Spectrophotometric Measurement of Calcium Carbonate Saturation States in Seawater

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Saturation, Seawater, Absorption, Ions, Inorganic carbon compounds

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Measurements of ocean pH and carbonate ion concentrations in the North Pacific and Arctic Oceans were used to determine calcium carbonate saturation states (ΩCaCO3) from spectrophotometric methods alone. Total carbonate ion concentrations, [CO32–]T, were for the first time at sea directly measured using Pb(II) UV absorbance spectra. The basis of the method is given by the following: where CO3β1 is the PbCO30 formation constant, ei are molar absorptivity ratios, and R = 250A/234A (ratio of absorbances measured at 250 and 234 nm). On the basis of shipboard and laboratory Pb(II) data and complementary carbon-system measurements, the experimental parameters were determined to be (25 °C) the following: The resulting mean difference between the shipboard spectrophotometric and conventional determinations of [CO32–]T was ±2.03 μmol kg–1. The shipboard analytical precision of the Pb(II) method was ∼1.71 μmol kg–1 (2.28%). Spectrophotometric [CO32–]T and pHT were then combined to calculate ΩCaCO3. For the case of aragonite, 95% of the spectrophotometric aragonite saturation states (ΩAspec) were within ±0.06 of the conventionally calculated values (ΩAcalc) when 0.5 ≤ ΩA ≤ 2.0. When ΩA > 2.0, 95% of the ΩAspec values were within ±0.18 of ΩAcalc. Our shipboard experience indicates that spectrophotometric determinations of [CO32–]T and ΩCaCO3 are straightforward, fast, and precise. The method yields high-quality measurements of two important, rapidly changing aspects of ocean chemistry and offers capabilities suitable for long-term automated in situ monitoring.

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Environmental Science & Technology, v. 47, issue 3, p. 1468-1477