Enhanced uptake of atmospheric CO₂ during freezing of seawater: A field study in Storfjorden, Svalbard

Abstract

[1] The waters of Storfjorden, a fjord in southern Svalbard, were investigated in late April 2002. The temperature was at the freezing point throughout the water column; the salinity in the top 30 m was just above 34.8, then increased nearly linearly to about 35.8 at the bottom. Nutrient and oxygen concentrations showed a minimal trend all through the water column, indicating minimal decay of organic matter. Normalized dissolved inorganic carbon, f CO 2 , and CFCs increase with depth below the surface mixed layer, while pH decreases. In waters below 50 m, there was an increase in dissolved inorganic carbon, corrected for decay of organic matter using the phosphate profile, corresponding to about 9 g C m À2 relative to the surface water concentration. We suggest this excess is a result of enhanced air-sea exchange of CO 2 caused by sea ice formation. This enhancement is suggested to be a result of an efficient exchange through the surface film during the ice crystal formation and the rapid transport of the high salinity brine out of the surface layer.

Inorganic carbon transport during sea ice growth and decay: A carbon pump in polar seas

“…Besides the physical impact of sea ice on ocean circulation, brine rejection also influences the release of dissolved gases and solutes into the water column [Glud et al, 2002;Papadimitriou et al, 2003]. Supersaturated pCO 2 levels below sea ice have previously been observed in the Arctic Ocean [Kelley, 1968;Semiletov et al, 2004] and in coastal Antarctic areas [Gibson and Trull, 1999] and high TCO 2 concentrations have been observed below Arctic sea ice [Anderson et al, 2004]. In addition, low total alkalinity (TA) and TCO 2 concentrations in sea ice brine relative to surface waters have been observed in the Antarctic during summer, whereas high TA and TCO 2 concentrations in sea ice brine relative to surface waters were observed during winter [Gleitz et al, 1995].…”

“…Sea ice formation likely also increases the turbulence at the air-sea interface during the initial growth phase when ice crystals form together with brine at the very surface. Anderson et al [2004] hypothesized that a highly efficient gas exchange takes place in conjunction with the heat exchange that occurs when ice is produced in open water with a partial pressure different from that of the overlying atmosphere. Thus very low temperatures in the salt-enriched water surrounding the ice crystals during formation could amplify this gas exchange.…”

Ikaite crystal distribution in winter sea ice and implications for CO<sub>2</sub> system dynamics

“…Dense brine production in the polynya region (Anderson et al, 2004) may provide a mechanism to deliver CO 2 below the mixed layer, making ikaite production in sea ice a "carbon pump" that removes CO 2 from the surface ocean to deeper water layers (Rysgaard et al, 2009. Low-density ice meltwater remaining at the surface will facilitate atmospheric CO 2 deposition as a result of ikaite dissolution.…”

“…The polynya formation at POLY I is predominantly governed by mechanical forcing caused by northerly gales; it has been classified as a winddriven shelf water system (Pedersen et al, 2010), where sea ice formation is continuous and rejection of CO 2 to deeper water layer with dense brine occurs (Anderson et al, 2004). Ikaite crystals trapped in the forming sea ice will be exported with the ice to melt elsewhere.…”

Climate relevant trace gases (N2O and CH4) in the Eurasian Basin (Arctic Ocean)

“…The dissolved gas contents depend on physical and microbiological processes. During the autumn-winter period of sea ice formation, gases are expelled from the sea ice and transported to the deeper layer or vented to the atmosphere; during spring-summer, melting ice dilutes the gas content of seawater, and may produce under-saturated conditions in the surrounding waters (Anderson et al, 2004;Kitidis et al, 2010). Therefore, surface water can behave as a source or a sink of gases.…”