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Glacial Silt Encased Some of Earth’s Best-Preserved Fossils
Surprise: Radioactive Mercury Decays Into Uneven Chunks
Track Record: Man-Made Footprints on Other Worlds

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

Posted: 03 Dec 2010 02:47 PM PST

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 is 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 six 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 the 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 Cape Town).

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, ultrafine layers of shale 1 millimeter 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 the 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 worldwide that preserve 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 rock can blow into the air and bounce across sea ice, ultimately into the ocean. There, phytoplankton and algal blooms drag the sand and dust particles to the sea floor and help preserve soft-bodied animals. Credit: Geology

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Surprise: Radioactive Mercury Decays Into Uneven Chunks

Posted: 03 Dec 2010 08:21 AM PST

More than seven decades after German chemists discovered nuclear fission — the splitting of an atom that is harnessed by nuclear energy and nuclear weapons — scientists still can't describe the process in detail. A paper to appear in Physical Review Letters underscores that knowledge gap with the report of a totally unexpected type of fission in the element mercury. Instead of splitting into two equal-mass chunks as theory predicts, this bit of mercury split into uneven chunks, one lighter and one heavier than expected.

Asymmetric fission, which results in daughter fragments with different masses, has been seen before. But these earlier examples all could be easily explained. Isotopes of uranium, for instance, like to fission into one large chunk of tin-132 along with a smaller chunk. Like apartment dwellers filling each apartment in a complex, the 50 protons and 82 neutrons of tin-132 completely fill shells, or energy levels, within the nucleus and hence make it extremely stable.

In the new experiments, researchers thought the isotope mercury-180 would split equally into blobs of zirconium-90, which has 40 protons and 50 neutrons that stably fill the shells in the nucleus. "Zirconium-90 plus zirconium-90 makes mercury-180," says Witold Nazarewicz, a theoretical physicist at the University of Tennessee in Knoxville and the Oak Ridge National Laboratory who was not involved in the work.

Yet that's not what the scientists saw in their experiments at the ISOLDE radioactive beam facility at CERN, Europe's particle physics laboratory near Geneva. The researchers, led by Andrei Andreyev of the University of the West of Scotland in Paisley, instead saw the mercury-180 fission unevenly into ruthenium-100 and krypton-80 — isotopes that don't have completely filled shells the way zirconium-90 does.

Not only were the products of mercury-180 fission asymmetric, but it's the first time researchers have seen asymmetric fission and not been able to explain it by the filled-shells theory. "It was a big surprise," says team member Piet Van Duppen, a nuclear physicist at the Catholic University of Leuven in Belgium. "This is a totally new form of asymmetric fission."

Puzzled, the scientists analyzed the energy it takes mercury-180 to fission. The most energy-efficient way turns out to be to split into ruthenium-100 and krypton-80 rather than equal parts of zirconium-90, Van Duppen says.

Other isotopes in the same part of the periodic table might also show the same uneven split, he says. The team has already tested a second isotope of mercury and seen asymmetric fission there.

Probing fission throughout the periodic table will get easier with a new generation of radioactive beam facilities coming online in the next decade, says Van Duppen. These include the Facility for Rare Isotope Beams at Michigan State University in East Lansing and the Facility for Antiproton and Ion Research at the GSI research center in Darmstadt, Germany.

"What we have here," he adds, "is a new experimental tool to really verify our understanding of the atomic nucleus."

Image: Mercury vapor glowing in an electrical discharge tube. Credit: Wikimedia Commons/Alchemist-hp

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Track Record: Man-Made Footprints on Other Worlds

Posted: 03 Dec 2010 04:00 AM PST

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Humans have scribbled "We were here" all over the solar system. From footprints on the moon to burnt-out landers on Titan, the visible records of our excursions past Earth are badges of pride — not to mention unique opportunities to do science.

"Aside from just a curiosity, it allows us to know exactly where all the lunar samples came from relative to one another," said planetary scientist Jeff Plescia of the Johns Hopkins University Applied Physics Lab, who uses the Lunar Reconnaissance Orbiter spacecraft to look for human artifacts on the moon.

But our tracks won't last forever. A new paper in the Journal of Geophysical Research traces how tracks left by the Mars rovers Spirit and Opportunity are wiped away by the wind, usually within a Martian year.

"It's a little humbling," said planetary scientist Paul Geissler of the U.S. Geological Survey, lead author of the new paper. "Mars will just clean up after us, and wait for the next visitors."

This gallery takes a quick tour through the solar system through the visible marks we've left behind.

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