ANSTO has collaborated with researchers at the University of Manchester (UK) on a new study published in Quaternary Science Reviews that details the timing of the dramatic collapse of one of the largest glaciers in the Himalayas. The findings overturn a long-held assumption about what sustains wet-climate (monsoon dominated) glaciers.
The research reconstructs the retreat and final collapse of a Himalayan glacier in unprecedented detail using the technique of cosmogenic radionuclide surface exposure dating carried out on the SIRIUS accelerator in the Centre for Accelerator Science..
The work provides the first dated glacial chronology of the Dri Valley Glacier, in Arunachal Pradesh, which is located in the remote and difficult to access far eastern Himalayas (north of Myanmar and east of Bhutan). The region is dominated by the Indian Summer Monsoon resulting in the highest precipitations across the Himalaya mountain arc.
Around 58,500 years ago, during a climatic glacial period in Earth’s paleoclimate history, the Dri glacier was nearly 100 kilometres long starting from an elevation of approximately 5300 meters and terminating at 1500 meters, lower than many of India’s modern hill stations in nearby Darjeeling and Sikkim, and among the lowest glacier termination elevations documented anywhere in the Himalayan-Tibetan region.
At the end of the last ice age about 12,000 years ago, the Dri glacier was restricted to elevations above 3500. Today its largest surviving remnant is less than five kilometres long.
The collaboration team included Prof Phil Hughes (Uni Manchester) and Dr David Fink (ANSTO). The study was part of the PhD research project of Shashank Nitundil (University of Manchester). Nitundil combined satellite mapping, digital elevation models and several months of fieldwork in dense, steep, monsoon-soaked terrain. Other contributors included Dr Christopher Darvill, Dr Abi Stone, and Matt Tomkins.
With logistical support from the Idu Mishmi community, the researchers identified the glacier’s fingerprints deposited in the valley landscape: U-shaped valley flanks, cirques, moraines, bedrock cut flat and smooth by the passage of ice and classic roche moutonnée bedrock forms, rounded on the upstream side and jagged downstream.
Reading time in the rocks
To date, the timing these glacial bedrock features and moraine deposits, the team turned to terrestrial cosmogenic radionuclide dating. When a glacier retreats and exposes a fresh rock surface, energetic cosmic rays which penetrate through the atmosphere and interact with earth’s surface, begin producing rare radioisotopes such as beryllium-10 (10Be) within the quartz mineral of the rock, effectively converting a nucleus of oxygen-16 to a nucleus of 10Be. The longer the surface has been exposed, the more 10Be it contains, making the 10Be a natural clock that dates glacial retreat.
“It’s like tick tock there’s a 10Be clock in my rock,” said Dr Fink.
The team collected 53 moraine boulders and 10 bedrock samples, which were shipped to ANSTO Australia. The samples were processed by PhD Nitundil who worked with Krista Simon at the cosmogenic sample preparation laboratory CAS. The difficult and time-consuming task of collecting the tens of millions of 10Be atoms from one kilogram of rock involved Isolating the quartz crystals, etching, then final dissolution using a combination of hydrofluoric acid and hot phosphoric acids.
The ¹⁰Be was then measured on the SIRIUS Accelerator, which has detection sensitivity for these radioisotopes at concentrations of just a few parts in a thousand trillion (1 in 1015).
ANSTO scientists Dr David Fink and Krista Simon are co-authors on the study, contributing to project management, participation in fieldwork, supervision in sample preparation at ANSTO and AMS measurement. Support for the cosmogenic dating capability at the Centre is provided through the National Collaborative Research Infrastructure Strategy (NCRIS). Field work was financially supported by a grant from the Walters Kundert Fellowship awarded by the Royal Geogrpahical Society to Dr Darvill.
A glacier that died in stages
The ¹⁰Be ages reveal a glacier that did not simply shrink steadily but retreated in distinct steps. At its maximum, before about 58,500 years ago, it stretched close to 100 kilometres. By around 44,800 years ago it had pulled back somewhat, and even at the global Last Glacial Maximum about 20,000 years ago, the coldest phase of the last Ice Age, it still extended roughly 80 kilometres. Then came a sharp decline: by about 12,600 years ago the glacier had collapsed to around 25 kilometres, with high bowl- shaped basins becoming ice-free by roughly 13,000 years ago.
That step-like pattern matters. As the authors note, it points to the potential for abrupt shifts in glacier extent rather than gradual wasting, shifts that can reshape river flow, sediment transport and downstream stability.
Temperature, not rainfall, holds the key
The most striking result challenges a common assumption. The eastern Himalayas are among the wettest places in High Mountain Asia, fed by the Indian Summer Monsoon, and it might be thought that abundant precipitation would protect their glaciers on the condition that the atmospheric temperature remained sufficiently low to deliver the precipitation as seasonal snow. The Dri Valley record shows otherwise.
Across the last glacial cycle, the glacier expanded and contracted primarily in response to temperature, with precipitation acting only as a secondary modulator. During cold phases, monsoon moisture fell as snow and the glacier grew to enormous size; as temperatures rose, that same moisture fell increasingly as rain rather than snow, cutting off accumulation while accelerating melt.
Crucially, the glacier remained large even through the relatively dry Last Glacial Maximum, demonstrating that it was warming, not a lack of rainfall, that ultimately drove its retreat.
In other words, monsoon-fed glaciers cross a temperature threshold beyond which heavy rainfall offers no protection at all, and in fact precipitation as rain increases the rate of glacial ice loss. As lead author Shashank Nitundil put it, "these wet Himalayan regions are among the most vulnerable to ice loss."
The findings carry a contemporary warning. As eastern Himalayan glaciers retreat, they leave behind depressions that fill with meltwater to form new lakes, and the eastern Himalayas now host a growing number of them.
When such lakes overtop or breach, the result can be a glacial lake outburst flood capable of devastating valleys downstream, an acute concern in a region carrying some of the highest concentrations of hydropower infrastructure anywhere in the world.
Robust reconstructions of how these glaciers behaved in the past also give climate scientists a benchmark for testing their models. The better a model can reproduce known glacier history, the more confidence it earns for projecting the future.
By filling a major geographic gap in the Himalayan record, the Dri Valley chronology, underpinned by accelerator measurements made in Sydney, adds a valuable new constraint for understanding one of the world’s most climate-sensitive mountain systems.
Thanks to Dr David Fink and Dr Mitra Safavi Naeini for their contribution to this article.
