Project: Collaborative Research: NSFGEO-NERC: A Thermodynamic Chemical Speciation Model for the Oceans, Seas, and Estuaries

Project Overview:
Ocean acidification due to increasing dissolution of carbon dioxide in seawater, and the effects of cumulative stressors (including acidification, pollution, warming, and anoxia) are among the top priorities for ocean research. Scientific progress requires accurate and consistent measurements across the globe to monitor and understand present effects, and modelling to evaluate future scenarios and methods of remediation. The work of observational scientists and modelers is linked by the need for an accurate knowledge of the chemical speciation of the inorganic carbonate system, pH, and nutrient and contaminate trace metals, in both natural waters and the reference materials and solutions used for instrument calibration. 

In this collaborative project between investigators in the U.S. and the United Kingdom, we have created a comprehensive modeling tool for determining the detailed chemical composition, or "speciation," of natural waters including seawater.

The work has enabled a step change in the capability of marine scientists to measure, interpret, and predict chemical speciation and pH in natural waters of varying composition by creating a speciation model based upon the Pitzer equations for the calculation of solute and water activities. The approach has a long track record of success in geochemistry. The equations are based upon the concept that interactions between pairs and triplets of dissolved solute species control activities. The values of the parameters for these interactions are determined from a wide range of measurements of solution properties. Work in this project has included the measurement of aqueous species activities, and the use of recent literature data, to improve and test the model; the computer coding and validation of the model and the development of methods to quantify the relationship between uncertainties in model-predicted speciation and those in the underlying measurements; and engagement with oceanographers internationally to help design practical speciation modelling tools and associated guidance for specific applications. 

The result of this ongoing project is a critically-evaluated thermodynamic model of natural waters, containing (in any proportion) the major and minor ions of seawater, seawater-related pH buffers, and the trace metals iron, zinc, copper, cadmium, and others. The model, known as MarChemSpec, is made available to the wider community through a web site and Zenodo archive (See "Project Outputs" section below). The academic beneficiaries are chemical oceanographers, aquatic scientists, and others working in the following areas: ocean acidification including polar brines (in seasonal ice); pore water chemistry diagenetic reactions; paleoceanography; metrology of pH in saline solutions such as seawater. 

Project outputs:

The outputs of this project a series of standalone programs (known as MarChemSpec) that are run on Windows, Linux, and Apple computers. There are also MATLAB and Python functions (for Windows and Linux) that do the same calculations. The data on which the programs and functions are based consist of sets of thermodynamic equilibrium constants and Pitzer activity coefficient model interaction parameters. These are listed in the Supporting Information to the published papers from the project.

Documentation and access to the MarChemSpec programs developed by this project can be found at:
Clegg, S. L., &; Turner, D. R. (2023). MarChemSpec (Marine Chemical Speciation Model)  (Version 1.01) [Computer software]. Zenodo. https://doi.org/10.5281/ZENODO.8373045.
*  This archival copy contains the description of the models as well as zip files containing all the programs. There is also a file containing all the manuals.
* MarChemSpec programs should be cited using the above Clegg & Turner (2023) reference as well as the open source papers listed in section "4. Papers and Other Documents" of that archival page.

Model development, including a programme of measurements, is ongoing and potential users can find the latest news at website:
"MARCHEMSPEC: Chemical Speciation Modelling in Seawater to Meet 21st Century Needs" SCOR Working Group 145.  http://marchemspec.org/  
* For a summary of MarChemSpec capabilities, and for downloads, see: marchemspec.org/software.

NSF Award Abstract:

Ocean acidification due to increasing dissolution of carbon dioxide in seawater, and the effects of cumulative stressors (including acidification, pollution, warming, and anoxia) are among the top priorities for ocean research. Scientific progress requires accurate and consistent measurements across the globe to monitor and understand present effects, and modelling to evaluate future scenarios and methods of remediation. The work of observational scientists and modelers is linked by the need for an accurate knowledge of the chemical speciation of the inorganic carbonate system, pH, and nutrient and contaminate trace metals, in both natural waters and the reference materials and solutions used for instrument calibration. In this collaborative project between investigators in the U.S. and the United Kingdom, a team of scientists will work to create a thoroughly updated modeling tool for determining the detailed chemical composition, or "speciation," of seawater.

The investigators propose to create a step change in the capability of marine scientists to measure, interpret, and predict chemical speciation and pH in natural waters of varying composition by creating a speciation model based upon the Pitzer equations for the calculation of solute and water activities. The approach has a long track record of success in geochemistry. The equations are based upon the concept that interactions between pairs and triplets of dissolved solute species control activities. The values of the parameters for these interactions are determined from a wide range of measurements of solution properties. Work in this project will include measuring activities and heat capacities, and using recent literature data, to improve and test the model; the computer coding and validation of the model and the development of methods to quantify the relationship between uncertainties in model-predicted speciation and those in the underlying measurements; and engagement with oceanographers internationally to help design practical speciation modelling tools and associated guidance for specific applications. The result of this project will be a critically-evaluated thermodynamic model of natural waters containing (in any proportion) the major and minor ions of seawater, seawater-related pH buffers, and the trace metals iron, zinc, copper, and cadmium, to be made available to the wider community through a project web site. The academic beneficiaries will be chemical oceanographers, aquatic scientists, and others working in the following areas: ocean acidification including polar brines (in seasonal ice); pore water chemistry diagenetic reactions; paleoceanography; metrology of pH in saline solutions such as seawater. A postdoctoral researcher will be supported and trained through this project.

This is a project that is jointly funded by the National Science Foundation's Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. 

NERC Award Abstract:
Ocean acidification due to the dissolution of anthropogenic CO2, and the effects of cumulative stressors (including acidification, pollution, warming, and anoxia) are among the top priorities for ocean research, requiring accurate and consistent measurements across the globe to monitor and understand present effects, and modelling to evaluate future scenarios and methods of remediation. The work of observational scientists and modellers is linked by the need for an accurate knowledge of the chemical speciation of the inorganic carbonate system, pH, and nutrient and contaminate trace metals, in both natural waters and the reference materials and solutions used for instrument calibration. Chemical speciation is defined as the distribution of a chemical element between different molecular and ionic forms in seawater, and determines its reactivity and bioavailability. Speciation depends on the value of the relevant thermodynamic equilibrium constant, and on the activities of each of the dissolved ions and molecules. These are complex functions of temperature, pressure, and salinity (or, more generally, solution composition), and cannot be predicted from theory. Many of the important reactions in seawater involve acid-base equilibria, which introduces pH as an additional variable. Despite the importance of chemical speciation, the available calculation tools are often only simple empirical equations that yield equilibrium constants for reactions as functions of salinity and temperature. Such equations cannot be used for many important natural waters whose composition differs from that of normal seawater (e.g., polar brines, estuaries, pore-waters, enclosed seas, and paleo-oceans). Furthermore, human-driven changes in seawater pH and carbonate chemistry in shelf seas and estuaries are complicated by the effects of eutrophication, upwelling, the dissolved solutes contained in river water, and changes in metal toxicity accompanying pH change. Consequently, despite the best efforts of physical chemists over the last several decades, there is not yet the ability to calculate the equilibria controlling the chemical factors impacting shellfish and a broader range of marine fauna in the brackish/mesohaline environments typical of many estuaries and coasts. We will create a step change in the capability of marine scientists to measure, interpret, and predict chemical speciation and pH in natural waters of varying composition by creating a speciation model based upon the Pitzer equations for the calculation of solute and water activities. The approach has a long track record of success in geochemistry. The equations are based upon the concept that interactions between pairs and triplets of dissolved solute species control activities. The values of the parameters for these interactions are determined from a wide range of measurements of solution properties. Work in this project will include measuring activities and heat capacities, and using recent literature data, to improve and test the model; the computer coding and validation of the model and the development of methods to quantify the relationship between uncertainties in model-predicted speciation and those in the underlying measurements; and engagement with oceanographers internationally to help design practical speciation modelling tools and associated guidance for specific applications. The completed model will enable the activities and speciation of all seawater components to be calculated within a unified framework that, (i) includes the major and trace elements in seawater and its mixtures with freshwaters, (ii) includes other saline environments of differing composition, and (iii) encompasses the buffers that are used to calibrate pH and other instruments, and. Our results will this advance the quantitative understanding of chemical speciation - from ocean measurements to ecosystem models - for an expanded range of natural water bodies and marine environments.