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

mean 2.4%), Polygonum (0%–1.4%, mean 0.2%), and Typha (0.5%–2.0%, mean 1.2%). Some other shrub and herb taxa were present but at low abundance, such as Chenopodiaceae (1.0%– 4.9%, mean 2.6%), Caprifoliaceae (0%–3.3%, mean 1.6%), Ranunculaceae (0%–6.2%, mean 1.9%), Thalictrum (0%–2.1%, mean 0.8%), Lamiaceae (0%–1.0%, mean 0.3%), Spiraeoideae (0%–1.6%, mean 1.2%), Caryophyllaceae (0%–3.6%, mean 1.3%), and Ephedra (0%–3.8%, mean 1.4%) [Fig 3]. Broad-leaved pollen (23.8%–40.9%, mean 29.3%) showed an increase relative to zone D and was dominated by Quercus (8.6%–16.2%, mean 11.0%), Castanea (8.0%–14.2%, mean 10.4%), and Betula (2.0%–7.1%, mean 3.6%). Ulmus (1.5%–3.5%, mean 2.3%) and Corylus (1.0%–4.0%, mean 2.2%) remained present but at low abundances. Liquidambar and Juglans occurred sporadically. Conifer pollen (6.6%–19.9%, mean 11.0%) was dominated by Pinus (5.5%–18.1%, mean 8.7%). Abies and Picea pollen were present but at highly variable abun- dances [Fig 3]. Fern spores and algae (6.5%–18.1%, mean 10.4%) showed minor variations and were domi- nated by Pediastrum (1.3%–8.6%, mean 5.2%) and monolete spores (1.4%–2.9%, mean 2.3%). Polypodium and trilete spores were sporadically present [Fig 3]. 5. Discussion The Xinmocun palynoflora was dominated by shrubs and herbs with lesser arboreal pollen and fern spores, and algae. Among the shrub and herb pollen, the hydrophilous herbs, domi- nated by Cyperaceae, Poaceae, Myriophyllum, Polygonum, and Typha, showed clear changes in abundance during the last deglacial period, consistent with previously published records and

factors that controlled the environmental conditions, as discussed below. 18.7–16.8 ka This period was characterized by a gradual increase in the abundance of hydrophilous herbs [Fig 4A], suggesting that the climate became more humid. In contrast, despite the slow strengthening of insolation during this period [Fig 4E] [32], climate cooling was indicated by a negative δ18O shift in the Greenland ice core [Fig 4B] [33] and increased precipitation was implied in stalagmite records from Hulu [Fig 4C] [11] and Haozhu [Fig 4D] [10] caves in China. This cooler climate and increased precipitation was inconsistent with the idea that increased ISM activity was linked to a warmer climate. Instead, it was more consistent with the cold humid climate of the middle Yangtze region inferred from low Sr/Ca ratios in stalagmites from Haozhu cave [Fig 4F] [10]. Accordingly, we consider that global cooling during this period was responsible for increased humidity at the eastern margin of the TP. 16.8–14.6 ka Hydrophilous herbs continued to increase in abundance during this period [Fig 4A]. A posi- tive shift in the δ18O record of the Greenland ice core [Fig 4B] [33] and of stalagmites in Hulu [Fig 4C] [11] and Haozhu [Fig 4D] [10] caves indicated a general warming and wetting trend during this period globally and in China, consistent with the continuous increase in insolation [Fig 4E] [32]. This was consistent with the traditional understanding that ISM activity increases with warming and wetting. A reduction in the Sr/Ca ratio of stalagmites from

Haozhu cave indicated that climate in the middle Yangtze region became more humid at this time [Fig 4F] [10]. At 16.8 ka, abrupt increases were recorded in the magnetic susceptibility (SUS) of loess at Yuanbo [12], the total organic carbon (TOC) content of a peat core from Hongyuan [14], and the redness of sediments at Lake Qinghai [34]. These changes suggest that 16.8 ka was an PLOS ONE Last deglacial pollen reocord from the eastern TP PLOS ONE | https://doi.org/10.1371/journal.pone.0232803 May 6, 2020 8 / 12