Sea-Ice Mass Balance Influenced by Ice Shelves

Mario.Hoppmann [ at ]


Floating ice shelves at the boundary between the Antarctic ice sheet and the Southern Ocean play a crucial role in Antarctica’s mass balance through iceberg calving and basal melting. The latter has strong implications for coastal Antarctic sea-ice properties through the formation of a sub-ice platelet layer. This matrix of intertwined individual ice crystals influences sea-ice properties, mass- and energy balance, and represents an important habitat for a highly productive and uniquely adapted microbial community. Although its potential significance has been recognized already 100 years ago, a comprehensive understanding of this system and associated processes is still not well established due to logistical and methodological difficulties. At the same time, snow precipitation over the the coastal margin and its redistribution through seaward winds may increase the amount of snow deposited on coastal Antarctic sea ice. The presence of a thick, highly reflective and insulating snow cover has manifold consequences for sea-ice mass- and energy balance. However, in situ observations of snow on Antarctic sea ice are sparse, and remote-sensing techniques still lack the ability to infer accurate information about the snow pack. The aim of this thesis was to overcome these limitations by the detailed investigation of a sea-ice regime heavily influenced by a nearby ice shelf. An ongoing monitoring program was developed and realized on the landfast sea ice of Atka Bay, a small embayment in front of the Ekström Ice Shelf in the eastern Weddell Sea, Antarctica. By combining measurements of sea ice, ocean and atmosphere over a period of five years, this work revealed important information about the seasonal cycle of the sea ice, its properties, and how it is influenced by the presence of thick snow on the surface and a several meter thick platelet layer underneath. This study showed that ice platelets emerge from the ice-shelf cavity in episodic events, and interact with the fast ice as early as June. The average annual platelet-layer thickening was 4 m, and the additional buoyancy partly prevented surface flooding and snow-ice formation despite a high snow load. The highly reflective snow cover and the thick platelet layer shielded the solid sea ice from increased radiative and oceanic heat fluxes in summer, respectively. A combination of drillhole measurements, sea-ice temperature profiles and model studies was used to calculate the ice-volume fraction in the platelet layer, a parameter necessary to estimate the overall contribution of ice-shelf processes to sea-ice mass balance. Results yielded ice-volume fractions between 0.18 and 0.35, consistent with earlier studies in other regions of Antarctica. We found that the contribution of ocean/ice-shelf interaction dominated the total sea-ice production, effectively accounting for up to 70 % of annual sea-ice growth. The total annual ice-platelet volume underlying Atka Bay fast ice was equivalent to more than one fifth of the annual basal melt volume under the Ekström Ice Shelf. In addition to the development of a sea-ice monitoring and the scientific outcomes outlined above, this work contributed to the advance of several innovative methodological approaches, which are expected to facilitate sea-ice research in the future: thermistor chains with active heating, multi-frequency electromagnetic induction sounding, and high-resolution X-band synthetic aperture radar (SAR) imagery from satellites. First, this study applied a comprehensive analysis to sea-ice temperature- and heating profiles over a period of 15 months in order to infer the seasonal cycle of sea-ice and platelet-layer properties. By doing so, we were able to show that, in contrast to the established acoustic sounding approach, a thermistor chain with an active heating of embedded resistors is well suited to derive the fast-ice mass balance in regions influenced by ocean/ice-shelf interaction. Our results further indicate that the active heating technique is able to detect the platelet-layer bottom through the steep gradient in current speed, similar to a hot-wire anemometer. Second, we used a ground-based, multi-frequency electromagnetic induction sounding device to quantify sub-ice platelet-layer properties. The combination of in situ data with theoretical responses yielded bulk platelet-layer conductivities of 1154 mS m-1 +- 271 mS m-1, corresponding to ice-volume fractions between 0.29 and 0.43. Detailed descriptions of calibration routines and measurement uncertainties were provided to facilitate future sea-ice studies with such an instrument. First results of platelet-layer thickness retrieval by geophysical inversion of our unique dataset suggest that this method is promising to efficiently map sub-ice platelet-layer thickness on a larger spatial scale than previously feasible. Third, we linked the evolution of X-band synthetic aperture radar (SAR) backscatter during spring/summer transition to changes in the seasonal snow pack. Between 75 % and 93 % of the spatiotemporal variations of the recorded backscatter signal were explained by combination of up to four snow-pack parameters: especially after the onset of early-melt processes and freeze-thaw cycles, the majority of the backscatter variations were influenced by changes primarily in snow/ice-interface temperature, snow depth and grain size. This study implies a great potential to retrieve snow physical properties from X-Band SAR backscatter, albeit more research is necessary to achieve this aim.

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Hoppmann, M. (2015): Sea-Ice Mass Balance Influenced by Ice Shelves , PhD thesis, Jacobs University Bremen.

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