Abstract:

The records of Mid-Pleistocene lake environmental change have been obtained by comprehensively analyzing characteristics of environmental proxies of lacustrine section in Youyun in Yellow River source region, such as granularity, carbonate content (C), magnetic susceptibility, TOC, nitrogen content (N) and C/N ratio. The results show that the paleaolake might have experienced five progresses of lake evolution from 465 kaB.P. to 219 kaB.P, i.e. deep lake, shallow lake, deep lake, shallow lake and deep lake. During the period of 465 kaB.P. to 4352 kaB.P., the paleaolake was in a deep lake environment with more precipitation and more runoff into the lake, and the climate is cold and wet. During the period of 435.2 kaB.P. to 367.3 kaB.P., the paleaolake was in a shallow lake environment with the reduction of precipitation and runoff into the lake, and the hydrodynamics of the lake water was weakened; the surrounding temperature was low; the nutrition of the lake was poor and the organic matters of terrestrial origin were in majority. During the period of 367.3 kaB.P to 284.6 kaB.P., the paleaolake was in a deep lake environment with gradually increasing temperature of surrounding; the nutritional status was improved; the volume of the lake water increased. During the period of 284.6 kaB.P.to 228.2 kaB.P., the paleaolake was in a shallow lake environment with low temperature, and the nutrition of the lake was poor; the water volume gradually increased afterwards, and the lake changed to deep water. During the period  of 228.2 kaB.P. to 219.0 kaB.P., the paleaolake was in a deep lake environment with low temperature; the nutrition of the lake was poor; the volume of lake water was large. The environmental proxies had two sudden changes in 370 kaB.P. and 250 kaB.P., and the two sudden changes corresponded to the change of MIS11 to MIS10 and MIS8 to MIS7 in the deep-sea oxygen isotope curve, respectively. This result reveals that the environmental evolution in northeastern Qinghai-Tibet Plateau area was in accordance with the global environmental evolution.

Key words: Yellow River source, lacustrine deposit, environmental proxy, paleoenvironment change, Qinghai-Tibet Plateau