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Tectonic evolution

Geologists study ancient Antarctic landscape as possible influence on climate

It seems a time machine would make geology so much easier to do.

Then scientists like John Isbell and David Elliot could spin back hundreds of millions of years ago to when Antarctica was sandwiched next to other landmasses in a supercontinent called Gondwana, which itself once formed an even larger continent called Pangaea.

They could then watch firsthand the tectonic processes unfold that scrambled plates of the Earth, shoved one block against another, stretched plains into basins, pushed mountains skyward and dumped sediments into ancient lakes and flood plains of large river systems.

The story of how a region of the central Transantarctic Mountains (CTAM) evolved would be cut and dry. All that’s left would be to write the paper for a journal like Nature or Science.

Instead, these researchers and their colleagues spent weeks this past austral summer at a remote field camp, just north of the mighty Beardmore Glacier that slices through the Transantarctic Mountains from East Antarctica into the Ross Ice Shelf.

From that large field camp, home to more than 70 people at one time, they flew day trips aboard helicopters and camped next to exposed rock outcrops and mountain peaks, hiking countless kilometers to examine the layers of sediments or to collect rocks for later analysis.

It's not an easy place to work: A katabatic windstorm tore through the team’s small tent camp at Bunker Cwm, a deep-walled cirque at Clarkson Peak, during an extended field excursion.

Yet one would assume that Isbell and Elliot would eschew a time machine any day for another chance to tramp through Antarctica’s backcountry as they continue to refine the story of its geological and glaciological history.

“Every time you come down, you learn new things,” said Elliot, who has made 25 visits to the Antarctic, beginning in the 1960s with the British Antarctic Survey. Today a professor emeritus at The Ohio State University, Elliot has done most of his research on the continent under the auspices of the U.S. Antarctic Program.

He and others in the 1960s and 1970s did much of the original footwork in the Transantarctics, describing the layers of sediments that serve as a dipstick through time. Colleagues like Peter Barrett, a well-known polar scientist from New Zealand, found the first land vertebrate fossil in Antarctica. John Mercer, an eminent scientist from The Ohio State University who first suggested the West Antarctic Ice Sheet might be unstable, found exceptionally preserved peat deposits that opened the door for paleobotany.

“Without the initial exploratory work of mapping, nobody would have known about the possibility of there being … all of these amazing organisms,” Elliot said. Such finds were the impetus for the first CTAM camp, often referred to as the Beardmore camp, in 1969-70. The 2010-11 camp was the fifth incarnation.

“There’s been a slow evolution over time, from basic mapping and exploration to problem-oriented [studies], which is what we’re doing at the moment,” Elliot added.

The particular problem that Isbell and Elliot are trying to understand is how this section of Antarctica evolved from the Devonian period some 400 million years ago through the Triassic, which ended about 200 million years ago.

During that time, the Earth went into a deep freeze that lasted from about 350 to 275 million years ago when the climate passed into a greenhouse world that persisted until about 40 million years ago, the beginning of the present ice age.

Part of the story that interests Isbell, a veteran of 15 Antarctic field seasons, concerns the tectonic extension under way that pulled apart the earth’s crust, forming great sedimentary basins. This was early Permian, about 290 million years ago, when much of the planet was still in an icehouse climate.  

Monstrous glaciers poured over Antarctica and extended into the other southern continents of Gondwana, including Africa, Australia, South America and India. The ice would have affected sea level and climate, according to Isbell.

But the story isn’t as simple as a giant ice sheet blanketing all of Antarctica, not based on studies of the ancient glacial deposits and the fossil floras that emerged in the Permian, indicating forests thrived. Isbell believes tectonic forces related to the formation of the basins played a big role in glaciation, probably leaving some areas ice-free.

“I think probably today is more glaciated than then, at least for Antarctica,” he said. “[Antarctica] was more tectonically active than previously thought. That tectonism probably played a role in the formation of those glaciers.”

And the basin tectonics continued to influence the climate into the Triassic, which marked a change in climate from warm and wet to warmer and drier. But even that conventional interpretation is being fine-tuned. “I think we’re all finding that it wasn’t as arid as people thought, and that the observed changes were likely due to basin forming,” Isbell said.

Elliot, Isbell and their team of graduate students are partly able to fine-tune the picture of tectonic evolution by examining the strata of the rocks — the sediments that eventually poured into the basin — to try to identify the environments in which they formed. That information provides clues about the physical conditions of the time, such as dry versus wet.  

In addition, structures in the rock can indicate from what direction the sediments were transported by rivers responsible for their deposition in the basin, which helps to track the material back to the source terrains. 

“[It] comes back to the basins, changing depositional environments and what they tell you about changing physical conditions on the surface of the Earth,” Isbell said.

The researchers can also learn something from the rocks back in the lab. Elliot, collaborating with Mark Fanning at the Australian National University in Canberra, will extract the mineral zircon from sandstones observed and collected in the field to get at the ages of the source materials for the sedimentary fill of the basin.

Zircon contains trace amounts of uranium that can be used to determine the age of the mineral due to its radioactive decay over time. The mineral is particularly attractive for these types of measurements because it is highly resistant to erosion and chemical weathering.

Other isotopic measurements using zircon can help match the Antarctic terrain to other landmasses that once abutted the continent. For example, working on time scales even deeper back in time, other researchers have been able to connect Antarctica with North America.

“Since we’re dealing with ice-covered areas, we’re starting to fill in information about what might be present underneath the ice sheet, which we otherwise can’t get to,” Elliot said.

But not just any old rock sitting above the glaciers and ice sheet will do. That’s where Isbell’s work on the sediments comes in. It’s a symbiotic collaboration, Elliot explained.

“It requires careful examination of the rocks to pick the right rock unit from which you extract a sample,” he said. “I can’t do my research without [Isbell’s] expertise in sedimentology. He can’t get at details of the source terrains, without my component of this project.”

NSF-funded research in this article: David Elliot, Ohio State University, Award No. 0944662; and John Isbell, University of Wisconsin-Milwaukee, Award No. 0944532.

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Curator: Peter Rejcek, Antarctic Support Contract | NSF Official: Winifred Reuning, Division of Polar Programs