Analytical Methods and Critical Analyses Supporting Thermodynamically Consistent Characterizations of the Marine CO 2 System

• Main Author: Sharp, Jonathan D.
• Format: Doctoral or Postdoctoral Thesis
• Language: unknown
• Published: Digital Commons @ University of South Florida 2020
• Subjects: Carbonate ion; Ocean carbon system consistency; Organic alkalinity; Seawater titrations; Total alkalinity; Ultraviolet spectrophotometry; Other Oceanography and Atmospheric Sciences and Meteorology; Carbonic acid
• Online Access: https://digitalcommons.usf.edu/etd/8588; https://digitalcommons.usf.edu/context/etd/article/9785/viewcontent/Sharp_usf_0206D_16421.pdf

Chemical equilibria describing the unique behavior of gaseous and ionic forms of dissolved carbon dioxide (CO2) in seawater comprise what is known as the marine CO2 (or carbonate) system. Observations of the marine CO2 system with high degrees of accuracy, reproducibility, spatial coverage, and temporal resolution are critical for evaluating natural cycles of carbon within the Earth system, as well as chemical and biological responses to anthropogenic CO2 emissions. One component of the CO2 system is the carbonate ion (CO2−3), a dissolved ion that is produced when carbonic acid (H2CO30) dissociates both its hydrogen ions. The carbonate ion is an important buffer against seawater pH changes and is vital for marine organisms that build shells and/or skeletons out of calcium carbonate. Concentrations of carbonate, [CO2−3], are typically inferred from measurements of other CO2 system variables followed by calculations using established thermodynamic relationships. This dissertation advances and evaluates a method for direct measurement of [CO2−3]. The method is based on observations of seawater absorbance in the ultraviolet spectrum after addition of dissolved lead (Pb2+). The absorbance is caused by lead carbonate and lead chloride species, and its magnitude is a function of the carbonate ion concentration. In this dissertation, an instrument-dependent artifact in [CO2−3] measurements is identified and corrected, improving differences between measured and calculated [CO2−3] from –2.78 ± 2.9 µmol kg–1 to –0.03 ± 1.9 µmol kg–1 for the datasets examined in chapter two (where μ ± σ represents the mean, μ, and one standard deviation, σ). An algorithm is introduced to convert [CO2−3] measured at laboratory conditions to [CO32−] at in situ ocean conditions. Aragonite saturation states appropriate to in situ conditions are determined from laboratory-measured [CO2−3] using this algorithm. The resulting saturation states show very good agreement with saturation states calculated from laboratory-measured pH and total ...