Bacteria darken Greenland’s glacial streams | Polarjournal
The thickness and extent of the Greenland ice sheet only really become apparent from above. A closer look reveals that the seemingly white surface is criss-crossed by numerous waterways and ponds These promote melting through heat absorption. Photo: NASA Earth Observatory

In many areas, the Greenland ice sheet does not consist of a completely closed blanket, but is criss-crossed by countless water outlets. These blue veins consist of meltwater that makes its way across the surface and disappears via glacial mills into the depths of the glaciers. The dark blue intensifies the melting process at the surface. Now, a team of researchers has discovered another melting factor in the clear streams and rivers: dark sediment accumulations, created by bacteria, probably further enhance the melting.

The team came to this conclusion when they flew over a melting stream with a drone in the southwest of Greenland in 2017, had taken measurements and sediment samples from the bottom of the flowing water and had analysed them, and incorporated the results into models of flow dynamics. “These streams can be seen all over Greenland,” explains the study’s lead author, Sasha Leidman, a graduate student at Rutgers University (USA). “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..”

The potential for enhancement of melting processes results from the amount of streams that lie on the ice surface of Greenland. If the flow dynamics are right, dark sediment can quickly accumulate over a large area and accelerate the melting processes due to the increased heat absorption of the dark surface. Image: Sasha Leidman

Melting streams and rivers on the surface of the Greenland ice sheet are caused by solar radiation on the surface and the resulting melting processes. Up to now, flowing waters have been regarded as an important factor contributing to the melting of the ice and the resulting rise in sea level. But sediment accumulations were excluded from the calculations because they were considered negligible. But this new study shows that up to a quarter of a river’s bottom can be covered in dark sediment. Bacteria, which feel at home in the sediments and, under suitable flow conditions, form entire sediment accumulations, are responsible for the formation. In doing so, they clump the material, thus influencing the flow of the water and its heat absorption capacity. “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,” Leidman continues. Due to the bacteria, the sediment clumps that are formed can grow up to 91 times larger than the original size. This is a considerable accumulation of sediment.

The video shows the research team’s drone footage and the extent of sediment deposition. The researchers attribute the bacterial growth to rising temperatures and less cloud formation over the ice sheet. Video: Sasha Leidman

When asked about the reason for the increased bacterial growth and the effects on the melting processes, the lead author explains that the bacteria are cyanobacteria, bacteria that also capable of photosynthesis. The reduction in cloud formation over Greenland, higher temperatures leading to more melt water and more sediment input from the air would favour conditions for cyanobacteria. “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,” Leidman continues. “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.”

Dr Michael Wenger, PolarJournal

Link to the study: Sasha Z. Leidman, Åsa K. Rennermalm, Rohi Muthyala, Qizhong Guo, Irina Overeem. 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, 2020; DOI: 10.1029/2020GL088444.

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