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

• We examine the distribution of dissolved calcium in the subtropical North Atlantic Ocean.
• Deep North Atlantic waters show more calcium than expected by CaCO3 dissolution plus organic matter oxidation.
• CaCO3 settling/benthic dissolution accounts for two thirds and riverine inputs for one third of the observed calcium excess.
• Shallower saturation horizon depths for calcite and aragonite are yielded with measured calcium than estimated from salinity.
• Seawater carbon software packages should allow the input of measured calcium, to reduce errors in the solubility ratio.

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°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°N systematically show a positive calcium excess of 20 ± 14 μmol·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 +]meas 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.