Columnar Ice versus Platelet Ice: Differences, Consequences, and Significance

Abstract

Antarctic land-fast sea ice (fast ice) is sea ice fastened to land or ice shelves. Fast ice is an important component of Antarctic coastal marine ecosystems, providing a prolific habitat for ice algal communities. Columnar ice is the usual mode of fast ice growth in relatively calm waters. However, near an ice shelf, pelagic ice crystals accumulate as an unconsolidated sub-ice platelet layer beneath the columnar ice (CI), where they are subsumed by the advancing sea ice interface to form incorporated platelet ice (PI). We have mapped the full crystallographic orientation of sea ice using electron backscatter diffraction (EBSD) to investigate the differences between CI and PI at the scale of ∼ 0.01 m. This is the first time EBSD has been used to study sea ice. Crystal preferred orientation in CI can be explained by ocean current as is well known from the literature. Analysis of misorientation between grains using EBSD data in PI indicates that the mechanical rotation of crystals at grain boundaries is the most likely explanation for preferred orientation in this case. We examine the consequences of the difference between CI and PI at the scale of ∼ 0.1 m. We demonstrate the feasibility of using temperature fluctuations as a proxy for fluid movement, a key process for supplying nutrients to Antarctic sea ice algal communities. CI and PI permeability distributions in the bottom 0.1 m of winter Antarctic sea ice are marginally different but their arithmetic means are both of order 10-9 m^2. We develop new observation-based algorithms to estimate Antarctic fast ice algal biomass and snow thickness from under-ice irradiance measurements. We analyse these high biomass measurements in CI and PI along transect lines (∼ km) at two contrasting fast ice sites, i.e., in McMurdo Sound and off Davis Station. These algorithms can be used for future non-invasive surveys for example by using moored sensors or underwater vehicles. Altogether the key message of this thesis is that we can apply the same parameterisation for CI and PI in thermodynamic sea ice models unless the crystal orientations are important. To demonstrate this we represent platelet ice processes in a one-dimensional model using the same permeability parameterisations for CI and PI. The results are in good agreement with observational data from an over-winter study in 2009 of McMurdo Sound. Ultimately, this model will improve our understanding of not only sea ice near ice shelves but also the biogeochemical significance of fast ice in Antarctic ecological system