Pollen record of climate change during the last deglaciation from the eastern Tibetan Plateau

characterized by frequent tectonic activity [1,2] and was settled by humans for agriculture rela- tively early [3,4], making it an interesting region for paleoclimatic and paleoenvironmental studies. Recent observations of spatiotemporal patterns in precipitation have indicated that the cli- mate of the eastern TP is dominated by the Indian summer monsoon (ISM) [Fig 1] [5–8]. Decadal/multi-decadal temperature discrepancy analysis along the eastern TP has suggested that long-term temperature and precipitation trends are coupled over the northern part of the eastern TP, but are decoupled over the southern part, and that the Zoige Plateau and adjacent regions represent a zone of dynamic climate that separates the northern and southern parts of the eastern TP [9]. Intriguingly, a speleothem record from the middle Yangtze region covering the last deglacial period recently indicates a wetter central eastern China during North Atlantic cooling episodes [10]. However, Wang et al. [11] argue that the East Asian Summer Monsoon activity increases with warming-wetting and decreases with cooling-drying. In addition, previ- ous studies concentrated mainly on climate change during the Holocene period and have reported only low-resolution or obscure trends of climate change during the last deglaciation [6, 12–15]. Therefore, the main factors that affected the climate of the eastern TP during the last deglaciation remain unclear. Pollen sequences are regarded as one of the most reliable continental archives of paleocli-

mate and paleoenvironment. Deposition occurs as pollen rain that can be characterized by the percentage of pollen components; i.e., the pollen spectrum. This spectrum provides a reliable record of regional vegetation and therefore regional climate [16,17]. The last deglaciation was a critical transitional period from the Last Glacial Maximum to the beginning of the Holocene. In this study, we investigate the well-dated Xinmocun lacus- trine section to reveal the climatic conditions of the last deglaciation by analyzing variations in the pollen record. This record is correlated with other contemporary records from the eastern Asia, and the controlling mechanism of climate change during this period is addressed. 2. Geologic and geographic settings The Xinmocun lacustrine section (32˚2.70N, 103˚40.10E; 2188 m above sea level) is ~11 m thick and is located in an intermountain basin at Diexi on the eastern TP [Fig 2A] [18]. The Diexi lake remains a lake today and our samples were collected from an outcrop well exposed in the field. No specific permissions were required for these locations and field studies did not involve endangered or protected species. The landscape of the basin is characterized by alpine valleys dominated by two major mountain ranges: the Min Shan and Longmen Shan. The study area varies in elevation from 870 m to 6253 m. The valleys along the Min River are usu- ally steep and narrow, with an incision depth of 800–3000 m [Fig 2A]. The climate in the study area varies from subtropical to cold-temperate with increasing lati-

tude, and from arid in the Diexi intermountain basin to humid in the high mountains. The Diexi intermountain basin is a sub-frigid, semi-humid highland with an overall dry and windy climate, cool summers, and cold winters. The mean annual temperature at the Maoxian mete- orological station (31.41˚N, 103.51˚E, 1590.1 m above sea level) is 11.2˚C and the mean annual precipitation is 490.7 mm, with 70%–90% falling between June and September. The mean annual latent evaporation is 1375.7 mm, which is 2.8 times the annual precipitation [18–20]. The vegetation cover in the upper reaches of the Minjing River includes Abies faxoniana, Picea asperata, Pinus densata, and Tsuga chinensis according to the Vegetation Map of China [21]. The vertical zonation of vegetation changes distinctly with increasing elevation: 1) arid valley scrubs (1300–2200 m a.s.l.), dominated by Caryopteris mongholica, Ajania potaninii, Leptodermis, and Caryopteris incana, etc. 2) Mixed evergreen and deciduous broad-leaved PLOS ONE Last deglacial pollen reocord from the eastern TP PLOS ONE | https://doi.org/10.1371/journal.pone.0232803 May 6, 2020 2 / 12 Competing interests: The authors have declared that no competing interests exist.