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 (CO2)-carbonate chemistry of sea-ice melt and co-located, contemporaneous seawater has rarely been studied in sea ice covered oceans. Here, we describe the CO2-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 nineteen 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 water in the underlying mixed layer. The partial pressure of CO2 (pCO2) in these melt ponds was highly variable (~ < 10 to > 1500 μatm) with the majority of melt ponds acting as potentially strong sources of CO2 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 to 10.7). All of observed melt ponds had very low (< 0.1) saturation states (Ω) for calcium carbonate (CaCO3) minerals such as aragonite (Ωaragonite). Our data suggests that sea ice generated "alkaline" or "acidic" melt pond water. This melt-water chemistry dictates whether the ponds are sources of CO2 to the atmosphere or CO2 sinks. Below-ice interface water CO2-carbonate chemistry data also indicated substantial generation of alkalinity, presumably owing to dissolution of calcium CaCO3 in sea ice. The interface waters generally had lower pCO2 and higher pH/Ωaragonite than the co-located mixed layer beneath. Sea-ice melt thus contributed to the suppression of mixed layer pCO2 enhancing the surface ocean's capacity to uptake CO2 from the atmosphere. Meltwater contributions to changes in mixed–layer DIC were also used to estimate net community production rates (mean of 46.9 ±29.8 g C m–2 for the early-season period) under sea-ice cover. Although sea-ice melt is a transient seasonal feature, above-ice melt pond coverage can be substantial (10 to > 50%) and under-ice interface melt water is ubiquitous during this spring/summer sea-ice retreat. Our observations contribute to growing evidence that sea-ice CO2-carbonate chemistry is highly variable and its contribution to the complex factors that influence the balance of CO2 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.

Response of Antarctic sea-ice algae to an experimental decrease in pH: a preliminary analysis from chlorophyll fluorescence imaging of melting ice

“…By the end of the incubation period, ice-melt and the biological activity of liberated microbes had increased the seawater pH in all treatments and the control, from 0.2 to 0.55 units. This provides a very basic illustration of the dynamic that occurs at the receding ice edge which has recently been described at an ecologically relevant scale in the Arctic Ocean as part of the NASA ICESCAPE project (Bates et al 2014). The carbonate chemistry of below-ice interface melt waters at 19 ice stations was variable and difficult to predict, but generally exhibited higher pH and lower pCO 2 compared to the co-located mixed layer beneath.…”

“…A general understanding of how sea ice impacts the carbon cycle and ocean-atmosphere CO 2 exchange in polar regions is now emerging (see Rysgaard et al 2011;Fransson et al 2015), but the associated changes in carbonate chemistry due to melting ice remain poorly characterized (Bates et al 2014). The bottom-ice algae that dominate the biological assemblages in Antarctic fast-ice undergo considerable physiological stress during the melt process.…”

Diazotroph Diversity in the Sea Ice, Melt Ponds, and Surface Waters of the Eurasian Basin of the Central Arctic Ocean

“…The generally low temperatures and the high dissolved oxygen in Arctic waters (Bates et al, 2014) support the long-held assumption that nitrogen fixation is unlikely in the Arctic Ocean. However, diazotrophs have developed strategies to withstand freezing and high salinity by producing antifreeze proteins (Schmidt et al, 1991), and to avoid oxygen by developing endo-symbiosis, like Rhizobiales , which include mainly Alpha-proteobacteria and Beta-proteobacteria and have been identified also in polar soils and frost flowers (Bordeleau and Prévost, 1994; Bowman et al, 2013).…”

Ocean Acidification in the Surface Waters of the Pacific-Arctic Boundary Regions

“…In the past decade, ice-free periods have lengthened by several days each year, and open water areas have substantially increased, particularly in autumn months (Stroeve et al, 2007;Comiso et al, 2008). This melt has been substantial enough to freshen the surface throughout the Canada Basin (Yamamoto-Kawai et al, 2009a), and has been shown to lower Ω, producing corrosive conditions in surface waters in some areas (Yamamoto-Kawai et al, 2009b;Mathis et al, 2011a;Cross et al, 2013;Bates et al, 2014).…”