THE GLACIAL CHRONOLOGY OF EASTERN LUNANA (NW BHUTAN–HIMALAYA)

Lunana, a heavily glaciated area in the northwest of Bhutan, acted as accumulation area for Pleistocene ice-stream nets and is the source area for recurrent glacial lake outburst floods during Holocene and recent times. The Quaternary sediments and glacial geomorphology have been examined in detail. Here we present a first glacial chronology for Eastern Lunana, constrained by radiocarbon age dating. Remnant sequences of Pleistocene lateral moraines and kame terraces can be traced downvalley and contain datable organic material. Using these ice-marginal deposits, a glacier advance of approximately 35 km can be reconstructed for the local LGM. A younger loess-covered moraine complex at an altitude of 4100 m (Thanza Stage) indicates a glacier advance of 4 km, with an approximate Equilibrium Line Altitude depression of 300 m (relative to the recent situation). This contains datable buried paleosols as witnesses of warm periods. The ages of the Neoglacial moraines were constrained using dendrochronology and radiocarbon age dating. The main valleys of NW Bhutan are open to the south and are situated just out of the rain–shadow produced by the Shillong Plateau. Thus the Indian summer monsoon can directly transport moisture to high altitudes, feeding glaciers in Lunana. The recent “Glacier Elevation Index” (as approximation of the Equilibrium Line Altitude) was calculated at 5300 m, some 250 m lower than in the southwestern Everest region, which probably reflects the more humid climatic conditions in NW Bhutan. According to our meteorological data, 80 to 90 % of the total annual precipitation (500 – 700 mm/yr) falls during March to October, which is strongly influencing the mass balance of Lunana´s glaciers. We assume that these glaciers, which are entirely summer accumulation–type glaciers, as well as their Quaternary sediments, reflect recent and paleo–monsoon dynamics, thus making Lunana a suitable place for testing whether the glaciers in the Bhutan–Himalaya region fluctuated synchronously with other northern hemisphere ice volumes or not.