Sea-ice melt CO<sub>2</sub>–carbonate chemistry in the western Arctic Ocean: meltwater contributions to air–sea CO<sub>2</sub> gas exchange, mixed-layer properties and rates of net community production under sea ice

Abstract

Abstract. The carbon dioxide (CO 2 )-carbonate chemistry of sea-ice melt and co-located, contemporaneous seawater has rarely been studied in sea-ice-covered oceans. Here, we describe the CO 2 -carbonate chemistry of sea-ice melt (both above sea-ice as "melt ponds" and below sea-ice as "interface waters") and mixed-layer properties in the western Arctic Ocean in the early summer of 2010 and 2011. At 19 stations, the salinity (∼ 0.5 to < 6.5), dissolved inorganic carbon (DIC; ∼ 20 to < 550 µmol kg −1 ) and total alkalinity (TA; ∼ 30 to < 500 µmol kg −1 ) of above-ice melt pond water was low compared to the co-located underlying mixed layer. The partial pressure of CO 2 (pCO 2 ) in these melt ponds was highly variable (∼ < 10 to > 1500 µatm) with the majority of melt ponds acting as potentially strong sources of CO 2 to the atmosphere. The pH of melt pond waters was also highly variable ranging from mildly acidic (6.1 to 7) to slightly more alkaline than underlying seawater (> 8.2 to 10.8). All of the observed melt ponds had very low (< 0.1) saturation states ( ) for calcium carbonate (CaCO 3 ) minerals such as aragonite ( aragonite ). Our data suggest that sea-ice generated alkaline or acidic type melt pond water. This melt water chemistry dictates whether the ponds are sources of CO 2 to the atmosphere or CO 2 sinks. Below-ice interface water CO 2 -carbonate chemistry data also indicated substantial generation of alkalinity, presumably owing to dissolution of CaCO 3 in sea-ice. The interface waters generally had lower pCO 2 and higher pH / aragonite than the co-located mixed layer beneath. Sea-ice melt thus contributed to the suppression of mixed-layer pCO 2 , thereby enhancing the surface ocean's capacity to uptake CO 2 from the atmosphere. Our observations contribute to growing evidence that sea-ice CO 2 -carbonate chemistry is highly variable and its contribution to the complex factors that influence the balance of CO 2 sinks and sources (and thereby ocean acidification) is difficult to predict in an era of rapid warming and sea-ice loss in the Arctic Ocean.

Iron in sea ice: Review and new insights

“…Recent work on modeled Arctic sea ice has shown that during ice growth, due to CO 2 flux from sea ice to the atmosphere, sea ice is likely basic (pH > 9). During ice melt, due to the flux of CO 2 from the atmosphere to sea ice, melting sea ice becomes likely acidic (pH < 6.5), which can lead to acidic melt ponds in the Arctic (Bates et al, 2014). The acidification of the sea-ice environment has not yet been observed in Antarctica, but it is possible that net primary productivity can change the sea surface pH and therefore Fe bio-availability over a seasonal growth/melt cycle.…”

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

“…Dissolution of ikaite within melting sea ice in the spring and export of this related high TA : TCO 2 ratio meltwater from the ice to the water column will decrease the pCO 2 and increase pH and aragonite of the surface layer seawater. Accordingly, during sea ice melt, an increase of aragonite in the surface water in the Arctic was observed (Chierici et al, 2011;Fransson et al, 2013;Bates et al, 2014). However, it was difficult to ascribe this increase to the legacy of excess TA in sea ice, ikaite dissolution or primary production.…”

Low &lt;i&gt;p&lt;/i&gt;CO&lt;sub&gt;2&lt;/sub&gt; under sea-ice melt in the Canada Basin of the western Arctic Ocean

“…The impact of sea-ice melt itself on pCO sea 2 was difficult to resolve only from our observations. 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.…”

“…Although dilution of the N. Kosugi et al: Low pCO 2 under sea-ice melt in the Canada Basin surface water with meltwater lowers the partial pressure of CO 2 (pCO sea 2 ), shoaling of the surface mixed layer would accelerate equilibration of the surface water with the overlying air. The input of meltwater is also likely to influence carbonate chemistry by altering the ratio of dissolved inorganic carbon (DIC) to total alkalinity (TA), although it is still unclear whether the addition of meltwater increases or decreases the DIC / TA ratio (Rysgaard et al, 2007;Bates et al, 2014). Cai et al (2010) reported unprecedented high pCO sea 2 (∼ 370 µatm) in the Canada Basin in summer.…”