Microstructure and electrical properties of marine ice and its relationship to meteoric ice and sea ice

A 215-m ice core has penetrated the central part of the Ronne Ice Shelf (76°59′S, 52°16′W). The core consisted of meteoric ice above 152.8-m depth; below this the ice was bubble free, and of seawater origin. The salinity of the layer below 152.8-m depth is less than 0.05 ppt, very much lower than typical sea ice. The ice represents an unusual source of ice intermediate in salinity and some physical properties between meteoric ice and sea ice. The structure of the ice from four different depths, where salinities vary by a factor of 4, has been investigated using a scanning electron microscope (SEM). The fabric and grain structures are unusual and seem to depend on the impurity concentration in the ice. Chemical impurity localization has also been investigated. The dielectric properties of the ice show that the ice has a transitional behavior from the linear dependence of conductivity on chloride concentration found in meteoric ice, to the less predictable sea ice behavior. The behavior may be interpreted in terms of the structure of the ice. The higher-salinity samples show convoluted grain boundaries, small grain size, and brine inclusions. There are indications that the brine inclusions were liquid at temperatures between −10°C and −30°C. By contrast, the lower-salinity samples possess larger grain sizes and show no evidence of brine inclusions. All samples show brine concentrated at triple grain junctions and also along two-grain boundaries in higher-salinity samples. The dielectric properties of the lower-salinity samples are well described by a Jaccard mechanism with L defects created in proportion to salinity, that also describes the behavior of meteoric ice. Higher-salinity samples exhibit lower conductivity than would be seen if L defects continued to be created in proportion to salinity. The dielectric and structural data are consistent with a solubility limit of about 300 μM for Cl in the ice lattice.

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