PALEOALTIMETRY OF THE OIYUG BASIN, SOUTHERN TIBET USING CLUMPED-ISOTOPE PALEOTHERMOMETRY

We present new mid-Miocene to Pliocene paleoelevation estimates of the Oiyug Basin located on the southern Tibetan Plateau using Δ₄₇ clumped isotope paleothermometry and traditional δ¹⁸O stable isotope paleoaltimetry. Δ₄₇ reflects the statistical overabundance of ¹³C-¹⁸O bonds in multiply-substituted isotopologues of CO₂ generated from phosphoric acid digestion of carbonate minerals (Eiler, 2007). Commonly applied techniques for reconstructing paleoelevation histories are based on paleobotany (paleoenthalpy derived from floral physiognomy, e.g. Spicer et al., 2003) the oxygen isotope record of meteoric and pedogenic minerals (e.g. Chamberlain and Poage, 2000), and compound-specific δD of organic matter (Polissar et al., 2009). The carbonate clumped isotope paleothermometer provides independent constraints on both the temperature and isotopic composition of ancient surface waters (Ghosh et al., 2006a), making it a potentially powerful paleoaltimeter (Ghosh et al., 2006b, Quade et al., 2007) when the original isotopic signature is preserved. A burial history reconstructed from lipid-δD measurements in the Oiyug Basin suggests that these carbonates have not been buried, and give us high confidence that the carbonates are unaltered and retain their primary isotopic signature. The pedogenic carbonates from the Oiyug Basin yield Δ₄₇ values of 0.74-0.78 in the Absolute Reference Frame, which correspond to temperatures of 13-22 °C using Huntington et al.’s (2010) modern temperature calibration. These temperatures are likely a record of the warm month soil temperature of the Oiyug Basin. Soil carbonates most often form during the warmest, driest times of the year due to groundwater evaporation. This study tests the use of multiply substituted CO₂ isotopologues as a paleoaltimeter for the Tibetan Plateau and provides another independent paleoelevation estimate demonstrating that high elevations (~5300-4600 meters) were attained in southern Tibet by at least ~28 million years ago. We also examine how isotopic exchange with detrital minerals (quartz, feldspar) and clay minerals found in lake carbonates can shift the clumping signal.