Monthly Archives: March 2017

Draining the Ice-Sheet: Crevasses vs Moulins

Posted by William Colgan on March 22, 2017
Climate Change, New Research / No Comments

We have an open-access study that explores the various ways by which meltwater drains from the Greenland ice sheet in the current volume of Journal of Glaciology1. While surface meltwater rivers and lakes are a conspicuous feature of the Greenland ice sheet, virtually all meltwater that drains from the ice-sheet margin is flowing either within or beneath the ice sheet, rather than on its surface. Where and how meltwater reaches the ice-sheet bed can have implications on ice dynamics. For example, the sharp pulses of meltwater transmitted to the bed by near-vertical conduits called moulins are believed to cause more sliding at the ice-bed interface than the subdued pulses transmitted by crevasses.

In the study, we simulated a portion of the ice sheet in West Greenland known as Paakitsoq, using a computer model. The model used the measured ice sheet topography, as well as observed locations of crevasses and moulins, to simulate how meltwater was produced and flowed across the ice sheet surface under the climate conditions of the 2009 average intensity melt season and the 2012 extreme intensity melt season. The model could also simulate catastrophic lake drainage events, known as hydrofracture events, when a surface lake becomes sufficiently deep that its pressure fractures the underlying ice and creates a new moulin.

Figure 1 – Location of the Paakitsoq study area. Black outline denotes the model domain. Blue dots denote moulin locations identified in satellite imagery. Black contours denote surface elevations. Base image is Landsat-8 from 4 August 2014.

Our simulations suggested that, during an average intensity melt season, crevasses drain almost half (47 %) of ice-sheet meltwater. The hydrofracture of surface lakes drained about 24 % of ice-sheet meltwater, the majority of which resulted from drainage into new moulins following hydrofracture events, rather than the hydrofracture events themselves. Previously existing moulins drained an additional 15 % of meltwater. (The remaining meltwater either reached the ice-sheet margin, remained stored within the model area, or drained to ice-sheet areas North or South of the model area.)

While our simulations suggest that crevasses now drain more meltwater from the ice sheet at Paakitsoq (47 %) than previously existing and newly hydrofractured moulins together (39 %), our 2012 extreme intensity melt season simulation suggests that this ratio may change. The proportion of meltwater drainage via moulins increases, and the proportion of drainage via crevasses decreases, in the 2012 extreme intensity melt season simulation, which may be characteristic of a warmer future climate. This increase in both relative and absolute moulin drainage under warmer conditions is due to an increase in moulins created by hydrofracture events, as meltwater production moves to higher elevations.

Figure 2 – Partitioning surface meltwater drainage into eight categories under the average melt (2009; R1) and extreme melt (2012; R11) simulations. “Lake” is the volume remaining in lakes at season end. “Remaining Flow” is the volume in transit at season end. “Lateral Outflow” is the volume that drains through North and South model domain boundaries. “Ice Margin” is the volume that reaches the ice-sheet edge. The volumes captured in “Crevasses” and “Moulins” are denoted. The volumes drained by lake hydrofracture induced surface-to-bed connections are “Lake Hydrofracture Lake” (LHL), while the subsequent drainage into the new surface-to-bed connection is “Lake Hydrofracture Moulin” (LHM).

Overall, our study provided insight on the space-time variation of pathways by which the vast majority of ice-sheet meltwater descends to the ice-sheet bed prior to reaching the ice-sheet margin. A better understanding of how meltwater travels through the ice-sheet can help improve scientific understanding of not only the ice-sheet mass loss caused by runoff, but also the implications of increasing meltwater production on the mass loss caused ice dynamics. Our simulations also suggest there is value in ice-sheet wide mapping of surface hydrology features like crevasses, rivers, lakes, and moulins, as computer models can use this knowledge to improve simulations of meltwater routing.

1Koziol, C., N. Arnold, A. Pope and W. Colgan. 2017. Quantifying supraglacial meltwater pathways in the Paakitsoq region, West Greenland. Journal of Glaciology. 1-13. doi:10.1017/jog.2017.5

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Firn Permeability: New Use of an Old Technique

Posted by William Colgan on March 06, 2017
Communicating Science, New Research / No Comments

We have a new study out this month in Frontiers in Earth Science1 that describes using an old-school hydrogeology method on the Greenland Ice Sheet. We used pump-testing, which has been conventionally used to measure soil permeability for groundwater flow, to infer the permeability of ice-sheet firn to meltwater flow. We wanted to quantitatively measure how massive ice layers formed by refreezing meltwater in the near-surface ice sheet firn could inhibit meltwater flow in subsequent years.

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Figure 1 – The low tech and low cost pump-testing device used to infer firn permeability on the Greenland Ice Sheet. A vacuum is applied at depth in the sealed vacuum borehole and the resulting pressure response is measured in the sealed monitoring borehole.

In conventional pump-testing, water is pumped out of a borehole at a controlled rate, and the groundwater level response, or drawdown is observed in a monitoring borehole located some distance away. We did something similar in the ice-sheet firn, pumping air out of a vacuum borehole and measuring the air pressure response is a sealed monitoring borehole about one meter away. We did pump tests at six ice sheet sites that had varying degrees of massive ice layers in the near-surface firn.

We found that vertical permeability between firn layers was generally much lower than horizontal permeability within a firn layer, and that vertical permeability decreased with increasing ice content. At the lowest elevation site, where meltwater production and refreezing is most prevalent, we drilled into an exceptionally massive ice layer the pump borehole was able to maintain an effective vacuum. In other words, thick massive ice layers are indeed impermeable to fluids. That was a little surprising!

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Figure 2 – Inferred horizontal (kr) and vertical (kz) firn permeability values at five ice-sheet sites. Horizontal blue lines indicate the depths of ice layers at each site. Vertical cyan and magenta shading represents inferred permeability limits.

While it may sound esoteric, the permeability of near-surface firn is an increasingly visible topic in ice-sheet research. Studies have shown that firn can act to either buffer sea level rise by absorbing meltwater2, or enhance sea level rise by forming impermeable refrozen ice layers3. As climate change increases meltwater production within the historical accumulation zone of the ice sheet, a greater area of ice-sheet hydrology will be influenced by refrozen ice layers. In future, higher vacuum pressures and repeated measurements should allow firn permeability to be measured over larger scales to improve our understanding of changing firn permeability.

For now, the proof-of-concept pump-testing device is relatively low tech and low cost. Aside from air-pressure sensors and a data logger, it was constructed by items you could find at your local hardware store; plastic PVC pipes channeling the power of a shop vacuum. Development of the firn pump-testing device was initiated by a University of Colorado Dean’s Graduate Student Research Grant to highly innovative lead-author Aleah Sommers, and it was deployed in collaboration with the FirnCover project during the 2016 field campaign.

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Figure 3 – Max Stevens and Aleah Sommers preparing to insert the pressure sensor and seal into the monitoring borehole at Saddle, Greenland, in May 2016.

1Sommers et al. 2017. Inferring Firn Permeability from Pneumatic Testing: A Case Study on the Greenland Ice Sheet. Frontiers in Earth Science. 5: 20.

2Harper et al. 2012. Greenland ice-sheet contribution to sea-level rise buffered by meltwater storage in firn. Nature. 491: 240-243.

3Machguth et al. 2016. Greenland meltwater storage in firn limited by near-surface ice formation. Nature Climate Change. 6: 390-393.

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