Resetting Southern Tibet: The serious challenge of obtaining primary records of Paleoaltimetry

Abstract

Over the last several decades a number of studies have attempted to reconstruct the rise of the Himalaya and Tibet since India-Asia collision began in the early Cenozoic, with rather contradictory results. Here, we evaluate the efforts at reconstructing the history of this major mountain-building event as archived in oxygen, carbon, and clumped isotope records from carbonates exposed on the southern Tibetan Plateau. We find that a number of potential isotopic records of paleoaltimetry from Tibet – using both conventional oxygen and clumped isotope systems–may have been reset during burial and heating. Without exception, the marine δ18O values of Cretaceous and Paleogene marine carbonates across the orogen have been reset from their primary values of 0 ± 4‰ (VPDB) to lower values between −5 and −20‰, most conspicuously in the Indo-Asian suture zone. For this and other reasons, we view isotopic records of paleoaltimetry from the suture zone and adjacent Gangdese arc with great caution, especially early Cenozoic basin sediments that have experienced similar burial and heating as the underlying marine limestones. Outside the suture zone/arc, marine carbonates retain the isotopic imprint of low-elevation, meteoric diagenesis, a result that supports paleoaltimetric reconstruction from these areas using younger non-marine carbonates. We are even more cautious about temperature reconstructions using clumped isotope analyses, which previous studies suggest are sensitive to resetting at modest burial temperatures. With a few possible exceptions, primary clumped isotope values have probably not withstood the elevated temperatures of the suture zone and magmatic arc, nor burial depths of >3 km in basins outside the suture. In light of susceptibility to alteration of oxygen isotopes in carbonates in orogenic belts, future paleoaltimetric reconstruction prior to the Miocene on the Tibetan Plateau should couple δ18O, Δ47, and Δ'17O analysis of carbonates with analysis of noncarbonate archives—such as silicates and organic matter— that are less susceptible to resetting.

Carbon, unlike oxygen, isotopic values from paleosol carbonate are well preserved from all periods, due to the very low C/O ratio of most altering fluids. Samples from within the suture zone yield a record of paleo-vegetation change covering much of the Cenozoic. Carbon isotopic values from paleosols have no analog among modern Tibetan soils and most resemble in appearance and chemical composition the vegetated soils in the lowlands of northern India today. Carbon isotopes from paleosols depict a major reduction in vegetation cover in the suture zone since 20 Ma, probably due to a combination of uplift of the suture zone and global cooling. Northward on the Lhasa and Qiantang terranes, the landscape was less vegetated in the Oligocene compared to the contemporaneous suture zone, but more vegetated and less arid than it is today.