Sea Ice

We all know that ice surrounds the Antarctic continent, as well as covering 99% of its surface (to a thickness of more than 3,000 m on average). The sea ice covers at its seasonal maximum some 20 x 106 km2 (and at its minimum, 2 x 106 km2), so over the course of 6 months a huge area of the Southern Ocean becomes ice covered.

What are the effects of this ice? First, it acts like a lid, reducing the exchanges of heat, radiation, materials, and water between the ocean and atmosphere. For example, under the ice in the Ross Sea, carbon dioxide increases to up to 460 parts per million (ppm) during winter as a result of the oxidation of organic matter produced during the summer. As the ice recedes, two things can happen to this "excess" CO2 (excess relative to atmospheric equilibrium)— it can be removed by phytoplankton uptake during spring growth, or it can outgas to the atmosphere. Undoubtedly both occur, but the extent of each will depend on a number of factors. At this time we can't say with confidence which process dominates in spring.

Ice also reduces heat flux from the ocean to the atmosphere. Since the ocean is warm relative to the air (-1.86°C in the ocean, and air temperatures often reach -60°C in the Ross Sea during winter), heat is always lost to the air. Ice, once formed, is a relatively solid sheet approximately 1 meter thick, and effectively stops heat losses from the ocean.

Ice also is a unique biological substrate for organisms, and is a unique oceanic environment. It is three-dimensional; often there is a layer of snow on top of the ice, which itself is relatively porous and perfused with brine. Microbes, such as bacteria and microalgae, grow in layers in the ice. In the Ross Sea during CORSACS as we break ice, we see a dark brown layer of algae on the bottom surface—the interface between the ocean and ice. Some call ice an "upside-down benthos," which can be an apt analogy. The interstitial environment is also unusual—dissolved organic carbon (DOC) levels are two orders of magnitude greater than those in the ocean, and as the ice/brine melts, the DOC is rapidly used by bacteria. No one knows why the ice bacteria do not, or cannot, use the DOC in ice. Some organisms have larvae that spend a full year within the ice, feeding on the algae, before changing into juveniles. Many fish live in close association with ice as well; hence many penguins and seals prey on the undersurface of the ice too.

During CORSACS a few of the groups are using the opportunity to conduct measurements within the ice by taking cores. Much of the material in ice is eventually released into the water column, and hence knowing the constituents is relevant to the CORSACS objectives. Soon we will be in the polynya and open water, and ice will surround us, but will also influence our work. It also provides a beautiful environment for us to view and observe the wonders of this Antarctic world we are visiting.