Spectrophotometric Measurement of Calcium Carbonate Saturation States in Seawater

Measurements of ocean pH and carbonate ion concentrations in the North Pacific and Arctic Oceans were used to determine calcium carbonate saturation states (Ω_CaCO3) from spectrophotometric methods alone. Total carbonate ion concentrations, [CO₃^2–]_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, e_i 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 [CO₃^2–]_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 [CO₃^2–]_T and pH_T were then combined to calculate Ω_CaCO3. For the case of aragonite, 95% of the spectrophotometric aragonite saturation states (Ω_Aspec) were within ±0.06 of the conventionally calculated values (Ω_Acalc) when 0.5 ≤ Ω_A ≤ 2.0. When Ω_A > 2.0, 95% of the Ω_Aspec values were within ±0.18 of Ω_Acalc. Our shipboard experience indicates that spectrophotometric determinations of [CO₃^2–]_T and Ω_CaCO3 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.