Links with the Tibetan uplift and the ice buildup in the northern hemisphere

Two main factors may be invoked to explain the desertification in the interior of Asia in the Late Miocene-Pliocene time. Climate models suggest that the uplift of the Tibetan Plateau may have played an important role in Asian aridification through modulating the atmospheric circulation and its barrier effect to moisture (Ruddiman and Kutzbach, 1989; Manabe and Broccoli, 1990; An et al., 2001). Another factor is the ongoing global cooling and the expansion of Arctic ice-sheet, which is likely to have had a major impact on the intensity of the winter Siberian highpressure cell, resulting in higher continental aridity in Asia (Ruddiman and Kutzbach, 1989).

Although the timing of the Tibetan uplift is still controversial (Sun and Zheng, 1998), the available data (e.g. Li et al., 1997; Sun and Zheng, 1998) suggest that an uplift of portions of the Plateau occurred at ~3.6 Ma BP, which is synchronous with a major increase in Asian continental aridity. Another strengthening of the Asian aridification occurred at ~2.6 Ma BP indicates a major uplift of the Tibetan Plateau at that time. These suggest that other factors have also played a major role on the development of Asian desertification.

Although ice-rafting history near the Arctic region may be traced back to ~12 Ma BP, a real icesheet in the Arctic region appeared at ~6 - 7 Ma BP in Greenland (Jansen and Sjoholm, 1991). The high Asian continental aridity from ~6.2 to ~5 Ma BP is correlative with intense icerafting events in the Arctic region, and with a cooler events indicated by the marine 8 18O record. The lower aridity from ~5.0 to ~3.6 Ma BP correlates with the lower intensity of ice-rafting events and warmer conditions. The strong aridification at ~2.6 Ma BP, indicated by the intense eolian deposition (the lower boundary of the loess-soil sequence) in northern China is synchronous with a sudden increase in the amount of ice-rafted debris.

The aridification history in the Late Miocene and Pliocene in East Asia is therefore highly consistent with the ongoing cooling and the development of the Arctic ice-sheet. The stepwise increases of loess accumulation rate at ~1.8 and ~0.9 Ma BP as recorded by the loess of the last 2.6 Ma also strongly support this interpretation. This suggests that the development of the northern hemispheric icesheets had played an important role in driving the Asian aridification over the past 6 Ma. The ~0.9 Ma age boundary would also be consistent with another uplift phase of the Himalayas-Tibetan complexes (Amano and Taira, 1992; Sun and Zheng, 1998; Lu et al., 2001).

The links between the Asian aridification and northern hemisphere cooling are likely to be explainable by two mechanisms. First, ocean cooling would provide less moisture to the continents, and thus lead to drier conditions in Asia. Second, cooler high latitudes, extended sea-ice and icesheets in the arctic region strengthened the Siberian high-pressure cell, which in turn, significantly intensify the continental aridity in the interior of Asia (Ruddiman and Kutzbach, 1989).

Both the Tibetan uplift and the ongoing global cooling/arctic icebuilding processes have played major role on the aridification in the interior of Asia during the in the late Cenozoic. Their roles are not mutually exclusive.

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