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 Snow Tourist by Charlie English | Book Review

I didn’t know what to expect when I picked up a popular science book about snow, especially when its tagline is ‘A search for the world’s purest, deepest snowfall’. I thought The Snow Tourist by Charlie English, a book exploring the places in the world with the most snowfall, could be quite intriguing. On reading it, I feel like this book was mis-advertised though. It wasn’t intriguing… it was much better than that. Charlie English effectively conveys an intense curiosity and obsession with snow that goes much farther than merely looking for the deepest snow. Snow is explored in the context of art, folklore, science and as a way of life. This book far surpassed my expectations. I still learnt about so many fascinating things even though my career is in the study of ice and snow. Literally anyone can pick up this book and find out something interesting about snow.

The Snow Tourist by Charlie English book cover (source: Book Depository)The Snow Tourist begins with author Charlie English declaring his ambitious adventure – to travel to some of the coldest places and see the best snow over the world. I felt uncertain about this endeavour at first – what constitutes the ‘best’ snow? how will the author steer away from being dry and repetitive when describing each region? is there enough to explore and discuss in relation to snow?

Over the course of each chapter, a new theme relating to snow is introduced with each region that he is exploring. He examines the portrayal of snow in art whilst he is in Vienna, for example. Stories of catastrophic avalanches and fatal snow storms are illustrated in East Sussex and New York (yes, there was actually an avalanche in East Sussex in 1836). The pioneers of skiing are reviewed in the Alps.

The tagline of the book (‘A search for the world’s purest, deepest snowfall’) somewhat misrepresents what English’s adventure amounts to. He’s not merely looking for the ‘best’ snow, instead examining snow from a variety of different perspectives that range from scientific to cultural to historic to artist and beyond. It is only in the Chapter 9 (whilst visiting Rainer, Seattle and Glacier) that English addresses the search for the deepest, ‘best’ snow. There are so many interesting aspects of snow to read about before and after that. Here are a couple of things I found fascinating to read about:

  1. Mary Shelley’s Frankenstein was partly inspired by her travels to Chamonix when she was just 19 years old.
  2. The first ski race across Greenland (a 220km race from coast to coast) was in 1884 (see image and caption for more info).
  3. Yuki-onna is the Japanese snow spirit that allegedly appears during heavy blizzards as a young woman with pale skin and dressed in a pure white kimono. She sometimes appears with a baby in her arms, which she asks travellers to hold for her. On taking the baby, they find that it is actually a lump of hard ice that freezes them to death.
  4. Monet was invited to Norway, where he painted many snowy landscapes. He would often sit outside and paint in temperatures as low as -30 degrees (Celsius) with icicles hanging off his beard.
  5. Given a microscope and a camera in the 1880s, a young Wilson Bentley spent most of his life photographing snowflakes up close. Whilst his family and neighbours thought he was mad, Bentley sparked massive interest among the scientific community and was generally regarded as the most dedicated observer of snow in history (see here for more on the life and works of Wilson Bentley)

To round off the book, English includes a Snow Handbook that summarises a lot of his interests surrounding snow that are discussed previously and also includes more detail such as how to build an igloo, a list of fiction books that include snow, and the legend of the abominable snowman.

Nordenskiöld's second Greenland exhibition in 1883 (source: Redbull Nordenskiöldsloppet)

A picture from Adolf Erik Nordenskiöld’s second Greenland expedition in 1883. During this trip, two of the expedition members explored the ice sheet’s inner regions which were unknown at the time. On returning, no one believed that they had skied the 460-kilometre round trip though. To prove that is was possible, Nordenskiöld announced a 24-hour ski race across the interior of Greenland in the following year. On 3rd April 1884, 18 skiers took part in the race and the winner crossed the finish line in 21 hours and 22 minutes, restoring Nordenskiöld’s good reputation. This race is still held today, aptly named Nordenskiöldsloppet. The total distance of the race (220 kilometres) makes it the world’s longest cross-country ski competition (source: Redbull Nordenskiöldsloppet).

Also in this is ten facts about glaciers. Now, this might be my only critique of the book – it needed more focus on glaciers. I understand that this is the bias opinion of a glaciologist, but hear me out. Glaciers and ice are an essential aspect when writing a book that is broadly exploring snow – how snow transforms into ice, how ice moves, where snow/ice is stored in the world etc. etc. By including glaciers, English could have visited many other places (such as Antarctica, Greenland, Svalbard or Iceland) to explain the scientific and cultural aspects of glaciology. Ten facts about glaciers barely scratches the surface. And in some instances, the facts are poorly explained, such as fact #9: ‘The fastest glacier whose speed has been clocked dashed 12 kilometres in three months’ – which glacier was this? when was this? how was this speed measured?

Basically, I was mesmerised by this book. English’s curiosity with snow is evident on every page. There are facts from this book that I will remember for a long time. I have already shared a lot of them with friends, beginning with ‘Fun fact! Did you know…’ (to any friend who has been on the receiving end of this, I’m sorry if you are sick of my ramblings by now). My only criticism is that I wanted more. English made snow so exciting and interesting that I wish he would do the same for glaciers, and I have no doubt that he could.