First estimates of the contribution of CaCO₃precipitation to the release of CO₂to the atmosphere during young sea ice growth

Abstract

[1] We report measurements of pH, total alkalinity, air-ice CO 2 fluxes (chamber method), and CaCO 3 content of frost flowers (FF) and thin landfast sea ice. As the temperature decreases, concentration of solutes in the brine skim increases. Along this gradual concentration process, some salts reach their solubility threshold and start precipitating. The precipitation of ikaite (CaCO 3 .6H 2 O) was confirmed in the FF and throughout the ice by Raman spectroscopy and X-ray analysis. The amount of ikaite precipitated was estimated to be 25 mmol kg À1 melted FF, in the FF and is shown to decrease from 19 to 15 mmol kg À1 melted ice in the upper part and at the bottom of the ice, respectively. CO 2 release due to precipitation of CaCO 3 is estimated to be 50 mmol kg À1 melted samples. The dissolved inorganic carbon (DIC) normalized to a salinity of 10 exhibits significant depletion in the upper layer of the ice and in the FF. This DIC loss is estimated to be 2069 mmol kg À1 melted sample and corresponds to a CO 2 release from the ice to the atmosphere ranging from 20 to 40 mmol m À2 d À1. This estimate is consistent with flux measurements of air-ice CO 2 exchange. Our measurements confirm previous laboratory findings that growing young sea ice acts as a source of CO 2 to the atmosphere. CaCO 3 precipitation during early ice growth appears to promote the release of CO 2 to the atmosphere; however, its contribution to the overall release by newly formed ice is most likely minor.

Winter observations of CO<sub>2</sub> exchange between sea ice and the atmosphere in a coastal fjord environment

“…Hence, the temperature sensitivity of carbon speciation in sea-ice brines supports the premise that thermochemical processes within brine exposed to the atmosphere facilitates an air-ice pCO 2 gradient, thereby linking CO 2 exchange to site energetics via brine carbon chemistry (Loose et al, 2011a, b). In theory, sea ice is permeable to vertical brine transport when brine proportion by volume in sea ice is in excess of ∼ 5 % (Golden et al, 1998). The brineatmosphere interface may be positioned at the sea-ice surface or at distance up into the snowpack as would be the case for brine-wetted snow.…”

“…Consequently, most carbon-cycle research has treated ice cover as areas of zero (or very low) exchange (Tison et al, 2002). This view has been challenged by reports of significant fluxes of CO 2 over first and multiyear sea ice during both spring/summer (Delille et al, 2007;Geilfus et al, 2012;Semiletov et al, 2004Semiletov et al, , 2007Zemmelink et al, 2006) and autumn/winter (Else et al, 2011;Geilfus et al, 2013;Miller et al, 2011a, b) and challenged by suggestions of a coupling between the carbonate system in sea ice, the underlying sea water and the atmosphere (Anderson et al, Figure 1. (a) Regional and (b) local overview of field sites in Young Sound, northeast Greenland.…”

“…The fact that CO 2 fluxes at ICEI were close to zero may be because (1) the calculated brine volume was just at the critical threshold for gas permeability V B = 5.1 % (Golden et al, 1998;Loose et al, 2011a, b), raising the possibility that brine transport was inhibited within the ice during that part of the experiment, and (2) the thick overlying snow cover prevented the free exchange of CO 2 in the absence of wind-induced ventilation. We discuss the latter issue below.…”

“…the notion that given sufficiently permeable sea ice (Golden et al, 1998;Loose et al, 2011a, b), brine concentration/dilution alters the pCO 2 gradient across the sea-ice surface and thus the potential for CO 2 exchanges (Geilfus et al, 2012;Killawee et al, 1998;Tison et al, 2002;Nomura et al, 2006;Papadimitriou et al, 2004);…”

Imaged brine inclusions in young sea ice—Shape, distribution and formation timing

“…Much of the young ice volume we sampled was cold enough (b− 2.2°C ) to produce ikaite crystals, and the observed brine drainage channel feature might offer them a mechanism by which to descend into the seawater below, taking their CO 2 -equivalent with them. The low concentrations of ikaite near the bottom of sea ice observed by Geilfus et al (2013a) may therefore be due to either the lack of crystal formation or ikaite crystal export downward to the water column through these imaged channels. The attending channels to the main brine drainage channel range from 0.8 to 1.5 mm in diameter, which is greater than the ikaite crystal sizes of b0.1 mm to 1 mm reported by Rysgaard et al (2012) in Arctic sea ice, consistent with work showing that ikaite removal from sea ice through brine drainage is a process affecting the alkalinity of under-sea ice seawater and consequently affects oceanatmosphere CO 2 exchange.…”