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In-Depth Information
and last manifestations of volcanism may span on the order of 5 to 10 Myr. But it
was found in the late 1980s that the main volumes of volcanism had been emplaced
over only
~
1 Myr. It was next found that a small number of climactic phases,
having lasted from 10 to 100 kyr, were separated by several hundreds of thousands
of years of quiescence. Finally, the analysis of paleomagnetic secular variation,
coupled with
field observations and petrologic analyses, has shown that single
eruptive events with volumes in excess of 1000 km
3
and up to 10,000 km
3
were
emplaced in remarkably short durations of 100 down to possibly 10 years and less!
During these climaxes, LIPs emplaced basaltic
fluxes up to 40 times (and more?)
the global production of all mid-ocean ridges (25 km
3
/yr).
These gigantic individual
flows must have injected enormous amounts of gases,
and notably CO
2
and SO
2
, into the atmosphere. One should add to these possibly
even larger gas emissions, which are due to contact metamorphism of sediments
intruded by dikes and sills. The injection sequence would have led to a compli-
cated thermal atmospheric response for each SEE: short-term (months to years)
cooling due to sulfate aerosols, longer-term (decades to centuries) warming due to
a greenhouse effect, and
finally much longer-term (100 kyr) cooling due to CO
2
removal and alteration of fresh basaltic lava. It is estimated that 1 km
3
of magma
could have released
10 Tg of CO
2
, 5 Tg of SO
2
and 1 Tg of HCl. The role of SO
2
is particularly important in producing early cooling phases and decreasing the
ef
~
cation of the rise of pCO
2
.
The actual climate changes would have involved the superimposed responses
of the sequence of SEEs erupted by the
ciency of silicate weathering, leading to an ampli
flood basalt. Our working hypothesis
remains that the details of the SEE sequences are the key factor of the severity of
the extinctions caused by these climate fluctuations. With SEEs separated in
time by less than a few thousand years, a runaway effect would have occurred,
leading to a phase of mass extinction.
It is unclear how we can hope to attain the kind of temporal resolution
required to test this hypothesis further. Some hope comes from climate modeling
of pulsed volcanism, of the type summarized in this paper. But the observational
database would greatly bene
t from concerted efforts to achieve the kind of
analyses now available for the Columbia, NAIP, Deccan, Karoo, CAMP and
Siberian traps to all other continental
flood basalts.
References
Alvarez, L., Alvarez, W., Asaro, F., and Michel, H., 1980. Extraterrestrial cause for the
Cretaceous
-
Tertiary extinction
-
experimental results and theoretical interpretation.
1108.
Armstrong, H.A. and Harper, D.A.T., 2013. End-Ordovician mass extinction (abstract). In
Proceedings of
Volcanism, Impacts and Mass Extinctions: Causes and Effects, March
27
Science
, 208, 1095
-
-
29, 2013, London, The Natural History Museum, p. 23.
