Geometric properties of platelet ice crystals

摘要

Sea ice that forms in McMurdo Sound, Antarctica, which is adjacent to the McMurdo and Ross Ice Shelves, exhibits the uncommon ice fabric, incorporated platelet ice. This ice fabric is the subject of a high resolution thin sectioning study, consisting of ten closely spaced horizontal sections bounded above and below by vertical sections. From these we investigate the crystallographic and geometric properties of the ice grains and find a relationship between c-axis orientation and the minor axis of an ellipse fitted to a crystal's grain boundary. Although accuracy is dependent upon the size and shape of the crystal, the technique provides a means of extracting crystallographic information from historical datasets for which only thin section photographs remain. Identification of common grains in multiple, 2-D sections allows us to reconstruct an approximation of the original 3-D crystal structures. These structures exhibit a variety of growth interactions, including upward directed crystal growth and intertwining crystals. We also construct hypothetical crystal grain boundaries that might be observed for a variety of vertical thin section orientations. These indicate that grain shape can depend on thin section orientation and illustrates some of the limitations of 2-D representations of 3-D crystals. Serrated grain boundaries are identified between neighboring crystals in several vertical sections. Geometric arguments and crystallographic observations lead us to propose a mechanism of layer nucleation within grain boundary grooves that occurs as supercooling exceeds a threshold value.