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Microbes in Meltwater Stream Sediment Responsible for Greenland Melting

According to Rutgers researchers, bacteria are the reason behind greater melting on the Greenland ice sheet, probably increasing the contribution of the island to sea-level rise.

A supraglacial stream and sediment floodplain in southwest Greenland. Image Credit: Sasha Leidman.

As reported in a first-of-its-kind study under the guidance of Rutgers University, published in the journal Geophysical Research Letters, the underlying reason for the sea-level rise caused by bacteria is that microbes make the sunlight-absorbing sediment to cluster together and get accumulated in the meltwater streams.

The researchers stated that the results of the study can be integrated with climate models, resulting in more precise forecasts of melting.

These streams can be seen all over Greenland and they have a brilliant blue color, which leads to further melting since they absorb more sunlight than the surrounding ice. This is exacerbated as dark sediment accumulates in these streams, absorbing even more sunlight and causing more melting that may increase sea-level rise.

Sasha Leidman, Study Lead Author, Department of Geography, School of Arts & Sciences, Rutgers University–New Brunswick

Leidman is a graduate student in the laboratory of co-author Åsa K. Rennermalm, who is an associate professor also from the Department of Geography.

The Greenland ice sheet is spread over an area of around 656,000 square miles—a major portion of the island and three times the size of Texas, as per the National Snow & Ice Data Center. If the thick ice sheet melted, the worldwide sea level would increase by about 20 feet.

Climate change causes an increase in sea level and coastal storms, which are a threat to cities, low-lying islands and lands across the globe.

A majority of the researchers do not consider the sediment present in glacial streams that develop on top of the Greenland ice sheet with the flow of meltwater to the ocean, but the Rutgers researchers intended to determine why so much sediment was accumulated. In 2017, researchers flew drones over a nearly 425-foot-long stream in southwest Greenland, performed measurements, and gathered sediment samples.

They discovered that nearly one-quarter of the stream bottom is covered by the sediment, much more than the approximate 1.2% that would occur if organic matter and cyanobacteria did not induce clumping together of sediment granules. Moreover, this shows that streams have more sediment compared to what hydrological models predict.

We found that the only way for sediment to accumulate in these streams was if bacteria grew in the sediment, causing it to clump into balls 91 times their original size. If bacteria didn’t grow in the sediment, all the sediment would be washed away and these streams would absorb significantly less sunlight. This sediment aggregation process has been going on for longer than human history.

Sasha Leidman, Study Lead Author, Department of Geography, School of Arts and Sciences, Rutgers University–New Brunswick

According to the study, the solar energy absorbed by streams is probably based on the longevity and health of the bacteria, and additional warming in Greenland might result in greater sediment deposits in glacial streams.

Decreases in cloud cover and increases in temperature in Greenland are likely causing these bacteria to grow more extensively, causing more sediment-driven melting. With climate change causing more of the ice sheet to be covered by streams, this feedback may lead to an increase in Greenland’s contribution to sea-level rise.

Sasha Leidman, Study Lead Author, Department of Geography, School of Arts and Sciences, Rutgers University–New Brunswick

By incorporating this process into climate models, we’ll be able to more accurately predict how much melting will occur, with the caveat that it is uncertain how much more melting will take place compared with what climate models predict. It will likely not be negligible,” added Leidman.

The co-authors of the study from Rutgers include graduate student Rohi Muthyala and School of Engineering Professor Qizhong (George) Guo. A researcher from the University of Colorado Boulder also worked on the study.

Drone flight over a supraglacial stream in Greenland

Video Credit: Rutgers University.

Journal Reference:

Leidman, S. Z., et al. (2020) The Presence and Widespread Distribution of Dark Sediment in Greenland Ice Sheet Supraglacial Streams Implies Substantial Impact of Microbial Communities on Sediment Deposition and Albedo. Geophysical Research Letters.


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