PS104 - Weekly Report No. 5 | 6 - 12 March 2017
End of a successful Expedition
A world of gigantic icebergs made the scenery which Polarstern passed through on its track from Pine Island Bay to the western Amundsen Sea Embayment. The icebergs are so abundant here because of a bathymetric ridge that extends northwards from Bear Peninsula and onto which the icebergs are grounded and move only slightly at the peaks of high tide.
This so-called Bear Ridge divides the continental shelf into two domains of which the glaci-morphological bedforms and sediments are characterised by two different glacial systems. Here, on the western shelf, the dominant lineations and pattern are not caused by past ice streams of the Pine Island, Thwaites and Smith Glaciers, but by those of the Kohler and Getz Glaciers. These two glaciers also contribute to the large ice mass loss of this region and are also affected by melt processes through warm deep-water currents. Previous seismic profiling indicates that sedimentary sequences exist on this shelf, which may document the transition from ice-free warm periods to times of early glacial advances of the West Antarctic Ice Sheet. Therefore, this area is also of great interest for our drill aims and the other research methods as we want to compare drilled sediment cores here with those of the eastern shelf and Pine Island Bay.
While Polarstern was on its way to the planned drill sites, the land geologists and geodesists used the good flight weather for collecting rock samples for the first time from outcrops of the Martin Peninsula of Marie Byrd Land. Similar to other outcrops in Pine Island Bay, lichens also grow here on some of the rocks.
The geophysicists Florian and Ricarda also took advantage of the good flight conditions and continued their helicopter-magnetic surveying programme. They already surveyed along a number of flight tracks in the eastern Amundsen Sea Embayment to close large gaps of a pre-existing grid of earlier survey flights we undertook in 2006 and 2010 and to cover previously unsurveyed areas. A magnetometer sensor bird is connected to the helicopter by 30 m long cable and measures the Earth’s magnetic field along straight survey flight lines. After proper data processing of the grid data, the resulting map will show the distribution of magnetic anomalies in the survey area, which is used to interpret tectonic structures of the Earth’s crust. In addition, these data are used to derive the geothermal heatflux, which is why these surveys are so important for us in connection with the temperature gradient measurements in sub-seafloor sediments by using the geothermal temperature probe.
We completed four drill holes with the MeBo drill device during this week. Two drill holes are on the same site, because we had to stop drilling already after 8 m drill depth. But we returned to this site for the last drill operation on this expedition and optimised our drill strategy – e.g. by using a different drill bit – with the knowledge gained in terms of rock composition (lithology). This decision turned out to be right, because now we drilled – after penetrating a layer boundary – into very hard sandy mudstone to 17.5 m depth and obtained a core recovery of 50%. Fossil diatoms are abundant in this rock core, but it is difficult to determine their exact age at this stage. More analyses are necessary. Of the other two drill sites, one was – from seismic analysis – in a stratigraphically younger sedimentary sequence, and we drilled down to 7.5 m depth to recover very consolidated diamicton, sediments of varying grain sizes. The other drill target was in a stratigraphically older sequence and recovered very hard, dark mudstone from down to 10 m drill depth. The core recovery of 40% was surprisingly good.
As explained before, the eastern and western shelves of the Amundsen Sea Embayment have been under the influence of different glacier systems with potentially different sedimentary processes. For a comparison of the stratigraphy of both regions it is important to link the drill sites with seismic survey profiles. Thus, we conducted here also a seismic survey by using a 600 m long hydrophone cable (streamer) which is towed at 5 knots speed behind the vessel at about 10 m water depth. Hydrophones, installed every metre in this cable, record acoustic waves similar to what microphones do. The recording unit of this streamer records the seismic waves that are emitted every 10 seconds by small airguns also towed behind the vessel. This seismic reflection technique allows us to image layers of deposited sediments down to depths of several kilometres below the seafloor.
In the early morning of Saturday we completed our research activities of this expedition and began the long transit back to Punta Arenas. With a BBQ party in the evening we celebrated the end of a successful scientific mission. All research teams are now busy packing and securing their gear, instruments and samples, because we certainly won’t have the calm sea-state conditions on this transit through the southeast Pacific as we enjoyed during the last 4 weeks on the continental shelf.
We will get back to you one last time with a résumé of this expedition in a concluding weekly report by the end of next week.
All are well and send their best regards and wishes