Environmental Engineering Reference
In-Depth Information
volatiles from sedimentary rocks by magmatic heating (Ganino and Arndt,
2009
).
Analogous long-term consequences of variability in the composition of
subduction-zone or other non-LIP magmatic gases due to variations in the arc
crustal compositions over geological time have yet to be explored. Other links
between volcanic gas composition and factors such as mantle oxidation state, volatile
cycling and average degassing pressure (e.g. Holland,
2002
;
2009
; Gaillard
et al
.,
2011
) have been linked to the oxygenation of Earth
'
s atmosphere; however,
our understanding of the exact nature of such controls on gas composition remains
incomplete.
It has been suggested that subduction of carbonate-rich sediments (analogous to
Central America today) could be linked to increased volcanic CO
2
outgassing
and periods of global warming. The deep sources of magmatic gases at subduction
zones include both volatiles from the mantle wedge, and components brought into
the mantle with the downgoing slab. For example, carbon may be carried into
the mantle in sediments (carbonates, or organic C) or in the hydrothermally altered
basaltic crust, and serpentinised peridotite. A poorly constrained portion of this
C is released by progressive metamorphism during subduction (Gorman
et al
.,
2006
). Therefore, the closure of the Tethys Ocean and subduction of large volumes
of marine carbonate has been postulated as a major factor contributing to late
Cretaceous to early Cenozoic elevated atmospheric CO
2
concentrations (Edmond
and Huh,
2003
;KentandMuttoni,
2008
; Johnston
et al
.,
2011
). Without a better
understanding of the factors controlling the ef
ciency of C recycling through
subduction zones, however, these ideas remain poorly understood (Gorman
et al
.,
2006
).
14.4 Summary
This chapter is intended to put the discussions of speci
c modes of volcanic
impacts on Earth
'
s atmosphere into context. Even the largest short-lived explosive
eruptions contribute little to atmospheric composition on geological timescales,
even though they represent signi
cant perturbations on decadal timescales. Due
to their scale and prolonged nature, LIPs are one manifestation of volcanism
that has the potential to impact the Earth
'
s atmosphere on geological timescales
(10
4
10
5
yr), comparable to the carbon-cycle perturbations such as ocean anoxic
events. There is still much to be done to understand how these signi
-
cant and
prolonged LIP events may affect the global environment. Equally, much remains
to be done to understand more subtle feedbacks between volcanism and the
environment, such as changes in glacial loading; and the consequences of spatially
and temporally varying
fluxes of volatile elements (including carbon and halogens)
through the world
'
s subduction zones. Improving our global geological eruption
