Carbonate oxygen isotope paleoaltimetry: evaluating the effect of diagenesis on paleoelevation estimates for the Tibetan plateau
Carbonate oxygen isotope paleoaltimetry is based on analysis of the d 18 O value of carbonate precipitated from surface water. Deciphering the diagenetic history is important for establishing whether particular carbonates are accurate recorders of paleosurface waters, which reflect paleoelevation. This study provides examples from southern, east–central, and northeastern Tibet of approaches aimed at evaluating the diagenetic history of lacustrine micrites and pedogenic carbonates. The most desirable technique for avoiding erroneous interpretations related to diagenetic overprinting is to analyze carbonates that are known to be primary, such as aragonitic shell material. In rocks that do not contain shell material, we have compared lacustrine micrites and pedogenic carbonates to diagenetic carbonate phases to determine the effects of diagenesis on the isotopic composition of primary carbonates. Where the potential effects of diagenesis are subtle or ambiguous, we have evaluated the fidelity of the carbonate record from systematic trends in C and O isotopes that agree with other interpretations of paleoenvironment, such as high frequency covariance in C and O that corresponds with changes in the Mg concentration of carbonates. Using these strategies, we have determined that diagenesis has not affected the isotopic composition of carbonates in the Late Miocene–Pliocene Thakkhola graben in southern Tibet and the Oligocene to Pliocene Linxia basin in northeastern Tibet. In the case of Paleogene basins in east–central Tibet, however, ambiguity in data precludes the determination of diagenetic effects. D 2004 Elsevier B.V. All rights reserved.