Sea-ice melt CO2–carbonate chemistry in the western Arctic Ocean: meltwater contributions to air–sea CO2 gas exchange, mixed-layer properties and rates of net community production under sea ice

[摘要] The carbon dioxide (CO₂)-carbonate chemistry of sea-ice melt andco-located, contemporaneous seawater has rarely been studied in sea-ice-covered oceans. Here, we describe the CO₂–carbonate chemistry of sea-icemelt (both above sea-ice as "melt ponds" and below sea-ice as "interfacewaters") and mixed-layer properties in the western Arctic Ocean in the earlysummer 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 lowcompared to the co-located underlying mixed layer. The partial pressure ofCO₂ (pCO₂) in these melt ponds was highly variable(∼<10 to >1500 μatm) with the majority of melt pondsacting as potentially strong sources of CO₂ to the atmosphere. The pH ofmelt pond waters was also highly variable ranging from mildly acidic (6.1 to7) to slightly more alkaline than underlying seawater (>8.2 to 10.8). Allof the observed melt ponds had very low (<0.1) saturation states (Ω) for calcium carbonate (CaCO₃) minerals such as aragonite (Ω_aragonite). Our data suggest that sea-ice generated alkaline oracidic type melt pond water. This melt water chemistry dictates whether theponds are sources of CO₂ to the atmosphere or CO₂ sinks. Below-iceinterface water CO₂–carbonate chemistry data also indicated substantialgeneration of alkalinity, presumably owing to dissolution of CaCO₃ insea-ice. The interface waters generally had lower pCO₂ and higherpH/Ω_aragonite than the co-located mixed layer beneath.Sea-ice melt thus contributed to the suppression of mixed-layer pCO₂,thereby enhancing the surface ocean's capacity to uptake CO₂ from theatmosphere. Our observations contribute to growing evidence that sea-iceCO₂–carbonate chemistry is highly variable and its contribution to thecomplex factors that influence the balance of CO₂ sinks and sources (andthereby ocean acidification) is difficult to predict in an era of rapidwarming and sea-ice loss in the Arctic Ocean.

[学科分类] 地球化学与岩石