Friday, June 24, 2011

Oasis



Lop Nur, Xinjiang, China:

NASA image acquired May 17, 2011

Located in China’s resource-rich but moisture-poor Xinjiang autonomous region, Lop Nur is an uninviting location for any kind of agriculture. It sits at the eastern end of the Taklimakan Desert, where marching sand dunes can reach heights of 200 meters (650 feet), and dust storms rage across the landscape.

Yet for all it lacks in agricultural appeal, Lop Nur offers something valuable to farmers the world over: potash. This potassium salt provides a major nutrient required for plant growth, making it a key ingredient in fertilizer.

The discovery of potash at Lop Nur in the mid-1990s turned the area into a large-scale mining operation. The Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite captured this natural-color image of Lop Nur on May 17, 2011. The rectangular shapes in this image show the bright colors characteristic of solar evaporation ponds. Around the evaporation ponds are the earth tones typical of sandy desert.

During the early and middle Pleistocene epoch, this area held a large brackish lake. Uplift of the northern part of the lake in the late Pleistocene created hollows that became receptacles for potash deposition. The main potash deposits found at Lop Nur today are brine potash, and this site is the second-largest source of potash in China.

Lop Nur slowly dried up in the Holocene. The area now receives average annual precipitation of just 31.2 millimeters (1.2 inches), and experiences annual evaporation of 2,901 millimeters (114 inches), according to a study published in 2008. The study found, however, that this area has experienced seven major climate changes since the end of the Pleistocene, including climatic conditions far more favorable to farming and settlement than today.

Examination of plant and mollusk remains at the lake, as well as studies of sediments, indicate that the Lop Nur region experienced a severe drought about 3,000 years ago, followed by wetter conditions. Between 1,250 and 400 years ago, Lop Nur likely experienced the conditions most favorable to farming and settlement, and red willow trees grew in the area. Pottery dating from the Tang and Song dynasties further testifies to welcoming conditions at the lake centuries ago.
Starting around 400 years ago, however, a more arid climate took hold, completely drying out Lop Nur. Today, by providing potash, the desiccated lake still supports agriculture, but it does so for farming efforts further afield.

NASA Earth Observatory image created by Jesse Allen and Robert Simmon, using EO-1 ALI data provided courtesy of the NASA EO-1 team. Caption by Michon Scott.

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