Friday Morning: Looking High and Low for Missing Lead
A scientific poster session can look very much like a large
high-school science fair project, except with more equations and fewer
hand-drawn graphs. Among today’s 55 sessions (with 30 posters each), I
set my sights on “Frontiers in Isotope Geochemistry IV,” a session
organized by Nobu Shimizu and Albrecht Hofmann in honor of one of the field’s pioneers, Stan Hart.
immediately I bumped into Hart himself, and he began to explain the
first of the Great Lead Paradoxes. Sketching a quick graph on my
notepad (see photo), Hart explained a problem that has been vexing
geologists for decades: They have calculated how much of two lead
isotopes (called lead-206 and lead-207) ought to be found in the Earth.
The paradox: In all parts of the Earth examined so far, most of the
lead-207 is missing. Where did it go?
Hart said one possible
location is deep in the Earth’s mantle, whose depth and intense heat
and pressure make it off limits to human exploration. Hart thinks that
dense sulfides, whose crystal structure can store lots of lead atoms,
carry the lead downward. A nearby poster showed
the first few experimental results supporting this explanation. But
still, Hart said, the inaccessibility of the mantle means the
explanation is hard to test and therefore not terribly convincing at
As a working hypothesis, though, the idea spurs
scientists to look for evidence of the Earth’s mantle in more
accessible places. Shimizu’s graduate student Jessica Warren went looking in a kind of rock called peridotite that they found on the Southwest Indian Ridge between Africa and Antarctica.
region of seafloor is spreading apart extremely slowly, bringing up
rock that normally lies much deeper. Dredging peridotite from this
ocean ridge is about as close as geologists can get to visiting the
center of the Earth with a collecting jar. Perhaps sulfides scattered
through the peridotite will lead geologists to discover a massive
lead-207 hideout deep in Earth’s mantle.
|Stan Hart and the Cloud of Eels|
While I had Hart’s attention, I asked him to tell his story about
being surrounded by eels at the tip of a volcano 700 meters (2,300
feet) under water.
The story made headlines last May when Hart investigated the Vailulu'u seamount off Samoa. Hart was diving in the Pisces V submarine when they discovered the eels.
were hiding in this porous rock,” Hart said. “We sat and looked at it
and saw their little heads in the rock looking out.”
A horde of
eels lurking inside a volcano isn’t an everyday sight, but after a
while the sub pilot got tired of looking at their noses. So he rammed
the rock (Hart described it as a “gentle ramming”). Suddenly “the eels
came crashing out, billions of them,” filling the water around the sub,
Hart said. “It was incredible.”
Hart said that after the word got
back to other scientists on the cruise, a brief bit of rock-nudging
became standard on the remaining dives.