Environmental Engineering Reference
In-Depth Information
for which CO
2
flux information is available. Given the paucity of available data,
and the wide range of emission
fluxes that have been measured, from
>
20 000
t/day for top emitters to
<
50 t/day (
Figure 6.6a
), the reported wide variation
in the estimates of the global volcanic CO
2
output of 65
-
540 Tg/year is not
surprising. Observations have been sporadic and discontinuous at most of
the volcanoes listed in Burton
et al
.(
2013
). The utility of such spot measure-
ments is unclear, especially given the conspicuous CO
2
-
ux variations demon-
strated at the few volcanoes where systematic observations have been made
(
Figure 6.6b
).
Hypotheses for the mechanisms underlying large variations in CO
2
emissions
include changes in magma supply rate from the mantle source (Poland
et al
.,
2012
), pulses of CO
2
-rich magma/gas transfer from deep to shallow magma
storage zones (Aiuppa
et al
.,
2007
; Patanè
et al
.,
2013
; Werner
et al
.,
2012
),
precursor leakage of a CO
2
-rich foam from a deeply stored magma (Aiuppa
et al
.,
2010
), and assimilation of limestone blocks into shallow magma (Goff
et al
.,
2001
). However, the utility of forecasting volcanic activity based on gas emissions
is supported by CO
2
flux variations reportedly occurring days to several weeks
before eruptions of different styles and magnitudes (Aiuppa
et al
.,
2010
; Poland
et al
.,
2012
; Werner
et al
.,
2012
)(
Figure 6.5b
).
The relatively narrow range of compositions revealed by Multi-GAS observa-
tions (
Figure 6.2
) for the strongest degassing arc volcanoes, all of which have
CO
2
/SO
2
molar ratios in the range of 1
-
ning the
global CO
2
emissions from arc volcanism. Although the strongest CO
2
arc
emitter (Popocatépetl) is not included in the Multi-GAS data set, its time-average
background CO
2
/SO
2
ratio might be close to the range of 1
-
5(Goff
et al
.,
2001
).
Then, even if the entire 13
5,mightbeusefulforcon
18 Tg/year SO
2
volcanic emissions derive from arc
volcanoes, the CO
2
/SO
2
ratio range of 1
-
-
5 would lead to an arc CO
2
flux of 9
-
61
Tg/year, with a best-guess range of 17.9
-
24.7 Tg/year based on the average
Multi-GAS ratio of CO
2
/SO
2
2in
Figure 6.2
. This is at the lower end of
estimates reported by Burton
et al
.(
2013
). For comparison, Fischer (
2008
)
calculated a total arc CO
2
~
flux of 85 Tg/year, which partially re
ected the use
of a higher SO
2
arc
flux of 20.2 Tg/year. Following the same line of reasoning,
the overall H
2
O/CO
2
ratio of
~
35
10 in the Multi-GAS data set would
xthe
global volcanic arc H
2
O
ux at 256
-
354 Tg/year, which is twofold less than that
quoted by Fischer (
2008
).
The large spread of CO
2
/SO
2
ratios evident in
Figure 6.2
means that similar
arguments cannot be extended to within-plate/rift volcanism. Burton
et al
.(
2013
)
reported that these volcanoes may have a combined CO
2
output of
30 Tg/year.
This, combined with our above estimate for the arc volcanism, would increase
the total CO
2
output from subaerial volcanism to 39
~
-
91 Tg/year.
