Paleoelevation Reconstruction using Pedogenic Carbonates

Abstract

Paleoelevation reconstruction using stable isotopes, although a relatively new science, is making a significant 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" (Δ_(47)) carbonate thermometer in carbonates from soils. Globally, the oxygen isotopic composition (δ^(18)O) of rainfall decreases on average by about 2.8‰/km of elevation gain. This effect of elevation will in turn be archived in the δ^(18)O 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 of these issues and find that evaporation commonly elevates δ^(18)O 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 fluctuations and from evaporation. The new "clumped-isotope" (Δ_(47)) carbonate thermometer, expressed as Δ_(47), offers an independent and potentially very powerful approach to paleoelevation reconstruction. In contrast to the use of δ^(18)O 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 Δ_(47) 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 of primary carbonate values first 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. Δ_(47) values constitute a relatively straightforward test, since any temperature in excess of reasonable surface temperatures points to diagenetic alteration. For δ^(18)O 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.

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