LOW LATITUDE TERRESTRIAL CLIMATE OF SOUTHEASTERN TIBET REVEALED THROUGH CLUMPED ISOTOPE THERMOMETRY: IMPLICATIONS FOR THE LATE CRETACEOUS ELEVATION HISTORY OF SOUTHEASTERN TIBET

Views range widely on the timing of surface uplift of the Tibetan Plateau to its current height of ~5 km, specifically, whether the current elevation of Tibet developed solely in response to the Cenozoic Indo-Asia collision, or was partially inherited from high topography developed in Mesozoic time is unclear. Here, we evaluate the Cretaceous elevation history of southeastern Tibet by studying pedogenic carbonate from an alluvial sequence in the Qamdo basin (~15° N paleo-latitude) using clumped isotopes as a paleoaltimetry tool.

We studied two stratigraphic sections through Cenomanian-Santonian strata of the Qamdo basin and find that pedogenic carbonate petrography, burial history and elemental composition are consistent with preservation of isotopic signatures and minimal influence on solid-state reordering.

Average soil temperatures (i.e., D₄₇-T) decreased from 45 ± 2°C in the Turonian to 35 ± 2°C by the Santonian, accompanied by a decrease in average δ¹⁸O_water values from ~-6.0‰ to ~-8.0‰. Contemporaneous tropical sea surface temperature (SST) estimates show that low-latitude SST during the Turonian-Santonian interval were relatively warm and constant at ~35°C, without evidence of significant global cooling episodes. Together, the observed changes in D₄₇-T and δ¹⁸O_water values exhibit a pattern that is consistent with surface elevation changes during Late Cretaceous time. Assuming temperature and isotopic lapse rates of 6.5°C/km and 1.5‰/km, respectively, the observed D₄₇-T and δ¹⁸O_water shifts correspond to a positive elevation change of ~1.5 km in southeastern Tibet during Late Cretaceous time. This inference is supported by synchronous changes in strontium isotopes, structure, and sedimentology that are consistent with elevation change during this time. Strontium isotope ratios of pedogenic carbonate increase up-section from 0.7106 to 0.7112, indicating enhanced weathering of old continental source areas. In addition, the development of growth strata and occurrence of thick-bedded conglomerate layers all implying active tectonics during the Turonian-Santonian interval. Together, these data suggest that the modern Tibetan Plateau may inherited some elevation from tectonic activity in Late Cretaceous time.