Friday, November 23, 2007

The Mackay Sea Valley Survey

At long last, I made my way out to the deep field camp of Granite Harbor for the geophysical survey of the Mackay Sea Valley. Granite Harbor is a small bay in the Ross Sea, nestled along the old granite cliffs that form the basement rocks of the Central Transantarctic Mountains. The setting is quite breathtaking! The Mackay Glacier is a large glacier which terminates in the Ross Sea. In past years, when the ice sheet was larger, the Mackay Glacier carved out a deep trough on the continental shelf. In subsequent years, as the climate warmed, this trough became an area where deep packages of marine sediments accumulated. Those marine sediments are the reason for this site survey.

The Ross Sea in this area is still frozen over – so our camp is built on frozen sea ice, approximately 2 meters thick. As we look out toward the sea, there is solid ice as far as the eye can see – with old icebergs from last year, or perhaps older, embedded in the sea ice.
They are quite monumental – standing sometimes hundreds of feet up above the ice surface and can be seen for many miles away. As the sun moves across the sky during the day, the illumination of the icebergs is constantly changing from bright white to deep blue to gray – to every shade in between.

Our deep field camp consists of 11 people. 7 Scott tents, 2 Rac tents, 2 snowmobiles, and 2 Pisten Bullys (oversnow vehicles).
The Scott tents are for sleeping, 2 per tent, and the Rac Tents are for eating and for science. The Rac tents have heat and generators for recharging electronics. It is remarkably comfortable and “cushy” in this remote camp!

Why are we here? We are in Granite Harbor to conduct a seismic survey of the seafloor and the rocks below the seafloor which accumulated in this large trough carved by the Mackay Glacier. Under the leadership of Dr. Ross Powell, of Northern Illinois University, we are working to get a picture of the rocks below the seabed where thick accumulations of Holocene (the past 7,000 years) sediments are located. These sediments are of interest because they can tell us information about what the climate has been like in this part of Antarctica for the past 7,000 years, and how it compares to the climate seen in atmospheric records and marine geological records for other parts of Antarctica during the same time period. The seismic survey will not drill for rocks, but will provide information for future drilling projects about where the sediments are located and where a future drill hole should be placed.

Seismic surveys operate using a sound blast, made by an airgun, which is set off below the ice. We drill a hole through the sea ice so that we can place the airgun, which is on a cable, down 8 meters in the water. We have a 1.5 kilometer long streamer that is dragged on the ice, behind the piston bully that carries the laboratory with all of the seismic recording devices. The streamer contains 60 geophones that record the sound waves after they have traveled out of their air gun, and traveled down through the different layers of rock on and below the sea floor, and been reflected back up. Different types of rocks give off different types of reflections, and require differing amounts of time to travel back up to the geophones. Computers in the mobile lab record the sounds as they bounce back to the geophones, and based on their time delay, they give an image of the different layers of rock in the subsurface. This type of work is typically done by ship with the geophones towed behind the ship. We are using a very new technology to conduct this survey over sea ice, and getting excellent results.

The survey was very successful in identifying where the pockets of sediment are. The seismic data was all collected digitally and will require continued processing back at Montana Tech University where Dr. Marvin Speece, the geophysicist on the site survey, teaches. Once processed, the seismic records will tell us in much greater detail, about the types and thicknesses of sediments – and allow scientists to determine the optimal site for future drilling.

Field work dominated life in Granite Harbor. But there were also wonderful moments of exploration and fun. Please visit my next blog, entitled "Reflections from a Remote Field Camp" to learn more about life and exploration in the Granite Harbor area.

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