The favourable weather conditions we had enjoyed for more than two weeks in our work area of the Bellingshausen Sea changed for the worse, but we were still able to continue with our seismic survey. In this issue of the expedition letter, we want to focus on the work of the seismic team as well as that of the land geologists. Both teams investigate the deep geological and tectonic formations that are of great significance for the dynamics of the West Antarctic Ice Sheet.
Scientific and public media have recently reported on the record seasonal minimum of the entire Antarctic sea-ice this southern summer. Last year’s southern winter sea-ice has largely disappeared; here in the Bellingshausen Sea it even disappeared completely. The extent of this disappearance has been observed for the first time since the beginning of satellite imagery, and it follows the multi-decadal trend of a retreating continental West Antarctic Ice Sheet. But how did these processes act in the geological past when climate changes occurred due to natural variations of the Earth’s orbital parameters without any influences by humans? How did the ice sheet behave, when did it grow, and when did it retreat? We know today that the interaction of the ocean, atmosphere and ice plays a significant role in this. Understanding and reconstructing these interacting processes of the past is our aim in order to distinguish between the natural and the present anthropogenic factors of climate change and to obtain better indications on to what extent the future sea-level rise will occur.
One approach for an improved understanding of the past interactions, e.g., between ocean currents and ice sheet dynamics, is to image the deep sedimentary sub-bottom of the continental shelf. The sediment sequences of the shelf provide indications over periods of many million years about the locations of early deep-water currents and how the ice sheet advanced and retreated. These sedimentary structures can be imaged with seismic records of the sub-bottom. Over many days, pulses of low-frequency signals are generated by a seismic source towed behind the ship. These pulsed seismic signals travel through the water and deep into the sub-bottom, where they reflect from geological layer boundaries, travel back and are recorded with a 3000-meter-long towed hydrophone cable (Fig. 1). The team shifts in the seismic lab (Fig. 2) check continuously the function of the seismic source and the quality of the recorded data (Fig. 3).
The seismic survey work is supported by a team of whale observers who watch out for marine mammals in the vicinity of the ship by visual observation and monitoring of an infrared camera. If a whale is detected within a safety zone, the seismic source is shut-down immediately to avoid that these acoustically sensitive animals are disturbed.
Also the land geology team aims to decipher the geological and tectonic underground of West Antarctica, because it is related to the development of the ice sheet. The largest ice mass loss in Antarctica happens in the Bellingshausen and Amundsen Sea sectors, where deeply incised troughs extend from the continental shelf to the continental interior. Fast flowing ice streams follow these throughs and transport large amounts of inland ice to the shelf. These are the areas where warm ocean deep-water causes most melting of the ice shelf from beneath, leading to a destabilization of the West Antarctic Ice Sheet. Geophysicists and geologists relate some of these troughs to the West Antarctic Rift System, a system of large tectonic depressions that cross the entire West Antarctic continent. But very little is known about the development of this rift and its role on the ice sheet formation and dynamics. The geological sampling is intended to fill some of the knowledge gaps to give clues about the local and regional landscape evolution. Analyses of the rock samples will show how tectonic processes may have influenced the ice sheet advances and retreats in the past.
Thermochronological age-dating of heavy minerals such as apatite and zircon help reveal the temporal sequences of tectonic phases. The resulting data show the cooling of the Earth’s crust as a result of its uplift and erosion. As soon as a critical temperature range is reached in this process, traces of radioactive fission products are generated in the mineral’s crystals that can be analysed with highly sophisticated methods in the lab. However, as the mineral composition of the various rock types differs enormously, it is difficult to find suitable rock outcrops in the field (Fig. 4). The coastal sector of the Bellingshausen Sea has a relatively flat topography, often with a thick ice cover on top, making it difficult to find rock outcrops. Therefore, geological fieldwork is carefully planned with a selection of potential sampling sites using high-resolution satellite images. Only then one of the Polarstern helicopters, operated by a very experienced team of pilots and mechanics, will take off. Unfavourable weather conditions make fieldwork often difficult. And so does the ice cover for direct rock sampling, which is the reason why rock deposits on the seafloor are also of interest for providing important information. In this case, the box corer is used to sample rock fragments that were transported and dropped by glaciers in near-coastal locations. These provide clues of the geology of the hinterland.
After collecting the rock samples, their preparation for analysis is a time-consuming procedure which includes the crushing into separate mineral grains. These grains are then ready for microscopic and chemical analyses to bring light into the hidden world beneath the ice.
In the next letter we will, among other topics, describe the biological research of our expedition in more detail.
Best regards and wishes
Gabriele Uenzelmann-Neben, Daniela Röhnert and Karsten Gohl
Further information on PS134:
Frequent short blogs: https://follow-polarstern.awi.de/
125-year anniversary of Belgica expedition: https://125yearsbelgica.com/