Holocene lake deposits of Bosten Lake,southern Xinjiang,China

A 9.25-m-long sediment core from Bosten Lake, Xinjiang, provides detailed information about changes in the water budget and biological acticity over the last 8400 calendar years. The chronology is constructed from six AMS radiocarbon dates on the terrestrial plant remains. Based on analyses of TOC, CO3, detrital compounds and biogenic SiO2, lake level fluctuations and periods of remarkably-negative water budget appeared at 8.4—8.2 cal ka, 7.38—7.25 cal ka, 5.7—5.5 cal ka, 3.7—3.4 cal ka and 3.3—2.9 cal ka, respectively. As they are in-phase with low lake levels at Sumxi Co and Bangong Co in western Tibet Plateau and with paleolakes in Inner Mongolia, a climate-induced change to somewhat drier and warmer conditions is inferred. A further drop in lake level after 1320 AD of about 200 yr duration may be attributed to a negative water balance prior to the main phase of the Little Ice Age. Deep and stable lake phases of 1500 yr and 1800 yr duration at 7.2—5.7 cal ka and 5.5—3.7 cal ka coincide with maximum moisture during the Holocene Megathermal in China. The long term trend towards aridity since about 4.3 cal ka can clearly be recognised. The reduced water budget of Bosten Lake from 640—1200 AD may be attributed to local effects.

High-resolution precipitation variations in the Northeast Tibetan Plateau over the last 800 years documented by sediment cores of Qinghai Lake

Short cores of about 80-cm retrieved from three main basins of the deepwater areas in Qinghai Lake, the largest inland enclosed lake in China, were studied. Stable isotopes of authigenic carbonates, grain-size, carbonate and organic matter content at 5-year resolution are used to reconstruct the climatic history over the last 800 years in the Northeastern Tibetan Plateau. Chronology was established according to 210Pb dating and 137Cs methods and the core correlation. It is found that cores from different deep basins of the lake can be well correlated. The sedimentary rate is highest in the western basin of the lake and lowest in the east. In the southern basin of the lake where the short core Qing-6 is located, the recent average sedimentation rate is 0.1004 cm/yr. Variations in effective precipitation recorded by the oxygen isotopes and grain size data during the last 800 years are consistent with the glacial accumulation record form the Dunde and Guliya ice cores. A dry climate lasted for 300 years from 1200 AD to 1500 AD, followed by a wet period from 1500 to 1560 AD. The two dry periods, 1560 to 1650 AD and 1780 to 1850 AD, were the results of southwest monsoon weakening. The effective precipitation generally increased since 1650 AD due to the strengthening of the Asian Southwest Monsoon, resulting in a wet period until the 1950s. Except the early stage, the Little Ice Age on the Plateau is characterized by increased effective moisture. Organic mat- ter content, with nearly 200-year cycles, shows similar trend with the atmospheric delta carbon-14 before the 1850s, indicating that the bioproductivity responds to solar activity.