Calibration and Application of the Tibetan Paleoaltimeter

We analyzed modern meteoric water (n = 170) and soil carbonate (n = 300) from southern and central Tibet in order to establish an isotopic framework for reconstructions of paleoelevation of the Tibetan Plateau. The oxygen isotopic composition of modern meteoric waters is not a simple function of elevation—as assumed by some previous reconstructions of paleoelevation—but increases markedly with distance north of the Himalayan massif (~3 per mil/degree latitude; r2 = 0.6). This decrease reflects the interplay of monsoonal moisture sources in the south with northern moisture sources possibly recycled from local lakes, bogs, and soils. The oxygen isotopic value of modern soil carbonate shows significant scatter due to the effects of evaporation, but the most negative, least evaporated oxygen isotopic values also increase northwards, following the pattern of meteoric waters but at a lower slope of 1.0-1.5 per mil/degree latitude. The latitudinal differences in the modern soil and water data sets may be due to the fact that the water samples represents a much shorter—and hence less representative—snapshot of long-term averages than soil carbonates. Any previous or future studies in central to northern Tibet that fail to take account these strong latitudinal effects will underestimate paleoelevation. We examined the oxygen isotopic composition of Mio-Pliocene age fossil shell from Zhada in southeast Tibet, and paleosol carbonates of Oligo-Miocene age from Nyima in central Tibet. Careful evaluation of the samples from the standpoint of potential diagenesis suggest that neither record has been altered. Comparison to the oxygen isotopic composition of local modern carbonates shows that that these areas of southern and central Tibet had attained elevations comparable to today's by the early Miocene.