[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.
The distributions of carbonate system parameters in Hudson Bay, which not only receives nearly one-third of Canada's river discharge but is also subject to annual cycles of sea-ice formation and melt, indicate that the timing and magnitude of freshwater inputs play an important role in carbon biogeochemistry and acidification in this unique Arctic ecosystem. This study uses basin-wide measurements of dissolved inorganic carbon (DIC) and total alkalinity (TA), as well as stable isotope tracers (δ 18 O and δ 13 C DIC ), to provide a detailed assessment of carbon cycling processes within the bay. Surface distributions of carbonate parameters reveal the particular importance of freshwater inputs in the southern portion of the bay. Based on TA, we surmise that the deep waters in the Hudson Bay are largely of Pacific origin. Riverine TA end-members vary significantly both regionally and with small changes in near-surface depths, highlighting the importance of careful surface water sampling in highly stratified waters. In an along-shore transect, large increases in subsurface DIC are accompanied by equivalent decreases in δ 13 C DIC with no discernable change in TA, indicating a respiratory DIC production on the order of 100 µmol kg −1 DIC during deep water circulation around the bay.
Geoengineering strategies to slow sea ice melting would affect not only Earth's climate but also the biology and chemistry of the oceans, atmosphere, and ice.
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