Geoscience Reference
In-Depth Information
GEOCHEMISTRY
The Haitian glassy spherules are of two kinds: more abundant ones
made of black glass, and rarer ones made of yellow glass. The chem-
ical composition of the glasses indicates that they could have been
derived from continental rocks of granitic composition, plus a minor
component of limestone and clay.
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This is consistent with an
impact onto a continental shelf, where limestones, muds, and sulfur-
bearing rocks are apt to be found, and which was the setting of the
Chicxulub region during late Cretaceous time.
Imagine again the impact of a meteorite 10 km in diameter, but
now focus on the target rocks and what happens to them. The tem-
perature at ground zero instantly far exceeds the point at which
limestone and sulfur-bearing rocks are converted into gases. These
gases, along with vaporized meteorite and other target rocks, are
lofted high into the atmosphere and distributed around the entire
earth. Estimates are that billions of tons of both carbon dioxide and
sulfur were injected into the atmosphere.
Not only did the chemical signatures of the Haitian glasses match
those of the rocks from Chicxulub, so did the isotopes of oxygen,
neodymium, and strontium. The isotopic measurements were made
by Joel Blum and Page Chamberlain of Dartmouth, the university of
Officer and Drake, showing that the geology department there
was not monolithic in its view of the Alvarez theory.
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Blum and
Chamberlain found the Chicxulub igneous rock and the Haitian
tektites to have identical isotopic ratios. The odds of this happening
by chance are vanishingly small, and therefore not only are the two
of the same age, as previously confirmed, they are linked by origin.
However it was that the Chicxulub melt rock and the Haitian tek-
tites formed, they come from the same source.
EJECTA DEPOSITS
Adriana Ocampo and Kevin Pope of the Jet Propulsion Lab, and
Alfred Fischer (who moved from Princeton to the University of
Southern California) have discovered the closest ejecta deposit to
ground zero.
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In a quarry on Albion Island in the Hondo River in
Belize, 360 km from Chicxulub, at the top of the Cretaceous, they
have come upon a double layer reminiscent of those found else-
where around North and Central America, except that here some of
the rock fragments are truly on a giant scale. The lower layer is
about 1 m thick and contains abundant rounded spherules, 1 mm to
20 mm in size, composed of dolomite, a magnesiated limestone.
Above it lies a 15-m layer containing broken fragments of a variety
of rocks of Cretaceous age; some of the chunks are as big as a car.
