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.