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October 16th, 2017

Muddy waters: First mapping of Greenland sedimentation rates shows a turbulent system

INSTAAR scientists Ben Hudson and Irina Overeem install river gauging equipment on the the Naujatkuat river, West Greenland. Photo by Katy Barnhart, 2017.

A new study has measured the sediment carried by Greenland’s rivers to the ocean, with implications for marine ecosystems, carbon in the ocean, and dynamics of the coastal zone. Led by INSTAAR researchers and published today in Nature Geoscience, the study is the first to quantify in detail the concentrations of sediment in rivers flowing from Greenland to the sea. In the rapidly changing environment of the Arctic, with faster glaciers boosting erosion rates, the study gives an important picture of sediment and associated nutrients that are draining off the Greenland ice sheet.

Overeem pulls up an oceanographic sensor that measures salinity and sediment concentration near the delta front of the Pakitsjup river. Photo by Katy Barnhart, 2017.

The researchers used satellite imagery to quantify the concentrations of sediment in rivers flowing from Greenland to the North Atlantic and Arctic Oceans. Their data set measures suspended sediment in 160 rivers and develops detailed maps of where sediment in fjords originates, giving ten times as much information on individual rivers as previously known.

The results, combined with calculations about the meltwater runoff from all of Greenland, give a good picture of sediment exported from the ice sheet. Although runoff from Greenland only amounts to a bit over one percent of the earth's freshwater flux, the Greenland ice sheet produces about 8 percent of sediment delivered from rivers to the sea. A surprising finding was how much sediment loads vary from one river to another, controlled mostly by glacial erosion. Deeply incised, fast-moving glaciers are hot spots of sediment export. Irina Overeem, lead author and research scientist at INSTAAR and the Community Surface Dynamics Modeling System (CSDMS), explains, "We are most intrigued by our finding that the pattern of the sediment flux—which rivers have the highest concentrations of sediments—is strongly determined by the movements and associated grinding of the ice sheet. Deep valleys under the ice that funnel lots of ice result in turbid rivers downstream."

Overeem and Hudson install a river gauging site near Pakitsjup delta, West Greenland. Photo by Katy Barnhart, 2017.

Together, just 15 percent of Greenland's rivers move 80 percent of sediment from the ice sheet to the sea. The study suggests that as Greenland warms and its ice sheet melts faster, sediment delivery from Greenland to its fjords and the nearby ocean will also accelerate. The authors conclude that the amount of sediment moved into the ocean from Greenland is about 56 percent higher today than it was during the period from 1961 to 1990 that was used as a baseline for the study.

The researchers developed a data analysis technique that makes it possible to measure suspended sediment concentrations in remote areas using satellite data. But the study began on the ground—and on the water—in Greenland. Overeem says, "This reconstruction of the sediment flux and its mechanism has only been possible with large Greenland-wide remote-sensing datasets, but it all started with collecting field observations to arrive at the understanding on a local scale."

Study authors include Overeem, former CU PhD student Ben Hudson, INSTAAR and CSDMS researcher James Syvitski, and researchers from the University of Washington, University of Copenhagen, Utrecht University, and University of California Irvine.