Growth, structure, and properties of sea ice.

In an experimental and theoretical study, we model a phenomenon observed in the summer Arctic, where a fresh-water layer at a temperature of 0°C floats both over a sea-water layer at its freezing point and under an ice layer. Our results show that the ice growth in this system takes place in three phases. First, because the fresh-water density decreases upon supercooling, the rapid diffusion of heat relative to salt from the fresh to the salt water causes a density inversion and thereby generates a high Rayleigh number convection in the fresh water. In this convection, supercooled water rises to the ice layer, where it nucleates into thin vertical interlocking ice crystals. When these sheets grow down to the interface, supercooling ceases. Second, the presence of the vertical ice sheets both constrains the temperature T and salinity s to lie on the freezing curve and allows them to diffuse in the vertical. In the interfacial region, the combination of these processes generates a lateral crystal growth, which continues until a horizontal ice sheet forms. Third, because of the T and s gradients in the sea water below this ice sheet, the horizontal sheet both migrates upwards and increases in thickness. From one-dimensional theoretical models of the first two phases, we find that the heat-transfer rates are 5–10 times those calculated for classic thermal diffusion.

The evolution of under-ice melt ponds, or double diffusion at the freezing point

Ice shelves are thick, floating ice masses most often associated with Antarctica where they are seaward extensions of the grounded Antarctic ice sheet and sources of many icebergs. However, there are also ice shelves in the Arctic, primarily located along the north coast of Ellesmere Island in the Canadian High Arctic. The only ice shelves in North America and the most extensive in the north polar region, the Ellesmere ice shelves originate from glaciers and from sea ice and are the source of ice islands, the tabular icebergs of the Arctic Ocean. The present state of knowledge and understanding of these ice features is summarized in this paper. It includes historical background to the discovery and early study of ice shelves and ice islands, including the use of ice islands as floating laboratories for polar geophysical research. Growth mechanisms and age, the former extent and the twentieth century disintegration of the Ellesmere ice shelves, and the processes and mechanisms of ice island calving are summarized. Surface features, thickness, thermal regime, and the size, shape, and numbers of ice islands are discussed. The structural-stratigraphic variability of ice islands and ice shelves and the complex nature of their growth and development are described. Large-scale and small-scale dynamics of ice islands are described, and the results of modeling their drift and recurrence intervals are presented. The conclusion identifies some unanswered questions and future research opportunities and needs.

Arctic ice shelves and ice islands: Origin, growth and disintegration, physical characteristics, structural-stratigraphic variability, and dynamics

Results of impulse radar, ice crystal c axis, and subice current measurements on the fast ice near Narwhal Island, Alaska, are presented The crystal structure of the ice was found to have a horizontal crystal c axis with a preferred azimuthal orientation This orientation was found to align with the direction of the current at the ice-water interface Impulse radar reflection measurements revealed that the preferred orientation of the sea ice crystal structure behaved as a microwave polarizer It was observed that when the antenna E field was oriented parallel with the c axis of the crystal platelets, a strong reflection of the radar signal from the bottom of the ice was obtained However, when the antenna E field was oriented perpendicular to the c axis, no bottom reflection was detected The results of this study fully support earlier reports of sea ice inhomogeneity and anisotropy in reference to both structure and electromagnetic energy transmission

Radar anisotropy of sea ice due to preferred azimuthal orientation of the horizontal c axes of ice crystals

The morphology, sediments, and processes associated with the construction of a moraine along the western margin of the ice shelf in George VI Sound, Antarctica, are discussed. The moraine occurs as a double ridge where the ice sheet grounds against promontories on Alexander Island and is approximately horizontal over a distance of 120 km. It consists of exotic rock debris carried into the ice shelf by Antarctic Peninsula glaciers and local rock debris derived from the grounding line on Alexander Island. As the coast steepens, so the proportion of exotic rocks increases. The transport of basal material from the peninsula implies that there can be little bottom melting beneath this part of the ice shelf. The moraine is modified by streams and marginal lakes which periodically drain into and through the ice shelf. Tidal lakes are impounded against the ice shelf in shallower embayments and consist of fresh water overlying sea-water. A conceptual model of the moraine is developed and may help to explain some features of puzzling horizontal moraines found in formerly glaciated areas.

/pdf/an-ice-shelf-moraine-george-vi-sound-antarctica-1dg7g8ru6t.pdf

An Ice-shelf Moraine, George VI Sound, Antarctica

Oxygen-isotope and chlorinity determinations, as well as petrographie observations, indicate that the basement we of the Ward Hunt Ice Shelf is largely composed of a unique brackish ice, which interdigitates with sea ice Some iced firn occurs near the top of the Basement Ice, below an unconformity stratification in brackish and sea ice represents annual increments to the bottom of the ice shelf The c-axis vertical orientation and small-angle grain-boundary relations in brackish ice are explained by nucleation and floating of ice dendrites from the undercooled brackish water zone to the bottom of the ice shelf, where they attach themselves sub-parallel to the plane of the undersurface Ice island T-3 did not come from a break-up of the main part of the Ward Hunt Ice Shelf, but probably originated in a nearby area to the west

/pdf/basement-ice-ward-hunt-ice-shelf-ellesmere-island-canada-wqcpkgahjf.pdf

Basement Ice, Ward Hunt Ice Shelf, Ellesmere Island, Canada

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Basement Ice, Ward Hunt Ice Shelf, Ellesmere Island, Canada