Interplay between seasonal arctic sea-ice processes and halocline stability - towards understanding arctic gas and matter fluxes

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As a result of global climate change the sea-ice cover in the Arctic has changed dramatically. In its present state, the Arctic ice cover plays an important role, as it shields the surface water, the so-called Arctic halocline (salinity stratification), from heating in the summer sunlight. In addition, the halocline is formed and stabilized by the salt, which is excluded from the crystal structure during the freezing process of seawater. At the same time, the halocline provides a barrier between the ice cover and the underlying warm Atlantic waters and contributes in return to the preservation of the Arctic sea-ice cover. This balance is now disturbed by an overall much thinner Arctic ice cover and its reduced summer extent. In addition seawater contains gases and important biogeochemical trace substances that are trapped, influenced and rejected by different sea-ice process. Thereby arctic sea ice influences the gas and matter fluxes between atmosphere, ice and upper water column. With the ice movement there is also a transport, e.g. in the so-called Transpolar Drift from the Siberian shelf areas, over the North Pole, southwards into the Nordic seas.In the light of the "new Arctic" it is assumed that the Arctic has already shifted to a different mode of operation. However, it is largely unknown how this new system works, evolves and how it will affect sea-ice formation processes and halocline stability and thus gas and matter fluxes. For such investigations we need samples of the upper water column and the sea-ice cover from the central Arctic over the course of the year, and the scientific initiative MOSAiC provides such an opportunity. Stable isotopes of the water (18O and D) taken from ice and upper water column contain information on the source waters and the freezing processes and these results will be directly linked to gas and biogeochemical investigations (from partner projects). Important might be e.g. storms, melting processes, snow cover, pond formation and aging of sea-ice. These process-oriented studies of seasonal sea-ice processes within sea-ice cores and the upper water column of the central Arctic will provide an important contribution to the understanding of the Arctic Ocean halocline stability as well as gas and matter fluxes. The discrepancy between non-conservative behavior of gases and substances relative to the conservative behavior of the isotopes offers the chance to derive further important information, e.g. on refreezing processes.