Paleoaltimetry of the Tibetan Plateau from D/ H ratios of lipid biomarkers

Abstract

The past elevation of the land surface provides a unique constraint on the underlying lithospheric structure during mountain and plateau formation. Development of new paleoaltimetry techniques that can be applied to a wide variety of sample types is therefore of continuing importance. This study evaluates organic substrates that preserve the δD ratio of surface waters as a new approach to reconstruct paleoaltimetry. We measured the hydrogen isotope composition of n-alkanes from epicuticular plant waxes preserved in lacustrine deposits to reconstruct the δD of precipitation in Cenozoic basins that have been elevated as part of the Tibetan Plateau. n-Alkane δD- and carbonate δ 18O-inferred water compositions from the Eocene–Miocene Lunpola Basin and Miocene Hoh-Xil Basin plot near or at enriched values relative to the global meteoric water line, as expected for evaporative lakewater and leafwater systems that have the same precipitation source. n-Alkane δD-based water compositions are nearly identical to the minimum carbonate δ 18O-based values, demonstrating that plant-wax δD is minimally affected by evaporation compared to lacustrine calcite δ 18O. This agreement strongly supports the presence of similar precipitation isotopic compositions in both archives despite different isotope systems, source water reservoirs, archive materials, modes of incorporation, and diagenetic processes. Paleoelevations for each basin and time period were calculated from precipitation isotope ratios using the isotope–altitude relationship derived from both a simple thermodynamic model and modern precipitation sampling from the Plateau region. Our new results from the Hoh-Xil Basin suggest 1700 to 2600 m of uplift may have occurred some time between the late Eocene and early Miocene. The timing of this uplift is consistent with late-Oligocene compressional deformation of the Hoh-Xil Basin and northward growth of the Tibetan Plateau however, the calculated uplift is not a unique solution from the paleoisotope data because of uncertainties in Eocene and Miocene moisture sources and isotope gradients for the northern plateau. Our results demonstrate the utility of lipid-based estimates of paleoelevation and expand the types of deposits amenable to paleoaltimetry analysis.