Sea Ice Biogeochemistry
Polar environments are diverse and undergo large dynamic changes due to seasonal atmospheric temperature fluctuations that extend well into the below-zero temperature range. Sea ice adds further complexity as a multi-phase medium and a biome to oceanic chemistry in high latitudes. Sea ice is a dynamic matrix of solid pure water ice, gas pockets, and numerous inter-connected or isolated micro-habitats of brines rich in dissolved sea salts and particles recruited from the frozen surface seawater. It has internal temperature and brine salinity gradients, which expand and contract during the seasonal cycle of formation and decay of this medium as a function of atmospheric temperature. Sea ice also displays structural complexity over wider scales. It is erratically fractured by leads of open ocean water separating large sea ice expanses (ice floes), which, in addition, can have structural gaps at sea surface level, where surface seawater intrusion occurs, and surface melt ponds. Sea ice covers 13% of the surface of the Earth at its maximum extent, equivalent to the areal extent of deserts and tundra. It is an ecologically diverse habitat and has been increasingly recognized in its key role in global biogeochemical cycles, including contribution to the primary productivity, as well as to salt and gas fluxes to the underlying ocean and the atmosphere, and the ventilation of the deep ocean at high latitudes (Thomas and Dieckmann 2010).
Generously supported by NERC and European funds, our recent research has focussed on the biological and chemical reactions that take place in the brine channels of sea ice. It includes field campaigns and large scale mesocosm artificial sea ice experiments in collaboration with colleagues from Germany (AWI, Bremerhaven) Belgium, Finland, Canada, Australia (Australian Antarctic Division), Denmark, UK (Essex University) and the Hamburgische Schiffbau- und Versuchsanstalt (HSVA) in Germany. Our research group has co-led biogeochemical field investigations in the Baltic Sea, Arctic and Antarctic Oceans in both pack and land-fast ice. These have produced detailed biogeochemical mapping of particulate and dissolved inorganic and organic carbon species and its isotopic composition in sea ice.
Our field and mesocosm investigations have documented the transformations and fate of carbon in sea ice. This work has resulted in several avenues of interrelated research looking a gas content of ice and underlying waters, bacterial activity and population composition the ice, inorganic nutrients, organic matter respiration, dissolved organic matter optical properties. Bringing these aspects together the team at Bangor have been instrumental in discovering that:
- Dissolved organic carbon in sea ice is enriched above that predicted by conservative behaviour, especially due to the production of extracellular polymeric substances by sea ice algae and bacteria. (Underwood et al., 2013; Zhou et al., 2016)
- primary production and dissolved CO2 degassing lead to carbonate mineral precipitation from brine inclusions in sea ice and the concentration and isotopic composition of dissolved inorganic carbon was modulated by biological production, dissolved CO2 degassing, and carbonate mineral formation (Papadimitriou et al., 2003).
These discoveries, initially from controlled experiments were corroborated by seasonal field investigations of the CO2 system, in conjunction with, dissolved oxygen, and macronutrient data, in the seasonal sea ice zone (SIZ) of the Southern Ocean in summer and in the transition from winter to spring(Papadimitriou et al., 2012). In summer sea ice we found strong modification of the CO2 system in internal sea ice brines and gap waters towards dissolved CO2 and macronutrient depletion, stable isotopic enrichment of particulate and the total dissolved inorganic carbon, and strongly alkaline pH values, consistent with internal biological productivity in a closed or semi-closed system. In this setting no distinctive or uniform relationship between carbon and the major dissolved inorganic nutrients was observed owing to varying stoichiometry of biological activity within a very small spatial scale.
We established that the sea ice cover of the high latitude oceans is a potent carbon reactor and found that the largest sinks for dissolved inorganic carbon in sea ice brines were carbonate mineral precipitation and dissolved CO2 degassing, because the magnitude of the biological (photosynthetic) carbon sink is limited by the size of the internal inorganic macronutrient pool. In the same field campaign, the hydrated carbonate mineral ikaite was collected and identified from the sea ice for the first time, confirming our concurrent and earlier mesocosm experiments. We have thus initiated research into carbonate and non-carbonate mineral dynamics in the cryosphere, as well as rigorous physical-chemical laboratory investigation into the chemical thermodynamics of the CO2 system at below-zero temperatures . We have shown that sea ice is an active interface in the interaction between the ocean and the atmosphere, through which dissolved, particulate, and gaseous species transform and migrate.