ANT-XXVI/3, Weekly Report No. 6

8 March - 14 March 2010

Day and night the Polarstern towed the 3 km long seismic cable. Every 12 seconds, a short blast was emitted from the airguns. The computer screens in the seismic lab were filled with images of the geological subsurface down to several kilometres beneath the seafloor. This week’s program is almost entirely filled with a large seismic survey campaign of the outer continental shelf of Pine Island Bay and the adjacent deep sea of the Amundsen Sea. Several scientific objectives are being addressed with this survey.

Between 90 and 85 million years ago, New Zealand which in geological terms can be taken as an independent continent broke from West Antarctica as a consequence of major expansion of the Earth’s crust. The crust of the present Pacific Ocean was formed along the seafloor spreading ridge. Data from a Polarstern cruise in 2006 already showed that the crust of the West Antarctic continental margin extended and thinned before the continents broke up and the first oceanic crust developed. As the continental crust is composed and structured differently compared to oceanic crust, we are able to identify the boundary between both crustal types with the aid of our geophysical measurements. Bryan, our collaborating partner from New Zealand, has seen similar indications for extended and thinned continental crust far into the deep sea off the New Zealand margin. He has been looking just as anxiously at the newly collected data as we have. The knowledge about continent-ocean boundaries is important not only for more accurate plate tectonic reconstructions but also for calculating water depths of earlier times in Earth history. The reconstruction of this so-called paleo-bathymetry of the Southern Ocean is a significant component of numerical simulations of ocean currents, as they are supposed to show the influence these currents have had on the climate history of the Earth.

Direct evidence of strong ocean bottom currents is revealed in our seismic data of the deep sea. Sediment drifts of several hundred metres in height, which look similar to sand dunes with a steep and a less inclined flank, are situated in the deep sea off Pine Island Bay. They indicate that since the initial beginning of Antarctica’s glaciation, suspended loads of fine sediment have been flowing from the West Antarctic shelf into the deep sea, to be transported over hundreds of kilometres by strong bottom currents. Mapping these drifts allows analyses of where and when large glacial cycles set in along the West Antarctic margin. Our seismic data show that the present outer continental shelf of Pine Island Bay is composed of several kilometres of sediments. In addition to our seismic cable, we have been deploying ocean-bottom seismometers (OBS) every 20 km to obtain additional information on physical properties of the sediments and the underlying bedrock basement. Their anchor weight drags them to the seafloor. After the survey, a short hydro-acoustic signal is sent from aboard which is supposed to release the instrument from its anchor. There is always a quiet anxiety on the bridge … will the OBS surface again?

We only have two weeks left before heading back along the transit route to Punta Arenas. Time is always running out too fast on the final lap of an expedition, and we still have so much work to do….

With best regards from all participants
Karsten Gohl