Senckenberg Forschung

QUASI

REKONSTRUKTION DES ÜBERREGIONALEN SIGNALTRANSFERS AUS SEDIMENTOLOGISCHEN LANGZEITARCHIVEN

In many respects, the northern foreland of the Tibetan Plateau, with the intermontane basins of Badain Jaran and Tengger Shamo, plays a key role in geological and geoecological studies of Central Asia:
1. The endorheic basins of Badain Jaran and Tengger Shamo are long-term terrestrial archives, recording more than 250 000 years of climate and environmental change.
2. Their sediments are major sources of dust transport over Central Asia and northern China. Hence, the basins are directly connected with the Loess Plateau, Asia's oldest terrestrial climate archive.
3. Both the formation and the recent evolution of the basins are due to the prograding indentation of the Indian subcontinent and thus record both the uplift of the Tibetan Plateau and the recent tectonic activity directed northward towards the Baikal rift system.
4. Three global wind systems interact at Badain Jaran and Tengger Shamo: the southeast monsoon, the northwest monsoon, and the westerlies. In particular, the boundary situation related to the waning of the southeast monsoon and the resulting rainfall variability make these basins uniquely sensitive archives of the late Quaternary monsoonal dynamics of Central and High Asia.
5. Because of its highly variable precipitation, this semiarid to hyperarid region is especially susceptible to the risk of desertification.
6. As well as these natural risks relating to water supplies, considerable water distribution problems are caused by population growth and a rapid rise in new industrial locations due to the increasing exploitation of available resources.
7. For more than 7000 years the river valleys of these basins have served as important trade routes between Central and East Asia. So this region is a key link in the cultural development of West, Central and East Asia. Especially the peoples of High Asia and the steppes of North Asia and central China have long inhabited this region.

All the above factors cumulate in the Gaxun Nur Basin (Ejina Basin, Hei River Basin). This is the endorheic base level of erosion of the Hei He and the Shule He, which rise in the Qilian Shan at more than 5000 m asl. The closed lakes of Gaxun Nur, Sogo Nur and Juyanze form a crescent at the northern margin of the basin and are the deepest points of the approx. 130000 km² catchment. The basin is bounded in the south by the foreland mountains of Heli Shan and Longshu Shan. A few kilometres to the southwest, the Altyn Tagh fault ends near the Shule He alluvial fan. The Beishan massif forms a sharp boundary in the west. In the north, the Gaxun Nur Basin is bounded by the pediments of the Gobi-Tienshan with the left-lateral Gobi-Tienshan fault. In the east is the Badain Jaran sand sea (shamo) with its more than 400 m high, barchanoid dunes. This sand sea is unusual, not only because of its large size: its southern part features frequent and closely spaced lakes that have developed between the huge dunes, along a big sub-dune valley coming from the Yabraishan. This valley system is recognisable on the SRTM elevation model and evidently constitutes the palaeorelief, which is oriented towards the southern Gaxun Nur Basin and may be the reason for the existence of the intradune lakes.
The basin itself contains up-to-300-m-thick Quaternary sediments overlying pre-Quaternary conglomerates. In the 1990s a drilling programme reached 230 m depth in the northern part of the basin, showing that fluvial and lacustrine sequences had been continuously deposited over the past 250 000 years (Wünnemann et al. 2007a, among others). Together with neotectonic finds (Hartmann 2003, Becken et al. 2007, Hölz et al. 2007, among others), the directly underlying conglomerates show a very late subsidence of the basin during the Pleistocene. The thickest sediment fills are located in the southeastern part of the basin in direct continuation of the valley crossing the Badain Jaran Shamo.
A key element of the proposed project is a deep borehole (approx. 300 m) to be drilled in the southeastern part of Gaxun Nur Basin to investigate climate and environmental evolution. This site was chosen because of its thick Pleistocene sediments and its location in a possible outflow area of the subdune valley. Sediment record descriptions are available from numerous groundwater prospections in the entire Gaxun Nur Basin, so the expected sediment sequences are already largely known, and therefore it is extremely likely that we will obtain appropriate sediments for the planned analyses. High-resolution ("centennial") sediment analysis will be employed to address the following questions:

1. When was the onset of regional basin evolution and hence of dust dynamics and mid-Pleistocene loess deposition?
2. What role did the Gaxun Nur Basin play as part of a supraregional sediment cascade from the glaciers of the Qilian Shan to the long-term archives of the Loess Plateau? What conclusions may be drawn from the sediments of Gaxun Nur Basin about monsoonal dynamics during the past 250 000 years?
3. Have non-climatic processes of basin evolution impacted dust flows and loess dynamics and so modified the external climate signal?
4. How did the water balance in the Gaxun Nur Basin function in extremely dry periods in the past? What are the implications for present-day desertification processes, what awaits the local population?
5. When did the Badain Jaran sand sea begin to form? What role did the subsidence of the Gaxun Nur Basin play? What consequences did the subdune valley have for the formation of the intradune lakes and their hydrology?

 

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