Spectrophotometric Measurement of Calcium Carbonate Saturation States in Seawater

Authors

Regina A. Easley, University of South Florida
Mark C. Patsavas, University of South Florida
Robert H. Byrne, University of South FloridaFollow
Xuewu Liu, University of South FloridaFollow
Richard A. Feely, NOAA Pacific Marine Environmental Laboratory, Seattle
Jeremy T. Mathis, University of Alaska Fairbanks

Document Type

Article

Publication Date

2013

Keywords

Saturation, Seawater, Absorption, Ions, Inorganic carbon compounds

Digital Object Identifier (DOI)

https://doi.org/10.1021/es303631g

Abstract

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.

Was this content written or created while at USF?

Yes

Citation / Publisher Attribution

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

Scholar Commons Citation

Easley, Regina A.; Patsavas, Mark C.; Byrne, Robert H.; Liu, Xuewu; Feely, Richard A.; and Mathis, Jeremy T., "Spectrophotometric Measurement of Calcium Carbonate Saturation States in Seawater" (2013). Marine Science Faculty Publications. 1648.
https://digitalcommons.usf.edu/msc_facpub/1648