Geotimes - December 2002 - Choked into extinction

Geotimes

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Ancient soils
Choked into extinction

Life reached the brink of complete annihilation at the end of the Permian, 251 million years ago. Roughly 85 percent of all marine species went extinct during the Late Permian, along with 70 percent of land vertebrates and a significant number of plants and insects. The extinctions cleared out ecological niches and opened the door for the rise of the dinosaurs. Despite the enormity of the event, few physical clues remain as to what caused the extinctions.

On Mt. Crean in Antarctica, an ancient soil provides a glimpse into what may have caused mass extinctions at the end of the Permian. A black seam (top, left) in between two lighter-colored units marks the Permian-Triassic boundary. Researchers found the rare mineral berthierine in soil layers that formed during the largest pulse of extinctions. They argue that the presence of the mineral indicates that many plants went extinct because they ran out of oxygen. Photo by Gregory Retallack.

A study in the October Geology reveals another piece of the puzzle. Geologists found the green mineral berthierine in soil layers that developed during the largest pulse of extinctions. A rare soil mineral, berthierine only forms in low-oxygen conditions. The authors argue that the presence of the mineral suggests that many land plants went extinct because they ran out of oxygen. And without plants to hold soil in place, huge amounts of sediment would flow into the coastal waters, explains lead author Nathan Sheldon, a graduate student at the University of Oregon. That sediment would cloud the water and lower its oxygen concentration, contributing to the decline of marine species.

“There aren’t very many ways to investigate the status of oxygen in the atmosphere. Putting forward a new method is a valuable contribution,” says paleoclimatologist Hope Jahren of Johns Hopkins University.

A team of researchers, led by Gregory Retallack of the University of Oregon, first found the brethierine in 1999 in ancient soils in Antarctica and Australia, but could not identify it. X-ray diffraction suggested berthierine, but the evidence was not convincing. Berthierine is so rare that the researchers thought they had found the much more common mineral chlorite. Chlorite forms under a range of oxygen conditions and so cannot constrain paleoclimates.

A couple of years later, Sheldon, funded by a grant from the National Science Foundation, re-examined the minerals. He used scanning electron microscopy to locate individual grains of the mineral within polished thin sections of the paleosols. Using an electron microprobe, he honed in on those grains, ranging from five to 20 microns in diameter, to determine their chemical composition. High iron and low magnesium indicated berthierine, not chlorite.

“The microprobe data nailed it,” Retallack says.

Sheldon and Retallack argue that the presence of berthierine supports the hypothesis that a catastrophic release of methane at the end of the Permian drew oxygen out of the atmosphere and soil. Methane naturally binds with oxygen to form carbon dioxide and, in the process, scrubs oxygen out of the air. Several researchers have argued for a methane pulse across the P-T boundary; Retallack and colleagues were the first to link that release to a significant drop in oxygen.

The methane that escaped had probably been trapped in permafrost ice and coastal shelves, Sheldon says. Today these so-called methane clathrates hold roughly 10,000 billion tons (gigatons) of carbon. They probably stored even more during the Late Permian, when the supercontinent Pangaea hosted more shelf and permafrost area.

But the event that triggered the release remains a mystery. “We just don’t know,” Retallack says. A meteorite impact or a large volcanic eruption may have melted or disrupted the clathrates enough to release their lode (Geotimes, November 2001).

Methane-driven oxygen depletion would have particularly devastated lowland, waterlogged soils, Retallack says. Those soils would already have been low in oxygen; further lowering would have driven them to anoxia. Retallack argues that the spatial distribution of anoxia helps explain why certain plant species survived the P-T boundary while others did not — influencing the trajectory of evolution. “The plants that make coals, the peat-forming plants, they basically didn’t survive. The plants that we do see making it are those that lived in upland, well-drained soils — for example conifers, seed ferns and ginkgos.”

Bob Berner, a geochemist at Yale University, agrees that oxygen dropped during the early Triassic, but disagrees that methane is the culprit. “The extremely low calculated concentration of oxygen [Sheldon] gets from his equilibrium equation can be achieved only after about 250,000 gigations of carbon, in the form of methane, are liberated into the atmosphere.” That amount is more than 50 times the amount Berner estimates to have escaped based on carbon isotopes across the P-T boundary, according to research published this year in Proceedings of the National Academy of Sciences. Instead, Berner argues, oxygen dropped because the rate at which organic matter was buried in sediments declined during the Triassic. Lower burial rates reflect greater decomposition of organic matter, a process that consumes oxygen.

While debate will continue, Sheldon is forging ahead with the research. He plans to look for berthierine in other paleosols that document the P-T extinctions, especially those in lower latitudes. His goal is to find out if the oxygen depletion was global, or just confined to high latitudes.

Greg Peterson

Read a related story from the November 2001 Geotimes, More end-Permian impact proof.

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