Schematic figure indicating biogeochemical processes at sea ice interfaces and within the sea ice matrix over the season. DIC indicates dissolved inorganic carbon, POC particular organic carbon, VOC volatile organic carbon, DMS dimethyl sulfide, and CCN cloud condensation nuclei)

Climate change is and will likely remain the strongest but also the most uncertain factor of change in polar regions. Earth system feedbacks, mostly related to temperature and albedo feedbacks and changes in atmospheric meridional heat transport cause amplified changes in polar regions (polar amplification) with wide-ranging impacts to be expected. Our understanding of these processes and adequate representation in models is still in its infancy. While sea ice should not completely disappear from polar regions within the next 100+ years, it will at the very least experience profound changes in seasonality, coverage, and thickness with associated alterations in its physical, chemical and biological properties. Observations over the past 20 years have identified numerous, previously unknown, biogeochemical processes occurring in the sea ice zone, which spans 10% of the world ocean. These processes can occur within sea ice itself, or involve terrestrial-atmosphere-sea ice-ocean exchange of biogeochemical compounds.

Sea ice was long assumed to inhibit air-sea gas and material exchange, but we now understand that sea ice is a biologically and chemically rich and complex system, that not only stores, produces or destroys chemical compounds, but also transports and exchanges them with both the atmosphere and the underlying water.. Sea-ice processes interact with the underlying seawater and remote water masses by releasing brine and meltwater, and flow-on effects on nutrient stocks and marine productivity. Because of both biotic and abiotic processes occurring during the freeze/melt cycles, sea ice actively participates in the biogeochemical cycles of many elements which modulate the surface ocean ecosystem As a result, sea ice contributes to substantial seasonal gas fluxes and possibly enhances long-term export and CO2 sequestration in deep waters. Sea ice processes also affect the atmospheric composition by altering air-sea fluxes of gases and aerosols with the potential for significant climate feedbacks through direct and indirect (cloud) forcing mechanisms. Air-sea gas exchange rates in partially ice-covered waters can be up to an order of magnitude larger than what would be expected from observations in ice-free waters, while stratification and the geochemistry of both ice melt and river runoff can substantially limit air-sea CO2 exchange in summer. Therefore, because of the annual sea-ice formation and melt cycle, coupled with increasing areas of open water in summer and first-year sea ice in winter, the polar oceans are very specific in their air-sea interactions.

To improve the model representations of air-sea fluxes in the presence of sea ice and of the role of stored and transported chemical elements by sea ice, we must understand not only sea-ice biogeochemistry, but also air-sea fluxes in ice-covered seas, as well as the impact of sea-ice melt and water column stratification in summer. In addition to controlling air-sea CO2 exchanges in polar waters, sea-ice dynamics impact CH4 emissions and acidification processes in as yet unpredictable ways. Because sea-ice formation regions contribute to deep water formation, the footprint of air-sea exchange processes in polar waters propagates throughout the global ocean eventually influencing acidification states at lower latitudes. Halogenated aerosols released during sea-ice formation and growth appear to play a crucial role in controlling ozone dynamics in the, and releases of sulfur and organic gel aerosols also likely impact other aspects of polar atmospheric chemistry, but the feedbacks into the marine ecosystem are still almost completely unknown.

Sea-ice biogeochemical compounds and processes vary over a wide range of scales. At the very small spatial scale, brine inclusions within sea ice are dominant, ranging from micrometers to centimeters in diameter. Vertical variations in concentrations of biogeochemical species with depth can be extremely strong, sea ice being typically up to a few meters thick. Large horizontal variations at subfloe-scales, over a few meters – associated with changes in ice thickness, snow depth, brine fraction and pond coverage – have been observed. For instance, the distribution of ice algae and associated primary production is highly variable at multiple spatial scales, and we have only begun to understand the potentially significant role of topographical features such as hummocks and ridges for ice algal production. We would also expect large-scale variations, which remain little understood.

The concentrations of biogeochemical species within the ice is highly dependent on species. For some of them (e.g. nutrients), concentrations are smaller than in the ocean, for others (e.g. chl-a, iron, …), they can be orders of magnitude larger than in the ocean. There are large uncertainties on sea-ice primary production, but we suspect it could dominate the water column integrated production in regions of permanent ice coverage. The dynamics of the sea-ice ecosystem directly affect those of the underlying waters down to the seafloor but may also affect the integrity of the ice, including porosity, fluid transport and radiative transfer. Sea-ice algal production directly impact species that rely on sea ice as their habitat and source of food, e.g., ice-associated zooplankton species, but also higher trophic levels of the water column such as polar cod Boreogadus saida and Antarctic krill Euphausia superba which play a primary role in the transfer of energy to the Arctic megafauna. Benthic ecosystem are also impacted by changes in sea ice cover. Sea-ice cover accumulates and stores atmospheric fallout and incorporates sedimentary loads of both nutrients, contaminants as well as micro- and nanoplastics during its formation in winter. When it melts, sea ice releases them to the water column in concentrated pulses, with flow-on effect on the marine ecosystem. Terrestrial processes can impact sea-ice and ocean processes by transporting nutrients and carbon via riverine input and coastal erosion.

The controls exerted by sea ice on air-sea exchange processes are intimately connected to the life-styles and livelihoods of people living in the Arctic. In particular, transport of contaminants (including mercury and persistent organic pollutants), air quality, and ecosystem structure and development

have immediate impacts on the quality of life in this extreme environment. In addition, as the Arctic Ocean becomes more accessible, with the loss of thick, multi-year ice, increases in shipping and fossil fuel prospecting will contribute to more contaminating materials into this region. Higher trophic levels of polar ecosystems are dependent on sea ice. Indigenous communities in the Arctic are affected by changes in sea-ice biogeochemistry through cascading ecosystem effects. Changes in sea-ice structure and integrity will themselves impact the use of sea ice for cultural purposes and transportation.

Despite significant progress, our understanding of marine biogeochemistry in the sea ice zone is still limited. Observations are sparse due to technical and logistic limitations, and satellite remote sensing is not really appropriate to the retrieval of sea-ice biogeochemical parameters. In addition, available historic observations have not all been compiled into consistent and easily-accessible databases, in particular in the Arctic. As a consequence, the representation of sea-ice zone biogeochemical processes in regional and Earth System Models (ESMs) is extremely simple and our confidence in understanding either the present importance of these processes or how they will respond to climatic change is limited.