Understanding under-ice water movement in Greenland

Joel Harper and research team drill bore holes in the Greenland ice sheet using an innovative drill designed by Neil Humphrey. Photo: courtesy Joel Harper In order to better understand how water beneath the ice sheet affects glacial movement in Greenland, scientists have developed and field tested a new way of drilling boreholes from the ice surface to the base rock in order to collect data that can help project future sea-level rise worldwide. The research is supported by the National Science Foundation.

During the summers of 2010-2012, University of Wyoming geology professor Neil Humphrey and colleague University of Montana geoscientist professor Joel Harper drilled 23 boreholes on the Greenland ice sheet using a custom-designed hot water/high pressure drill. In some spots, the ice was up to one kilometer thick.

Heat and pressure

Humphrey designed and constructed the drill, which pumps cold surface water through a series of heaters and high-pressure pumps. The combination of scalding water and high pressure can drill a hole at a rate of 120 meters per hour. After drilling, the scientists installed sensors to monitor water flow and ice motion.

The data collected will help scientists understand how meltwater beneath the ice sheet impacts the sheet’s rate of movement. This is significant because the unique “plumbing” beneath the ice sheet impacts the rate at which ice breaks off into the ocean, which has the effect of causing sea levels to rise. More, as the ice sheet migrates to lower elevations, there is a higher chance it will melt or break off at a faster rate.

Complicated plumbing

However, the basal hydraulic system beneath the ice sheet is complex, and scientists aren’t certain how water drains, the impact of under-ice water flow on ice movement, and more. Part of the difficulty is accessing the bedrock, which can be thousands of feet beneath the ice, to document and measure the movement of melted water.

The predominant theory has been that the water drainage under the Greenland ice sheet is comparable to that of mountain glaciers. But in a paper published in the journal Science this month entitled, “Basal Drainage System Response to Increasing Surface Melt on the Greenland Ice Sheet,” Harper and Humphrey, along with their co-authors, suggest that such a view overlooks important processes specific to the ice sheet.

Under the ice

Each summer surface meltwater migrates through thousands of meters of cold ice to reach a subglacial drainage network to the ice edge. This system is in a constant state of flux as it adjusts to input variations from the surface. The ability of the draining system to move the meltwater impacts how much the ice sheet moves in a season by modulating the speed at which the ice slides over the bed.

Looking beneath the surface

Until recently, accessing the bedrock under the ice has been challenging because large-scale ice coring projects can take years to complete and produce only a single borehole. These studies collect ice samples and analyze them for paleoclimate records.

The novel hot water/high pressure drill gave the scientists unprecedented access to the hydraulic system beneath the ice sheet. This allows scientists to more fully understand how the drainage system responds to surface ice melt. Previously, scientists interpreted Greenland’s velocity changes based on what they know about mountain glaciers. But because of geometric differences between the Greenland ice sheet and mountain glaciers, Humphrey’s and Harper say the systems are not analogous to one another.

Rather than the melting ice draining out from under the ice sheet within a matter of days – as generally happens with mountain glaciers -- the melting ice from the Greenland ice sheet moves much more slowly, as ice melts from everywhere on the ice sheet and meltwater moves underneath into a distribution system.

Ultimately, by understanding how the ice sheet will accommodate changes in surface melt and how ice sheets interact with groundwater systems, scientists will be better able to forecast the impacts from ice melt, including predicting rates of sea level rise. More, Harper and Humphrey’s research highlights the benefits of innovation in research methods to fully understand the mechanisms of the Greenland ice sheet and its response to past, present or future climate conditions.  —Rachel Odell Walker