Sea-Ice Biogeochemistry
It was long assumed that if the ocean is covered with sea ice, there is no air-sea gas exchange. This assumption was based on experience with freshwater ice, which is indeed a solid, nearly impermeable substance. However, anyone who has worked with sea ice knows that it is a very different kind of material – extremely porous and with a lot going on in it. After more than 20 years of banging our heads against this problem, we’re finally starting to get a reasonable picture of what’s happening.
Photo: T;. Juul-Pedeersen
Through extensive field expeditions in collaboration with Tim Papakyriakou, who directly measures CO2 fluxes and in and out of the ice; Kristina Brown; and Nes Sutherland, who after an illustrious career in oceanographic research has now retired onto her sailboat in the Gulf Islands, we've discovered that gases are mobile in sea ice. In autumn, when the ice is initially freezing, it releases CO2 to both the atmosphere and the underlying water, and that release continues through the winter, although at a much slower rate. As winter wanes and spring advances, the CO2 fluxes reverse: the top of the ice absorbs CO2 from the atmosphere due to abiotic dissolution of calcium carbon in the ice brines and dilution, whereas the bottom of the ice absorbs CO2 from the water, as ice algae bloom.
To pin down exactly how much CO2 is released to the water when sea ice freezes, we retreated to the IOS cold lab, where we have a 1000-L model Arctic Ocean that is heated from the bottom. For her master's thesis, ETH Zürich graduate student Daniela König found that whereas more carbon is exported from slowly forming ice (i.e., at higher air temperatures), the carbon is exported deeper into the water column when the ice forms faster (at lower air temperatures). That means that in the future, if the ice cover is still solid but warmer, it may export more carbon to the water, but that carbon will stay at the surface. On the other hand, with a more broken, mobile ice cover in the winter, more ice might be forming at lower temperatures, which could contribute more carbon to the global solubility pump.
Kyle Simpson in the IOS cold lab
The most interesting carbon biogeochemistry seems to arise in young, first-year ice, and therefore, the processes we’ve discovered are likely to become more important in the Arctic Ocean, as the multi-year ice melts and is replaced by a larger area of seasonal ice.
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Quite a few others have also gotten interested in this problem, and together, we've formed an international research network on Biogeochemical Exchange Processes at Sea-Ice Interfaces (BEPSII). This group started out as a SCOR working group, but is now a much larger and stronger entity jointly sponsored by SOLAS, CliC, and SCAR. A follow-up SCOR working group focussed on testing and intercalibrating some of the different methods used to study sea-ice biogeochemistry (ECV-Ice). A new working group composed of both atmospheric and oceanic polar scientists is now trying to figure out how sea ice biogeochemisty influences polar cloud formation (CIce2Clouds).
​Sea-ice restoration as a climate intervention tool. A number of proposals are being floated to artificially restore the sea ice over large areas of the Arctic Ocean, a geoengineering tactic that would increase the planetary albedo, reduce the absorption of solar radiation, and limit atmospheric warming. Evaluation of these proposals has mainly focused on the radiative effects, neglecting potential feedbacks on the ecosystems and biogeochemical cycles, again assuming that sea ice is an inert cap on the ocean. In reality, wide-spread implementation of these proposals will have broad impacts on the light available for sea-ice and oceanic primary production, the production of atmospheric aerosols, drawdown and sequestration of carbon in the polar oceans, and the availability of nutrients downstream, in the Atlantic Ocean.
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