Abstract

The timing and pattern of Tibetan Plateau rise provide a critical test of possible mechanisms for the development and support of high topography, yet views range widely on the history of surface uplift to modern elevations of ∼4.5 km. To address this issue we present clumped isotope thermometry data from two well-studied basins in central and southwestern Tibet, for which previous carbonate δ18O data have been used to reconstruct high paleoelevations from late Oligocene to Pliocene time. Clumped isotope thermometry uses measurements of the 13C-18O bond ordering in carbonates to constrain the temperature [T47)] and δ18O value of the water from which the carbonate grew. These data can be used to infer paleoelevation by exploiting the systematic decrease of surface temperature and the δ18O value of meteoric water with elevation, provided samples record original depositional conditions and appropriate context exists for interpreting T47) and δ18O values.

Previous calcite δ18O and δ13C values for Oligocene-age marls from the Nima Basin in central Tibet are thought to reflect original depositional conditions; however, T47) values exceed Earth-surface temperatures, indicating that the samples have been diagenetically altered. Maximum burial temperatures were not high enough to cause solid-state C-O bond reordering. Instead, the elevated T47) and water δ18O values are consistent with recrystallization of the samples in a rock-buffered system.

Miocene–Pliocene aragonitic gastropod shells from the Zhada Basin in southwestern Tibet record primary environmental temperatures, which we interpret in the context of modern shell and tufa T47) values and lake water temperatures. Modern shell and tufa T47) values are similar to warm-season water temperatures. The ca. 9–4 Ma shell temperatures are significantly colder, suggesting a 9 ± 3 °C (2σ) average increase in warm-season lake water temperatures since the late Miocene. If the temperature increase is due entirely to elevation change and the modern June-July-August (JJA) surface water lapse rate of 6.1 °C/km applies, it implies >1 km of elevation loss since the late Miocene–Pliocene—corresponding to an average basin floor paleoelevation of 5.4 ± 0.5 km (2σ). A warmer mid-Pliocene climate would make this a minimum estimate. Our finding of cold paleotemperatures contrasts with previous conclusions based on Pliocene snail shell T47) data interpreted in the absence of modern shell and water temperature data, but is consistent with δ18O-based paleoaltimetry and with paleontological and isotopic data indicating the presence of cold-adapted mammals living in a cold, high-elevation climate. We suggest that late Neogene elevation loss across the Zhada Basin catchment probably related to local expression of east-west extension across much of the southern Tibetan Plateau at this time.

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