Project: Ocean Acidification: Collaborative Research: Investigation of seawater CO2 system thermodynamics under high pCO2 conditions

NSF abstract:

Assessments of the effects of ocean acidification in a high CO2 world require well-constrained models that interrelate measurable carbon system parameters: total dissolved inorganic carbon (CT) and total alkalinity (AT), CO2 fugacity (fCO2), pH and carbonate ion concentration ([CO3 2-]). Current thermodynamic models that relate CT and AT to fCO2 using carbonic acid dissociation constants (K1' and K2') developed for seawater analyses have been shown to provide good results at current fCO2 values ca. 380 uatm, but break down at the elevated CO2 concentrations (>500 uatm) that are likely to occur before the end of this century. Studies of the effects of elevated CO2 on organismal and ecosystem functions require robust thermodynamic models to ensure that results, which occasionally involve measurements of only two system-variables, provide accurate depictions of the investigated system in its entirety. Because accurate predictions of the consequences of ocean acidification are critical to guide management and policy decisions, development of accurate thermodynamic CO2 system models is essential.

In this project, researchers at the University of South Florida and SRI International will perform best practices measurements of CT, AT, fCO2 and pH, and subsequently assess the magnitudes of K1', K2' and KB' that produce an internally consistent thermodynamic model of the marine CO2 system. The proposed work is facilitated by (a) the recent development and characterization of purified indicators for precise and accurate seawater pH measurements and (b) the development of accurate borate to salinity ratios that provide an improved account of the contributions of boric acid to the buffer intensity of seawater. The study will include investigations of the AT contributions of uncharacterized seawater protolytes (e.g. from dissolved organic matter) as well as the possible interactions between carbonate and borate ions that may influence CO2 equilibria under high pCO2 conditions. The proposed work will additionally be promoted by the recent development of UV spectrometric procedures for direct measurements of carbonate ion concentrations in seawater.

Broader Impacts: The OCB Ocean Acidification Principal Investigator Workshop Report (2011) identified a near-term need to "determine the consequences of large pH change on the carbonate system; as pH shifts the carbonate system may respond in ways different from the range we customarily measure". Since interpretation of the results of CO2 system perturbation experiments depend on accurate knowledge of the carbon system parameters under which the experiments were performed, development of improved carbon system equilibrium relationships, especially at high CO2 levels, is vitally important to the study of ocean acidification.

Graduate and undergraduate students involved in the project will benefit from learning carbon system best practices as well as obtaining a comprehensive understanding of CO2 system thermodynamics. The field component will provide hands-on experience in at sea measurements and statistical analysis of CO2 system relationships. Furthermore, we intend to develop an ocean acidification classroom and laboratory module that will be taught at a local high school serving an ethnically-diverse student population. The lesson plans developed during this activity will be presented to teachers at the annual District-wide Training for High School Science/Math for Pinellas County schools.