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

12.9–11.6 ka This period corresponds with the Younger Dryas (YD) identified in other paleoclimate rec- ords. During this period, hydrophilous herbs decreased to their lowest mean abundance (9.3%) in the entire section [Fig 4A], coeval with a negative shift in the δ18O record of the Greenland ice core [Fig 4B] [33] and cave stalagmites [Fig 4C and 4D] [10, 11]. This suggests that the driest climate in the study area was coincident with a cold period in the Northern Hemisphere, in spite of the high and slowly increasing insolation during this time [Fig 4E]. In contrast, the low Sr/Ca ratios of the Haozhu cave stalagmites indicate a humid climate in the middle Yangtze region during this period [Fig 4F] [10] in spite of global cooling during the YD. The high humidity was possibly due to a lengthening of the Meiyu rainy period and short- ening of the post-Meiyu stage [10]. 11.6–10.6 ka This period is characterized by an abrupt recovery in hydrophilous herbs to typical values of the sequence [Fig 4A], implying a humid climate in the study area. There is also a positive shift in the δ18O records of the Greenland ice core [Fig 4B] and cave stalagmites [Fig 4C and 4D], suggesting a general warming and more humid climate [10, 11, 33], which is consistent with the high insolation at this time [32] [Fig 4E]. Such a humid and warm climate is consistent with increased ISM activity. However, the high Sr/Ca ratio of Haozhu cave stalagmites suggests that the climate of the middle Yangtze region was comparatively dry [Fig 4F] [10], possibly related to the high insolation [Fig 4E] [32].

6. Conclusion Pollen analysis of the Xinmocun lacustrine sediments at Maoxian revealed the climate change during the last deglaciation. The results indicate that shrubs and herbs were the dominant veg- etation types. Among these, hydrophilous herbs showed a clear change which can be correlated with existing records. The period from 18.7 to 16.8 ka was marked by a cold and humid cli- mate in the study area. During 16.8–14.6 ka, the abundance of hydrophilous herbs increased and δ18O records show an increase, indicating a warming climate and increased precipitation in the eastern TP. Between 14.6 and 14.0 ka, the highest abundances of hydrophilous herbs in the entire section correspond to relatively high δ18O values. From ~14.0 to 12.9 ka, hydrophi- lous herbs show a decreasing trend. Intriguingly, longitudinal temperature gradients in the Northern Hemisphere increased during this period and consequently drove the southward retreat of the ISM circulation. This was probably responsible for the decline of hydrophilous herbs in the study area and increased drought in the middle Yangtze region from 14.0 to 12.9 ka. During 12.9–11.6 ka (the YD period), the study area was characterized by a cold and dry climate. Subsequently, hydrophilous herbs show a significant recovery between 11.6 and 10.6 ka, implying a warm and humid climate in the study area. Thus, the ISM began to affect cli- mate in the eastern TP from the onset of the Northern Hemisphere deglaciation at ~16.8 ka. Supporting information S1 Table. Two AMS 14C ages of the Xinmocun section at Diexi, Sichuan, East Tibet. (PDF)

S2 Table. OSL ages of the Xinmocun section at Diexi, Sichuan, East Tibet. (PDF) PLOS ONE Last deglacial pollen reocord from the eastern TP PLOS ONE | https://doi.org/10.1371/journal.pone.0232803 May 6, 2020 10 / 12