Impact of sea‐ice processes on the carbonate system and ocean acidification at the ice‐water interface of the Amundsen Gulf, Arctic Ocean

Abstract

[1] From sea-ice formation in November 2007 to onset of ice melt in May 2008, we studied the carbonate system in first-year Arctic sea ice, focusing on the impact of calcium-carbonate (CaCO 3 ) saturation states of aragonite (XAr) and calcite (XCa) at the ice-water interface (UIW). Based on total inorganic carbon (C T ) and total alkalinity (A T ), and derived pH, CO 2 , carbonate ion ( [CO 3 22 ]) concentrations and X, we investigated the major drivers such as brine rejection, CaCO 3 precipitation, bacterial respiration, primary production and CO 2 -gas flux in sea ice, brine, frost flowers and UIW. We estimated large variability in sea-ice C T at the top, mid, and bottom ice. Changes due to CaCO 3 and CO 2 -gas flux had large impact on C T in the whole ice core from March to May, bacterial respiration was important at the bottom ice during all months, and primary production in May. It was evident that the sea-ice processes had large impact on UIW, resulting in a five times larger seasonal amplitude of the carbonate system, relative to the upper 20 m. During ice formation, [CO 2 ] increased by 30 mmol kg 21 , [CO 3 22 ] decreased by 50 mmol kg 21, and the XAr decreased by 0.8 in the UIW due to CO 2 -enriched brine from solid CaCO 3 . Conversely, during ice melt, [CO 3 22 ] increased by 90 mmol kg 21 in the UIW, and X increased by 1.4 between March and May, likely due to CaCO 3 dissolution and primary production. We estimated that increased ice melt would lead to enhanced oceanic uptake of inorganic carbon to the surface layer.

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

“…As a result, the concentration of dissolved salts, including inorganic carbon, increases within the brine and promotes the precipitation of calcium carbonate crystals such as ikaite (CaCO 3 q 6H 2 O) (Marion, 2001). These crystals have been reported in both natural (Dieckmann et al, 2008;Nomura et al, 2013;Søgaard et al, 2013) and experimental sea ice (Geilfus et al, 2013b;Rysgaard et al, 2014) and have been suggested to be a key component of the carbonate system Fransson et al, 2013;.…”

“…Release of CO 2 from sea ice to the atmosphere has been reported during sea ice formation from open water (Geilfus et al, 2013a) and in winter Fransson et al, 2013), while uptake of CO 2 by sea ice from the atmosphere has been reported after sea ice melt onset (e.g., Semiletov et al, 2004;Nomura et al, 2010Nomura et al, , 2013Geilfus et al, 2012Geilfus et al, , 2014Geilfus et al, , 2015Fransson et al, 2013). In combination, these works suggest that the temporal cycle of sea ice formation and melt affects atmospheric CO 2 uptake by the ocean in variable ways.…”

“…The main air-sea fluxes of CO 2 and TCO 2 are driven by brine rejection to the underlying seawater and its contribution to intermediate and deep-water formation (Semiletov et al, 2004;Rysgaard et al, 2007Rysgaard et al, , 2009Fransson et al, 2013) or below sea ice in ice tank studies (e.g., Killawee et al, 1998;Papadimitriou et al, 2004). As sea ice thickens, reduced near-surface ice temperatures result in reduced brine volume content, increased brine salinity and increased solute concentration in the brine.…”

Low <i>p</i>CO<sub>2</sub> under sea-ice melt in the Canada Basin of the western Arctic Ocean

“…Bates et al (2014) found both basic (i.e., DIC / TA < 1) and relatively acidic (i.e., DIC / TA > 1) melt ponds in the Canada Basin. To study the impact of meltwater on carbonate chemistry, direct sampling of sea ice into gastight bags (Fransson et al, 2013) will be required.…”

“…DIC SIM and DIC RRO were assumed to be 300 µmol kg −1 (Fransson et al, 2013) and 800 µmol kg −1 (Tank et al, 2012), respectively. For the PSW in which salinity ranged from 29.3 to 31.5, f SIM = 0.01, and f RRO = 0.06, preformed DIC observed was 2027 µmol kg −1 .…”