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 (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: -log[CO32-]Tspec = log{(CO3β1)/e2} + log{(R – e1)/(1 – Re3/e2)} 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). Based on shipboard and laboratory Pb(II) data and complementary carbon-system measurements, experimental parameters were determined to be (25 °C): log{(CO3β1)/e2} = 5.513 – 5.358×10-2S + 5.166×10-4S2 e1 = 0.2293 – 5.554×10-4S + 8.440×10-5S2 e3/e2 = 3.091 – 8.730×10-2S + 9.363×10-4S2 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.

Easley R. A., Patsavas M. C., Byrne R. H., Liu X., Feely R. & Mathis J. T., in press. Spectrophotometric measurement of calcium carbonate saturation states in seawater. Environmental Science & Technology. Article (subscription required).

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