Feng Zhaodong

Holocene environmental variations recorded by organic-related and carbonate-related proxies of the lacustrine sediments from Bosten Lake, northwestern China

We reconstructed a multiproxy record of the Holocene climate changes from a 9.53 m core from Bosten Lake in the southern Xinjiang of northwestern China based on a chronology supported by nine AMS 14C dates in the hope that the previous reconstructions can be improved. Our data exhibited three timescales of variation in the water salinity of the lake. First, the upward-increasing parallel trends of major salinity proxies suggest that the salinity has been gradually increasing in the past ~8000 years. This constant increase in the salinity is most likely to have resulted from the shrinking of glacial cover in the central Tianshan Mountains where the inflowing river (i.e. Kaidu River) originated. Second, the C/S ratio and carbonate-based salinity proxies suggest that the lake was formed around ~8060 cal. yr BP and reached its maximum depth around ~7250 cal yr BP. The lake level then declined to the mid- and late-Holocene average level around ~6370 cal. yr BP and has varied drastically around a relatively constant mean during the past ~6370 years. Third, the past ~8060 yr history of the Bosten Lake can be divided into five salinity stages. That is, three high salinity intervals are separated by two low salinity intervals. The two low salinity stages occurred between ~6370 and ~5170 cal. yr BP and between ~3000 and ~2170 cal. yr BP. Each one of the three high salinity stages can be further divided into less saline and more saline substages. Specifically, less saline time intervals are: substage 2-1 (~8060—~7250 cal. yr BP), substage 4-2 (~4370—~3830 cal. yr BP) and substage 6-2 (~1250—~0 cal. yr BP). Also, saltier time intervals are: substage 2-2 (~7250—~6370 cal. yr BP), substage 4-1 (~5170—~4370 cal. yr BP), substage 4-3 (~3830—~3000 cal. yr BP) and substage 6-1 (~2170—~1250 cal. yr BP). It should be stressed that the phase relationships between the carbonate-based salinity proxy and diatom-based temperature proxy suggest that the salinity was mainly controlled by temperature. Finally, the comparison between the Bosten Lake salinity record and the high northern latitudinal climate pattern suggests that the lower salinity and cooler periods in Bosten Lake area correspond with abrupt climatic events (i.e. cooling events) in the high northern latitudes.We reconstructed a multiproxy record of the Holocene climate changes from a 9.53 m core from Bosten Lake in the southern Xinjiang of northwestern China based on a chronology supported by nine AMS 1...

Land Cover Change and its Environmental Impact in the Upper Reaches of the Yellow River, Northeast Qinghai-Tibetan Plateau

Abstract Many studies have focused on desertification processes in northern China in an attempt to understand desertification-related ecological problems. Desertification on the Qinghai-Tibetan Plateau has received little attention, even though desertification resulting from overgrazing is a widespread phenomenon there. This study focuses on the Gonghe Basin in the upper reaches of the Yellow River, located on the northeast Qinghai-Tibetan Plateau, to shed light on desertification processes and associated environmental impacts during a 10-year period from 1987 to 1996. We first used 2 thematic mapper images to classify land cover for 2 summers (1987 and 1996) into 9 land cover types related to desertification. To assess the severity of desertification and to calculate the total soil carbon emission caused by desertification, we first defined severity by comparing land cover differences between 1987 and 1996 and then determined the differences in organic carbon content of the soil in grasslands during different stages of desertification between 1987 and 1996. The results showed that about 240 km2 of grasslands was lost to agricultural encroachment and about 190 km2 became sand-covered area between 1987 and 1996. During the same period, desertification affected 569.6 km2 of grasslands, with desertification assessed as very severe for 41.8%, severe for 15.1%, and moderate for 43.1% of the area. The results also show that carbon emissions from grassland deterioration caused by desertification amounted to 2.06 × 106 tC during the 10-year period, the average annual emission rate being 0.206 × 106 tC.

GIS-assisted FEFLOW modeling of groundwater moving processes within the Minqin oasis in the lower reach of the Shiyang River, Northwest China

Oases are specific landscapes and play very important economic roles in arid Northwestern China. Yet, the peripheral ecological systems around the oases are equally important to the oasis economic systems in terms of protecting the oasis from desertification and ensuring sustainable development of oases. However, the rapid increases in the population and steady progress in the society and technology since 1950s have greatly expanded irrigated agricultural production in the oases, resulting in a dramatic decrease in stream flow and associated increase in the groundwater withdrawal in arid Northwestern China. The overexploitation of groundwater formed groundwater table depletion cones and eventually lowered the groundwater table not only in the oases but also in the periphery of the oasis, resulting in an overall decrease in soil water content. In response to the soil moisture decrease in the periphery, some plant species relying on groundwater resource have been dying out and other more mesic species have been replaced with more xerosere species. In this paper, the groundwater flowing processes and the water-table fluctuating processes during the past 40 years were simulated to obtain the spatial and temporal distribution of groundwater table by means of the GIS-assisted FEFLOW modeling based on the hydrogeologic data obtained in the Minqin Basin. Several conclusions can be drawn from the simulated results. Firstly, the discharge and recharge of groundwater retained balanced and the groundwater table depth in the periphery area of the oasis maintained 2-3 m deep during 1960-1965. Secondly, the utilization of the groundwater dropped the groundwater table within the oasis to 5-10 m, leading to the formation of big groundwater depletion cones that started to influence the groundwater table of the periphery area in the 1970s. Thirdly, the further overexploitation of the groundwater within the oasis has dropped the groundwater table to 10-20 m in the oasis and 7-10 m in the periphery since the middle 1980s

Grassland desertification and its impacts on carbon cycle in the source region of the Yellow River, northeastern Qinghai-Tibetan Plateau by remote sensing

This study deals with desertification and its impacts on environment in the source region of the Yellow River. Our objectives were to reveal and quantify desertification changes from 1990-2000 by remote sensing and GIS, and discuss its impacts on carbon cycle. The results indicate that the study area is one of the most desertified regions in northeast Qinghai-Tibetan Plateau. The study area is now a carbon source. The desertification would have contributed to CO/sub 2/ emission from grasslands of Qinghai-Tibetan Plateau. Otherwise, there are great potential to resequestration carbon through controlling grazing, restoring ecosystem, and reducing desertification.