LARISSA - LARsen Ice Shelf System, Antarctica, a NSF-funded project.

LARISSA - LARsen Ice Shelf System, Antarctica, a NSF-funded project.
We are conducting an integrated, multi-disciplinary field program to address the rapid and fundamental changes occurring in the Antarctic Peninsula region as a consequence of the abrupt collapse of the Larsen B Ice Shelf in the fall of 2002. A profound transformation in ecosystem structure and function is occurring in coastal waters of the western Weddell Sea. This transformation appears to be yielding a redistribution of energy flow between chemoautotrophic and photosynthetic production, and to be causing the rapid demise of the extraordinary seep ecosystem discovered beneath the ice shelf, providing an ideal opportunity to test fundamental paradigms in ecosystem evolution.

Tuesday, May 8, 2012

Plowing through the sea-ice


Entry 4/8 - 4/15 - After finishing our seafloor and water-column studies in the Larsen A Embayment on 8 April, we headed northward through the Prince Gustav Channel, planning to nearly circumnavigate James Ross Island to sample potential cold-seeps sites in in Admiralty Sound.  Cold seeps are sites were methane or other energy-rich chemical seeps from the seafloor, producing rich communities of animals related to those found at hydrothermal vents and at whale falls.   Although active cold seeps have been photographed on the seafloor in Antarctica, no living communities of animals have ever been sampled.   
Once again, the unusually heavy sea ice in the Weddell Sea has thwarted our sampling efforts.  We have spent the 8th through the 12th of April backing and ramming through sea-ice composed of large, multi-year flow, some 1.5 – 2 m thick!  The heavy sea-ice blocked our path around James Ross Island to the cold seep site; after four days backing and ramming, we turned north to avoid getting trapped in the sea ice, which continued to thicken under the very cold conditions (- 16 C).   We thus turned northward and crunched our way northward into the Antarctic Sound. The racket caused by the shipped ramming and crunching sea ice resounded in the galley, limiting any conversation during meals.   



The NB Palmer breaking through a hodge-podge of new and multiyear sea ice. For scale, the ice ball next to the ship protrudes about 3 m (10 feet) above the water.


video



Map of our cruise area and 36 stations.  It took us four days to break 50 miles through sea ice from Station 34 in the Prince Gustav Channel to the Station 35 in the Antarctic Sound.


We finally broke free of sea ice and entered the Antarctic Sound in the wee hours of April 13th, planning to recover the whale-bone lander which has been at the 1000 m deep seafloor for 2 years.  The lander is designed to capture the special animals that live on whale skeletons at the deep-sea floor, which include bone eating worms, bristle worms that graze on bacterial mats, and a large diversity of other invertebrates.  The bone lander could only be recovered in daylight, so we first conducted  a Blake trawl, and a yoyo-camera survey to in the Antarctic Sound, an embayment never before sampled by benthic ecologists.  The Blake trawl contained a remarkable fauna of holothurians, sea spiders, and giant polychaete worms. The trawl reeked of sulfide and was full of black sediment. The yoyo-camera survey revealed extraordinary concentrations of macroalgae throughout the Sound floor, indicating substantial horizontal nutrient subsidies from benthic habitats in euphotic zone.  The Antarctic Sound floor is a surprisingly food rich setting compared to most communities at 1000 m. 



Yoyo camera picture of the Antarctic Sound seafloor at 1000 m depths, showing abundant seaweed (macroalgal) detritus, as well as anemones and numerous brittle stars.  The amount of macroalgal detritus is extraordinary for 1000 m depths.


After daybreak at 800 am on the 13th, we communicated acoustically with the whale-bone lander, commanding it to drop its 120 kg lead weight, allowing it to float to the sea surface.   When we sent the release command, the sea surface above the lander was free of sea ice for a ¼ mile radius. However, in the half hour required for the lander to float to the sea surface, sea-ice drifted over the site, to the lander came up under an ice floe.  After three hours of smashing floes with our intrepid icebreaking ship, we finally spied the lander under a translucent ice flow, much to the relief of David Honig, whose dissertation depends on recovering this lander experiment.  Within an hour, we had the bone lander on deck.

The whale-bone lander on deck after 28 months on the seafloor. Whale ribs bones can be seen attached the at right.  The nylon mesh bin in the aluminum frame also contains whale bones and thousands of colonizing worms. The whale array smells like dead whale.


We then spent 24 hour picking and identifying the diversity of life living on the whale bones, preserving animals for genetic, food-web and biodiversity analyses.  The bone lander contained thousands of dorvilleid polychaetes, amphipods and other animals attracted to whale bones, and promises to provide exciting data for David Honig dissertation concerning food-web structure in Antarctic ecosystems.
By the time the benthic ecology group had finished processing the bone-lander samples late in the evening of April 14th, we had completed our geological sampling, collecting a jumbo piston core 18-m long just north of the Antarctic Sound. The sea ice was extremely heavy, forcing us to head north  towards across the Drake Passage by 200 am on April 15th.  The Antarctic Ice Gods would not let us leave easily – we spend another 16 hours backing and ramming through heavy sea ice before we finally broke into open water just southeast of King George Island.  Now we are “speeding” at 12 knots across the Drake Passage, in the first open ocean swell we have felt for more than one month.   If the fair weather holds, we should be in Punta Arenas by late on April 19th, to unload the ship and head homeward.