At least for the Cretaceous extinction, there is good evidence that an
asteroid impact played at least some, if not the major, role. In that an
asteroid impact creates quite a massive shock wave, that there might be
enough geological disruption at the antipode (or near to it) to have
resulted in the unusual burst of volcanic eruptions that occurred in the
Indian subcontinent seems to me a plausible hypothesis for connection of
the two suggested causes of the sharp extinction that occurred, and this
linkages seems to me to be a more reasonable hypothesis than assuming a
chance coincidence in timing of the two very massive and unusual events.
With the land surface and ocean depths being recreated over time as the
tectonic plates shift, evidence for such linkages for older extinctions
may well be very hard to pin down, but, while I am not an expert in the
field or well read in it, I've not read or heard of a good explanation
for why there might from time to time be such massive volcanic eruptions
suddenly starting absent some triggering cause like a massive asteroid
impact.
Thus, might it be that the extinction events are really being caused by
the joint actions of an asteroid impact and a subsequent extensive
massive volcanic event? If so, then the potential for considering the
Ordivician extinction (or other extinctions) as a possible analog for
stratospheric climate intervention would seem to require a good deal
more research into the extinction events to get a much better sense of
what caused what and how.
Mike MacCracken
On 5/31/17 10:51 AM, Renaud de_Richter wrote:
*Anyone concerned by the idea that people might try to combat global
warming by injecting tons of sulfate aerosols*
*into Earth's atmosphere may want to read an article in the May 1,
2017 issue of the journal Geology.*
https://www.sciencedaily.com/releases/2017/05/170530082345.htm
http://geology.gsapubs.org/content/early/2017/05/01/G38940.1.full.pdf
In the article, a Washington University scientist and his colleagues
describe what happened when pulses of
atmospheric carbon dioxide and sulfate aerosols were intermixed at the
end of the Ordivician geological period more than 440 million years ago.
The counterpart of the tumult in the skies was death in the seas. At a
time when most of the planet north of the
tropics was covered by an ocean and most complex multicellular
organisms lived in the sea, 85 percent of marine
animal species disappeared forever. The end Ordivician extinction, as
this event was called, was one of the five
largest mass extinctions in Earth's history.
Although the gases were injected into the atmosphere by massive
volcanism rather than prodigious burning of fossil
fuels and under circumstances that will never be exactly repeated,
they provide a worrying case history that reveals
the potential instability of planetaryscale climate dynamics.
Figuring out what caused the end Ordivician extinction or any of the
other mass extinctions in Earth's history is
notoriously difficult, said David Fike, associate professor of earth
and planetary sciences in Arts & Sciences and a coauthor on the paper.
Because the ancient atmospheres and oceans have long since been
altered beyond recognition, scientists have to
work from proxies, such as variations in oxygen isotopes in ancient
rock, to learn about climates long past. The
trouble with most proxies, said Fike, who specializes in interpreting
the chemical signatures of biological and
geological activity in the rock record, is that most elements in rock
participate in so many chemical reactions that a
signal can often be interpreted in more than one way.
But a team led by David Jones, an earth scientist at Amherst College,
was able to bypass this problem by
measuring the abundance of mercury. Today, the primary sources of
mercury are coalburning power plants and
other anthropocentric activities; during the Ordivician, however, the
main source was volcanism.
Volcanism coincides with mass extinctions with suspicious frequency,
Fike said. He is speaking not about an
isolated volcano but rather about massive eruptions that covered
thousands of square kilometers with thick lava
flows, creating large igneous provinces (LIPs). The most famous U.S.
example of a LIP is the Columbia River Basalt
province, which covers most of the southeastern part of the state of
Washington and extends to the Pacific and into Oregon.
Volcanoes are plausible climate forcers, or change agents, because
they release both carbon dioxide that can produce longterm
greenhouse warming and sulfur dioxide that can cause shortterm
reflective cooling. In addition,
the weathering of vast plains of newly exposed rock can draw down
atmospheric carbon dioxide and bury it as
limestone minerals in the oceans, also causing cooling.
When Jones analyzed samples of rock of Ordivician age from south China
and the Monitor Range in Nevada, he
found anomalously high mercury concentrations. Some samples held 500
times more mercury than the background
concentration. The mercury arrived in three pulses, before and during
the mass extinction.
But what happened? It had to have been an unusual sequence of events
because the extinction (atypically)
coincided with glaciation and also happened in two pulses.
As the scientists began to piece together the story, they began to
wonder if the first wave of eruptions didn't push
Earth's climate into a particularly vulnerable state, setting it up
for a climate catastrophe triggered by later eruptions.
The first wave of eruptions laid down a LIP whose weathering then drew
down atmospheric carbon dioxide. The
climate cooled and glaciers formed on the supercontinent of Gondwana,
which was then located in the southern hemisphere.
The cooling might have lowered the tropopause, the boundary between
two layers of the atmosphere with different
temperature gradients. The second wave of volcanic eruptions then
injected prodigious amounts of sulfur dioxide
above the tropopause, abruptly increasing Earth's albedo, or the
amount of sunlight it reflected.
This led to the first and largest pulse of extinctions. As ice sheets
grew, sea level dropped and the seas became
colder, causing many species to perish.
During the second wave of volcanism, the greenhouse warming from
carbon dioxide overtook the cooling caused by
sulfur dioxide and the climate warmed, the ice melted and sea levels
rose. Many of the survivors of the first pulse of
extinctions died in the ensuing flooding of habitat with warmer,
oxygen poor waters.
The takehome, said Fike, is that the different factors that affect
Earth's climate can interact in unanticipated ways
and it is possible that events that might not seem extreme in
themselves can put the climate system into a
precarious state where additional perturbations have catastrophic
consequences.
*"It's something to keep in mind when we contemplate geoengineering
schemes to mitigate global warming," said*
*Fike, who teaches a course where students examine such schemes and
then evaluate their willingness to deploy them.*
Story Source:
Materials provided by Washington University in St. Louis. Original
written by Diana Lutz. Note: Content may be
edited for style and length.
Journal Reference:
1. David S. Jones, Anna M. Martini, David A. Fike, Kunio Kaiho. A
volcanic trigger for the Late Ordovician
mass extinction? Mercury data from south China and Laurentia. Geology,
2017; G38940.1 DOI: 10.1130/G38940.1
_http://geology.gsapubs.org/content/early/2017/05/01/G38940.1.full.pdf _
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