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

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... total alkalinity (TA; Introduction Sea-ice is one of the largest biomes on the planet. It has a strong impact on climate through its modulation of heat, water vapour and momentum exchanges, and via the physics (stratification, mixing) and biology (sea-ice and pelagic communities) of the polar regions (Thomas and Dieckman, 2010). In the last decade, seasonal sea-ice loss, associated with atmospheric warming, air-sea-ice feedbacks and climate modes of variability has increased significantly in the Arctic Ocean since 2007 (e.g. ACIA, 2005; Serreze and Francis, 2006; Maslanik et al., 2007; Wang and Overland, 2009). Rapid loss of summertime sea-ice, combined with other synergistic impacts such as hydrological changes (e.g. increased melt and fresh water inputs to polar waters), has profound implications for the biology and physico-biogeochemical con-