Scientists climbed into moulins that drain meltwater from the ice sheet to better understand how volume is related to ice motion.
Holes that carry meltwater on the surface to the bottom of the Greenland ice sheet, called moulins, are much larger than previously thought, according to a new study based on observation and first-hand study by a team including a geologist from the University of Arkansas.
The extra volume can affect the stability of the Greenland ice sheet and how fast it slides towards the sea.
The team studied the relationship between the size of the mills and the daily variation in the water depth in them during the summer melting season. Scientists believe that increased water depth, and therefore presses inside the moulins butter the bottom of the ice sheet and increases the speed of its movement towards the sea, such as an ice cube sliding easily on a thin film of water. But until now, not much was known about the actual size of moulins and how much water they can hold.
“We compared our models with on-site observations of the water level in the field, and it seemed like we really needed huge amounts inside the moulins to produce the relatively smaller water variations we saw,” said Matt Covington, associate professor of geoscience. and first author of the study published in the journal Geophysical research letters. “When we then went back the following year and explored a moulin, it was huge. It was a case where the model predicted and we went out into the field and it turned out to be right. ”
The team made two trips to the Greenland ice sheet in October 2018 and October 2019. During each trip, they used ropes and other climbing equipment to rappel 100 meters in two separate mills that almost reached the water level.
“It’s scary,” said Covington, an experienced cave explorer. “You come back over the edge and you just see bluish ice going down as far as you can see, and then it’s blackness, and there are also occasional sounds of broken ice, which is pretty nervous.”
Researchers have long observed that Greenland’s ice sheet is moving and theorized that warmer summer melting seasons could accelerate this movement due to climate change. But scientists have little data to help them understand the interplay between meltwater and the bottom of the ice sheet. The team’s findings add to knowledge about how water interacts with the bottom of the ice sheet.
“We are trying to understand how the meltwater interacts with the ice movement, and the most important thing we found is that the water pressure in these mills is not as variable as it was previously observed, and that this seems to be a result of really large quantities in the mills, ”said Covington.
Reference: “Moulin Volumes Regulate Subglacial Water Pressure on the Greenland Ice Sheet” by MD Covington, JD Gulley, C. Trunz, J. Mejia and W. Gadd, October 9, 2020, Geophysical research letters.
DOI: 10.1029 / 2020GL088901