Tropical Weathering in Tibet and PreLGP Interglacials

Remains of a widespread Hipparion fauna in Middle Pliocene sediments of central Tibet are indicators of a warm-tropical steppe climate and show that the altitude of the winter snow zone had not yet been reached at this time (Chen 1981, Ji et al. 1981).

From 2.5 Ma onwards did sediment begin to accumulate on the loess plateau of China. This is interpreted as indicator of the onset of winter monsoon circulation (Kukla and An 1989; An et al. 1990; Ding et al. 1992).

Tibet's climatic impact first became effective when uplift raised the plateau to the level of the seasonal snowline from ~2.5 MaB.P. onwards, and subsequently to the level of maximum glaciation starting at ~1 MaB.P. The progressive glaciation of the Tibetan plateau may thus have been the decisive terrestrial factor causing orbital variations to translate into global ice ages.

During warm interglacials in Tibet the conditions had most likely been considerable warmer and wetter than today. On the eastern edge of Tibet (30°45'N/103°20'-40'E; Fig.1, Nos. 51, 57, 58), west of the town of Chengdu, at approximately 600 m asl there are mud flow-, or debris flow sediments or even ground moraines with a thickness of several (2-15) meters. They stretch over an area of about 40x50 km. Their origin as older than the LGP is suggested by the intensive degree of tropical weathering. Li Tianchi from the Geographical Institute in Chengdu confirms the glacial origin of the sediments and interprets these material as even older than the second but last glacial period. Li Tianchi and coworkers suggest that the debris flows and moraines belong to the Early or Middle Pleistocene (ca. 500 ka, pers. comm. August 1991) without, however, an absolute date being available. This implies that in East Tibet tropical conditions prevailed at that time. Together with an ice-covered Arctic Ocean considerable latitudinal temperature gradients are interpreted to have existed in this time, particularly in winter. The extension of tropical Pleistocene and Pliocene sediments in Tibet needs to be determined quantitatively by drillsites. Near the older ground moraines and a few kilometres from the edge of the plateau is the Chung Leh Shan (moutain massif) at about 4500 m asl. There is a moraine and linked debris flow indication, dated by weathering and position, of Early to Middle Pleistocene age. It documents a piedmont glaciation. Because it extends down into present warm subtropical climatic conditions, the glacier must have flown from an already highly uplifted Tibetan Plateau in order to have been able to reach down so far. The lowest of the Last Glacial glacier terminal moraines in this area reached down to 1300-1500 m asl (see v. Loczy 1893; Li Tianchi 1988). This means that at the time of that older piedmont glaciation the ELA must have been 350-450 m lower than during the LGP assuming a similar altitude for the Tibetan Plateau and surrounding mountains.

As the moraines appear in an area that is today characterized by subtropical conditions, comparable conditions can be inferred for pleistocene interglacials. Most likely (see Vostok ice-core) the conditions had been warmer. This is consistent with considerable latitudinal temperature gradients in times preceding a glaciation.

Judging from a world-wide comparison of ELA differences between the LGP and older glacial periods, the snowline of these earlier ice ages was only about 100 m lower than during the LGP. This means that the height of the base of the catchment area (the height of the plateau) must have been some 300 m greater than during the Last Glacial period. The lowest ice margin, related to the LGP, must have been 800 m higher than at the time of the older piedmont glaciation (see above).

The preservation of old moraines implies a stagnation in the uplift, if not subsidence of Tibet, at least during the Late Pleistocene. This points to a glacio-isostatic compensation or overcompensation of tectonic uplift in Tibet (Kuhle 1993). This supports the above-mentioned alternative 4.7.2.: reduced or nearly non-existing uplift in the Pleistocene, pleistocene interglacials too short to allow for large uplift (mantle viscosity), pronounced uplift since the Holocene.

This has implications for the boundary conditions for warm climates, both in the Pliocene and in the future. In the Pliocene, after about 2.75 Ma, large scale northern hemisphere glaciations began after the Tibet-Plateau reached a high-altitude. As various other factors changed also around this time, the uplift of the Tibet-Plateau is regarded as one important co-factor. The mechanism to terminate a warm climate are explained above. During the Pleistocene a considerable isostatic depression is inferred. An inland-ice existed. During the Holocene no reglaciation occurred. The uplift that is observed today is interpreted to be caused partially by isostatic uplift. Depending on its surface characteristics, particularly glaciated versus non-glaciated (measured incoming radiation close to the solar constant for that latitude), the Tibet-Plateau contributes to stabilizing the climate system: Either in glacial conditions or in non-glacial conditions. It is thus inferred that the Tibet-Plateau will contribute to stabilizing the climate system in the present non-glacial conditions until an elevation of ca. 5600 m will be reached. Other conditions, such as pronounced latitudinal temperature gradients are expected to modify this value.

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