Sedimentology of Glacial Lake Core from Gokyo Valley, Everest Region, Nepal

Abstract:

Glacial lakes are a suitable depocenter to record sedimentation history for more than a millennial development in the Everest region of Nepal. The research was done as part of the National Geographic and Rolex Perpetual Planet Expedition to Everest in spring 2019. In an effort to understand environmental change as well as tectonic activity in Gokyo area, two lake cores of the “Taboche” lake situated at 4712m were recovered from the sediment-water interface at depth using a Unicorer gravity-based percussion corer with a hammer attachment deployed from inflatable boats. The 58cm long core recovered at 95m offshore distance at a 19m-deep part of the lake was CT scanned. 14C radiocarbon dating of organic matter sampled from the same core were performed at six different sediment depths. The second 56.2cm long core sampled at 158m offshore distance from 25m depth in the same lake was used for grain size analyses using laser diffraction technique. Sediments in the lake are comprised by light to dark grey silty sands with silty laminae, light grey sandy silts with sandy laminae, tan and blackish clayey silts with sandy laminae and blackish, brownish to grey clayey silts having sandy laminae. The lake sediment reveals particle sizes ≤ 1.9mm in diameter. The proportions of sand-, silt- and clay-size particle distribution range from 29.5% to 95%, 65% to 4.2% and 0.4% to 5.5%, respectively in the lake deposit.

The youngest age found in the core sample is 85 ±25 BP at 12-13cm core depth and 1795 ±15 BP at 50.5-51.5cm depth. The 58cm long core records ∼2000 years of sedimentation history. Syn-sedimentary deformational structure, depositional structures and slumped structures preserved in the lake core were examined from CT scan tomography. In this presentation, we discuss sedimentology and the origin of the structures looking evidences related to climatic triggering forces or tectonic forces for a span of 2000 years period in the glacial environment of High Himalaya.