaporation are small, (3) the effects of climate change can be accounted for, and (4) the isotopic composition of the carbonate is not diagenetically altered. We review data from modern soils to evaluate some these issues and fi nd that evaporation commonly elevates δ18O values of carbonates in deserts, an effect that would lead to underestimates of paleoelevation. Some assessment of paleoaridity, using qualitative indicators or carbon isotopes from soil carbonate, is therefore useful in evaluating the oxygen isotope-based estimates of paleoelevation. Sampling from deep (> 50 cm) in paleosols helps reduce the uncertainties arising from seasonal temperature fl uctuations and from evaporation. The new “clumped-isotope” (Δ47) carbonate thermometer, expressed as Δ47, offers an independent and potentially very powerful approach to paleoelevation reconstruction. In contrast to the use of δ18O values, nothing need be known about the isotopic composition of water from which carbonate grew in order to estimate of temperature of carbonate formation from Δ47 values. Using assumed temperature lapse rates with elevation, paleoelevations can thereby be reconstructed. Case studies from the Andes and Tibet show how these methods can be used alone or in combination to estimate paleoelevation. In both cases, the potential for diagenetic alteration 03_Quade_etal.indd 1 7/3/2007 12:41:09 AM 2 Quade, Garzione, Eiler of primary carbonate values fi rst has to be assessed. Clear examples of both preservation and alteration of primary isotopic values are available from deposits of varying ages and burial histories. Δ47 values constitute a relatively straightforward test, since any temperature in excess of reasonable surface temperatures points to diagenetic alteration. For δ18O values, preservation of isotopic heterogeneity between different carbonate phases offers a check on diagenesis. Results of these case studies show that one area of south-central Tibet attained elevations comparable to today by the late Oligocene, whereas 2.7 ± 0.4 km of uplift occurred in the Bolivian Altiplano during the late Miocene.