Inland Aridification of NW China Since the Late Middle Eocene: Stable Isotope Evidence from Western Qaidam Basin

Abstract:

Cenozoic paleoclimate reconstructions of China, based on pollens, fossils and sedimentary deposits, show a change from planetary aridity to inland aridity of NW China by the early Miocene. However, the initiation of this paleoclimate transition is not well-documented and might be much earlier. The surface uplift of the Tibetan Plateau, the retreat of the Para-Tethys sea, and global cooling have all been suggested to influence the establishment of this inland aridity, although their relative significance remains obscure. This paper presents a stable isotope study of a 4435 m long sedimentary section from the western Qaidam Basin, northern Tibetan Plateau, that spans from the late middle Eocene to late Miocene. The lowermost and uppermost parts of the section are dominated by fluvial and alluvial fan deposits, while the majority of the middle of the section represents palustrine, lower fan delta and marginal to shallow lacustrine fine-grained sediments intercalated with coarse sandstone and conglomerate. Our isotope data show sporadic aridity events in the late middle Eocene to early Oligocene, which might mark the transition from planetary aridity to, or the initiation of, inland aridity in NW China, due to the retreat of the Para-Tethys sea, a process that might be significantly influenced by the early topographic growth of the south-central Tibetan Plateau. A negative shift in oxygen isotope values around 19 Ma is also in accordance with other geological evidence suggesting the Oligocene–early Miocene growth of the Kunlun mountains south of the Qaidam basin. Later intensification of aridity occurred at ~12 Ma that corresponds with a regional climate change event, which we attribute to the upward and outward growth of the northern Tibetan Plateau. The final establishment of extreme inland aridity that is comparable to present day was most likely established at ~3.1–2.6 Ma in the Qaidam basin, and therefore global cooling and northern hemisphere glaciation is a major candidate for the cause.