Sensitivity of altitudinal vegetation in southwest China to changes in the Indian summer monsoon during the past 68000 years

摘要

Reconstructing vegetation changes during a glacial-interglacial cycles in southwest (SW) China is crucial for understanding the amplitude of palaeoclimatic dynamics associated with orbital forcing and evolution of the Indian summer monsoon (ISM). However, few studies on the relationship between paleoclimate and ecosystem changes have covered all of the last glacial-interglacial cycle (or any other), which limits understanding of the ISM's variability and its impact on mountain flora. In particular, knowledge of responses of vertical vegetation belts in margins of the high Tibetan plateau to glacial and subglacial (MIS3) climate changes is poor. To improve understanding, we have studied a continuous core extending back to MIS4 from Tengchong Qinghai volcanic lake in Yunnan Province. Identification of Quercus pollen by scanning electron microscopy showed that most of the abundant oak pollen in the core can be assigned to mountainous taxa of sclerophyllous Quercus living today on the high Tibetan plateau. Their current distribution indicates that the maximum downward shift of vertical vegetation belts during the last glacial period corresponds to a similar to 5.5 degrees C drop in temperature, based on altitudinal lapse rates, in accordance with mean annual temperature (MAT) reconstruction from biomarkers in the same core. Our data clearly suggest that conditions were wet during MIS3, and humidity highest at 38-56 kyr. The wetting appears to have started after the Last Glacial Termination, indicating that the initial deglacial ISM intensification lagged behind rises in temperature and summer insolation. The highest charcoal concentrations and evidence of expansion of open vegetation coincide with the apparent onset of deglaciation (18-16 kyr). These observations indicate that this was the driest period, likely linked with reductions in the polar ice volume and delayed Tibetan glacier melting, which absorbed massive amounts of atmospheric heat and caused regional aridity. Our pollen data clearly correlate with rapid changes such as Younger Dryas and Heinrich Events, and marine records from the Indian Ocean, confirming climate teleconnection through ice volumes and global ocean circulation, and that the ISM is a key factor driving south-north transportation of energy and moisture. The multi-proxy data reveal with high confidence that the Holocene optimum in the region was between 9 and 4 kyr BP, and followed by weakening of the ISM accompanied by increasing anthropogenic disturbance of the vegetation. (C) 2020 Elsevier Ltd. All rights reserved.