Sikkim floods: Glacial lakes hold paleoenvironmental traces. What happens to them during outbursts?

The outburst of the South Lhonak glacial lake in north Sikkim in the early hours of October 4, 2023 caused violent floods downstream, washing away major highways and settlements. Glacial lakes are formed due to glacial retreat and hold valuable geological information, including fossils and landmass formation. 

Glaciers are compressed layers of snow formed at high altitudes that move down mountain slopes as they become too heavy. During the course of this movement, they scrape up the mountain surface, causing the surrounding land to wrinkle up. 

The debris displaced by the glaciers called moraine often accumulate to form natural barriers that prevent meltwater from the tail of the glacier to flow out. This gives birth to moraine-bound glacial lakes, like the South Lhonak glacial lake that burst to cause the disaster in Sikkim. 

Glacial lakes can also be bound by ice when a moving glacier slides into a deepened land and settles there. The water that melts off the stationary glacier is blocked out by the higher surrounding landmass on some sides and the ice of the glacier on the rest, thus forming a lake. 

The stability of all kinds of glacial lakes depend on the meteorological conditions of the region as well as physical triggers such as earthquakes. But in case of moraine-dammed glacial lakes, the loose debris easily shifts, giving way to leakages and, in some cases, outbursts which cause massive flooding downstream. 

But till such events occur, lakes formed by glaciers can exist for thousands of years and store valuable information about the soil and climate of the past.

The bed of these lakes are rich with mineral and biological sediments carried by the glacier during its journey. These are preserved for ages and may be lost to weathering as the water drains out due to lake outbursts. 

Glacial lakes are great sediment traps as the still water helps the residues settle. These can give us valuable information about the past environment. 

“Moraine-dammed and ice-dammed lakes can contain archives such as varves. These are layers of sediment laid down annually. We can count the layers, much like a tree ring, and this can tell us how long the lake has been in existence,” Bethan Davies, senior lecturer in physical geography, Newcastle University, England, United Kingdom, told Down To Earth. 

These varves can give us information about past climate and glacier dynamics since the varve thickness is related to these variables, she added. Most proglacial lakes still in contact with the glacier tend to be quite barren of life and do not hold too many fossil traces, the glaciologist noted. 

But lakes formed by Pleistocene ice may have ancient treasures buried underneath than can open a world of information about the earlier world. 

“Fossils can be very useful in understanding past environmental change; this can be linked to past glacier fluctuations so that we can understand what the environment or climate was like when glaciers in the past grew larger or smaller,” Davies wrote in the website AntarcticGlaciers.org.

“For example, fossil pollen from plants can be found in cores from lake sediments or peat bogs. This can tell us about past vegetation, which is linked to regional climate. Chironomids, which are a kind of non-biting midge, can tell us about past temperatures. In the ocean, tiny animals like foraminifera can tell us about past ocean conditions,” the glaciologist added. 

Striations and other physical weathering patterns on the ancient rock formations that hold the lakes also provide paleoclimatic information. 

In 2010-11, thousands of bones of large Pleistocene-era vertebrates along with many plant fossils were accidentally unearthed from the Ziegler Reservoir fossil site in Snowmass Village, Colorado, United States. 

“Thousands of well-preserved bones of late Pleistocene megafauna, including mastodons, mammoths, ground sloths, horses, camels, deer, bison, black bear, coyotes, and bighorn sheep” were recovered from the site, according to a 2014 report in the journal Quaternary Research.

“In addition, the site contained more than 26,000 bones from at least 30 species of small animals including salamanders, otters, muskrats, minks, rabbits, beavers, frogs, lizards, snakes, fish, and birds,” the authors of the report added.

The site situated at an elevation of 2,705 metres was the basin of a lake formed by a glacier flowing down Snowmass Creek Valley. 

Almost all glacial lake basins hold such treasures. Apart from remains of ancient animals, they also contain fossil wood and pollen in large quantities as plants were the first to grow on the moist land left behind by glacial retreat. 

In a warming world, the world is losing glaciers at an unprecedented rate, thus giving rise to more glacial lakes that can be unstable and cause such disasters frequently. Fast melting glaciers also cause sea-level rise, putting Earth’s low-lying states at risk of vanishing under water. 

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