Spatial patterns of soil n-alkane δD values on the Tibetan Plateau: Implications for monsoon boundaries and paleoelevation reconstructions

Between 2010 and 2011, this project collected and analyzed forty-nine superficial soil samples, for wax-derived n-alkane delta D values (delta D-wax), along a south to north transect of the Tibetan Plateau, from the southern Plateau and Nepalese Himalayas, passing through the Nam Co basin, to Qilian Mountains. Twenty-two paired river water samples were also collected from northeastern Tibet during this period and analyzed for delta D (delta D-RW). The delta D-RW and delta D-wax values become progressively more negative northward from similar to 27.5 degrees N, and reach a minimum at similar to 30.6 degrees N (Nam Co basin). North of the Nam Co basin, to 35 degrees N, isotope values increase, due to increasing contributions from the year-round westerlies and recycled moisture from the Plateau. Relatively high and constant delta D-wax and delta D-RW values prevail in areas north of similar to 35 degrees N in northeastern Tibet. Results show that these delta D-wax values vary considerably with location and relate closely to the influences of the summer monsoon and circulation changes. These changes track the spatial variability of isotopes from modern river water and precipitation at large spatial scales. Paleoelevation reconstructions should take into account the impact of mixing between continental and monsoon-derived moisture on the relationships with elevation and delta D-wax (and linear isotopic lapse rates) for central northeastern Tibet. Based on extended and more sensitive (relative to delta O-18) delta D-wax and delta D-RW values, we infer that in the past the westerlies reached further south, to the north piedmont of the Nyainqentanglha Range, and that Indian Summer Monsoon moisture pushes across the Tanggula Mountains and approaches the Kunlun Mountains.