Dr Jenine McCutcheon, a member of the Black and Bloom project, discusses algal growth, mineral dust and black carbon deposits on the Greenland Ice Sheet.
A postdoctoral research fellow on the NERC-funded Black and Bloom project, Dr Jenine McCutcheon operates from the School of Earth and Environment at the University of Leeds, UK. McCutcheon is also a member of the European Association of Geochemistry and the Geological Association of Canada, and is affiliated to Leeds University’s Earth Surface Science Institute (ESSI).
Held in Paris, France, the Goldschmidt2017 conference which ran from the 13-18 August featuring McCutcheon as a speaker on several topics including: ‘The Great Melting: The Unstoppable Contest between Snow Physics, Soot, Mineral Dust and Microbes’ and ‘Particulate Geochemistry and Algal Growth as Factors Driving Melting of the Greenland Ice Sheet’. Pan European Networks attended the conference where McCutcheon discussed her participation in the Black and Bloom project.
Black and Bloom
Black and Bloom – black representing the dark particles, and bloom signifying microbial processes – is run in collaboration with a number of universities. Motivating the project is the significant melting of the Greenland Ice Sheet over the last few decades. “We know that snow and ice properties are not enough to explain darkening, so we need to look for other factors that may be contributing to this change in algal bloom,” McCutcheon explained. The project is focused on the southwestern area of Greenland where darkening is prevalent. She added: “We’re looking here because melting in this area provides a glimpse of what could happen if mechanisms start melting, so we’re wondering what is causing this darkening.”
Black and Bloom is studying the presence of light-absorbing impurities (LAI) which include black carbon, fragmented algae and mineral dust. Going further than this, McCutcheon said that there was a need for understanding “where they’re coming from, what concentrations they are present in, and whether or not they’re influencing albedo and to what extent they’re doing so”. To answer such large questions, contemporary atmosphere adjustment measurements of LAI are being taken on the ice sheet. These data measurements will be used in parallel with weather data and ozone logging in order to attempt to form an understanding of where air masses bringing LAI are originating from.
McCutcheon discussed how her work focuses upon the presence of black carbon – an “amorphous carbonaceous by-product of hydrocarbon-based combustion”. However, complications can arise in measuring black carbon, and carbon more generally, as measurements are dependent on technique, varying between fields. Black carbon has an ability to lower snow and ice albedo. Its presence, despite being in low concentrations, needs to be understood, McCutcheon added, saying, “We want to get a handle on where it’s coming from. What does it contain? Is it accumulating on the surface?”
Sample and understanding
The project is focusing on sampling measurable materials from the sky by collecting them in samplers which are positioned on the ice sheet surface. “In addition to collecting physical samples, we’re also doing real-time **it in situ black carbon monitoring. For this we’re using ethylometers, which measure transmitted light absorption at 880nm,” McCutcheon said.
Looking “at black carbon nanoparticles, we find that they are made up of these 50-800nm aggregates of carbonaceous nanoparticles with a turbostratic structure” which presents itself in a ring-like structure. To further the accumulation of data, Black and Bloom will be conducting spectral fingerprinting of nanoparticles to determine where the nanoparticles are originating from, as well as their light absorption properties. They anticipate that the latter of these scientific discoveries is vital in understanding how they could potentially be influencing albedo – the measurement of how much light hitting a surface is reflected, as opposed to being absorbed.
In addition, “we need to look into brown carbon, which is [a] form of organic carbon with wavelength-dependent light absorption”. A major source of brown carbon in Greenland is biomass combustion. “Fire, such as that taking place in British Columbia right now, can produce large amounts of carbon capable of long-range transfer, and people have been able to see this over the Greenland Ice Sheet,” McCutcheon added. One of her collaborators revealed that back-trajectories of air-mass movement can be used to understand where brown carbon may be coming from, and confirm that “air-mass moving over the mainland at this time does seem to be coming from western Canada”. However, wildfires in Greenland itself could be a secondary cause of deposits of brown carbon on the ice sheet.
To understand how much of the carbon deposited onto the ice sheet is compromised by black carbon, Black and Bloom performed thermo-optical carbon measurements which divide total carbon into organic and elemental fractions. McCutcheon said: “In this the elemental carbon can be more or less considered equivalent to black carbon in terms of measurement, and for this our air sampling – which involves sampling 700,00 litres of air in a 24-hour period – saw that elemental (or black) carbon made up about 2-3% of the total carbon coming down. The rest was made up of organics.” McCutcheon said that although black carbon deposition is slow, we do have surface accumulation over a period of time.
Researchers hope to gain understanding of what organic carbon fractions contain, where they currently know that at least a portion is microbial. They are employing the use of a sampling technique which samples 300,000 litres of air per minute into sterile water. Further, through a high volume short interval sampling method researchers are able to constrain time points because they understand what air mass they have sampled. As a result, researchers concluded that there is a simultaneous delivery of black carbon microbes onto the surface.
“We do see this delivery of LAI onto the surface, but we don’t see darkening everywhere,” McCutcheon said. She added: “Rather, we see a progressive variable darkening over time. We were wondering if the differences between these types of surface cover are due to a difference in surface bio-geochemistry.” McCutcheon also explained how dust could be an important factor in influencing albedo as mineral grains, although transparent and not as absorbent as black carbon or pigmented algae, can act like small magnifying glasses, and with this potentially enhance the absorption of black carbon by 50% to 60%.
“We want to get an understanding of what dust is coming out of the ice, and that, plus the aerosols, will give us the full picture of the particulate loading on the surface that darkens over the growth season.” McCutcheon added: “I’d like to introduce the interactions that we’ve looked at so far – a few samples of dark ice that we took this season. We see that these are composed of complex aggregates of intermixed organic and inorganic materials. These aggregates are usually less than a millimetre in size [and] contain a whole range of algal cells, bacteria, mineral dust [and] probably black carbon.”
In analysing the attachment of microbial cells and mineral grains, researchers are unaware whether this is playing a role in helping or restricting microbial growth. “What we know so far is that we have aerosol delivery of black carbon, microbes and mineral dust onto the surface from the atmosphere. We also see a progressive variable darkening over the summer melt season and growth of pigmented algae occurring within these complex aggregates of light-absorbing impurities, and, together, they’re causing darkening on the surface.” McCutcheon concluded by proposing that a potential area of focus would be for the project to quantify how each type of LAI contributes to darkening – algae, mineral dust, black carbon – or some combination of all three. She expects that will provide a formulated reasoning as to how this changes, or will change, albedo and, as a result, melting rates.
This article will appear in Pan European Networks: Science & Technology 24, which will be published in September.