Calcium distribution in the subtropical Atlantic Ocean: Implications for calcium excess and saturation horizons

This study constituted the first attempt to measure dissolved calcium ([Ca2+](meas)) in the subtropical North Atlantic, during a zonal transoceanic cruise along 24.5 degrees N (WOCE A05 section), in summer 1992. [Ca2+](meas) was obtained in 20 full-depth equidistant stations along the section, showing that their gradients are rather sensitive to horizontal and vertical water mass distribution. Deep waters along 24.5 degrees N systematically show a positive calcium excess of 20 +/- 14 mu mol.kg(-1), i.e., more [Ca2+] than expected by CaCO3 dissolution and organic matter oxidation (estimated by total alkalinity and nitrate). CaCO3 settling and benthic dissolution accounts for 70% and the North Atlantic plus Arctic riverine inputs of HCO3- for the remaining 30%. Combining [Ca2+](meas) and CO2 data sets, carbonate mineral saturation states distributions for aragonite and calcite can be obtained. The two solubility ratios resulted, on average, 0.5% smaller than if conservative behavior for Ca2+ was assumed (an approach widely followed when [Ca2+]meal is unknown). As a result, shallower saturation horizon depths for both carbonate states (19 dbar for aragonite and 10 dbar for calcite) are yielded if [Ca2+](meas) is taken into account instead than estimated from salinity. (C) 2016 Elsevier B.V. All rights reserved.