Modern-like elevation and climate in Tibet since the mid-Miocene (ca. 15 Ma)

Abstract When the modern-like geomorphology and climate pattern of the Himalaya-Tibetan Plateau were established still remains unclear. In this study, we apply paired stable isotope compositions of carbonate (δ18Oc) and leaf waxderived n-alkanes (δ2Halk) from the upper Gazhacun Formation in the Namling Basin to reconstruct the middle Miocene elevation and climate of southern Tibet. Depositional age of the upper Gazhacun Formation has been precisely constrained to between 15.5 Ma and 15.1 Ma by zircon U-Pb ages of dacite interlayers. Paired carbonate derived δ18Ow values (–17.9 ± 1.3‰ to –18.3 ± 1.3‰) and leaf-wax derived δ2Hw values (–131.5 ± 20‰ to –145.7 ± 20‰) plot on or very close to the global meteoric water line suggesting that these samples experienced little evaporation enrichment and isotopic alternation. Based on these two independent proxies, paleoelevation estimates for the Namling Basin are consistently between 4.6+0.7/-0.8 km and 5.2+0.7/-0.8 km, supporting a high elevation for southern Tibet in the mid-Miocene. Integrated with published paleoelevation estimates for the Himalaya, central and northern Tibet in literature, a near-present elevation across the whole Himalaya-Tibetan Plateau has already been established since the middle Miocene (ca. 15 Ma). Besides, stable isotopic values across the Himalaya-Tibetan Plateau show that the δ2Hw values gradually increase northward from the Himalaya to northeastern Tibet, quite similar to that of the present day. This pattern suggests that during the middle Miocene, the Himalaya-Gangdese system may have blocked southerly monsoonal moisture from reaching northern Tibet. Westerlies or local recycling of moisture might be the dominant moisture sources across northern Tibet, with enriched δ18Ow and δ2Hw precipitation values that could lead to erroneous paleoelevation estimates over central and northern Tibet.