Mineral Dynamics in Equilibrium Sea Ice Brines
Our research in the laboratory is used to evaluate the role, presence, and distribution of sea ice minerals in Arctic and Antarctic sea ice. Such behaviour would otherwise remain elusive from fieldwork findings due to the isolated nature (both geographically and microscopically) of the environment these minerals exist within. As research progresses, it is becoming clear that mineral precipitation in sea ice is not only temperature dependent, but also dependent upon the micro-organisms that inhabit the ice, and the fundamental structure of the brine pockets that evolve as the ice cools and warms.
The composition of seawater is dominated by 6 major ions; Na+, Cl-, Mg2+, SO42-, Ca2+ and K+. Together these ions account for >99% of the dissolved salt in seawater. Crucially, the ratio of these major ions to one another throughout the world's oceans is constant, which gives rise to the concept of oceanic 'salinity': a simple measure for the mass of salt per unit mass of seawater (figure 1).
Seawater typically has a salinity (S) of ˜35 g/kgsolution. The presence of this salt has particularly profound effects when seawater freezes. When seawater (S = 35) begins to freeze at -1.8°C, the extraordinary resistance of ice to incorporating dissolved ions into its crystal lattice, results in the concentration of any brine that remains. In sea ice, these cold, concentrated seawater brines become trapped within microscopic pores and channels that permeate the microstructure. As the temperature of sea ice decreases, the icy walls of the brine pockets freeze further, and advance upon the ever more concentrated brine trapped within them, therefore the temperature and brine composition co-evolve in sea ice when it warms or cools. At certain temperatures, these coupled changes result in the brine becoming supersaturated with respect to a sequence of major minerals, resulting in the precipitation of countless microscopic salt crystals throughout the sea ice. Several paradigms exist that predict the sequence and composition of mineral precipitates in frozen seawater as a function of temperature. However, to date there has been an absence of detail in describing the mineral dynamics in the context of sea ice. Our research concentrates upon the dynamics of three major minerals that precipitate at temperatures experienced in Arctic and Antarctic sea ice.
These include; mirabilite (Na2SO4.10H2O), gypsum (CaSO4.2H2O) and hydrohalite (NaCl.2H2O). To assess the dynamics of these minerals within the sea ice environment, our approach uses equilibrium solubility experiments (Butler et al., 2016), as well as short (Butler and Kennedy 2015) and long duration synchrotron powder X-ray diffraction studies at Diamond Light Source, Harwell .. These minerals affect the composition, freezing point and volume of brine within the sea ice microstructure, which consequentially has macroscopic effects to the physical properties of the ice that contribute towards climatic forcings in polar regions. Furthermore, when minerals precipitate in sea ice, they result in significant changes to the constant ratios of seawaters major ions to one another, which compromises the concept of salinity in this environment.