HOLOCENE ENVIRONMENTAL CHANGES IN THE SHIYANG RIVER BASIN, ARID CHINA BASED ON POLLEN ANALYSES

The Shiyang River Basin (at 37°£02°¯ -39°£17°¯ N, 100°£57°¯ -104°£57°¯ E, and elevations of 5000-1200m) is an internal river system located in the arid lands of NW China. The Shiyang River originates in the Qilian Mountains and disappears into the nowrthwestern Tengger Desert. During the late Pleistocene and Holocene there was a palaeo-lake at the northern end of the drainage supplied by the Shiyang River, but by the 1950°¯s the paleao-lake was dry because human use of river water increased. The results of pollen analyses of the SJC section, at the edge of the paleolake, indicate that the Holocene can be divided into three periods: a wetter early Holocene (11.6-7.1 Cal BP), a dry Middle Holocene (7.1-2.4 Cal BP), and a moderately dry Late Holocene influenced by human activities (2.4-0 Cal BP). In the wetter early Holocene, uplands were covered by dense forests and the Paleolake expanded. However, during several dry events (9.65-9.25 Cal BP, 8.75-8.2 Cal BP, 7.9-7.75 Cal BP, 7.25-7.1 Cal BP) the upland forest deteriorated and the palaeolake shrank. In the early Middle Holocene (7.1-3.8 Cal aBP), the paleolake became gradually shallower as river flow decreased. The driest climate occurred in the late Middle Holocene when the palaeolake was drastically reduced and the SJC section site became dry land, indicating that desiccation processes began prior to human use of inflowing water. Effective moisture in the late Holocene was higher than that in the middle Holocene and lower than the early Holocene. However, from 2100 Cal BP (Xihan dynasty) to the present, there were three periods of agriculture development in the Shiyang River drainages: 1) from Xihan dynasty to Donghan dynasty; 2) Tang dynasty; 3) Ming dynasty to present. Because most river waters were diverted to irrigated lands during these intervals, the paleolake became desiccated, ground water levels dropped, and vegetation deteriorated.