CO2 and CH4 in sea ice from a subarctic fjord under influence of riverine input

We present the CH4 concentration [CH4], the partial pressure of CO2 (pCO2) and the total gas content in bulk sea ice from subarctic, land-fast sea ice in the Kapisillit fjord, Greenland. Fjord systems are characterized by freshwater runoff and riverine input and based on dδ18O data, we show that > 30% of the surface water originated from periodic river input during ice growth. This resulted in fresher sea-ice layers with higher gas content than is typical from marine sea ice. The bulk ice [CH4] ranged from 1.8 to 12.1 nmol Lg-1, which corresponds to a partial pressure ranging from 3 to 28 ppmv. This is markedly higher than the average atmospheric methane content of 1.9 ppmv. Evidently most of the trapped methane within the ice was contained inside bubbles, and only a minor portion was dissolved in the brines. The bulk ice pCO2 ranged from 60 to 330 ppmv indicating that sea ice at temperatures above -4 °C is undersaturated compared to the atmosphere (390 ppmv). This study adds to the few existing studies of CH4 and CO2 in sea ice, and we conclude that subarctic seawater can be a sink for atmospheric CO2, while being a net source of CH4

Evidence of Freezing Pressure in Sea Ice Discrete Brine Inclusions and Its Impact on Aqueous-Gaseous Equilibrium

Sea ice in part controls surface water properties and the ocean-atmosphere exchange of greenhouse gases at high latitudes. In sea ice, gas exists dissolved in brine and as air bubbles contained in liquid brine inclusions or as bubbles trapped directly within the ice matrix. Current research on gas dynamics within the ocean-sea ice-atmosphere interface has been based on the premise that brine with dissolved air becomes supersaturated with respect to the atmosphere during ice growth. Based on Henry's law, gas bubbles within brine should grow when brine reaches saturation during cooling, given that the total partial pressure of atmospheric gases is above the implicit pressure in brine of 1 atm. Using high-resolution light microscopy time series imagery of gas bubble evolution inside discrete brine pockets, we observed bubbles shrinking during cooling events in response to the development of freezing pressure above 3 atm. During warming of discrete brine pockets, existing bubbles expand and new bubbles nucleate in response to depressurization. Pressure variation within these inclusions has direct impacts on aqueous-gaseous equilibrium, indicating that Henry's law at a constant pressure of 1 atm is inadequate to assess the partitioning between dissolved and gaseous fractions of gas in sea ice. This new evidence of pressure build-up in discrete brine inclusions controlling the solubility of gas and nucleation of bubbles in these inclusions has the potential to affect the transport pathways of air bubbles and dissolved gases within sea ice-ocean-atmosphere interface and modifies brine biochemical properties

Inorganic carbon dynamics of melt-pond-covered first-year sea ice in the Canadian Arctic

Melt pond formation is a common feature of spring and summer Arctic sea ice, but the role and impact of sea ice melt and pond formation on both the direction and size of CO2 fluxes between air and sea is still unknown. Here we report on the CO2-carbonate chemistry of melting sea ice, melt ponds and the underlying seawater as well as CO2 fluxes at the surface of first-year landfast sea ice in the Resolute Passage, Nunavut, in June 2012. Early in the melt season, the increase in ice temperature and the subsequent decrease in bulk ice salinity promote a strong decrease of the total alkalinity (TA), total dissolved inorganic carbon (T CO2) and partial pressure of CO2 (pCO2) within the bulk sea ice and the brine. As sea ice melt progresses, melt ponds form, mainly from melted snow, leading to a low in situ melt pond pCO2 (36 μatm). The percolation of this low salinity and low pCO2 meltwater into the sea ice matrix decreased the brine salinity, TA and T CO2, and lowered the in situ brine pCO2 (to 20 μatm). This initial low in situ pCO2 observed in brine and melt ponds results in air-ice CO2 fluxes ranging between -0.04 and -5.4 mmolm-2 day-1 (negative sign for fluxes from the atmosphere into the ocean). As melt ponds strive to reach pCO2 equilibrium with the atmosphere, their in situ pCO2 increases (up to 380 μatm) with time and the percolation of this relatively high concentration pCO2 meltwater increases the in situ brine pCO2 within the sea ice matrix as the melt season progresses. As the melt pond pCO2 increases, the uptake of atmospheric CO2 becomes less significant. However, since melt ponds are continuously supplied by meltwater, their in situ pCO2 remains undersaturated with respect to the atmosphere, promoting a continuous but moderate uptake of CO2 (∼-1 mmolm-2 day-1) into the ocean. Considering the Arctic seasonal sea ice extent during the melt period (90 days), we estimate an uptake of atmospheric CO2 of -10.4 Tg of Cyr-1. This represents an additional uptake of CO2 associated with Arctic sea ice that needs to be further explored and considered in the estimation of the Arctic Ocean's overall CO2 budget

Replacement of multiyear sea ice and changes in the open water season duration in the Beaufort Sea since 2004

The last decade has witnessed the nine lowest Arctic September sea ice extents in the observational record. It also forms the most recent third of the long-term trend in that record, which reached -13.4% decade-1 in 2015. While hemispheric analyses paint a compelling picture of sea ice loss across the Arctic, the situation with multiyear ice in the Beaufort Sea is particularly dire. This study was undertaken in light of substantial changes that have occurred in the extent of summer multiyear sea ice in the Arctic inferred from the passive microwave record. To better elucidate these changes at a sub-regional scale, we use data from the Canadian Ice Service archive, the most direct observations of sea ice stage-of-development available. We also build upon the only previous sea ice climatological analysis for Canada's western Arctic region by sea ice stage-of-development that ended in 2004. The annual evolution of sea ice by stage of development in Canada's western Arctic changed dramatically between 1983 and 2014. The rate of these changes and their spatial prevalence were most prominent in the last decade. In summer, total sea ice loss occurred via reductions in old and first-year sea ice over increasingly large areas and over more months per year. Resultant delay of thermodynamic freeze up has increased the annual open water duration in the study region. The winter sea ice cover was increasingly composed of first-year sea ice at the expense of old ice. Breakup timing has not significantly changed in the region

Estimates of ikaite export from sea ice to the underlying seawater in a sea ice-seawater mesocosm

The precipitation of ikaite and its fate within sea ice is still poorly understood.We quantify temporal inorganic carbon dynamics in sea ice from initial formation to its melt in a sea ice.seawater mesocosm pool from 11 to 29 January 2013. Based on measurements of total alkalinity (TA) and total dissolved inorganic carbon (TCO2), the main processes affecting inorganic carbon dynamics within sea ice were ikaite precipitation and CO2 exchange with the atmosphere. In the underlying seawater, the dissolution of ikaite was the main process affecting inorganic carbon dynamics. Sea ice acted as an active layer, releasing CO2 to the atmosphere during the growth phase, taking up CO2 as it melted and exporting both ikaite and TCO2 into the underlying seawater during the whole experiment. Ikaite precipitation of up to 167 μmolkg-1 within sea ice was estimated, while its export and dissolution into the underlying seawater was responsible for a TA increase of 64.66 μmolkg-1 in the water column. The export of TCO2 from sea ice to the water column increased the underlying seawater TCO2 by 43.5 μmolkg-1, suggesting that almost all of the TCO2 that left the sea ice was exported to the underlying seawater. The export of ikaite from the ice to the underlying seawater was associated with brine rejection during sea ice growth, increased vertical connectivity in sea ice due to the upward percolation of seawater and meltwater flushing during sea ice melt. Based on the change in TA in the water column around the onset of sea ice melt, more than half of the total ikaite precipitated in the ice during sea ice growth was still contained in the ice when the sea ice began to melt. Ikaite crystal dissolution in the water column kept the seawater pCO2 undersaturated with respect to the atmosphere in spite of increased salinity, TA and TCO2 associated with sea ice growth. Results indicate that ikaite export from sea ice and its dissolution in the underlying seawater can potentially hamper the effect of oceanic acidification on the aragonite saturation state (ωaragonite) in fall and in winter in ice-covered areas, at the time when ωaragonite is smallest

Temporal dynamics of ikaite in experimental sea ice

Ikaite (CaCO3 · 6H2O) is a metastable phase of calcium carbonate that normally forms in a cold environment and/or under high pressure. Recently, ikaite crystals have been found in sea ice, and it has been suggested that their precipitation may play an important role in air-sea CO 2 exchange in ice-covered seas. Little is known, however, of the spatial and temporal dynamics of ikaite in sea ice. Here we present evidence for highly dynamic ikaite precipitation and dissolution in sea ice grown at an outdoor pool of the Sea-ice Environmental Research Facility (SERF) in Manitoba, Canada. During the experiment, ikaite precipitated in sea ice when temperatures were below -4 °C, creating three distinct zones of ikaite concentrations: (1) a millimeter-to-centimeter-thin surface layer containing frost flowers and brine skim with bulk ikaite concentrations of >2000 μmol kg-1, (2) an internal layer with ikaite concentrations of 200-400 μmol kg -1, and (3) a bottom layer with ikaite concentrations of <100 μmol kg-1. Snowfall events caused the sea ice to warm and ikaite crystals to dissolve. Manual removal of the snow cover allowed the sea ice to cool and brine salinities to increase, resulting in rapid ikaite precipitation. The observed ikaite concentrations were on the same order of magnitude as modeled by FREZCHEM, which further supports the notion that ikaite concentration in sea ice increases with decreasing temperature. Thus, varying snow conditions may play a key role in ikaite precipitation and dissolution in sea ice. This could have a major implication for CO2 exchange with the atmosphere and ocean that has not been accounted for previously

Gases in sea ice: Update of recent findings, caveats and open questions

Sea ice exchanges gases with the atmosphere including climate and ozone-depleting gases We will rapidly present a state of the art of current large scale assessment of spring and summer uptake of atmospheric CO2. We will challenge these assessments with 1) new evidence of significant winter CO2 release for winter experiments 2) role of bubbles formation and transport within sea ice 3) impact of biofilm. Finally, comparison of air-ice fluxes derived from automated chamber and micrometeorological method and, mechanistic and box models show significant discrepancies that suggest that the contribution of sea ice to the air-ocean fluxes of CO2 remain an open question. We will also sea ice contribution to the fluxes of other gases as CH4, N2O, DMS and VOC

Gas transport processes in sea ice: How convection and diffusion processes might affect biological imprints, a challenge for modellers

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