Journal focuses on Antarctic sea ice research

The results from Australia’s major Antarctic sea ice research program conducted during the International Polar Year (2007–2009), were published in a special issue of Deep-Sea Research Part II: Topical Studies in Oceanography in May.

Scientific authors from the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC) were represented on 13 of the 20 peer-reviewed papers, which detailed the results of the Sea Ice Physics and Ecosystem eXperiment (SIPEX).

This voyage was conducted in the sea ice off East Antarctica (115–130°E) during spring (September–October, 2007) and involved more than 50 scientists from 13 countries working on a suite of projects to characterise the physical, biological and biogeochemical characteristics of the pack-ice zone, and their interactions (Australian Antarctic Magazine 14, 2008). A similar sea ice voyage run by the United States Antarctic Program took place in the Bellingshausen Sea, on the other side of Antarctica, at approximately the same time (the results of which are also reported in the journal). Both voyages employed a range of measurement techniques for in situ (in the ice) and underway (ship-based) observations. They also focused on a calibration and validation program for NASA’s Ice Cloud and land Elevation Satellite (ICESat), which rescheduled its Geoscience Laser Altimeter System operations to coincide with the timing of the voyages.

The focus of SIPEX was to investigate the relationships between the physical sea ice environment and the sea ice biology, in particular the presence of algae and krill under the ice. A range of novel instruments and techniques were used including, for the first time, radar and laser altimetry from a helicopter, to measure the snow thickness and ‘freeboard height’ (elevation) above sea level. In addition, an instrumented Remotely Operated Vehicle was used for under-ice observations; a custom-built trawl was deployed for catching krill under the ice; and trace-metal clean sampling equipment was developed to determine the concentration and distribution of iron in sea ice. An overarching goal of the project was to understand the links between sea ice physical, chemical and biological parameters and their importance for sea-ice zone productivity in the Australian Antarctic Territory off East Antarctica.

The ice conditions encountered during the voyage were particularly difficult at times, not only for navigation but for conducting scientific work on the ice. We had difficulty finding ice floes that were suitable to work on, and helicopter reconnaissance flights were needed to navigate through the heaviest ice, which in some areas was up to 10m thick — some of the thickest reported in the East Antarctic region. The weather conditions were unfavourable at times, with three blizzards during the voyage. The third of these resulted in four consecutive days of white-out conditions that prevented flying operations just when they were needed most for the ICESat satellite validation program, thus preventing coincident airborne and satellite altimetry measurements. However, airborne data collected during SIPEX enabled aircraft measurements to be validated over drilled thickness lines on the ice. It also improved our knowledge of the issues surrounding long-base line GPS positioning that complicate the data analysis.

As well as underway sampling and helicopter-based observations, we established 15 ‘ice stations’ on ice floes to collect data to characterize the sea ice environment. Some of the ice station measurements were completed within one day, others were completed over two days to collect measurements over a full diurnal cycle, and some were completed in just a couple of hours, with the main aim to collect ice cores with high ice-algal biomass for the various biology groups onboard.

Sea ice communities can serve as an important food source for Antarctic krill (Euphausia superba), and the distribution of krill is closely linked to sea ice extent in many regions of the Southern Ocean. One research paper in the journal reports major differences in growth, diet and condition of larval and post-larval krill sampled from open water and the under-ice environments. This indicates that different over-wintering strategies are used by different life-cycle stages, and highlights the role of sea ice biota as a food source for krill larvae off East Antarctica. These findings were supported by under-ice observations with the Remotely Operated Vehicle, showing juvenile krill feeding at the sub-surface of the sea ice and in cracks in areas of rafted ice at many of the ice stations. Altogether, the multi-disciplinary sampling program highlighted the structuring role of sea ice in East Antarctic marine ecosystem function and biogeochemical cycling.

It is clear from recent studies that the distribution of sea ice around Antarctica has changed in recent decades. While a net increase in extent of 1.2% per decade has been reported between 1979 and 2008, far greater regional changes have occurred. In the Bellingshausen Sea some regions now experience an annual sea ice season that is three months shorter than in 1979, whereas parts of the Ross Sea have an annual sea ice season that is two months longer. Critical to understanding behavior in sea ice thickness, and the potential impacts of this on both the physical and biological Southern Ocean environment, are field studies that measure and monitor critical sea ice processes and provide the necessary ground validation data for hemispheric to global-scale satellite monitoring.

This research is published in Deep-Sea Research Part II, Vol 58, Issues 9–10, May 2011. (DOI: 10.1016/j.dsr2.2011.01.001)

Tony Worby1 and Klaus Meiners2

1SIPEX Voyage Leader, 1,2ACE CRC