THE INFLUENCE OF THE MONSOON ON PALEOGLACIATIONS ON THE TIBETAN PLATEAU

The Tibetan Plateau has a strong potential to influence the Earth’s climate on a regional to hemispheric scale because of its high elevation at a low geographic latitude and with a large area. The glacial deposits of the Tibetan Plateau reflect not only climate signals originating in the North Atlantic region, but also changes in the intensity of the Indian monsoon, the main moisture source of the area. It is not clear, though, what the nature of the forcing mechanisms of glaciations on the plateau is. Due to the scarcity of datable material, the establishment of absolute chronologies is very difficult. Available data from different parts of the plateau and the Himalaya mountains are not consistent with each other and difficult to relate to glacial chronologies in other parts of the earth. This situation has improved recently with the advent of surface exposure dating (SED) using in-situ terrestrial cosmogenic nuclides. We investigated a set of large terminal moraines in Nyalam County, near a 4300 metres high mountain pass between Tibet and Nepal, with SED. The study aimed at shedding light on the relative importance of North Atlantic and monsoonal climate signals. The climate in this region today is dominated by the monsoon as the wet air masses cross the Himalaya on their way from India to the Tibetan Plateau. Hence, the mountain glaciers in this area respond to variations of the summer monsoon intensity. The moraines are located about 15 km in front of the present-day glaciers. For dating the moraines we used the in-situ produced cosmogenic nuclides Be-10 and Ne-21. To control the non-cosmogenic signal, we analysed the shielded bottom of a sample alongside with its exposed surface wherever possible. First results indicate deposition of the Nyalam moraines at around 60 ka BP, i.e. during early Marine Isotope Stage 3, a time of enhanced summer monsoon, and an interglaciation in the North Atlantic record. This implies that increased precipitation from a strong summer monsoon triggered glacier advances in the Himalaya during interglacial times. A smaller succession of moraines, about 5 km upvalley from the first, is about to be dated. It is expected to yield information about the behaviour of the glaciers during times of northern hemisphere cooling. The data will be available at the conference.