Assessing seasonal changes in carbonate parameters across small spatial gradients in the Northeastern Chukchi Sea

AbstractObservations of the marine carbonate system were made in 2010 in the northeastern Chukchi Sea to constrain the seasonal progression of carbonate mineral saturation states (Ω) throughout the water column and determine the air–sea flux of carbon dioxide (CO2). As sea ice retreats from the Chukchi Shelf, primary production consumes dissolved inorganic carbon (DIC) in the euphotic zone causing pH and carbonate mineral saturation states to increase. Throughout the summer and early autumn months of 2010, saturation states for calcite and aragonite ranged from 2.5 to 4.0 and 1.5 to 2.5, respectively, well about the saturation horizon of 1.0. Much of the organic matter produced during the bloom was vertically exported from the relatively small study area leading to an uptake of CO2 from the atmosphere of at least 340,000kg-C. The exported organic matter settled near the bottom and was remineralized back into DIC, causing concentrations to increase sharply, particularly in autumn months, driving down pH to as low as 7.75 and suppressing the concentrations of important carbonate minerals to the point that aragonite became undersaturated. The data showed a definitive seasonal progression of this process with aragonite becoming partially undersaturated along the bottom in September, and broadly undersaturated in October. While carbonate saturation states would naturally be suppressed by the high rates of export production and the accumulation of DIC near the bottom, the penetration of anthropogenic CO2 into the water column (ocean acidification) has caused these observed undersaturations, which will likely expand as CO2 levels in the atmosphere continue to rise in the coming decades