Abstract
We elaborate the need for a quality-controlled chemical speciation model for seawater and related natural waters, work which forms the major focus of SCOR Working Group 145. Model development is based on Pitzer equations for the seawater electrolyte and trace components. These equations can be used to calculate activities of dissolved ions and molecules and, in combination with thermodynamic equilibrium constants, chemical speciation. The major tasks to be addressed are ensuring internal consistency of the Pitzer model parameters (expressing the interactions between pairs and triplets of species, which ultimately determines the calculated activities), assessing uncertainties, and identifying important data gaps that should be addressed by new measurements. It is recognised that natural organic matter plays an important role in many aquatic ecosystems, and options for including this material in a Pitzer-based model are discussed. The process of model development begins with the core components which include the seawater electrolyte and the weak acids controlling pH. This core model can then be expanded by incorporating additional chemical components, changing the standard seawater composition and/or broadening the range of temperature and pressure, without compromising its validity. Seven important areas of application are identified: open ocean acidification; micro-nutrient biogeochemistry and geochemical tracers; micro-nutrient behaviour in laboratory studies; water quality in coastal and estuarine waters; cycling of nutrients and trace metals in pore waters; chemical equilibria in hydrothermal systems; brines and salt lakes.
Highlights
Ocean composition is changing at an unprecedented rate as a result of anthropogenic pressures, with important implications for the health of the oceans and for economic activities
Chemical changes need to be modeled accurately to evaluate future scenarios and remediation strategies. These requirements are linked by the need to understand chemical speciation—for example that of carbonate and trace metal micronutrients—both in natural waters and in the reference materials and solutions used for analytical method verification and instrument calibration
We have proposed a set of core components that are essential for all marine science applications, followed by the specific requirements for each application
Summary
Ocean composition is changing at an unprecedented rate as a result of anthropogenic pressures, with important implications for the health of the oceans and for economic activities. Changes in ocean chemistry may be small on an annual basis, but are modulated on short time and space scales by physical parameters, such as salinity, temperature, and irradiance, as well as variations in upwelling and ocean currents These chemical changes need to be measured accurately and consistently across the globe to monitor and understand contemporary processes. Chemical changes need to be modeled accurately to evaluate future scenarios and remediation strategies These requirements are linked by the need to understand chemical speciation—for example that of carbonate and trace metal micronutrients—both in natural waters and in the reference materials and solutions used for analytical method verification and instrument calibration. The major tasks for the Working Group include: ensuring self-consistency of the chemical speciation model and traceability of its parameters to thermodynamic measurements, establishing quantitatively the uncertainties in the calculated speciation, and identifying important data gaps to be addressed by new measurements
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