ANT-XXVI/3, Weekly Report No. 5

1 March - 7 March 2010

Hopeful thinking played a big role this week. We were positioned in the inner Pine Island Bay in front of the 30-50 m high ice-shelf edges of the mighty Pine Island- and Thwaites Glaciers. Max, the meteorologist and Hans, the chief pilot gave their thumbs up for a helicopter flight to the volcanic Hudson Mountains during a narrow good-weather window. The team of geologists collected rocks which would document the time of the retreat of the ice-sheet. The next day in thick mist, Polarstern was slowly approaching the Becker Island group next to the Pine Island Glacier mouth, for which satellite images show outcropping hard rock formations. A few hours later, another hole in the low cloud cover was used to have the geologists flown to the islands. They returned with loads full of rock samples. The geodesists left their GPS instruments installed on their sites for measurements during the following days.

Within the last two decades, glaciers draining into Pine Island Bay have shown the most dramatic changes in ice flow dynamics of the entire Antarctic Ice Sheets. The glaciers are characterized by thinning and flow acceleration, and the grounding line of at least one of the glaciers (Pine Island Glacier) is currently retreating. This dynamic behaviour of the glaciers could indicate the start of a collapse of this part of the West Antarctic Ice Sheet, which would raise the global sea level by ca. 1.4 meters. So far, however, it is unclear, 1) if the recent changes are a consequence of modern global warming or a continuation of an ice-sheet retreat that started at the end of the last ice age, and 2) if the current glacier retreat is only a temporary phenomenon or will continue well into the future. Marine geologists onboard “Polarstern” are trying to answer these questions by investigating sedimentary sequences, which were deposited on the inner shelf in Pine Island Bay during approx. the last 12 thousand years.

This week they successfully recovered several long (up to 10 m) sediment cores that should reveal information about the long-term dynamics of the glaciers draining into Pine Island Bay. At least two of the cores contained calcareous shells of marine organisms, which lived in the water column or within the sediment near the seafloor surface. Finding such shells in Antarctic shelf sediments is quite unique since calcareous organisms are a very rare find in the cold waters of the Southern Ocean, as their shells are easily dissolved. Because the shells can be accurately dated using radiocarbon dating techniques, the geologists onboard were quite excited and compared their discovery to “finding gold”. With a reliable age control, it will be possible to constrain the past behaviour of the glaciers draining into Pine Island Bay much better than in previous studies. Another important discovery of the marine geologists was the recovery of a black muddy substance (that unfortunately smelled like rotten eggs) on top of one of the sediment cores. Such black mud can only form if the concentration of oxygen in the bottom water is very low. Its occurrence in a small basin on the inner shelf indicates that not the entire seafloor in Pine Island Bay is affected by flooding with relatively warm Circumpolar Deep Water. This deep water mass protrudes from beyond the shelf edge far onto the inner shelf and is held responsible for the present dramatic thinning of the floating part of the glaciers draining into Pine Island Bay.

Again, meteorologist Max was optimistic for a gap to develop in between the many low-pressure systems. These lows are being found unusually southward this year and seldom open the clouds to reveal blue sky, but they in turn are the reason that this region is relatively free of sea-ice. At sunrise, the team and pilots stood in front of the weather station. An hour later, they flew to the mighty Mt. Murphy and the Kohler Range a bit farther west. In the meantime, the seafloor-mapping program continued in the innermost part of the Crosson Ice-Shelf trough, which had not been open that far south as ships travel here. The crew also took advantage of the good weather and performed an obligatory exercise with the lifeboats.

A deeply incised circular structure was already noticeable on the sea floor on our way into the southern Pine Island Bay. Could this be an active mud-volcano? We gave this feature the appropriate name ‘Neptun’s Bottle’. However, water and sediment samples from various sites did not give any indications that fluids or gases (e.g. methane) escape from the sea floor. It is possibly a subglacial bedform, which developed during the last ice-sheet retreat.

Neptun was not pleased with these activities in his domain. His visit on Saturday was merciless for some ….

With best regards from all participants
Claus-Dieter Hillenbrand, Gerhard Kuhn and Karsten Gohl