THE PALEOALTIMETRY OF TIBET: AN ISOTOPIC PERSPECTIVE

Stable isotopes provide a valuable perspective on the timing of elevation change of the Tibetan Plateau. We begin our paper by looking in depth at isotopic patterns in modern Tibet. We show that the delta(18)O value of surface waters decreases systematically up the Himalayan front in central Nepal by about 2.8 parts per thousand/km, in agreement with the patterns documented and modeled by previous research. On the Tibetan plateau itself there is no apparent correlation between elevation and the delta(18)O value of flowing surface waters. Both surface waters and soil carbonates display a northward increase in delta(18)O values, of about 1.5 parts per thousand/degrees north of the Himalayan crest, even though elevation increases modestly. The isotopic increase with latitude reduces the isotope-elevation gradient for water in the northernmost plateau to 1 to -2 parts per thousand/km. Carbonates in both soils and lakes form at higher temperatures than assumed by previous studies on the plateau. Temperature estimates from clumped-isotope (Delta(47)) analyses of modern soil carbonates significantly exceed mean annual air T and modeled maximum summer soil temperatures by 15.8 +/- 2.8 degrees and 9.7 +/- 2.5 degrees C, respectively. Similarly elevated temperatures best account for the delta(18)O values observed in modern soil and lake carbonates. We recalculated paleoelevations from previous studies on the plateau using both higher formation temperatures and latitude-corrected isotopic values. With one notable exception, our revised model produces paleoelevation estimates very close to previous estimates. The exception is the reconstruction from late Eocene age deposits at Xoh Xil, for which we calculate elevations that are higher and much closer to the current elevation than previously reconstructed. Therefore, there is no evidence for northward progression through time of Tibetan elevation change. Instead, the available but admittedly very scanty evidence suggests that much of Tibet attained its modern elevation by the mid-Eocene. A truly robust test of the various geodynamic models of uplift await expansion and replication of isotopic records all across Tibet, especially in the center and north and for >15 Ma.