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Plate tectonics
may have had
an early start
Ancient zircon crystals imply
activity during Hadean eon
By Sid Perkins
The chemical composition of ancient
crystals bolsters the notion that tectonic
plates may have jostled across Earth’s surface more than 4 billion years ago.
New clues about the planet’s early history have emerged from zircons in rocks
from Western Australia’s Jack Hills,
scientists report in the Nov. 27 Nature. The
tiny crystals — chemically inert bits of zirconium silicate — are remnants of some of
Earth’s first rocks, says Mark Harrison of
the University of California, Los Angeles.
A remarkable implication of the work
is that processes making Earth habitable
were established early in the planet’s history, comments Stephen Mojzsis of the
University of Colorado at Boulder.
Scientists call the first 600 million
years of Earth’s history the “Hadean eon”
because of the presumably hellish tem-
peratures on the freshly coalesced and
largely molten planet. Additional heat
came from the decay of radioactive isotopes within the Earth. Previous studies
suggest that the heat flow from the young
planet’s interior was three to five times
higher than it is today, Harrison says.
Harrison and colleagues examined
stray bits of mineral, called inclusions,
in zircons that had crystallized between
4. 19 billion and 4.02 billion years ago.
Seven inclusions enabled the researchers
to infer the temperatures and pressures at
which the host zircons cooled.
In six inclusions made of the mineral
muscovite, levels of titanium ranged
from 3 parts per million to 9 ppm, a sign
that the zircons formed at temperatures
between 665° Celsius and 745°C. The
ratio of silicon to aluminum in the muscovite suggests the zircons formed about Tectonic activity would explain the dis-
25 kilometers below ground. An inclusion crepancy: The only regions today where
made of hornblende, a group of silicate the planet’s heat loss is so much lower
minerals, yielded similar results. than average is above subduction zones,
The findings indicate that the rate of where one tectonic plate collides with and
heat flow to the surface overlying the area is shoved below another. As the plate dives
where these zircons formed was about into the mantle, it cools the mantle mate-
75 milliwatts per square meter. That’s rial, allowing zircons to crystallize and
slightly higher than Earth’s average heat stifling heat flow to the surface. The new
loss today but only one-third to one-fifth analyses suggest that similar processes
the rate expected for the Hadean eon. may have operated in the Hadean.
Rocks hold ancient zircon crystals (inset)
that host tiny mineral inclusions (arrow).
Arctic freeze triggers big squeeze
Methane release linked to wetlands covering permafrost
By Sid Perkins
The annual freeze of wetland soils lying
atop permafrost in many high arctic
regions may trigger the long-noted, yet
mysterious rise of atmospheric methane
concentrations over those areas each fall,
a new study suggests.
The bacteria-aided decomposition of
organic material in high-latitude wetlands
in large part depends on soil being warm.
During the summer, the breakdown process generates prodigious amounts of
methane. As autumn slides toward win-
ter, methane emissions should wane.
But for decades scientists have detected
an unexplained autumn uptick in atmospheric methane at arctic latitudes, says
Torben Christensen, a biogeochemist at
Lund University in Sweden.
In the Dec. 4 Nature, he and his
colleagues speculate that as winter
approaches, the freezing of the soil overlying permafrost boosts the autumn
methane emissions. “Most of the methane is produced during the warm summer
months, but not all of it is emitted then,”
Christensen says.
Monitoring wetlands in northeastern
Greenland, he and his collaborators have
found that summer emissions of methane
roughly track soil temperatures, peaking
in July and then dropping off into early
September. Observations in 2007 showed
that methane emissions began to rise
again in mid-September and remained
high for several weeks.
At the Greenland site, only the upper
30 to 50 centimeters of soil thaws each
summer. In fall, the top layer of soil freezes
and expands, pressurizing the soil beneath,
Christensen contends. Because the underlying permafrost is impermeable, methane that accumulated in the thawed soil
during the summer is squeezed out and
forced to the surface.
