RECONSTRUCTING LATE HOLOCENE GLACIAL ADVANCES IN THE NW HIMALAYA, NORTHERN INDIA BY RESOLVING THE PROBLEM OF TOO-OLD (INHERITED) SURFACE EXPOSURE AGES ON YOUNG MORAINES

Reconstruction of glacial chronostratigraphies at the sub-millennial to century timescale requires high precision dating of geological materials. Due to limitations associated with the availability of suitable organic materials and partial bleaching of quartz grains in the high-altitude Himalayan glaciated environment, both radiocarbon and optically stimulated luminescence dating techniques, respectively, are not widely applicable. Terrestrial cosmogenic nuclide (TCN) surface exposure dating of moraine boulders is most suitable in such geomorphic settings. However, largely due to variable inventory of in situ TCNs inherited from prior exposures, the young moraine boulders manifest large scatter in their apparent ages. Forty-one percent of the 93 published and new Holocene moraines across the Himalayan-Tibetan orogen show reduced chi-squared values >1 and are positively skewed, indicating the likelihood of inheritance of TCNs in glacial boulders. This increases the uncertainties in moraine age calculation using community standard statistical methods and hinder the reconstruction of high-resolution late Holocene glacial chronostratigraphies in the Himalaya and Tibet. Ten supraglacial boulder samples were randomly collected from active ice surfaces in the Urgos and Chhota Shigri valleys of the NW Himalaya, northern India for TCN analysis (using ¹⁰Be) to test this problem. In addition, five boulder samples were also collected from a very young moraine in front of Gangotri glacier, northern India, which has been historically documented by early explorers to date to AD 1780. The young moraines in these valleys were also mapped in the field aided by GPS and remote sensing. Additionally, twenty boulder samples were collected from these moraines for ¹⁰Be dating. Theoretically, the supraglacial boulders should have zero exposure ages and boulders on the young Gangotri moraine of AD 1780 should have exposure ages at around ~200 yr. Greater than zero ages of supraglacial boulders and >200 yr ages of the young moraine samples should be treated as inherited surface exposure ages. These will subsequently be used to correct the scatter in apparent exposure age distribution of young moraines in these valleys using Chi-Square Goodness of Fit test.