Abstract
Lop Nur, a playa lake located on the eastern margin of Tarim Basin in northwestern China, is famous for the “Ear” feature of its salt crust, which appears in remote-sensing images. In this study, partial least squares (PLS) regression was used to estimated Lop Nur playa salt-crust properties, including total salt, Ca2+, Mg2+, Na+, Si2+, and Fe2+ using laboratory hyperspectral data. PLS results for laboratory-measured spectra were compared with those for resampled laboratory spectra with the same spectral resolution as Hyperion using the coefficient of determination (R2) and the ratio of standard deviation of sample chemical concentration to root mean squared error (RPD). Based on R2 and RPD, the results suggest that PLS can predict Ca2+ using Hyperion reflectance spectra. The Ca2+ distribution was compared to the “Ear area” shown in a Landsat Thematic Mapper (TM) 5 image. The mean value of reflectance from visible bands for a 14 km transversal profile to the “Ear area” rings was extracted with the TM 5 image. The reflectance was used to build a correlation with Ca2+ content estimated with PLS using Hyperion. Results show that the correlation between Ca2+ content and reflectance is in accordance with the evolution of the salt lake. Ca2+ content variation was consistent with salt deposition. Some areas show a negative correlation between Ca2+ content and reflectance, indicating that there could have been a small-scale temporary runoff event under an arid environmental background. Further work is needed to determine whether these areas of small-scale runoff are due to natural (climate events) or human factors (upstream channel changes).
Highlights
The salt crust of a playa lake is a complex landscape, which records the drying process of an arid land surface and the continuous evolution of natural water
A total of 78 samples were analyzed for the laboratory spectra, of which three samples were identified as outliers and 75 samples were used in partial least squares (PLS) modeling
Twenty-five of these samples were randomly selected for validation, and 50 remaining samples were used for calibration to estimate individual salt-crust property contents
Summary
The salt crust of a playa lake is a complex landscape, which records the drying process of an arid land surface and the continuous evolution of natural water. The type and abundance of salt-crust properties in a playa lake contain substantial paleoclimatic records and indicate the whole process of environmental evolution because climatic conditions were recorded in the lacustrine sediments as geochemical signatures. Lop Nur Lake dried up before 1972 according to Landsat MSS images and early field reconnaissance carried out by Chinese scientists [1]. In the gradual drying process of Lop Nur, because of strong evaporation, freezing, and thawing, salt crystallization occurred to form a salt crust 30–100 cm in depth in different land types. Lop Nur became famous around the world for its eastern part, which appears as “the big ear in the western part of China” in Landsat MSS images. The “Ear” feature of the salt crust appears in remote-sensing images as light rings alternating with dark rings in a roughly circular distribution
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