A HIGH-ELEVATION TERRESTRIAL RECORD OF ORBITAL CYCLICITY DRIVEN ENVIRONMENTAL CHANGE FROM THE ZHADA BASIN, SOUTHWESTERN TIBETAN PLATEAU

The Zhada basin is a large Neogene extensional sag basin in the Tethyan Himalaya of southwestern Tibet. A long-term environmental change observed in the Zhada basin involves a decrease in abundance of arboreal pollen in favor of nonarboreal pollen and a decrease in mega-faunal abundance and diversity. This has been attributed to the uplift of the basin and an associated colder, drier climate. However, recent stable isotope work suggests that the basin has been at or above its current elevation since ~ 9 Ma. In order to determine the driver of environmental change in the Zhada basin, we examined the sequence stratigraphy and isotope stratigraphy of the basin-filling Zhada formation.

Sequence stratigraphy reveals a long-term tectonic signal in the formation and filling of the Zhada basin, as well as higher-frequency cycles, which we attribute to Milankovitch forcing. The record of Milankovitch cycles in the Zhada basin implies that global climate drove lake and wetland expansion and contraction in the southern Tibetan Plateau from the Late Miocene to the Pleistocene. Sequence stratigraphy shows that the Zhada basin evolved from an overfilled to underfilled basin, but continued evolution was truncated by an abrupt return to fluvial conditions. Isotope stratigraphy shows distinct drying cycles, particularly during times when the basin was underfilled.

We conclude that environmental change in the southwestern Tibetan plateau in the Late Miocene-Pliocene was the result of global or regional climate change rather than the result of uplift. We hypothesize that environmental change reflects strengthening and weakening of the monsoon. We further hypothesize that floral and faunal populations responded to this environmental change; driving the observed decrease in diversity of mega-fauna and increase in nonarboreal pollen.