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The Tibet plateau and the origin of loess

24 April 2012

At http://phys.org/print254405062.html … when did the desertification as a result of the uplift of the Tibetan plateau happen. Geologists are divided. One view is that it occurred 22 million years ago, at the onset of the Miocene, when it seems the plateau began to rise – slowly. The plateau, as it rose, stopped the flow of moist air from the south, is the basic ingredients of the theory, leading to increasing amounts of desertification. It seems that the desert conditions are required in order to explain another theory, on the origin of loess by the wind. There are lots of loess fields in China, and almost everywhere else in the world. Loess deposits can be seen as surface or near surface deposits in various parts of SE England for example, particularly in river valley systems. So, did the wind blow the loess across a dry tundra environment. It seems this idea might be up for grabs. Now, it seems that some geologists are thinking that the arid region SE of the Tibet plateau may actually have been full of lakes and bogs until as recently as 8 million years ago (based on the geochronological dating of the particular loess deposit that is found downstream in the Tianshui Basin in central China). The new findings concern a reinterpretation of sediments at Tianshui – were they transported by wind or water? If loess was carried by wind it would support the 22 million year date as it takes time for the desert conditions to develop and the wind to blow the stuff so far away. The new research, by Peng et al, has found hydrocarbons from waxy plant material in the Tianshui loess which has an origin in plants from an aquatic or wetlands flora, as well as pollen grains and algae with a similar origin anhd suggests that during the Miocene, at least, the Tianshui Basin was a lake or flood plain and not arid loess. The research is confined to a narrow region of China and none of the other loess deposits there has been subject to the same kind of analysis. So, were Allen and Delair on the right tack? They claimed loess was laid down by water and not by wind.

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