PS118 - Weekly Report No. 4 | 4 - 10 March 2019
Northward journey and the first station
During the last week, we travelled for more than 60 nautical miles to the north and only stopped our journey for the first sampling station where we deployed most of our scientific equipment. Since then, we are busy to analyse water, sediment and benthos samples or to preserve them for later onshore analyses.
We also deployed the Ocean Floor Observation and Bathymetry System (OFOBS) at this station. Equipped with very high definition cameras and multibeam and sidescan sonars, OFOBS has continuously recorded seafloor imagery that combined with the sonar data, allow for modelling a 3D representation of the seafloor. After two days of station work, we set sail again to continue our travel northward. Since then, Polarstern ploughs her way through ice. The ice that makes our travel cumbersome is, however, a delight for the sea ice group that in the following introduces itself and its activities.
Therefore, I would like to send regards in the name of all expedition participants at this point and hand over to Christian Haas of the sea ice group.
Boris Dorschen - Chief Scientist
Sea Ice Group
Since entering the sea ice zone of the north-western Weddell Sea on February 22, the sea ice group has started their observations on, in, under, and above the ice. We have landed on ice floes or carried out airborne surveys whenever the weather allowed. Our core team includes Ilka Peeken, Erika Allhusen, Stefanie Arndt, Kerstin Jerosch, and Christian Haas, all from AWI. We aim to observe the regional variability of the sea ice thickness distribution; the role of snow melt on the ice mass balance, for satellite radar signals, and for primary production in the ice; and the abundance and composition of sea ice organisms, microplastic, and other bio-geochemical substances in the ice.
As now is the end of the summer melt season, our observations will provide important information on melt processes during the summer and how these affect the survival of the ice and its role for polar ocean carbon fluxes. Also, as this is the first time since 2006 that sea ice observations are again carried out in this part of the Weddell Sea, we hope to observe whether ice thickness and the extent of melt have changed in the decade since.
Because we have not been granted any ship time to enter the ice from the ship, we heavily rely on the use of our helicopters to reach the ice where we carry out snow and ice thickness measurements, and extract ice cores and water samples for bio-physical analyses back on the ship. In addition, we perform airborne ice thickness surveys with our EM Bird, a torpedo-shaped ice thickness sensor towed 20 m below the helicopter and 15 m above the ice. With the great support of the hard working helicopter pilots and engineers, and despite some poor weather days, we have already sampled eight ice floes and carried out four EM thickness surveys. Evenings and weather days are filled with the processing of our ice samples in the cold laboratory containers on board.
First, preliminary results showed that the snow is only 20 to 30 cm thick and highly metamorphic, and much has been transformed into superimposed ice. This is much thinner than the 70-100 cm of snow typically found in this region in late winter. Below the surface layer of metamorphic snow and superimposed ice we found hints of gap layers and rotten ice, with moderate abundances of sea ice algae. However, these were much less developed than typical for other regions of the Weddell Sea with thinner and younger ice, and attest to the severity of sea ice conditions in our study region. Local ice thicknesses at our coring sites chosen on level ice ranged between 1 m and 2.5 m. However, the airborne surveys showed that ice thicknesses between 3 and 4 m were quite typical overall, including heavily ridged ice. Those large thicknesses are similar to thicknesses observed in 2004 and 2006. They also explain why Polarstern struggled so much to penetrate farther to the south, as the ice was not only so thick but also pressed against the coast of the Antarctic Peninsula by southern winds.
An absolute highlight of our work was the opportunistic re-visit of an ice floe that was already sampled by one of us, Steffi, a year ago in the southern Weddell Sea, during the Polarstern cruise PS111. The floe was equipped with an autonomous snow accumulation measuring station that transmitted its data and position by satellite. The floe happened to drift right towards our cruise track and we ended up only six miles away from it. After a short helicopter flight to the last reported position, and a few turns to search for it from the air, one of us spotted it and we found a landing site nearby. While the floe was huge and level during the station deployment last year, it was now heavily ridged and unrecognizable, after having travelled over a thousand kilometers within more than a year.
At the site we repeated the same snow measurements as were done a year ago which gives us the unique opportunity to observe the changes happening during a full year of ice drift with its seasonal freezing and melting conditions. As we left the station on the ice, we could follow the parallel drift of the buoy and our ship nearby as it was beset in the ice. A dance formation of a buoy and a ship shifted by the winds and tides - with points deduced from Polarstern for the attempted ice breaking and escape from this formation.
Despite these first successful 1.5 weeks of sea ice sampling we are very disappointed that we are not able to cover larger regions further to the South and East including the Larsen B and C regions where we were expecting to see differences in sea ice melt and biology. There we had plans to study snow on the large iceberg A68, platelet ice indicative of ice shelf melt, and a unique area of multiyear landfast ice covering the inner parts of the Larsen B bay with the biological highly productive platelet ice. However, we are hopeful that we will now be able to continue our observations along a gradient from the inner part of the sea ice cover to the marginal ice zone in the northeast. Here we expect strong increases in the amount of accumulated melt and metamorphic snow, gap layer abundance, and a strong increase of the sea ice associated ecosystem.