QUASI - Supra-regional signal pathways and long-time archives: Quaternary monsoon dynamics at the northern margin of the Tibetan Plateau

 

The joint project QUASI focusses on the terrestrial sediment-archives of the northern foreland of the Tibetan Plateau, namely the intermontane basins of the Badain Jaran and Tengger Shamo, recording more than 250 000 years of climate and environmental change. Three global wind systems interact at Badain Jaran and Tengger Shamo: the southeast monsoon, the northwest monsoon, and the westerlies. 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. 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.

The Gaxun Nur basin is located in the lowlands of the Qilian Shan (Shan = mountain), the eastern part of the Nan Shan highlands. The basin is bounded to the south by the foreland mountains of Heli Shan and Longshu Shan. It is the endorheic base level of erosion of the Hei He (He= river) and the Shiule He, which both rise in the Qilian Shan at more than 5000 m asl. The Gaxun Nur basin contains up-to-300-m-thick Quaternary sediments overlying pre-Quaternary conglomerates. The thickest sediment fills are located in the southeastern part of the basin in direct continuation of the valley crossing the Badain Jaran Shamo. Together with neotectonic finds, the directly underlying conglomerates show a very late subsidence of the basin during the Pleistocene. In the 1990s one of the applicants conducted a drilling programme to 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.

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 which formulate the central issues of the proposed project for a large-scale evaluation of a long-term terrestrial archive of climate change in a highly sensitive and highly complex key location in Central Asia.

When was the onset of regional basin evolution and hence of dust dynamics and mid-Pleistocene loess deposition? What role did the Gaxun Nur Basin play as part of a supra-regional 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? Have non-climatic processes of basin evolution impacted dust flows and loess dynamics and so modified the external climate signal? 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?

 

Involved institutes and working groups

RWTH Aachen, Geographisches Institut

Alfred Wegener Institut für Polar und Meeresforschung

FU Berlin, Institut für Geographische Wissenschaften

IFM Geomar

Nanjing University