From desert to monsoon: irreversible climatic transition at\u2009~\u200936\xa0Ma in southeastern Tibetan Plateau

Hongbo Zheng,Huan Xu,Qing Yang,Akihiro Kano,Aki Sakuma,Shuo Cao,Mengying He,Peter D Clift,Fred Jourdan,Ryuji Tada

doi:10.1186/s40645-022-00470-x

Abstract

Although there is increasing evidence for wet, monsoonal conditions in Southeast Asia during the late Eocene, it has not been clear when this environment became established. Cenozoic sedimentary sequences constrained by radiometrically dated igneous rocks from the Jianchuan Basin in the southeast flank of Tibetan Plateau now provide a section whose facies and climatic proxies determine this evolution. Semi-arid conditions had dominated the region since Paleocene controlled by the northern sub-tropical high pressure system, culminating in mid Eocene when desert dunes developed. From 36 Ma, the basin began to accumulate swamp sediments with coals, together with synchronous braided river deposits and diversified pollen assemblages, indicating significant increase in precipitation. This remarkable transition from dry to wet conditions precedes the Eocene/Oligocene boundary at 34 Ma, thus excluding general global cooling as the prime driver. We propose that uplift of Tibetan Plateau might have reached a threshold elevation by that time, operating through thermal and dynamic forcing, causing the inception or significant intensification of monsoonal rains to penetrate into this downwind locality.

Highlights

The Asian monsoon is the global type example of how the solid Earth and atmosphere interact with one another in order to drive significant climate change (Webster et al 1998; Wang 2010) (Fig. 1)
It is clear that there are several processes that affect the intensity of the Asian summer monsoon, there is little doubt that the present strong intensity is at least partly related to the uplift of the Himalaya in the case of the South Asian monsoon (Boos and Kuang 2010), as well as the elevation and broadening of the Tibetan Plateau in East Asia (Molnar et al 2010; Tada et al 2016)
For a long time, the sediments in this basin were believed to date from the Paleocene up to the Pliocene recent re-dating of the section (Gourbet et al 2017), supported by new constraints presented here, demonstrated that some of the youngest sedimentary rocks in the basin are Upper Eocene

Summary

Introduction

The Asian monsoon is the global type example of how the solid Earth and atmosphere interact with one another in order to drive significant climate change (Webster et al 1998; Wang 2010) (Fig. 1). For a long time, the sediments in this basin were believed to date from the Paleocene up to the Pliocene recent re-dating of the section (Gourbet et al 2017), supported by new constraints presented here, demonstrated that some of the youngest sedimentary rocks in the basin are Upper Eocene These data constrain the deposition of the Shuanghe Formation (coal series) to being between 36. We concur with Sorrel et al (2017) that deposition of the Shuanghe Formation represents sedimentation under relatively humid conditions, as shown by the presence of coals interbedded with medium-bedded fluvial sandstones and shales, likely of a meandering river facies, based on the presence of channelized sands and tabular muddy overbank facies (Smith 1987) (Fig. 2B). The hematite /goethite ratios of Yunlong and Baoxiangsi Formations are much higher than that of Jinsichang and Shuanghe Formations (Fig. 7B)

Discussions

Conclusions