Holocene climate changes over the desert/loess transition of north-central China

A 2-m trench section at the desert/loess transition of north-central China was analyzed at 2-cm intervals for grain-size distribution (M-d, mum) and total organic carbon (TOC, %). AMS C-14 time series of the M-d and TOC records provides direct information on variations of the East Asian monsoon climate during the last 10000 yr. Md is considered to represent the wind strength and high Md reflects strong winter monsoon winds. TOC denotes the degree of vegetation cover, a function of the intensity of summer monsoon circulations. From ca. 10000 to 7000 cal yr BP, the climate was dominated by a strong winter monsoon and a very weak summer monsoon as indicated by high Md and low TOC values. Subsequently climatic conditions became warmer gradually from ca. 7000 to 5500 cal yr BP. The period between ca. 5500 and 2700 cal yr BP displayed the highest TOC and lowest M-d values over the last 10000 yr, marking a stable state of a warm, humid climate, i.e. the Holocene climatic optimum of China. During the interval of ca. 2700-1500 cal yr BP, the expansion and intensification of agrarian activities could have led to desertification and acceleration of deposition of coarse-grained particles, resulting in a decreased organic carbon accumulation rate. However, the TOC during this interval still has values as high as during the episode of ca. 7000-5500 cal yr BP, reflecting a relatively stronger summer monsoon circulation. Since ca. 1500 cal yr ago, high-amplitude fluctuations of TOC that fluctuate in phase with the clay-size fraction imply that the climate experienced three major oscillations with a prominent peak of enhanced summer monsoon circulation at ca. 1300 cal yr ago and with two intervals of increased winter monsoon strength occurring around ca. 1000 and 300 cal yr ago. The M-d and TOC records over the past 5000 yr demonstrate a good correlation with the archaeological, phenological and historical records of China. Climate changes in the East Asian monsoon region were not only related to the changing seasonality of solar insolation resulting from progressive changes in the Earth's orbital parameters but were also controlled by the pattern and intensity of the atmospheric circulation. (C) 2002 Elsevier Science B.V. All rights reserved.