Saylor Part of Team Evaluating Changes in Surface Elevation of the Tibetan Plateau

Saylor Part of Team Evaluating Changes in Surface Elevation of the Tibetan Plateau
Group Used Stable Isotope Analysis of Snail Shells

Tibetan PlateauJoel Saylor, assistant professor in the UH Department of Earth and Atmospheric Sciences, was part of a team that used stable isotope analysis of snail shells to evaluate changes in surface elevation of the Tibetan Plateau. The team was led by Kate Huntington from the University of Washington.

The Tibetan Plateau is the world's largest high-elevation terrane, and scientists have debated the mechanism and timing of surface uplift for decades.

Using the temperature-sensitive bonding between heavy isotopes of carbon and oxygen, the team showed that there had been significant warming in the Zhada Basin in the SW Tibetan Plateau, consistent with the conclusion that this large portion of the Tibetan Plateau lost 1-2 km (3,300-6,600 feet) of elevation in the last 3.5 million years. This independently confirms previous work by co-authors Saylor, Jay Quade and colleagues based on the ratio of heavy to light isotopes of oxygen from snail shells in the Zhada Basin.

The work was addressed in the article, "High late Miocene–Pliocene elevation of the Zhada Basin, southwestern Tibetan Plateau, from carbonate clumped isotope thermometry," which published in the Geological Society of America Bulletin and was featured in several news outlets including The Times of India.

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 [T(Δ47)] 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 T(Δ47) 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, T(Δ47) 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 elevatedT(Δ47) 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 T(Δ47) values and lake water temperatures. Modern shell and tufa T(Δ47) 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 T(Δ47) 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.