Late Holocene Climatic Changes Recorded At Lake Lugu, Northwestern Yunnan Province And Their Links To Global Climate

Knowledge of the climatic change during the late Holocene, especially for the last 2,000 years, is important for understanding the global climatic dynamics. Many proxy-derived temperature records reveal that in the Northern Hemisphere, temperature variations generally exhibit similar trends on centennial timescales during the last 2,000 years, including three important climatic events: Medieval Warm Period(MWP), Little Ice Age(LIA), and Current Warm Period(CWP). In contrast, hydroclimatic records show significant regional contrast over the last 2,000 years, particularly during the MWP and LIA. To understand this regional hydroclimatic variability and associated underlying mechanisms it is necessary to reconstruct more well-dated and high-resolution hydroclimatic records during the last 2,000 years from different regions. The Asian summer monsoon(ASM) precipitation variations affect more than 60% of the world’s population over much of South and East Asia. Numerous studies have focused on the ASM variations during the past 2,000 years using various archives. However, knowledge of the spatial patterns in the ASM precipitation variations on centennial timescales during the last 2,000 years, especially for some typical periods like MWP and LIA, is currently imcomplete.The northwestern Yunnan Province lies in the southeastern margin of Tibetan Plateau. It is dominated by the Indian summer monsoon(ISM), which is responsible for up to 80% of the annual precipitation. Many studies have attempted to reconstruct the climatic changes in this region mostly using lake sediment records. However, continuous and well-dated records with high resolution and climatic significance during the late Holocene are still lacking. Because of the deep water depth, large lake surface area and relatively weak human impacts Lake Lugu is an ideal site for paleoclimatic and paleoenvironmental studies.In this study, firstly, we establish our age model based on the 210 Pb, 137 Cs, 14 C dating methods; Secondly, we present ISM precipitation variations at Lake Lugu covering the last 2,900 years using multiple proxy records(including grain size, C/N ratio, TOC, carbonate content, magnetic susceptibility); Thirdly, through the comparison with the proxy records from other lakes in the northwestern Yunnan province, we understand the climatic history in the northwestern Yunnan Province during the late Holocene; Lastly, through large-scale climatic comparisons, we understand the spatial patterns of the Asian summer monsoon precipitation during the last 2,000 years and associated mechanisms. In this study, we have achived some conclusions as follows:(1) The age model of the Lugu12-1-3 core was generated from a combination of the 137 Cs time-marker and the corrected AMS 14 C ages, covering the last ~2,900 years with an average time resolution of ~23 years.(2) The proxy records show significant centennial variations in the ISM precipitation, with two remarkably dry intervals from 750 to 1167 AD and from 1733 AD to the present and two relatively wet intervals from 898 BC to 750 AD and from 1167 to 1733 AD.(3) The multi-proxy records indicate consistent trends of increased soil erosion associated with enhanced human activity during the last ~100 years.(4) The comparisons among the precipitation record from Lake Lugu, the proxy records(e.g., the pollen, grain size, TOC, and Rb/Sr ratio) from Lake Erhai, and the historical documentary record and lake terrace research from Lake Chenghai suggest that dry climate conditions prevailed during the MWP and over the last 100–200 years, whereas relatively humid contions prevailed during the LIA in the northwestern Yunnan Province.(5) These climatic characteristics are generally similar to those of the climate patterns in extensive areas dominated by the ISM(e.g., southern China, southwestern China, southeastern and southern Tibetan Plateau, northeastern India-Himalayas, southern Oman, and equatorial eastern Africa), but anti-phased with the climate patterns in the wider areas dominated by the East Asian summer monsoon(EASM)(e.g., north-central China, northeastern China and north-central Japan).(6) We speculate that both the variations in the sea surface temperature(SST) over the tropical Indo-Pacific Ocean, the ocean-atmosphere coupling processes and the southward-northward migration of the mean position of the Intertropical Convergence Zone(ITCZ) should be responsible for the hydroclimatic contrasts over the ASM region on centennial timescales during the last 2,000 years.