Our new discussion paper, led by Black and Bloom PDRA Andrew Tedstone, examines in detail why there is a stripe of dark, fast-melting ice on the Greenland Ice Sheet, particularly in the south-west. This ‘dark zone’ is clearly visible in satellite imagery of the Greenland Ice Sheet and is important because darker ice melts faster. It is crucial to understand what causes the ice to be dark there because if it grows or darkens in a warming climate then we can expect the deglaciation of Greenland to accelerate more than is currently predicted. There are two main competing hypotheses that could explain the presence of the dark zone: 1) dust melting out from ancient ice is darkening the ice; 2) algae are growing on the ice sheet and changing its colour.
The paper shows that the dark zone changes its shape, size and duration each year. This appears to be most strongly controlled by the sensible heat flux (air temperature) between June and August, number of days with air temperatures above zero, and timing of the snow-line retreat.
These findings provide some insights into which surface processes are most likely to explain the dynamics of the dark zone. The spatial distribution of the dark ice is best explained by the melting out of dust particles from ancient ice, although these particles are not dark enough to explain the colour change of the dark zone. However, these dusts may be crucial nutrients and substrates for ice algae, suggesting that the dusts control where the dark zone is, and the algae determine how dark it gets. Our other recent TCD paper showed how algae can darken ice and snow; however, there are also meteorological conditions required for algal growth including sufficient sunlight and liquid water. We suggest in the paper that the most likely hypothesis is that dust melts out from ancient ice and stimulates the growth of algae when meteorology allows it. Algae need the dust to grow, and the dust is not dark without the algae.
On Wednesday last week I traveled to the University of Bristol to give a seminar at the Centre for Glaciology. I presented a new physical model for the spectral albedo of ice with algal growth, along with some field data from 2016. Preparing for the talk, discussions with fellow researchers and insightful questions in the Q&A all reinforced some key issues that remain unresolved in bioalbedo studies – fundamental questions that have proven difficult to answer. First, do algae darken ice? Second, are they widespread enough to have ice sheet scale impact?
The answer to the first question is a clear yes. That dark materials contaminating an ice surface lower its albedo is not surprising. However, the crucial follow-up question is “by how much?” and this is much more challenging to answer; however, physical modelling provides a clear framework for determining the impact of an algal bloom on ice albedo. With sufficient information from empirical lab and field studies, we can quantify the bioalbedo effect and characterize its variability over space and time.
Standing in the so called ‘dark zone’ on the Greenland ice sheet, the answer to the second question also seems to be a clear ‘yes’. The ice surface is dark for as far as the eye can see in all directions, and wherever ice is sampled and examined under the microscope, it is found to be teeming with algal cells. However, what is visible from standing in the dark zone and what is important at the ice-sheet scale are two different things. To quantify algal coverage over the ice sheet we need to be able to detect blooms remotely, ideally from space using spectral data from satellites. This method of mapping is routine for terrestrial vegetation and algal blooms in the ocean; however, there are specific challenges to doing the same for algal blooms on ice.
The most common way to identify photosynthetic life in satellite reflectance data is to apply the ‘red-edge’ biomarker. This refers to a sharp rise in the reflectance spectrum of a surface due to vegetation because of efficient absorption by chlorophyll and very little absorption at near-infrared wavelengths (which has been suggested to be the result of evolutionary pressure to avoid overheating, or alternatively a side-effect of the evolution of cell-spacing in early aqueous plants). This has also been proposed as a spectral feature that could be used to map photosynthetic life on other planets. Amazingly, the red-edge has been detected in Earth-shine (light that has reflected multiple times between the Earth and moon and faintly illuminates the dark part of crescent moons), which provides a hemisphere-integrated reflectance signal for our planet. Since ice algae is photosynthetic, it follows that it could be mapped using the red-edge biomarker.
However, there are several issues that may complicate matters and increase the risk of a ‘false-positive’ result from applying the red-edge biomarker to Earth’s ice. These are
1. Carotenoids obscuring chlorophyll
Ice algae produce photoprotective carotenoid pigments that absorb over a wide range of visible wavelengths. They have a strong but broad absorption spectrum (which is why they protect the algae from ‘sunburn’). This could obscure the chlorophyll ‘bump’ near 500 nm and make interpretation of the red-edge more difficult. While the carotenoids themselves might provide a diagnostic reflectance spectrum, they too are hard to distinguish from other reflectance-reducers on ice.
Dust also absorbs strongly in visible wavelengths and also reflects effectively at red wavelengths, leading to a pseudo-red-edge feature in the reflectance spectrum. The precise shape of the reflectance spectrum varies for each mineral, and actually no mineral exactly replicates the vegetation red-edge signal. However, dust on ice is not composed of a single mineral, and both the dust and any biological impurities are mixed together and set in a complex ice matrix with its own reflectance spectra. It is feasible that the slope of the red-edge might be diagnostic of biological impurities, but this requires truly hyperspectral (i.e. spectral resolution of 1-2 nm) and will not be achievable using current satellite data. These issues combined lead to a high chance of a false positive result from the application of the red-edge biomarker to ice surfaces. This is especially important for explaining the ‘dark ice’ on the Greenland ice sheet since the two leading hypotheses are biological growth and outcropping dust.
3. Spatial integration reducing signal
An additional important issue is that any biomarker signal will be diluted by spatial integration over the viewing footprint of a satellite sensor. The presence of clean ice, ponded water, cryoconite, abiotic impurities or roughness elements will decrease the signal to noise ratio, probably further obscuring the red-edge signal.
These issues do not necessarily prohibit the use of the red-edge biomarker, but they do necessitate robust correction for abiotic impurities (particularly dusts) and rigorous ground truthing to validate the application of the biomarker to satellite data. There was a fascinating discussion in the planetary sciences in the early-mid twentieth century surrounding a reflectance signal detected on Mars which spread to cover wider areas each spring. This was proposed to be evidence of Martian plant life (e.g. Lowell, 1911); however, this hypothesis was discredited by further spectral analysis (Millman, 1939) and was then shown to be due to blowing dusts (Sagan and Pollack, 1969).
While physical modelling paired with ground reflectance measurements and sample analysis can answer the first fundamental question (do algae darken ice?), the second question (are they widespread enough to have an albedo-lowering effect at the ice sheet scale?) may prove challenging to answer robustly.
We are now well into planning 2017 field work so I revisited some archive footage from previous trips. The short clip below provides a good summary of the great diversity of microbial habitats that exist, even within a very small area of ice. These include cryoconite holes, a cryo-pond (the big cryoconite and water filled pool), algal blooms on the ice surface, dispersed cryoconite, streams, cryoconite ‘alluvium’ stranded on the stream banks, weathered ice and the snowpack. The clip also shows how hummocky and non-uniform the ice surface is near the margin of the ice sheet.
To get a better idea of how these habitats are arranged spatially we also flew a small UAV (unmanned aerial vehicle) with a downwards-looking HD camera. The clip below shows some of the footage. The winds were pretty strong and you can actually see the landing gear bow into shot every so often. We’ll have a more sophisticated UAV system in Greenland in 2017 that will collect images at specific wavelengths of light.
Finally, here is a short clip of the 2016 team at the S6 camp enjoying a beautiful full moon over the ice sheet. This site is well into the ‘dark zone’ where impurity loading is very high. We’ll be back there this summer to measure the effect of this on the reflectivity and therefore melt rate of the ice sheet.
Here is a brief field report from our 2016 field season which i also posted on the Arctic Club website (here).
2016 Greenland Field Work Report
Our field work aimed to deepen our understanding of the processes darkening the Greenland Ice Sheet. This is important because the colour of the ice sheet is one of the main drivers of its melt rate because it controls how much sunlight the ice sheet reflects or absorbs. The more sunlight absorbed, the more energy is available for melting ice.
In 2016 a team of researchers from Bristol, Sheffield, Leeds, Potsdam, Aberystwyth and NASA JPL camped on the ice sheet throughout the summer melt season in order to measure and monitor the changing colour of the ice and determine the causes of the darkening. The camp was inhabited in two month-long shifts. The first team comprised Joseph Cook (University of Sheffield), Chris Williamson (University of Bristol), Johan Nilsson (NASA JPL), Ewa Sypianska (Cardiff University), Tom Gribbin (Bristol University), Tris Irvine Fynn (Aberystwyth University) and Jim McQuaid (University of Leeds). Three weeks in, we were joined by Liane Benning, Steffi Lutz and Jenine McCutcheon (all University of Leeds). The team and all the camping and scientific kit was delivered in two flights on an Air Greenland Sikorsky S-61 helicopter.
The camp was built around two large Mountain Hardware “Space Station” tents, one of which was used as a mess tent (with a dining table and chairs, gas hob and food storage) and the other was a laboratory (kitted out with microscopes, spectrometers, filtration units, gas analysers, and all the usual lab consumables). The lab tent was also our power station, with the batteries, inverters and tracking system for our solar array. The long daylight hours and low temperatures helped the solar arrays to perform extremely well and we were able to charge all our scientific equipment, as well as laptops and satellite phones any time without issue. We were even able to run extension cables from the solar array to the mess tent to provide power across the camp! Around these two large tents were our own sleeping tents. Each person had a 3-man tent to provide room for bags and belongings.
A big problem is that the tents can melt the underlying ice, so we pitched on top of layers of white ‘polfelt’ and plyboard that both insulated the floor and provided a flat(ish) surface to walk on. However, this insulation also meant that after a few days the tents rested upon large ice pinnacles so needed to be repitched regularly!
For most of the season the weather was very friendly, with clear skies and very little precipitation – typical of summer on the SW Greenland ice Sheet. However, there was a significant rainfall event early on that washed away the crunchy, weathered ice layer and left a slick, slippery surface that was impossible to walk on without sharp crampons. It is also hard to dry out wet clothes and equipment in cold, overcast conditions. The rain also caused lots of glacier surface sediment (called ‘cryoconite’) to be washed onto the ice surface, instead of being held at the bottom of ‘cryoconite holes’. The combination of washed cryoconite and the loss of the crunchy, white ice made the surface noticeably darker.
We were particularly interested in the role of algae on the colour of the ice, and therefore our microbiology team was hard at work characterising the biology of the ice surface, including identifying the species present, their productivity, abundance and colouration. It seems that algae can bloom very densely and have a severe darkening effect on the ice surface. Coupled with this were detailed measurements of the reflectivity of the surface and the deposition of dark particulates from the atmosphere.
After the first month, the ‘in’ team decamped and was replaced by the project’s head-honcho Martyn Tranter, Alex Anesio, Alex Holland and Andrew Tedstone. Jenine also stayed out there with the second team. By the end of the season, the temperature had dropped significantly – large streams were freezing up completely every evening and remaining frozen until the middle of the day. What were almost 24 hour days at the start of the season became shorter and shorter and the team was treated to spectacular sunrises and sunsets over the ice sheet. In the far distance was a plume of water that, upon close inspection in the helicopter, turned out to be spray from a huge meltwater river crashing round a tight bend. Cryoconite holes grew, coalesced, divided and migrated around the camp.
The field season was successful in terms of the science and the team also reported feeling both awestruck at the scale of the ice sheet and simultaneously surprised by its sensitivity. The growth of microscopic algae and deposition of nanoscale particles of dust and soot influence the rate at which the vast ice sheet melts, and may therefore amplify climate changes and accelerate sea level rise. The challenge now is to quantify these processes and integrate them into future melt predictions.
It was Johan who first noticed, during a patrol of our Ice Surface Observatory, a tall jet of water bursting from the ice surface several kilometers to the West, punctuating the otherwise flat horizon. Out first thought was that this could be spray from water gushing into a huge moulin.
We then noticed this spray kicked off in the afternoon every day, shortly after the day’s peak melt, supporting our moulin spray hypothesis. As it turned out, the helicopter pilot flying our first ‘dash’ was as interested as we were and agreed to an impromptu flyover of the site. This solved the mystery – there was indeed a huge moulin at the site, but this was not the source of the jet – it was spray from a huge volume of fast-flowing meltwater cascading down a step and into a sharp bend in a supraglacial stream.
I was lucky enough to capture this film of the flyover…
Elizabeth described how cryoconite granules form when mineral particles and other debris are “ensnared… in the sticky arms of cyanobacteria” on ice surfaces, having spoken to Prof. Nozomu Takeuchi. I spoke to Elizabeth about the accelerated melting of ice beneath patches of these granules to form cryoconite holes. Krzyztof Zawierucha provided information about the microbes that inhabit the cryoconite holes, including cyanobacteria, heterotrophic bacteria, algae, fungi, protozoans and several invertebrates.
The article then discussed the ‘biocryomorphology‘ of cryoconite, focussing upon the remarkable process of ice-sculpting to maintain comfortable conditions for microbial activity on the hole floor. Potential impacts of cryoconite as amplifiers of the ice-albedo feedback was then examined, including comments from Andy Hodson (Sheffield).
The article is recommended to anyone looking for a popular science ‘quick-read’ introduction to cryoconite – Elizabeth has presented the basics and some of the complex biotic-abiotic feedbacks in a very accessible and engaging way.
The article is available to view here or in print in June 2016 issue of Discover Magazine.