Ostracodes on the Roof of the World: Timing the Uplift of the Tibetan Plateau Using Stable Isotope Paleoaltimetry and Clumped Isotope Paleothermometry

The timing of the uplift of the Tibetan Plateau has occupied the tectonics community for much of the past quarter century. The effort to elucidate the paleoelevation history has refined a more precise method of estimating paleoaltimetry—coupling stable isotopes (oxygen and deuterium; Rowley et al., 2001) with paleotemperatures derived from Δ₄₇ clumped isotope paleothermometry. However, geologists using this method on the Tibetan Plateau are correctly concerned with the preservation of a primary δ¹⁸O_w signal in the thermally altered rocks commonly inherent to collisional zones. Burial by both overlying strata and tectonics can lead to thermal alteration and isotopic resetting, eliminating the possibility of constraining the elevation history using the stable isotope record.

Lin et al. (2014) suggest preservation of a primary depleted oxygen signal in the 55 My carbonates of the Nianbo Fm. and argue against the previously accepted diagenetic resetting of lacustrine and paleosol carbonates found in the Linzhou (Penbo) Basin. The Lin et al. study reports recovery of seasonal isotopic variation in an ostracode using in situ analysis by NanoSIMS, which suggests the host carbonates have not reached sufficiently high temperatures for resetting of the primary δ¹⁸O signal. This result prompted further Δ₄₇ analyses at Chicago of Penbo carbonates initially determined to have high paleotemperatures (>60°C) using older, less-refined clumped isotope techniques five years ago. Results of our clumped isotope study will examine whether or not seasonal δ¹⁸O_c variations are preserved in the Nianbo Fm. Δ₄₇-derived paleotemperatures will reveal whether the ostracodes preserve an original oxygen signal or are thermally overprinted. The combination of paleotemperatures and δ¹⁸O_w will allow for the paleoelevation history of the Tibetan Plateau to be extended to the Eocene using the robust Δ₄₇-derived paleotemperature proxy.