Environmental Engineering Reference
In-Depth Information
the end-Permian, the Toarcian, and the end-Triassic events (Svensen
et al
.,
2009
;
McElwain
et al
.,
2005
; Ruhl and Kürschner,
2011
;Svensen
et al
.,
2007
). During
the last decade, gas hydrate dissociation, impacts, sluggish ocean circulation and
anoxia, lava CO
2
degassing and contact metamorphic CH
4
degassing are among the
hypotheses that are discussed in the scientific community as triggers for ancient
rapid global warming events (e.g. Wignall,
2001
; Knoll
et al
.,
2007
; Cohen
et al
.,
2007
;Cui
et al
.,
2013
). We emphasize that there is a current consensus that Siberian
Traps volcanic and metamorphic processes triggered the end-Permian crisis.
Here we show that contact metamorphism in volcanic basins has the potential to
generate a mass of CH
4
,CO
2
,SO
2
, and halocarbons in the
>
10
3
10
4
Gt range, and
that the gases produced in these provinces have been released to the atmosphere
via vertical pipe structures and basin seepage processes. The large volumes and
compositions of gases produced by contact metamorphism indicate that volcanic
and metamorphic processes may have
-
first-order in
uence on environmental
change and mass extinctions.
12.2 The anatomy of continental LIPs
During the formation of continental LIPs, magma is commonly emplaced as sills
and dikes in the upper crust. When the magma volumes are high and the emplace-
ment takes place within sedimentary basins, these basins are classi
'
volcanic
basins
'
. Volcanic basins are present along rifted continental margins and on
lithospheric cratons (Coffin and Eldholm,
1994
)(
Figure 12.1
). Sill thicknesses
can exceed 350 m, sandwiched between sedimentary sequences and associated
contact metamorphic aureoles. Here we present some of the key aspects on the
structure and anatomy of volcanic basins associated with three of the largest
LIPs, as summarized in
Figure 12.2
. We focus on the sediment compositions in
the basins, the distribution of sills, the formation of degassing pipes and the
composition and volume of the gas formed.
ed as
12.2.1 The Siberian Traps and the Tunguska Basin
The Siberian Traps covered several million square kilometres of Siberia with
kilometre-thick stacks of lava over a period of 0.5
1 Myr (Kamo
et al
.,
2006
;
Mundil
et al
.,
2004
). On the way to the surface, the lavas in Siberia passed through
a series of thick sedimentary sequences collectively known as the Tunguska Basin
(
Figure 12.3Aa
). The basin contains enormous volumes of Cambrian evaporites,
with up to 2.5-km-thick sequences of halite-rich strata, anhydrite and carbonates
across a 2 million km
2
area (Petrychenko
et al
.,
2005
; Zharkov,
1984
).
-
