Clim. Past, 16, 2255–2273, 2020 https://doi.org/10.5194/cp-16-2255-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Aridification signatures from fossil pollen indicate a drying climate in east-central Tibet during the late Eocene Qin Yuan1,2,3,4, Natasha Barbolini5,6, Catarina Rydin5,7, Dong-Lin Gao1,2, Hai-Cheng Wei1,2, Qi-Shun Fan1,2, Zhan-Jie Qin1,2, Yong-Sheng Du1,2, Jun-Jie Shan1,2,3, Fa-Shou Shan1,2, and Vivi Vajda4 1Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, China 2Qinghai Provincial Key Laboratory of Geology and Environment of Salt Lakes, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, China 3University of Chinese Academy of Sciences, Beijing, China 4Department of Palaeobiology, Swedish Museum of Natural History, Stockholm, Sweden 5Department of Ecology, Environment and Plant Sciences and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden 6Department of Ecosystem and Landscape Dynamics, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, the Netherlands 7The Bergius Foundation, The Royal Swedish Academy of Sciences, Stockholm, Sweden Correspondence: Natasha Barbolini ([email protected]) Received: 7 November 2019 – Discussion started: 20 January 2020 Revised: 1 October 2020 – Accepted: 7 October 2020 – Published: 20 November 2020 Abstract. Central Asia experienced a number of signif- caused widespread long-term aridification across the region. icant elevational and climatic changes during the Ceno- To better distinguish between local climatic variation and zoic, but much remains to be understood regarding the tim- farther-reaching drivers of Central Asian palaeoclimate and ing and driving mechanisms of these changes as well as elevation, we correlated key palynological sections across the their influence on ancient ecosystems. Here, we describe the Tibetan Plateau by means of established radioisotopic ages palaeoecology and palaeoclimate of a new section from the and biostratigraphy. This new palynozonation illustrates both Nangqian Basin in Tibet, north-western China, dated as Bar- intra- and inter-basinal floral response to Qinghai–Tibetan tonian (41.2–37.8 Ma; late Eocene) based on our palynolog- uplift and global climate change during the Paleogene, and ical analyses. Located on the east-central part of what is to- it provides a framework for the age assignment of future pa- day the Tibetan Plateau, this section is excellently placed for lynological studies in Central Asia. Our work highlights the better understanding the palaeoecological history of Tibet ongoing challenge of integrating various deep time records following the Indo-Asian collision. Our new palynological for the purpose of reconstructing palaeoelevation, indicat- record reveals that a strongly seasonal steppe–desert ecosys- ing that a multi-proxy approach is vital for unravelling the tem characterized by drought-tolerant shrubs, diverse ferns, complex uplift history of Tibet and its resulting influence on and an underlying component of broad-leaved forests ex- Asian climate. isted 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 re- 1 Introduction flected in our record by a temporary increase in regional trop- ical taxa and a concurrent decrease in steppe–desert vegeta- A series of major geological events occurred during the tion. In the late Eocene, a drying signature in the palynolog- Cenozoic, which led to a fundamental change in the global ical record is linked to proto-Paratethys Sea retreat, which climate (Zachos et al., 2001). The most important events in- clude the formation of the polar ice cap (e.g. DeConto and Published by Copernicus Publications on behalf of the European Geosciences Union. 2256 Q. Yuan et al.: Aridification signatures from Eocene pollen in east-central Tibet Pollard, 2003; Pagani et al., 2011), regression of the proto- Tibetan region since the collision of the Indian and Asian Paratethys Sea from Eurasia (Abels et al., 2011; Bosboom tectonic plates (Gupta et al., 2004; Molnar, 2004; Wang et et al., 2014; Caves et al., 2015; Bougeois et al., 2018; Kaya al., 2001). Previous palynological studies from this part of et al., 2019; Meijer et al., 2019), and uplift of the Qinghai– the plateau have revealed a relatively dry climate with brief Tibetan region (Dupont-Nivet et al., 2007, 2008; Molnar et humid intervals in the late Eocene, dominated by drought- al., 2010; Miao et al., 2012; Hu et al., 2016; Li et al., 2018). tolerant (xerophytic) and salt-tolerant (halophytic) steppe– Today the Tibetan Plateau (TP) is the highest elevated plateau desert vegetation (Wei, 1985; Yuan et al., 2017). in the world, with a complex uplift history beyond a sim- This climate and palaeoflora were very similar to contem- ple collision between the Indian and Asian continents (Mol- poraneous plateau ecosystems further to the north, such as nar and Tapponnier, 1975; Aitchison and Davis, 2001; Wang the Xining (Dupont-Nivet et al., 2007, 2008; Hoorn et al., et al., 2008; Xia et al., 2011; Aitchison et al., 2011; Zhang 2012) and Hoh Xil (Liu et al., 2003; Miao et al., 2016) et al., 2012; Wang, 2014; Spicer et al., 2020). In this paper, basins, demonstrating the potential for these successions to the term “Tibetan Plateau” is used to denote the geographic be biostratigraphically correlated. Furthermore, oxygen iso- extent occupied by the modern plateau, but it should not be tope records indicate that both northern and east-central Tibet taken to imply that an elevated expanse of low-relief topog- received moisture dominantly via the westerlies, which have raphy existed across this region in the Eocene (Spicer et al., maintained a semi-arid to arid climate in Central Asia since 2020). the early Eocene (Caves et al., 2015; Caves Rugenstein and Previous studies indicate that retreat of the proto- Chamberlain, 2018). This suggests that aridification across Paratethys Sea and the uplift of Tibet as well as other ranges this part of Tibet in the Eocene was related to large-scale to the north, such as the Altai, Sayan, and Hangay (Caves et atmospheric transport and justifies a comparison of palyno- al., 2014), may have been responsible for monsoon intensi- logical records in the northern and central parts of the TP. fication and aridification across the Asian continental inte- In contrast, south-eastern Tibet seems to have experi- rior in the Paleogene, although the timing of these mecha- enced a more humid climate hosting widespread conifer and nisms, and their roles in forcing climate dynamics, are still warm-temperate broad-leaved forests (Li et al., 2008; Su et debated (Caves et al., 2015; Spicer, 2017). In particular, a al., 2018), likely influenced by a Paleogene intertropical- lack of consensus exists regarding the onset of Asian aridifi- convergence-zone-driven monsoon system similar to the cation, whether it was a Paleogene or Neogene phenomenon, modern Indonesia–Australia monsoon (I-AM; Spicer, 2017). and its relationship with Tibetan uplift (e.g. Dupont-Nivet Today this wet-summer, dry-winter monsoonal regime pre- et al., 2007; Xiao et al., 2010; Miao et al., 2012; Caves et sides over a biodiversity hotspot in southern Asia; simi- al., 2015; Liu et al., 2016; Wang et al., 2018; L. Li et al., larly seasonal climates in the past are thought to also have 2019; Paeth et al., 2019). Aridification in north-eastern Tibet stimulated high biodiversity (Spicer, 2017). Southerly mois- appears to have intensified after the middle Eocene climatic ture has probably rarely extended northward of the central optimum (MECO; 40 Ma), a short-lived warming event doc- TP (Caves Rugenstein and Chamberlain, 2018); moreover, umented in marine records globally. The drying climate af- southern Tibetan Eocene floras display a modern aspect (e.g. ter this event is primarily linked to the second regression of Linnemann et al., 2018) that is quite different to more ances- the proto-Paratethys Sea, which reduced moisture supply via tral steppe vegetation hosted in the northern TP. the westerlies to Central Asia (Kaya et al., 2019). In north- The extent and timing of mechanisms that promoted eastern Tibet, the regional disappearance of perennial lakes, somewhat different floras south and north of the Tibetan– accompanied by an increase in pollen from xerophytic plants, Himalayan orogen remain poorly understood, with Licht et marks a permanent aridification step in the Asian terrestrial al. (2014) reporting marked monsoon-like patterns in both record after ∼ 40 Ma (Bosboom et al., 2014); however, these regions during the Eocene, utilizing records from north-west climatic trends are yet to be identified in central Tibet. China and Myanmar. The role of Qinghai–Tibetan uplift also The uplifting, large-scale thrusting, and striking of Tibet remains unclear, with contrasting models of plateau evo- caused several Paleogene intra-continental basins to form lution supported by various tectonic, isotopic, modelling, within the northern and central Qinghai–Tibetan region, in- and biological evidence (e.g. Mulch and Chamberlain, 2006; cluding the Nangqian Basin. Situated in the Yushu area Rowley and Currie, 2006; Ding et al., 2014; Li et al., 2015; (Fig. 1), this basin lies directly above the Lhasa Terrane, Jin et al., 2018; Botsyun et al., 2019; Su et al., 2019; Valdes et which comprised part of north-eastern Gondwana in the Late al., 2019; Shen and Poulsen, 2019; and summaries in Spurlin Triassic to Early Jurassic and formed through a subduction– et al., 2005; Wang et al., 2014; and Spicer, 2017). Accord- accretion process similar to that of the later Indo-Asian col- ingly, further stratigraphic and palaeoenvironmental studies lision (Liu et al., 2009). Subsequent to its formation, the of the sedimentary successions within these basins are nec- Nangqian Basin was infilled with non-marine sedimentary essary to provide clarification on local vs.