Climate-driven environmental change in the Zhada basin, southwestern Tibetan Plateau

Abstract

The Zhada basin is a large Neogene extensional sag basin in the Tethyan Himalaya of southwestern Tibet. In this paper we examine environmental changes in the Zhada basin using sequence stratigraphy, isotope stratigraphy, and lithostratigraphy. Sequence stratigraphy reveals a long-term tectonic signal in the formation and fi lling 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 overfi lled to underfi lled basin, but continued evolution was truncated by an abrupt return to fl uvial conditions. Isotope stratigraphy shows distinct drying cycles, particularly during times when the basin was underfi lled.A long-term environmental change observed in the Zhada basin involves a decrease in abundance of arboreal pollen in favor of nonarboreal pollen. The similarity between the long-term environmental changes in the Zhada basin and those observed elsewhere on and around the Tibetan Plateau suggests that those changes are due to global or regional climate change rather than solely the result of uplift of the Tibetan Plateau.

Late Neogene environmental changes in the central Himalaya related to tectonic uplift and orbital forcing

“…Alternatively, if meteoric water d 18 O w remained the same, the more negative d 18 O c values for the period of $5.5-3.2 Ma could reflect a reduced E/P ratio, implying either (1) a wetter climate or a stronger summer monsoon in the region during this time period compared to the previous interval of 7.2-5.5 Ma or (2) a decrease in water residence time in the lake due to faster passage of stream water through the lake. However, the inference of a wetter climate is inconsistent with other evidence for a drying climate in Asia since $8-9 Ma (e.g., Barry and Flynn, 1990;Barry et al, 2002;Quade et al, 1989Quade et al, , 1995An et al, 2001;Saylor et al, 2010). A change in residence time is also unlikely cause because this d 18 O c shift occurred over a much longer time scale (Fig.…”

High late Miocene–Pliocene elevation of the Zhada Basin, southwestern Tibetan Plateau, from carbonate clumped isotope thermometry

“…However, a broader evaluation of paleoelevation on the Tibetan Plateau is needed to determine whether the loss of elevation observed in the Zhada Basin is a regional phenomenon. If regional elevation loss were observed, it might be explained by the attainment of a limiting elevation and the onset of east-west extension in the Tibetan Plateau (e.g., Molnar and Tapponnier, 1978;Molnar and Lyon-Caen, 1988;Kapp and Guynn, 2004) or ductile escape of material from beneath the plateau (e.g., Clark and Royden, 2001) rather than by local structural control (e.g., Murphy et al, 2009;Saylor et al, 2010).…”

“…The need to assume the temperature of carbonate formation contributes to the uncertainty in δ 18 O mw values and in the resulting estimates of high paleoelevation and cold, arid conditions (Murphy et al, 2009;Saylor et al, 2009Saylor et al, , 2010. Estimates of lake carbonate formation temperature typically are based on mean annual or growing-season air temperatures (Horton et al, 2004;Dettman et al, 2005;Saylor et al, 2009;Chamberlain et al, 2012).…”