Be‐10 dating of ice‐marginal moraines in the Khumbu Valley, Nepal, Central Himalaya, reveals the response of monsoon‐influenced glaciers to Holocene climate change

The dynamic response of large mountain glaciers to climatic forcing operates over timescales of several centuries and therefore understanding how these glaciers change requires observations of their behavior through the Holocene. We used Be-10 exposure-age dating and geomorphological mapping to constrain the evolution of glaciers in the Khumbu Valley in the Everest region of Nepal. Khumbu and Lobuche Glaciers are surrounded by high-relief lateral and terminal moraines from which seven glacial stages were identified and dated to 7.4 ± 0.2, 5.0 ± 0.3, 3.9 ± 0.1, 2.8 ± 0.2, 1.3 ± 0.1, 0.9 ± 0.02, and 0.6 ± 0.16 ka. These stages correlate to each of the seven latest Holocene regional glacial stages identified across the monsoon-influenced Himalaya, demonstrating that a coherent record of high elevation terrestrial palaeoclimate change can be extracted from dynamic mountain landscapes. The time-constrained moraine complex represents a catchment-wide denudation rate of 0.8–1.4 mm a−1 over the last 8 kyr. The geometry of the ablation area of Khumbu Glacier changed around 4 ka from a broad, shallow ice tongue to become narrower and thicker as restricted by the topographic barrier of the terminal moraine complex.

Key Points
Ice-marginal moraines in the upper Khumbu Valley represent a complete record of regional glacier change between 7.4 ka and the present day

Moraine building modified the response of Khumbu Glacier to climate change after 8 ka

Holocene moraine volume indicates a catchment-wide denudation rate of 0.8–1.4 mm per year

Plain Language Summary
Satellite observations indicate that glaciers in the monsoon-influenced Himalaya are changing rapidly in response to climate change. However, understanding why glaciers are changing requires observing glacier behavior over longer timescales using the glacial geological record. We mapped the geometry and measured the ages of ice-marginal moraines built by two adjacent glaciers in the Everest region of Nepal. The moraines represent a complete record of glacier expansion during cold periods in the monsoon-influenced Himalayan region over the last 8,000 years. Moraines formed by the large Khumbu Glacier and the smaller, steeper Lobuche Glacier also reveal differences in how these glaciers have changed since the last Ice Age. The results are useful to understand how monsoon-influenced Himalayan glaciers respond to climate change, and to improve projections of their future behavior.