Environmental Changes During The Last Millennium Recorded By Lake Sediments From Gaxun Nur In Ejina Basin

The ecological environment deterioration of Ejin oasis at the lower reaches of Heihe River has drawn abroad attentions. The Gaxun Nur lake is situated the inland arid area in northwestern China,being sensitivity to global climate changes in this region. A2m-long section,GXN was obtained from Gaxun Nur for multiproxy analyses. Based on the lithology, chronology, and the variations of the proxies, three stages of environmental changes were recorded by lake sediments during the past millennium.The lower part of the section (200～110cm, corresponding to874～1364AD), has obvious river-binary structure, caused by river diversion of the lower reaches of Heihe River, resulting in fine-grained components deposited on the flood plain. The characteristics of the grain-size frequency curves of the lake sediments indicate that there are mixed depositions of fluvial and wind. Shallow lake deposit mingled with the aeolian sands. The climate was drier, corresponding to the Medieval Warm Period (MWP); The middle part of the section (110～20cm:1364～1853AD) indicated the environment changed from off shore shallow water to stable deep water. The lake area increased, and the climate became wetter, corresponding t the Little Ice Age(LIA); The upper section (20～0cm:1853～1962AD) indicated that the runoff reduced and the lake shrunk, finally dry up in1962.Regional comparison shows that climate events(MWP、LIA and the20century warming period) of the past millennium were well documented by the lake sediments of GXN section. The humid LIA in this region corresponds well with the characteristic of the LIA reported in northwestern China. The lake sediments of the past millennium reveals both natural environment change and human activities, since entering20centuries.