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When an asteroid smashed into the Yucatan peninsula near Chicxulub, Mexico,
65 million years ago, the resulting fireball vaporized thousands of gigatons
of rock, much with high sulfur content. The blast launched the rocky debris
high into the atmosphere, where it produced sulfate aerosols and precipitated
acid rain. Geologists partially credit the tremendous impact (called the K-T
impact) and its many consequences with the mass extinction that marked the end
of the Cretaceous. Yet while many marine and terrestrial species, including
the dinosaurs, went extinct, others somehow managed to escape unharmed —
including many freshwater organisms that should have been particularly susceptible
to acid rain, just as their descendants are today. This paradox has proved problematic
for most theories of impact extinction.
Now geochemists Teruyuki Maruoka, of Washington University in St. Louis, Mo.,
and Christian Koeberl, of the University of Vienna in Austria, have revisited
the longstanding question of how some freshwater species could have survived
rain with a pH potentially as low as battery acid.
In an article published in the June issue of Geology, Maruoka and Koeberl
suggest that the mineral larnite (Ca2SiO4) formed in the
vapor plume following the K-T impact and neutralized enough acid to make water
safe for freshwater species within hours of the blast.
Larnite is a member of the orthosilicate group, the fastest acid buffers of
all the silicates. It forms when carbonate rock becomes metamorphosed by exposure
to magma or when crystals precipitate out of a high-temperature melt that has
undergone rapid cooling, such as a vapor plume.
According to the authors, shortly after the blast, spherules of larnite would
have condensed out of the melt, falling to Earth and sinking to the bottom of
water bodies. In shallow freshwater environments such as lakes, ponds and streams,
as well as in near-shore marine environments, the larnite would have remained
close enough to the surface to buffer acid rain. In the deep ocean, however,
larnite would have sunk below the mixing zone and offered little buffering capacity
for acid rain near the surface. On land, green plants would have gained no protection
from acid rain.
“The new model proposed by Maruoka and Koeberl is thus very interesting,”
says Lionel Cavin, a paleontologist at London’s Natural History Museum,
“because it can explain one of the weaknesses of the (impact extinction)
hypothesis, that is, the absence of strong extinctions in freshwater environments
due to acidification.”
Because the impact occurred on siliceous continental crust overlain by a 3-kilometer-thick
sequence of carbonates and evaporites, the vapor plume would have been enriched
with calcium and sulfur. “The impact penetrated this sequence, and, therefore,
the vapor plume also incorporated the materials of the basement rock,”
Maruoka says. “The SiO2 [silicate] for the larnite was supplied by the
projectile and the basement rock.”
To test their theory, Maruoka and Koeberl considered the speed of acid neutralization
in the worst-case scenario for freshwater life — where all of the sulfur
liberated by the impact was immediately converted to sulfuric acid and fell
back to Earth.
Using data from previous studies of the melt chemistry of the plume they assumed
330 to 650 gigatons of larnite were present as spherules. According to their
calculations, this amount would have been more than enough to consume most sulfuric
and nitric acid related to the K-T impact and raise the pH of freshwater environments
to levels safe for life.
Researchers have not yet found larnite at the Chicxulub impact site. However,
they have found it at other impact sites that involved a carbonate-dominated
target like Chicxulub.
Sara Pratt
Geotimes contributing writer
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