December 7, 2010, 6:24 PM

On the Trail of Antarctica’s Geological Secrets

By JOHN GOODGE

John GoodgeA view of the lower Skelton Glacier, showing the converging flow of ice streams from valley glaciers into large “outlet” glaciers that drain ice through the Transantarctic Mountains.

John Goodge, a professor of geological sciences at the University of Minnesota-Duluth, and Jeff Vervoort, an isotope geochemist from Washington State University, will be writing occasional posts from their research expedition in Antarctica.

Wednesday, Dec. 3

After a few years away, it’s hard to believe I’m back on “the ice” again. It feels natural to be back at McMurdo Station in Antarctica, where I’m organizing my 11th geological research expedition into the Transantarctic Mountains. Our field group has been here almost two weeks already, after traveling through New Zealand, and we’ve been frantic with various training sessions and packing gear into the cargo system here.

The “system,” financed by the National Science Foundation and logistically coordinated by Raytheon Polar Services Corporation, is immense. On the upside, there is a process and a pile of material to tap into; on the other hand, it’s daunting to get all the details checked off our list. Although all the pieces are scattered in the various metal buildings around town, the system actually works well to help us organize food, test radios and tents, prep our gear for air cargo, and schedule flights to outlying camps. Once we leave McMurdo, we’re basically on our own.

John GoodgeOne of the many odd vehicles driving around town in dusty McMurdo.

The McMurdo population is peaking this season around 1,300 people, which is way more than a normal summertime population. As a result, everywhere it’s crowded — dormitories, labs and the galley. “Mactown” has everything from cargo loaders to construction workers, mechanics, science techs, air crews, cooks, janitors and meteorologists. Plus some scientists. The ratio of support staff to scientists is about 5 to1! It creates a lively and diverse but close-knit community. Folks here work hard and play hard, but they really help us get our work done.

We’ve been able to get in a couple of helicopter trips to distant field sites for some preliminary sampling, and now we’re awaiting a flight on a ski-equipped LC-130 Hercules cargo plane to our base camp near the head of Beardmore Glacier. The glacier is about halfway between McMurdo and the South Pole. Unfortunately, the weather has deteriorated both in McMurdo and at our put-in site, so we’re on a delay of three days. Fortunately, the dessert bar in the galley is open for both lunch and dinner.

John GoodgeA glacial moraine along the side of the Boomerang Range. Mark Fanning, who arrived by helicopter, is scanning the clasts for signs of old basement rocks.

While we wait, there’s time to review our project goals. Our main objective is to sample material from rock outcrop and glacial deposits that can help build a better picture of the continent hidden beneath the polar ice cap of Antarctica. Because about 98 percent of Antarctica is ice-covered, we see little of its geology. Good rock exposure can be found in coastal areas and in the Transantarctic Mountains, but otherwise we are left to come up with clever ways to probe at the geology under the ice.

NOAAA satellite-based radiometer image mosaic showing surface features of Antarctica. The white line outlines the general area of East Antarctica where there is no rock exposure.

We can extrapolate the geology from nearby continents that were once joined with Antarctica — Australia, India and Africa. We have also used geophysical tools to help us “see through” the ice to the underlying rock with varying magnetic and gravity properties. In this project, our team will be sampling granitic rocks exposed in the mountains and glacial boulders stranded in the ice next to the mountains that have been scraped off of the East Antarctic continent.

Granites are produced by the melting of existing crust. In this case, the melts formed as a result of subduction of an ancient Pacific oceanic plate beneath Antarctica, like in the Andes or Japan today. Granites made in this way give us geochemical tracers of the rocks that were melted, and that we cannot see at the surface.

BEDMAPAn ice thickness map of Antarctica, in which warmer colors are thicker ice and cooler colors are thinner ice. Heavy black lines are divides separating the ice cap into sectors of different flow toward the coast (shown by the thin lines). Ice in East Antarctica runs up against the high-standing Transantarctic Mountains, which have relief of up to 14,000 feet. (Click on the map for a larger version.)

The East Antarctic ice sheet is a huge lens of ice with an average thickness of over 8,000 feet. In places it is over 12,000 feet thick! There is so much ice covering eastern Antarctica that parts of the continent below are depressed below sea level. As the ice in this lens spreads laterally, it erodes rock of the continent below and carries this debris to the edge of Antarctica. Where the ice flows up against the Transantarctic Mountains, it carries these rocks along with it, and the ice ablates by wind erosion, leaving behind a lag of rock clasts. Glacial boulders transported from the interior of the continent by ice flow thus represent pieces of the hidden Antarctic crust that were eroded by glaciers flowing from the spreading ice sheet. Rock samples we collect from glacial moraines can give us geochemical clues about what is otherwise hidden by the ice cap.

Once home, the group will use a variety of instruments in the lab to learn what the rock compositions are, how old they are, where within the earth they formed, and the conditions they experienced in terms of pressure and temperature. Because the group will be sampling over an area greater than 1,500 kilometers long, we can use patterns in the samples to build a better picture of what the Antarctic continent looks like.

To do this, I’ve enlisted several newcomers to Antarctica. They include Jeff Vervoort, an isotope geochemist from Washington State University, Mark Fanning, a U-Pb geochronologist from Australian National University, and Tanya Dreyer, a new Ph.D. student at the University of Minnesota-Duluth who comes from Cape Town, South Africa. Our field party is rounded out by Dylan Taylor, a mountain guide based in Boulder, Colo., who is integral to planning the field program and getting us safely around the actively glaciated terrain.

Antarctica is bigger than the United States and Europe combined, but its geology is very poorly known. Why do we care about the geology of the Antarctic continent? Part of it is basic scientific curiosity, because we know from limited outcrop that parts of East Antarctica are as old as 3.8 billion years. It can tell us about the long evolution of continental crust formation as the earth had undergone chemical differentiation. Antarctica was a key piece in Pangea, Gondwana and Rodinia (huge supercontinents formed by the assembly of many of today’s familiar continents at roughly 250, 500 and 1,000 million years ago), and knowing more about its geologic architecture can help to refine the picture of global paleogeography as far back as 1 billion years ago. Lastly, because the polar ice cap and glaciers in Antarctica are critical to understanding past climate and ongoing processes of climate change, knowing more about the substrate for the earth’s largest ice cap and reservoir of fresh water will help to determine the stability and future fate of the ice sheet in the face of ongoing warming.