Wednesday 17 October
In my last blog post I talked about sea ice ‘diamonds’. Well today I saw a whole spectrum of sea ice ‘jewels’ – sapphires, emeralds, amethysts and more.
What look like jewels to my eyes though are actually sea ice crystals, which a cross-polarised filter allows you to see in great detail. The shape, orientation and size of the crystals tell scientists how the sea ice formed and whether any snow has been incorporated into the ice column. The structure also reveals brine pockets and channels in which highly saline water accumulates before draining out of the ice, or where brine or algae may travel up in the ice. This ice structure data allows scientists to put other measurements like ice temperature, salinity, and nutrient and sea ice algae concentrations, into context. Examining multiple ice cores from each of our sea ice stations, in this way, is a critical part of many scientists’ research.
To prepare a core for analysis a vertical thin section (about 1mm thick and 10cm wide) from the centre of the core is extracted using a bandsaw. This sample is thinned further with a microtome (a very fine, knife-like instrument), cleaned and then photographed under a cross-polarising filter.
Long, columnar crystals indicate that the ice formed in a calm environment, generally underneath an existing layer of sea ice, giving the crystals the opportunity to form long, unbroken fingers as they grow downwards into the water column. Ice formed in turbulent weather in the upper ocean, in contrast, tends to be more granular, as ‘frazil ice’ crystals coalesce. (Frazil crystals are millimetre- to centimetre-sized platelets of ice that float upwards in the upper water column and accumulate on the ocean surface.)
A different type of granular ice can also form when snow on the ice surface becomes saturated with sea water. This creates a slush layer that then gets incorporated into the ice when it freezes, forming ‘snow ice’. Distinguishing between frazil ice and snow ice in sea ice cores is important. Snow ice formation is a widespread phenomenon in the Antarctic sea ice zone, as the region receives substantial snowfall from transiting low pressure systems. These transfer moisture from the surrounding warmer open ocean areas. A thick layer of snow can depress the sea ice surface below sea level, leading to flooding and the formation of snow ice. Sea ice physicist Dr Petra Heil, from the Australian Antarctic Division and the Antarctic Climate and Ecosystems Cooperative Research Centre, estimates that snow ice may contribute some 15–20% of the Antarctic sea ice mass balance (total gain and loss of ice).
It’s difficult to differentiate snow ice from frazil ice based on visual inspection of the crystal structure in an ice core, so scientists melt short sections of the remaining core and analyse them for stable oxygen isotopes (and salinity). The ratio of different oxygen isotopes (different molecular forms of oxygen) provides information on whether the sea ice layer is atmospheric or oceanic in origin — and therefore whether it is composed of snow ice.
Dr Heil says vertical layers identified in the crystal structure of ice cores also allow scientists to identify stacked slabs of ice, which are the result of rafting or ridging events. These events occur in response to converging winds or ocean currents.
“Sea-ice dynamics is an efficient way to increase the ice volume over a very short (near instantaneous) time span, and is the only way to yield ice thicker than it would be possible due to thermodynamics ice growth alone,” Dr Heil says.
“It is expected that with predicted increasing storminess over the Southern Ocean, such dynamic processes may lead to an increase the ice volume per area of Antarctic sea ice.”