PROXY FIDELITY ASSESSMENT IN ROBUST ENVIRONMENTAL AND TECTONIC RECONSTRUCTIONS: A CASE STUDY FROM THE HIGH ELEVATION NORTHERN MARGIN OF THE LHASA TERRANE (Invited Presentation)

The elevation history of the northern margin of the Tibetan Plateau has been historically less well constrained than the southern margin, but workers have approached it using an array of techniques, each with their own biases and uncertainties. Some studies employing the same or similar proxies on coeval substrates yield discrepant elevation estimates. We posit that interpretations of proxy data can be biased based on incomplete or selective sampling, and that a full consideration of proxy data in the literature is necessary to inform a complete interpretation. We consider the current knowledge of the development of high elevation across the northern Lhasa block as recorded in basin sedimentation, paleo-ecology and stable isotopes through the lens of proxy assessment and the employment of multiple proxies. For example, we address the disagreement over stable isotopic compositions from the Lunpola basin that have received criticism for suspected high-temperature burial alteration by providing low clumped isotope-derived temperatures (~5-9°C) and preserved primary carbonate fabrics.

We reconstruct Eocene to Miocene elevations of the Lunpola basin on the northern Lhasa terrane, the southernmost crustal block of the Tibetan Plateau, using stable isotope paleoaltimetry and clumped-isotope paleothermometry on lacustrine and pedogenic carbonates. We use an additional precipitation proxy, water δD, which was calculated from plant-wax δD assuming an ε_aof -121±10‰; δ¹⁸O_wwas calculated by using a direct temperature measurement for the Miocene carbonate sediments using clumped isotopes (T(∆₄₇)=5.3±4.2°C). The use of multiple precipitation proxies in strongly evaporative systems—such as lakes in high-altitude, arid climates—provides a quantitative method of reversing apparent evaporative fractionation to reconstruct primary isotopic values. We use the evaporation slope to reconstruct primary compositions of the precipitation source to the catchment, which we use in our ∆(δ¹⁸O_w) paleo-elevation calculations. Paleo-elevation estimates for the Lunpola basin (~4.4 to 5 km) support previous evidence that the northern edge of the Lhasa Block attained high elevation by at least the late Eocene, and refutes studies invoking a low elevation interpretation based on higher oxygen isotope values.