Aridification signatures from fossil pollen indicate a drying climate in east-central Tibet during the late Eocene

Qin Yuan,Hai-Cheng Wei,Yong-Sheng Du,Zhan-Jie Qin,Jun-Jie Shan,Dong-Lin Gao,Vivi Vajda,Fa-Shou Shan,Qi-Shun Fan,Natasha Barbolini,Catarina Rydin

doi:10.5194/cp-16-2255-2020

Abstract

Abstract. Central Asia experienced a number of significant elevational and climatic changes during the Cenozoic, but much remains to be understood regarding the timing and driving mechanisms of these changes as well as their influence on ancient ecosystems. Here, we describe the palaeoecology and palaeoclimate of a new section from the Nangqian Basin in Tibet, north-western China, dated as Bartonian (41.2–37.8 Ma; late Eocene) based on our palynological analyses. Located on the east-central part of what is today the Tibetan Plateau, this section is excellently placed for better understanding the palaeoecological history of Tibet following the Indo-Asian collision. Our new palynological record reveals that a strongly seasonal steppe–desert ecosystem characterized by drought-tolerant shrubs, diverse ferns, and an underlying component of broad-leaved forests existed in east-central Tibet during the Eocene, influenced by a southern monsoon. A transient warming event, possibly the middle Eocene climatic optimum (MECO; 40 Ma), is reflected in our record by a temporary increase in regional tropical taxa and a concurrent decrease in steppe–desert vegetation. In the late Eocene, a drying signature in the palynological record is linked to proto-Paratethys Sea retreat, which caused widespread long-term aridification across the region. To better distinguish between local climatic variation and farther-reaching drivers of Central Asian palaeoclimate and elevation, we correlated key palynological sections across the Tibetan Plateau by means of established radioisotopic ages and biostratigraphy. This new palynozonation illustrates both intra- and inter-basinal floral response to Qinghai–Tibetan uplift and global climate change during the Paleogene, and it provides a framework for the age assignment of future palynological studies in Central Asia. Our work highlights the ongoing challenge of integrating various deep time records for the purpose of reconstructing palaeoelevation, indicating that a multi-proxy approach is vital for unravelling the complex uplift history of Tibet and its resulting influence on Asian climate.

Highlights

A series of major geological events occurred during the Cenozoic, which led to a fundamental change in the global climate (Zachos et al, 2001)
On the basis of palynological assemblages, we conclude that the rocks of the Ria Zhong (RZ) section (Nangqian Basin) are Bartonian (41.2–37.8 Ma; late Eocene) in age
They record a strongly seasonal steppe–desert ecosystem characterized by Ephedra and Nitraria shrubs, diverse ferns, and an underlying component of broad-leaved forests

Summary

Introduction

A series of major geological events occurred during the Cenozoic, which led to a fundamental change in the global climate (Zachos et al, 2001). Today the Tibetan Plateau (TP) is the highest elevated plateau in the world, with a complex uplift history beyond a simple collision between the Indian and Asian continents (Molnar and Tapponnier, 1975; Aitchison and Davis, 2001; Wang et al, 2008; Xia et al, 2011; Aitchison et al, 2011; Zhang et al, 2012; Wang, 2014; Spicer et al, 2020). The term “Tibetan Plateau” is used to denote the geographic extent occupied by the modern plateau, but it should not be taken to imply that an elevated expanse of low-relief topography existed across this region in the Eocene (Spicer et al, 2020)

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Discussion

Conclusion