Using meltwater plumes to infer subglacial hydrology at tidewater glaciers

PhD update: January 2017. Meltwater plumes are the upwelling of fresh water in front of a tidewater glacier. These are known to influence submarine melt rates, which are suggested to have a significant impact on the calving rate of glaciers that terminate in sea water. Recent work has suggested that meltwater plumes can also be used to infer the subglacial hydrology at the front of a glacier.

At land-terminating glaciers, water is evacuated via flow outlets which form large rivers on the adjacent land. It is therefore relatively straightforward to measure the amount of water leaving the glacial system. Things are a bit more complicated at glaciers which terminate in water (i.e. a fjord, sea, or ocean). Fresh water exits from the glacier at depth and interacts with the salty seawater. The fresh water moves upwards due to the density difference between freshwater and saltwater, forming a turbulent column of mixing water. This is a meltwater plume (and can also be referred to as a ‘submarine plume’, or simply just a ‘plume’).

An example of a meltwater plume at Tunabreen, a tidewater glacier in Svalbard

An example of a surfacing meltwater plume at Tunabreen, a tidewater glacier in Svalbard. Note the distinctive shape and the dark colour (indicating sediment content) of the surface expression.

The freshwater in a meltwater plume will continue to flow up through the water column and entrain surrounding saltwater until it is thoroughly mixed (i.e. there is no difference in the density between the plume and the surrounding water). At this point, a meltwater plume will reach its neutral buoyancy and the water will cease flowing upwards and flow horizontally away from the glacier front.

A meltwater plume can reach the sea surface if the neutral buoyancy exceeds the depth of the fjord. The surface expression of a meltwater plume is normally very distinctive, distinguished by its sediment-laden colour and turbulent flow away from the glacier. We have lovely images of meltwater plume activity at Tunabreen, a tidewater glacier in Svalbard, showing a surfacing plume which has entrained very rich red/brown sediment.

The neutral buoyancy point of a meltwater plume is influenced by a number of factors:

  1. The temperature/density difference between the freshwater in the plume and the surrounding saltwater
  2. The geometry of the fjord, such as how deep it is
  3. The stratification of the surrounding saltwater
  4. The rate at which meltwater is exiting the glacier (also referred to as discharge)

The first three of these listed influences undergo relatively little change compared to discharge over short time-scales (e.g. a summer season). Assuming this, the activity of a meltwater plume can be used as a signal for the rate at which meltwater is exiting a glacier over the course of a melt season.

Meltwater typically exits into a fjord/sea/ocean at the bed of a glacier. The meltwater can either be directed through a given number of big channels or a series of intricate, small cavities. Channels can typically accommodate large volumes of meltwater, hence they are known as an efficient drainage system. Linked cavities are not as effective at transporting meltwater and tend to hold water at the bed for much longer durations, so they are aptly referred to as an inefficient drainage system.

Kronebreen (centre) viewed from the west. Kronebreen shares its southern (right) margin with Kongsvegen, a slow-moving surge-type glacier that has been fairly inactive for the past couple of years. The glacier adjacent to Kronebreen, separated by the mountain Collethøgda (left), is called Kongsbreen. Kongsbreen has been retreating from the fjord onto land since approximately 2014 (September 2016)

A meltwater plume at the front of Kronebreen, a fast-flowing tidewater glacier in Svalbard. The surfacing plume  is situated on the north side of the plume (left side of the terminus in this image). This plume entrains sediment which gives it a red/brown colour. A plume also surfaced intermittently on the south side of the terminus during the melt season of 2014 (not pictured here). Photo taken: September 2016.

Timeline of surfacing plume activity at Kronebreen, Svalbard, monitored from time-lapse imagery. Plumes P1, P2 and P3 were present at the north side of the terminus, with P1 being active for the entire monitoring period (gaps are where there was no visibility in the images). Plume P4 surfaced at the south side of the terminus, showing intermittent activity throughout the melt season.

Timeline of surfacing plume activity at Kronebreen, Svalbard, monitored from time-lapse imagery. Activity began on the 23 June and continued through till the end of September. Plumes P1, P2 and P3 were present at the north side of the terminus. P1 (pictured in the above image) was active for the entire monitoring period (gaps are where there was no visibility in the time-lapse imagery). Plume P4 surfaced at the south side of the terminus, showing intermittent activity throughout the melt season.

An efficient drainage system can quickly channel a large volume of meltwater into the adjacent sea water. It is therefore likely that the neutral buoyancy of a meltwater plume from an efficient drainage system can exceed the depth of the fjord, so the plume will surface and will be visible. An inefficient drainage system is much more limited in the rate at which it can deliver meltwater into the adjacent sea water. It is therefore likely that the neutral buoyancy of a meltwater plume from an inefficient drainage system will be at depth, so the plume will not surface and will not be visible. We can thus infer what type of drainage system is present at the front of a glacier by monitoring meltwater plume activity over short durations.

We have been monitoring meltwater plume activity at the front of Kronebreen, a fast-flowing tidewater glacier in Svalbard. Two sets of plumes were present over the 2014 melt season, on the north and south side of the terminus. It is assumed here that a meltwater plume is likely to surface in the fjord if a channel is active based on the known fjord depth (∼80 m) and modelled runoff outputs. The set of plumes on the north side of the terminus persistently surfaced throughout the melt season, whereas the plume on the south side only surfaced intermittently.

A plume may not be able to consistently surface because meltwater is not leaving the glacier through a stable efficient drainage system. This could suggest that two different drainage systems preside at the north and south side of the glacier – a stable efficient drainage system on the north side, and an unstable system that switches between efficient and inefficient drainage on the south side.

Velocity map of Kronebreen over an 11-day period in April 2014 (Luckman et al., 2015)

A velocity map of Kronebreen over an 11-day period in April 2014 (Luckman et al., 2015). These velocities are derived from feature tracking between image pairs, and these images are TerraSAR-X satellite images. Higher surface velocities are present at the central/south side of the terminus compared to the north side. This is possibly related to a difference in subglacial drainage beneath these two regions. Source: UNIS.

In this situation, you would expect to see other differences between the north and south side of the terminus such as surface velocity. A large amount of subglacial meltwater is in contact with the bed in an inefficient drainage system, which enhances lubrication at the bed and promotes ice sliding. In an efficient drainage system, the water is channelled through a discrete area of the glacier and thus there is less basal lubrication as a smaller amount is in contact with the bed.

Surface velocities over the 2014 melt season show a distinct difference between the north and south side of the glacier terminus – the south is much faster flowing than the north, with the south exceeding velocities of 4 metres per day whilst the north remains relatively slow (see an example velocity map above). It is likely that a difference in drainage efficiency could facilitate this difference in surface velocities. The presence of an inefficient drainage system at the south side of the glacier tongue may be promoting faster velocities.

This idea is being further explored with additional datasets to better understand glacier hydrology and dynamics. The main take-home message from this post is that meltwater plume activity could be a reliable signal for meltwater outflow. This activity can be effectively monitored using time-lapse photography. Observations of plume activity can help us to diagnose the nature of subglacial drainage beneath tidewater glaciers, which is not accessible for direct measurements at this time. Kronebreen appears to have two different drainage systems active near the glacier terminus, as reflected in the differing plume activity, and this could be facilitating fast velocities in discrete areas of the glacier.


Further reading

Slater et al., 2017 – A newly-published study looked at meltwater plume activity at Kangiata Nunata Sermia (KNS) in Southwest Greenland using an in-situ time-lapse camera. They predicted from model simulations that a meltwater plume from a single channel should be able to surface in the adjacent fjord water, knowing the rate of discharge through the drainage system. However, the time-lapse imagery showed that the meltwater plume was only visible for brief periods throughout a melt season (May to September 2009). They argued that a plume was not consistently surfacing because meltwater may not leaving the glacier through a stable efficient drainage system. An efficient drainage system may not be able to persist at the front of KNS because it could be repeatedly disrupted by basal deformation, which is facilitated by the fast-flowing nature of the glacier. This paper has been neatly summarised by ice2ice.

Time-lapse sequences from Kronebreen. Note the visible plume activity seen from cameras 1 and 2 through the melt season.

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The Expedition: Solving the Mystery of a Polar Tragedy by Bea Uusma | Book Review

Normally the last thing I want to do after a hard day’s work focused on a polar subject is… read yet more polar-related stuff. There are multiple books gathering dust on my bookshelves that come under this category – Antarctica, polar explorers, Arctic history and folklore, popular science – lovingly given to me by friends and family who presumed that my interest was unrelenting. A small challenge I have now set myself is to read these, and this was encouraged by one book, which I picked up on a whim from the museum in Svalbard because I had nothing else to read at the time.

The Expedition: Solving the Mystery of a Polar Tragedy by Bea Uusma tells the true story of three Swedish men who vanished in 1897 whilst attempting to be the first to cross the North Pole in a hot air balloon. Thirty three years later, their bodies were stumbled upon at the shores of the harrowing White Island, which lies to the north-east of Svalbard. There was no concluding evidence to suggest how they had ended up there and why they died – they were found with ample provisions, warm clothing, functioning weapons and plenty of ammunition. Many speculated on the cause of death, from eating polar bear liver (known to be toxic) to carbon monoxide poisoning to suicide. But there has never been consensus on which theory is more likely, with little supporting evidence for each one.

The crashed balloon in 1897 (source: Wikipedia)

The crashed balloon in 1897, with expedition members Salomon August Andrée and Knut Frænkel. Photographed by the third expedition member, Nils Strindberg. Photography from the expedition was recovered from White Island in 1930 by a Norwegian group who stumbled across remains whilst on an expedition studying the glaciers of the Svalbard archipelago (source: Wikipedia).

Bea Uusma’s first encounter with the story occurred in the nineties when she began reading a book at a boring party about the subject. The author’s obsession grew from this as she spent fifteen years trawling through museums to find missing clues, compiling past theories, visiting White Island… and eventually providing her own theory with convincing evidence.

The narrative jumps from details of the expedition to Uusma’s own experiences trawling through museum after museum for evidence, and documenting her several attempts to visit White Island. The level of her obsession is obvious, with documented diary entries and thorough record of their diets in the lead up to their deaths. This is nicely broken up with images, tables, maps and diagrams. It kept me captivated and I was continually wondering whether she could conclude what really did happen rather than just add another theory to the mix. The ending conclusion is worth sticking around for. In my opinion, her evidence is very conclusive and a much stronger argument than any of the other theories out there. I’m bursting to write about it but I don’t want to spoil it for anyone else!

Frænkel (left) and Strindberg (right) with their first polar bear kill (source: Wikipedia)

Frænkel (left) and Strindberg (right) with their first polar bear kill. The three expedition members encountered many polar bears on the pack ice and on the shores of White Island. They lived off a diet of polar bear and seal for the majority of their time after the crash, which first began speculation that they died of dietary-related issues such as Trichinosis (a parasite found in undercooked meat), Vitamin A poisoning (from eating polar bear liver), lead poisoning (from their canned food), scurvy and diarrhoea (source: Wikipedia)

Overall, this book has a great balance of detailed documentation and the author’s personal exploration. Uusma gracefully navigates the trap of producing a very dry record of events. As I said, this book has really sparked an interest in reading more about early polar exploration, which I am quite embarrassed to say that I know little about (I hang my glaciology head in shame). Hopefully whatever I read next will be as good as this!

Key time-lapse studies into glacier dynamics

Camera sites 8a and 8b at Kronebreen, Svalbard (May 2015)

Two of our stereo time-lapse cameras (cameras 8a and 8b) at Kronebreen glacier (Svalbard), which were installed as part of the CRIOS (Calving Rates and Impact On Sea level) project. These cameras were focused on the front of the calving front to look at surface velocities at the glacier terminus in comparison to calving rate (May 2015)

I have been nearing the end of writing the first chapter of my thesis which is an overview of photogrammetry techniques in glaciology with particular focus of time-lapse photogrammetry. Whilst writing this chapter, I have had to review all previous studies which use time-lapse photography. I found that still a large proportion of studies are developing photogrammetry techniques, and there are only a few studies which actually use the techniques to answer the big questions in glaciology. I thought I would share a list of the key papers that use time-lapse photogrammetry to examine different aspects of glacier dynamics… (if there are studies you think I have missed off this list then please contact me, I’m always looking out for more!)

  1. Dietrich et al. (2007): Examined links between vertical displacement and tidal levels at Jabobshavn Isbræ (Greenland) to determine how much of the glacier tongue was free-floating.
  2. Ahn and Box (2010): Captured daily images of several glaciers in Greenland (Rink Isbræ, Store Gletscher, Umiamako, Jakobshavn Isbræ) to examine links between surface velocity and calving rate. Concluded that velocity speed-ups were caused by large calving events e.g. calving event at Umiamako caused speed-up (17% acceleration) over the subsequent six days.
  3. Kristensen and Benn (2012): Captured daily images during the 2003-05 surge of Skobreen-Paulabreen. Observed intensely crevassed ice at the front and lateral margins during the surge, but little crevassing behind the front. Concluded that the surge was almost entirely driven via basal motion (sliding/deformation), facilitated by trapped pressurised water at the bed.
  4. Danielson and Sharp (2013): Monitored water levels in supraglacial lakes on Belcher Glacier (Canada) using hourly time-lapse images. Linked drainage events in these lakes to four glacier acceleration events (determined using GPS). The time-lapse imagery was also used to determine lake drainage typology, classed by the lake constraints (crevasse or surface topography), connection to the basal hydrology system and the speed of lake drainage.
  5. Rosenau et al. (2013): Determined changes in the grounding line of Jakobshavn Isbræ (Greenland) over time, using vertical displacements (measured from time-lapse images acquired in 2004, 2007 and 2010) as a measure of flexure. They also examined the position of previous pinning points of the glacier front.
  6. James et al. (2014): This was the first study to use a high-frequency time-lapse sequence (one image every 10 secs) to observe a large calving event at Helheim glacier (Greenland) (see time-lapse video here). Vertical displacements at the glacier front showed that the rotation calving event was caused by differences in the fjord bed topography which promoted uplift at the north side of the glacier margin and a depression at the south part of the margin, creating an unbalanced buoyancy equilibrium.

From all of this, I have found that the future of terrestrial time-lapse photogrammetry is trending towards its valuable ability to examine different aspects of the glacier system simultaneously – glacier velocity, fjord dynamics, surface lake drainage, calving dynamics… and others. These can be studied using different image capture frequencies and over different lengths of time, and I think we will begin to see much more high-frequency time-lapse sequences based on the useful information gained from them so far.