Challenges in quantifying ‘bioalbedo’

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.

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A field camp in the ‘dark zone’ on the Greenland ice sheet, where the surface is darkened by expansive, dense algal blooms along with other impurities.

 

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.

rededge
The ‘red-edge’ in the reflectance spectrum for green vegetation. This diagram is from Seager and Ford (2002)

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.

2. Dust

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.

Refs:

Arnold, (2008) https://link.springer.com/article/10.1007%2Fs11214-007-9281-4

 

Lowell, P. (1911) The cartouches of the canals of Mars. Lowell Obs. Bull. 1(12), 59–86.

Millman, P.M. (1939) Is there vegetation on Mars? Sky 3, 10–11.

Sagan, C. and Pollack, J.B. (1969) Windblown dust on Mars. Nature 223, 791–794.

Seager and Ford (2002): https://arxiv.org/abs/astro-ph/0212550

Seager et al (2005) https://www.cfa.harvard.edu/~kchance/EPS238-2012/refdata/Seager-red-edge-2005.pdf

Iceland Fieldwork 2016

In April this year I had the pleasure or working with a group from Manchester Met and Derby Universities in Iceland. There was opportunity for some useful skills-swapping: I provided some insights into albedo measurements on the ice surface and the MMU team gave me a tour of the mysterious basal ice.

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One of the field sites: Svinafellsjokul
Kvíárjökull glacier research
The field team: L-R David Elliott, Robin Sen, Simon Cook, Joseph Cook, Mario Toubes Rodrigo (ph. David Elliott)
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An exposed englacial melt channel

The trip was focussed upon the team’s regular field sites – standard practice for them but refreshing new territory for me – especially since the focus was on ice that had been pushed up from the base of the glaciers (‘basal ice’).

Ice inspection
Robin explaining the properties of basal ice – with hand specimens! (ph. David Elliott)

There were several very rewarding outcomes of the trip: first, I got to see a new research group at work and observe their approach to glacier microbiological studies. Second, I had the opportunity to chat to the team at length about their experimental design – hopefully I was able to make some positive contributions as well as learning about their science. Third, I got to visit some wonderful new sites and learn about subglacial processes, including the microbiology of several basal ice ‘facies’. Finally, I got to talk to the team about some of the processes operating on the ice surface and introduce them to spectral reflectance measurements – great training for me prior to deploying these methods in Greenland later in summer.

Glacier spectroscopy
Making some spectral reflectance measurements on the ice surface (ph. David Elliott)

The team’s research is fascinating and I’m really looking forward to seeing the data and working with them more as the project develops. Mario, the team’s PhD student has been hard at work generating big datasets that should shed some light onto the dark underside of these Icelandic glaciers.

Jökulsárlón
Atmospheric shot of the famous iceberg lagoon at Jokulsaron (ph. David Elliott)

Supraglacial Biogeochemistry Chapter

I recently wrote a chapter on the application of biogeochemical measurement techniques to glacier surfaces which will hopefully be of interest to students and fellow early career researchers, especially now during MSc project proposal time!

Link here

Arwyn Edwards and I applying some of the techniques described in the chapter in Greenland in 2014.
Arwyn Edwards and I applying some of the techniques described in the chapter in Greenland in 2014.

This chapter contributes to the British Society for Geomorphology’s “Geomorphological Techniques” textbook which is rapidly growing and is a fantastic resource for field scientists. Browse here.

Discover Magazine: Ecosystem Engineers on ice

In June’s Discover Magazine, science writer Elizabeth Preston explored the mysterious world of icy microbes, focussing on cryoconite. I was lucky enough to chat to Elizabeth several times and provide some photos for the article.

Cover of June's Discover Magazine
Cover of June’s Discover Magazine

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.

Cryoconite researchers on the Greenland Ice Sheet
Cryoconite researchers on the Greenland Ice Sheet

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).

Tris Irvine-Fynn and I studying cryoconite on the Greenland Ice Sheet (ph. A Edwards)
Tris Irvine-Fynn and I studying cryoconite on the Greenland Ice Sheet (ph. A Edwards)

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.

Biocryomorphic evolution on the Greenland Ice Sheet

Our new paper, “Metabolome induced biocryomorphic evolution promotes carbon fixation in Greenlandic cryoconite holes” came out this week. The main finding is that cryoconite holes can change their shape in three dimensions to maintain comfortable conditions for microbial life – an example of biocryomorphology in action. Here’s a summary of the main points:

  1. Cryoconite holes change their shape and size according to environmental conditions. A mechanism for this, driven by nonuniform arrangement of cryoconite granules or receipt of solar radiation, is presented.
  2. Changes in hole shape are accompanied by changes in metabolic processes in microbial communities on the hole floors
  3. Cryoconite systems tend to evolve towards wide, flat floored shapes where cryoconite granules are spread out and able to photosynthesise more. This means cryoconite holes naturally maintain conditions conducive to capturing carbon.
  4. When these equilibrium states are disturbed, the microbes become stressed, send molecular signals to each other and quickly employ metabolic survival strategies.
  5. A possible mechanism for the migration of cryoconite holes away from shade implies biocryomorphic regulation of hole floor conditions for populations of holes.
IMGP1914
Making cryoconite hole measurements with co-author Tris Irvine-Fynn (ph. A Edwards)

This paper indicates the potential for combining ice physical, biogeochemical and molecular (in this case metabolomic) analyses in gaining a mechanistic understanding of Earth’s ice as a ‘living landscape’. Another recent paper by Bagshaw et al (Cardiff Cold Climate) examining cryoconite responses to light stress at the other end of the planet is available here.

Video: What Lives on Ice and Snow?

Here’s a link to Krzysztof Zawierucha’s (Adam Mickiewicz University, Poznan) new film about the biology of ice and snow, including cryoconite holes, snow and ice surfaces. The film clearly shows why Earth’s ice and snow represents a threatened store of biodiversity and illustrates some of the feedbacks between physical and biological processes operating in the cryosphere. See also Krzysztof’s paper in Journal of Zoology for more information about what can live in cryoconite holes specifically (and for more information on cryoconite holes, our cryoconite review).

GRIS 15 Diary: Part 4

Thanks to British Society for Geomorphology, Gino Watkins Memorial Fund, Gilchrist Educational Trust, Mount Everest Foundation, Andrew Croft Memorial Fund, Scottish Arctic Club and Gradconsult for supporting this field work. Thanks also to the GRIS15 field team: Ottavia Cavalli, Michael Sweet and Arwyn Edwards.

The team working at the field site, ca. 3 km from the margin of the SW Greenland Ice Sheet, captured using the DJI Phantom Vision 2 + drone.

July 13th

Despite the mosquitoes, Greenland is a beautiful place. The rocks glisten with flecks of pyrite, the lake waters are beautifully clear, the rivers are turbid with glacial flour and as the season progresses the green land is becoming freckled with blooms of cotton flowers. We have seen Arctic foxes and reindeer. The ice is spectacular, changing colour throughout each day as the melt rate waxes and wanes. Melt pools grow and shrink, cryoconite holes deepen and shallow, supraglacial streams swell and shrink and migrate across the ice surface. The colours are whites and blues to greys and greens. It is a magical, beautiful place and we are very lucky to be working here. Today’s field work went well. We are ahead of schedule on our science goals and the data is looking good. No sign of the weather changing at the moment either, so we are putting our heads together to come up with more ideas to extend the science programme and make the very most of our time here. When we arrived back at camp to find fellow glacier researchers Marek Stibal, Karen Cameron, Jakub Zarsky and Tyler Kohler (collectively @CryoEco) at camp. It was good to catch up and find out a little about their field season over at Leverett Glacier.

 

A Greenland lake in summer bloom (ph. M Sweet)
A Greenland lake in summer bloom (ph. M Sweet)
Cotton grass near the ice margin (ph. M Sweet)
Cotton grass near the ice margin (ph. M Sweet)
Midnight sun over a glacier-fed river
Midnight sun over a glacier-fed river

July 14th

Today was another productive day in relatively good weather. Another solid day’s worth of data was recorded by all members of the team. Everything ran pretty much according to plan. I had a look over the data so far and am hopeful of some good results, but it will require some deep analysis once back in the UK. I have been sleeping badly since we got here, largely due to the midnight sun and tonight was especially bad. I walked down to the river and read my book in the early hours and it felt like midday.

Cavalli, Edwards and Cook en route to the site (ph. M Sweet)
Cavalli, Edwards and Cook en route to the site (ph. M Sweet)
Observing a cryopond at the site
Observing a cryopond at the site

July 15th

My initial science objectives were met today! The weather has been extremely kind to us thus far and our productivity has been higher than expected. I plan to continue to make further measurements and expand the dataset, whilst also establishing some associated extension experiments. Today was hard going though. The katabatic winds were right back up to full strength and it was bitterly cold at the site, especially once my hands had been in a few cryoconite holes! We are all starting to feel tired after a long stretch of continuous field work, but the end of the first observation period is in sight and everyone’s primary science objectives should be in the bag in the next couple of days.

Otti at work (ph. A Edwards)
Otti at work (ph. A Edwards)

July 16th

Another hard day weather-wise. It is really the wind that makes things difficult and slows us down. It’s also hard work to stabilise the drone in the wind, and I doubt we will have much useful imagery from these very windy days. Thankfully, there have been enough calm days to ensure sufficient data capture, and more importantly, we haven’t lost or broken the drone! Again, I decanted samples into falcon tubes to process back at camp, and the mosquitoes made it very unpleasant. Still, it got done and as a team our minimum science aims have now been met. This is quite a weight off our minds, since data collected from here on in is largely bonus and if the weather or logistics turned against us from tomorrow onwards, we can still be assured of returning home with some science achievements and data to work up in the autumn.

Otti and Arwyn doing lunch
Otti and Arwyn doing lunch
Mile and I: drone selfie
Mile and I: drone selfie

July 17th

We finally took a bit of a rest day today, and gained a new recruit to our camp. Leo Nathan is an MSc student at Aberystwyth University who is working under the supervision of Prof. Alun Hubbard. Leo is flying fixed-wing UAVs over long transects to generate Digital Elevation Model data of several of the rapidly melting glaciers in this region. We visited his original camp, up near Point 660, where he has been building and launching the drones. It was all very impressive stuff, and Leo was very knowledgeable and happy to talk about the project, and made a welcome addition to the team.

Field site panorama (M Sweet)
Field site panorama (M Sweet)
The team by one of many dramatic and picturesque cryoponds (M Sweet)
The team by one of many dramatic and picturesque cryoponds (M Sweet)

July 18th

Today was a final day of measurements at the original field site and was relatively routine. Leo cooked dinner tonight and it was a damn fine spaghetti bolognese (although our resident Italian may disagree)!

Dr Edwards in a bag
Dr Edwards in a bag

July 19th

Today we pulled our equipment out ready to change field site. This meant dismantling the loggers we had set up, collecting in pieces of equipment and markers, and generally leaving the place as pristine as we found it. This took the morning and we were off the ice just after 1pm. We had some lunch and then went to another nearby glacier to scope out possible access points for obtaining some basal ice samples. On the way we encountered a family of six musk oxen, including two very small calves. Mike and I took a walk over to another nearby glacier and watched the calving ice for a while before dinner. The sun is starting to get lower in the sky at night now, and this evening was especially beautiful down at the river. I sat there and read until it was late and eventually too cold to be out of a tent.

One of the bigger musk ox (M Sweet)
One of the bigger musk ox (M Sweet)
Musk ox family (M Sweet)
Musk ox family (M Sweet)

July 20th

Today was Otti’s final day in Greenland with us. To make it a good one, we took a trip to Russell Glacier, where we watched the glacier calve. This site has changed dramatically since my last visit in 2014, having undergone some major calving and slumping. If there is some out there, I’d love to see some time lapse imagery of this piece of ice. We had some lunch and did some reconnaissance for a future research idea before walking out. Back at camp, we had a good sort out of our field kit, rearranged the tents and packed up gear that Otti would take back to the UK. We also organised the equipment that the remaining team members would need for the rest of the trip and nailed down some further research plans for the final leg of the trip. I stupidly fell asleep out in the open and woke up having been feasted upon by mosquitoes – my face looks like a sheet of bubble wrap!

The calving face of Rusell Glacier
The calving face of Rusell Glacier
The ice cave and calving face of Russell Glacier
The ice cave and calving face of Russell Glacier

July 21st

Today was not a good day. We awoke as usual and ate breakfast, then piled into the truck to drive Otti to town in time for her flight. About 12 km from Kangerlussuaq we were involved in a collision with another vehicle and had to evacuate to KISS. Thankfully nobody was hurt, but there was damage to both vehicles. A police report was filed and the rest of the day was spent trying to contact relevant insurance agencies and our university contacts.

July 22nd:

Today we necessarily stayed in KISS to try to sort out the vehicle issues. While we wait, the last of pour funds are evaporating in accommodation costs, plus food etc and we are without a vehicle to get to a field site to extend our science! We also have the additional problem that our camp is still established at the ice margin… Late in the evening two cancellations were made for tomorrow’s flight out of Kangerlussuaq, so Arwyn and I snapped them up. With Otti already home safe and sound, and Mike’s flights only two days away anyway, this was seen as the most prudent damage limitation option. An extremely kind offer of a lift out to decamp by a University of Essex research group meant we could quickly get our kit packed up in time to bail tomorrow.

July 23rd:

It is with heavy heart and light wallet that we leave Greenland today. However, we managed to achieve our primary science aims before disaster struck, and everyone is leaving injury free. So overall, although we are a few days early retreating from Greenland, we have the data we need to produce our manuscripts as planned and have loads of images and footage for outreach and analysis. We have met some great folks and seen an incredible part of the planet, and should produce some good publications as a result. However, two secondary objectives that were scheduled for the last few days were not met: depth sampling in a crevasse and bulk sampling of cryoconite. Things could have been a whole lot worse and we are now looking forward to getting stuck into analysing and writing up our findings!

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Thank you’s:

Another huge thank you to our funders  British Society for Geomorphology, Gino Watkins Memorial Fund, Gilchrist Educational Trust, Mount Everest Foundation, Andrew Croft Memorial Fund, Scottish Arctic Club and Gradconsult for supporting this field work.

I also thank Professor Alun Hubbard, Leo Nathan, Johnny Ryan and the team from the University of Essex for their company and/or collaboration.

Finally, my thanks go out to the GRIS15 team: Ottavia Cavalli, Michael Sweet and Arwyn Edwards.