Abstract
AbstractPure ice, brine and solid minerals are the main contributors to sea ice mass. Constitutional changes with salinity and temperature exert a fundamental control on sea ice physical, chemical, and biological properties. However, current estimation methods and model representations of the sea ice phase composition suffer from two limitations—in a context of poorly quantified uncertainties. First, salt minerals are neglected. Second, formulations are inconsistent with international standards, in particular with the International Thermodynamic Equation of Seawater (TEOS‐10). To address these issues, we revisit the thermodynamics of the sea ice phase composition by confronting observations, theory, and the usual computation methods. We find remarkable agreement between observations and the Gibbs‐Pitzer theory as implemented in FREZCHEM, both for brine salinity (RMSE = 1.9 g/kg) and liquid H2O mass fraction (RMSE = 8.6 g/kg). On this basis, we propose expanded sea ice phase composition equations including minerals, expressed in terms of International Temperature Scale 1990 temperature and absolute salinity, and valid down to the eutectic temperature (−36.2 °C). These equations precisely reproduce FREZCHEM, outcompeting currently used calculation techniques. We also suggest a modification of the TEOS‐10 seawater Gibbs function giving a liquidus curve consistent with observations down to the eutectic temperature without changing TEOS‐10 inside its original validity range.
Highlights
Sea ice is composed of pure ice, liquid brine, hydrated salt minerals, and gas bubbles (Hunke et al, 2011; Light et al, 2003; Weeks & Ackley, 1986)
We revisited the thermodynamics of sea ice phase composition by confronting observations, theory, and classical computation methods, from a revised formulation of the problem and a thorough account for available sources of information
We focused on two important diagnostics: brine salinity and liquid H2O fraction
Summary
Sea ice is composed of pure ice, liquid brine, hydrated salt minerals, and gas bubbles (Hunke et al, 2011; Light et al, 2003; Weeks & Ackley, 1986) These multiple phases render sea ice structurally, thermodynamically, biologically, and chemically different from freshwater ice (Thomas, 2017). Of all these constituents, brine is the most studied next to ice (see, e.g., Notz, 2005) and affects the ice thermal regime and seasonal cycle of ice thickness (e.g., Bitz & Lipscomb, 1999; Untersteiner, 1961; Vancoppenolle et al, 2005; Wiese et al, 2015) and, in turn, the seasonal evolution of ice extent and volume (Semtner, 1984; Turner & Hunke, 2015; Vancoppenolle et al, 2009). Sea ice models represent brine inclusions from highly parameterized (Semtner, 1976) to more and more explicit approaches (e.g., Bitz & Lipscomb, 1999; Griewank & Notz, 2013; Moreau et al, 2015; Turner et al, 2013), whereas biogeochemical field-based sea ice studies often include brine inclusions as part of their characterization of the sea ice environment (Miller et al, 2015).
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