February 27, 2017

A River Runs 40 km Across the Greenland Ice Sheet

Posted by Mauri Pelto

Supraglacial stream, on July 26, 2016 Landsat image, stretching 40 km across the ice sheet from the transient snowline, which marks the boundary between the percolation zone and the wet snow zone,  west toward the ice sheet margin, note black arrows.  

The Greenland Ice Sheet has experienced a significant increase in surface melt.  This is due both to warmer temperatures and enhanced melt due to a reduction in reflectivity-albedo. The expansion in melt area, duration and intensity (NSIDC, 2015)  has also generated large volume of meltwater transported via supraglacial streams.  Recent work by Tedesco et al (2016) and Kintisch et al (2017) illustrate three key reasons for the albedo change in the melt zone.

1) Upon melting and refreezing, ice crystals lose their branched shape, grow larger and rounder, which reduces the reflectivity of the snow by as much as 10%.

2) Satellite data show that the margins of the ice sheet have darkened by as much as 5% per decade since 2001. Dust trapped over the centuries has become concentrated at the melting edge of the ice sheet.

3)   The combination of algae and bacteria with dust generates a sludge—known as cryoconite. This dark material gathers in depressions decreasing albedo. Black and Bloom is a project focused on how dark particles (black) and microbial processes (bloom) darken and accelerate the melting of the Greenland Ice Sheet

Tedesco et al (2016) noted the negative trend in albedo is confined to the regions of the ice sheet that experience summer melting. They also observed no trend during the 1981–1996 period. Their analysis indicates the albedo decrease is due to the combined effects of increased air temperatures, which enhances melt promoting growth in snow grain size and the expansion of bare ice areas, and to increasing concentration of dark impurities on ice surfaces. Kintisch et al (2017) noted the same mechanisms with warmer summers also enhancing microbes and algae growth on the wetter surface of the ice, producing more cryocontie, that reduces albedo absorbing more solar energy. Cryoconite is more spatially limited than the other mechanisms. They also observed that soot and dust that blow in from lower latitudes and darken the ice are also increasing.

The darker surface enhances melt which generates more meltwater largely drained in the melt zone by supraglacial streams. Smith et al (2015) documented the surface drainage in the ablation zone of the southwest GIS. They focused on documenting the distribution of over 500 high order stream channel networks in a 6812 square kilometer region, inland from Kangerlussuaq.  All of the stream networks terminated in moulins before the ice sheet edge (NASA, 2015).  This indicates that moulins are common, important and sparse.

Poinar et al (2015) observe the longest streams in the 30-50 km range. Here we examine two streams one in detail using Google Earth that is 30 km long and a 40 km long surface stream in 2016 observed in Sentinel 2 and Landsat images. That the surface rivers can travel this distance across the surface before draining via a moulin indicates that the glacier is not structurally like Swiss cheese (Pelto, 2015).  The Google Earth detailed view illustrates both the darker surface, the maturity and hydrologic efficiency of the thermally incised meltwater streams.

The stream observed in Google Earth in its mid-reach has an average of 15 m in width.  The slope of the ice sheet is 1/120 in this region, with the river beginning at 1320 m and ending at 1070 m.  Gleason et al (2016) examined numerous supraglacial streams and noted that supraglacial streams with a width of 15-20 m and slopes of 1/100 to 1/200 had a depth of 1.5-2.0 m and velocity of ~0.5 m/sec.  This suggests the stream here has a discharge  of 7-10 cubic meters per second. The darkness of the ice surface indicating a low albedo is also apparent.  The ice is not nearly as dark when standing directly on it as it is in the macro-scale.

The second stream is seen in a Sentinel  image from July 15 and a Landsat image from July 26. The black arrow indicate the stream that is 40 km long.  The stream extends from 110 km from the edge of the ice sheet to within 75 km. The stream begins near the transient snowline at 1650 m and ends near 1400 m, creating a slushy valley above the local percolation zone. The stream in early July flows through the wet snow zone. By the end of the July the lower section of the stream becomes a bare ice region, the upper remains in the  wet snow zone.

Supraglacial stream in mid-July Sentinel images stretching 40 km across the ice sheet from the transient snowline west toward the ice sheet margin.