PALEOTOPOGRAPHY FROM STABLE ISOTOPES AND CLUMPED ISOTOPE THERMOMETRY

The timing and pattern of surface uplift provides a critical test of the possible underlying mechanisms for the rise of orogens and plateaus, yet paleoelevation remains extremely difficult to quantify from the geologic record. Stable isotopic evidence has assumed a key role in paleoelevation reconstructions, taking advantage of the sensitive relationship between the stable oxygen (δ¹⁸O_mw) and deuterium (δD_mw) composition of average meteoric water and elevation. The isotopic composition of paleowater can be reconstructed from the isotopic compositions of geologic archives such as carbonates (δ¹⁸O_cc) and waters of hydration in volcanic glass, provided that the relevant fractionation factors are known. For carbonates, fractionation factors are temperature dependent, and therefore temperatures of carbonate formation must be assumed to reconstruct δ¹⁸O_mw. Moreover, it must be demonstrated that original δ¹⁸O_cc values have not been altered by post-depositional diagenetic resetting. By constraining the temperature of carbonate growth or isotopic resetting directly, carbonate clumped isotope thermometry can shed light on both of these issues, and document changes in environmental temperature that reflect elevation and/or climate change. Clumped isotope thermometry is based on the thermodynamic tendency of the heavy isotopes of carbon and oxygen to bond, or “clump” together in carbonate minerals, with a single measurement providing independent estimates of the formation temperature (T(D₄₇)), δ¹³C and δ¹⁸O_cc values of the carbonate, from which the δ¹⁸O_mw value can be calculated. Although the analytical methods are challenging, a growing number of studies apply clumped isotope thermometry to paleoelevation problems. Examples from the Colorado Plateau, Andes and Tibet suggest how confidence in the results can be improved via careful evaluation of the potential for diagenetic resetting, the fidelity of the proxy based on analyses of modern carbonates, and the influence of both topography and global climate on past and present regional climate.