PS104 - Weekly Report No. 3 | 20 - 26 February 2017
The MeBo drilling program and stations on land
An exciting week has passed with changing plans as becoming part of the daily routine. Why is it impossible to follow a many days’ long work program in this region? Our work is mainly focussed on the drilling program with the MeBo.
But never in the past has there been any drilling in this region, which is why we are always anxious if the drilling is successful and what the core will reveal. How deep can the drill bit penetrate the subsurface below the seafloor? How much sediment core can be recovered in the drill barrels? What is the cored sediment material made of? And of particular importance is the question of how long a drill operation can last before is has to be stopped due to approaching icebergs, large ice floes, a technical problem, or an unexpected early wearing of the drill bit?
We decided in the beginning of this week that we leave the southern Pine Island Bay for another series of drill sites on the middle shelf. The reasons for this decision are the frequent iceberg approaches, which do not allow a long positioning at one site for the ship, and the type of sediments drilled at the two sites that can hardly be used for age dating. The sedimentary basin near Pine Island Glacier is likely filled with meltwater-transported sediments from beneath the glacier. The next drill site will target a different type of sediments. We want to collect samples from a stratigraphic sequence that fill the 7 km thick sedimentary basin of the middle and outer Amundsen Sea shelf. Seismic reflection data collected some years ago show clearly where the older, dipping (1-2°) layers of this sequence outcrop at the seafloor. Our sub-bottom profiler (Parasound) records indicate that only a thin veneer of young glacial- marine and postglacial drape covers these dipping sequences, so that we can drill into these. Our first MeBo drill station in this area was a success. About 31 m deep drilled the drill string. Although many of the drill barrels returned to the deck empty or only partially filled, excitement rose in the geology lab when the lowermost drill barrels arrived and where pushed out of their inner pipes. Beneath solid sandstone, a sharp contact to dark, hard mudstone was drilled. A first analysis revealed carbonaceous material. This means we indeed drilled into sedimentary rock that was likely deposited in a warm climate (Cretaceous to Eocene?). The age can only be determined after a more thorough analysis, e.g. by dating of pollen.
At the next drill site 15 hours later, we deployed MeBo to drill into a sedimentary sequence, which should be stratigraphically much younger (Miocene?) according to the seismic reflection data. But a jammed inner core barrel forced us to abandon the drilling operation after only 5.1 m drill depth. In addition, an iceberg was approaching. After every drill operation, MeBo must be maintained and prepared for the next deployment, which takes at least 12 hours. We always use this maintenance time for other sampling and profiling work with the ship. Back at the drill site, we undertook a second attempt. This time, the drill string went 10 m deep into the rocks, stopped by a technical failure inside the drill-head assembly. However, beneath a top layer of massive glacial till, we recovered mudstones of possible Miocene (?) origin.
Two groups have been busy in the last 10 days using helicopter transport almost daily to fly to the mainland or small islands of Pine Island Bay. The research project of geodesists Mirko Scheinert and Benjamin Ebermann of TU Dresden comprises precise GPS measurements at selected sites on outcropping rock. Highly accurate point coordinates – with an accuracy of 2-4 mm horizontally and 5-7 mm vertically – are measured over several days. By comparing data collected over many years, they can measure differences in the position coordinates and derive temporal changes and deformations from these. The vertical deformation is in particular important as this provides indications on how much the Earth’s crust is uplifted due to the glacial isostatic adjustment (GIA). This GIA effect can be explained by linking past and present ice mass changes with the rheology (material property) of the Earth’s crust and mantle. In our work region, we measured GPS records on point coordinates on Mt. Manthe and on an island north of Pine Island Glacier. These were repeat measurements from earlier measurements conducted in 2006 and 2010. Additional coordinate points were marked and first-time measured, e.g. on the northernmost of the Backer Islands and on Clark Island. We hope that the GPS instrument on Clark Island will continue functioning a while, because the present program of Polarstern will allow us to return to this island for instrument de-installation.
The second group of geologists Max Zundel (University of Bremen) and Yani Najman (Lancaster University, UK) have also used the good weather conditions to land on many of the island in Pine Island Bay. They sample crystalline basement rocks and erratic blocks, which are rocks transported large distances to the islands by movement of glaciers. By age dating of basement rock samples, they can derive the regional cooling history of the rocks, which is an indication of erosion, sedimentation and uplift of this region of the last 100 million years. This information is used to reconstruct the long-term surface development, providing important clues on the beginning and the behaviour of the West Antarctica glaciation. The erratic blocks are age-dated to reveal the time when these rocks, and therefore the islands, were uncovered from ice. This will help to reconstruct the ice sheet retreat in Pine Island Bay for the last 15,000 years.
We all feel that the days are flying by quickly, also because of the permanent re-planning of our research activities. Yesterday was middle time of the expedition, which we celebrated with a traditional ‘Bergfest’. A fantastic BBQ was set on deck by the excellent kitchen team.
The daily arriving satellite images of the German Aerospace Center (DLR) have indicated it already: A large ice-floe field has been drifting into our work area by constant easterly winds. We had to stopped our drilling here and set course to a previously little investigated part of the embayment east of Burke Island. There we will go for another drill target with a different objective …
All are well and send their best regards and wishes
Mirko Scheinert, Max Zundel and Karsten Gohl