Holocene proglacial loess in the Ranwu valley, southeastern Tibet, and its paleoclimatic implications

Abstract

Glaciers and aeolian sediments are widespread on the Tibetan Plateau. Although glaciations have played an important role in the formation of loess in many glaciated regions in the Northern Hemisphere, proglacial loess on the Tibetan Plateau is rarely reported, and its origin, source, and paleoclimatic implications remain unclear. We have found a small area of proglacial loess deposits in the Ranwu valley, southeastern Tibet. Grain-size, geochemical and geomorphologic evidence, as well as observations of local dust storms, reveal that the outwash sediments of the Laigu glacier (∼30 km long) are the primary source for the local loess. We also found that the glacial wind from this glacier is the main transport vector, and thus speculate that the onset of the Ranwu loess, at ca. 6.6 ka, correlates with the early Neoglacial advance of the Laigu glacier, which generated glacial winds and local dust storms in the closed and deep-incised glacial valley. This is supported by the basal radiocarbon ages of two ice cores and a synthesis of Holocene glacial stadials on the Tibetan Plateau. Loess layers and paleoclimatic proxies indicate two drought stages at ca. 6.6–6 ka and ca. 2.9 ka to the present, corresponding to two well-documented weakened stages of the Indian Summer Monsoon. This study highlights that, although local aeolian sediments always cover a small area, they also have potential to record some clues of regional climate changes in some alpine regions. Much more detailed studies should be done to reveal their specific formation mechanisms and paleoclimatic implications.