Welcome To Tanelorn: Glacial Silt Encased Some of Earth’s Best-Preserved Fossils

Friday, December 3, 2010

Glacial Silt Encased Some of Earth’s Best-Preserved Fossils

via Wired: Wired Science by Dave Mosher on 12/3/10

Some of the rarest and most detailed fossils on Earth owe their stunning preservation to dust blown out to sea by glacial winds.

Soft-bodied creatures usually rot or get eaten before sediment can bury and fossilize their fragile tissues. Yet a zoo of squishy animals that swam 435 million years ago are exquisitely preserved in the Soom Shale, a thick deposit that curves along the southern tip of Africa.

“This deposit preserved details in fossils you don’t normally get,” said Sarah Gabbott, a paleontologist at the University of Leicester. “Most often you see fossils of hard parts, but here you get muscles, eyes, organs and other tissues that decay away. It’s because of the windblown sediment.”

Gabbott and others, who describe their discovery in the December issue of Geology, regard it as the oldest case of a windblown fossil-making machine. The find could aid searches for similarly rich soft-bodied fossil beds that cover other loosely understood spans of prehistory.

“If you look at modern marine community, 90 to 99 percent of animals are soft-bodied. If we didn’t get these deposits, we’d be missing most of the life,” Gabbott said.

Around 445 million years ago, Earth’s familiar landmasses were all part of two supercontinents called Gondwana (Africa, Antarctica, Australia, South America) and Laurasia (Eurasia, North America). A chilly climate covered most of Gondwana with thick glaciers. As the sheets of ice moved, they ground up surface rock below into fine sand and dust.

When the glaciers receded, their cold winds rolled toward the ocean and blew the exposed, ultra-fine grit into the air, onto sea ice and ultimately into the 325-foot-deep water.

“It’s about the only plausible and geologically realistic interpretation that I can think of,” said Cliff Atkins, a sedimentologist at Victoria University of Wellington who wasn’t involved in the study. “It’s exactly what we’ve been finding in the modern environment like Antarctica, where I just spent 6 weeks off the coast collecting and analyzing airborne dust.”

Glacial dust blowing into the ocean, however, is only half the story. When silt particles landed on the water, they were rich in iron and other minerals that could produce phytoplankton and algal blooms.

The bursts of microscopic life that grew on the particles eventually weighed them down, sinking them to the seabed. There, the organic matter rotted, depleting oxygen from the water. These anoxic conditions prevented the decay of dead soft-bodied animals that sunk to the floor.

The resulting 30- to 50-foot-thick Soom Shale bends along the southern tip of Africa like a 560-mile-long hockey-stick, starting in the citrus groves and vineyards of Keurbos, meandering near Cape Town and banking east to Port Elizabeth. Gabbott and her team have unearthed fossils there for close to 20 years, primarily in a region near the Cedarberg mountains (about 150 miles north of Capetown).

It’s a continuous race against time for the scientists to save bug-eyed conodonts, crawly eurypterids (or sea scorpions) and yet-to-be-classified creatures.

“The farmers there dig this rock out and put it on the roads because it breaks down to make a good road stone,” Gabbott said. “Of course what they’re doing, perhaps unknowingly, is destroying the fossils.”

The scientists were suspicious of assumptions that sediment moved by storms, rivers and ocean currents preserved the specimens.

“It’s made of clay minerals, like most shale, but also clusters of silts,” Gabbott said of the sediment’s composition. “The only way to get that is from a landscape devastated by glaciation.”

Identifying such wind-blown processes in the geologic record is extremely difficult because turbid waters and scuttling sea creatures mix the sediment up beyond the point of recognition. But the anoxic sediment chemistry, ultra-fine layers of shale 1 to 10 millimeters thick, and a microscopic analysis revealing unusual specks of silt ruled out other explanations.

“We now have a nearly complete picture of the sea floor there over the thousands and thousands of years it took to deposit, and the only kind of deposition we can pin down is wind,” Gabbott said. “It’s really unique.”

Peter Van Roy, a paleobiologist at Yale University (also not involved in the study), said the model explains soft tissue preservation in a very plausible way.

“How a fossil is made tells us something about where and how the animal lived,” Van Roy said. “In short, it helps you interpret fossils correctly. It’s important work to be doing.”

With a definitive case pinned down, Gabbott said the next step is to start seeking out similarly formed shales to fill gaps in the fossil record.

“There are numerous black shales formed during other glaciations, like the Carboniferous period 300 million years ago,” she said, noting a few locations in Cape Province, South Africa. “I’d love to go out there and have a look.”

Images: 1) A eurypterid (sea scorpion) from the Soom Shale, South Africa. This fossil is approximately 440 million years old. It is so well-preserved that you can see its muscle blocks, gills and paddles that it used for swimming. Credit Dick Aldridge
2) Cape Province in South Africa, where the Soom Shale (gray) and its basin are located. Credit: Geology
3) Reconstruction of eurypterid (sea scorpion) chasing a conodont (early vertebrate). The Soom Shale is one of only two deposits world-wide that preserves complete conodont animals including their muscles, eyes and notochord (stiffening rod). Conodonts are some of our earliest vertebrate ancestors. Credit: Alan Male
4) Ground-up can blow into the air and bounce across sea ice, ultimately into the ocean. There, phytoplankton and algal blooms dragged the particles to the sea floor and help preserve soft-bodied animals. Credit: Geology

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