Palaeo-climatic Changes Since Late Glacial Inferred From Sediment Records Of Lake Lugu, SW China

Lake Lugu is a bordering lake of Yunnan and Sichuan Provinces, situated in southwesten China and to the north-eastern margin of the Tibetan Plateau. The climatic regime nowadays is predominantly controlled by the Indian summer monsoon (ISM) or the so-called the south-western monsoon. In this article, a sedimentary core (date back to late glacial) was geochronologically studied before it was applied for multiple variable analysis, of which the aim is to deduce ISM variations since late glacial on both orbital and sub-orbital time scales. We also compared our records with those from the south-eastern monsoon regimes as well as from the north Atlantic regions, in order to better understand the driving force of ISM and the relationships with global changing mechanisms.We present a record of ISM variations since the late glacial, which is inferred from the geochemical profiles of a sediment core (LG08-600,6meters in length) from Lake Lugu, SW China. The age model is derived from11radiocarbon ages of bulk organic sediment and terrestrial macrofossils. Accelerator mass spectrometry (AMS) radiocarbon dating was conducted on bulk sediment (after removal of inorganic carbon) from8horizons and land-derived macroscopic plant remains from3horizons. The age-depth relationship based on the bulk sediment dates was not consistent with that on the plant dates. The apparent older bulk sediment dates indicate the presence and variation of the so-called "lake reservoir effect"(1467～2962years). The dates on plant remains, however, provide absolute age controls and allow for adjustment of the older dates for correction of the "lake reservoir effect". Parallel to bulk sediment dates, the absolute dates were calculated by linear interpolation of the plant dates, which were further used for correction of the temporally variable "lake reservoir effect". The corrected bulk sediment dates plus the plant dates were calibrated into calendar ages to construct the age-depth model for LG08-600.We made down-core geochemical measurements of grain size, susceptibility, total organic carbon (TOC), total nitrogen (TN) and loss on ignition at950℃(LOI950). These data have been showed to be proxies for lake hydrological conditions and the strength of the ISM. We found that the ISM began to significantly intensify at the Bolling/Allerod (B/A) period, indicated by the sharp decrease of carbonate precipitation from the lake water (decrease in LOI950content) from high values (>10%) to as low as ca.2.5%and a significant enhancement in the lake productivity (denoted by increase in TOC content). The ISM continued to intensify after the B/A period until3.6cal ka BP, as reviewed by the increasing trend in lake productivity and the stable low values in carbonate contents. Thereafter, the ISM generally weakened since3.6cal ka BP, as proved by the elevated and variable carbonate contents. The overall ISM variations observed in our records exhibit responses to August insolation changes at boreal low latitudes. Superimposed on these general variations are a series of abrupt oscillations on a centennial/millennial time scales that is in good agreement with the Ice-rafting events6,5,4,3,2,1and0of the North Atlantic. Our late glacial to the Holocene record is in coherency with the archives from the Indo-Asian monsoonal regions and has teleconnection with records from the North Atlantic on both orbital and centennial/millennial scales, suggesting that the North Atlantic was an important driving source for these climatic changes.