Environmental Engineering Reference
In-Depth Information
waters to depth without recycling. It is also possible, but currently unknown, that
organic material may adhere to ash particles during settling. Given the strong
ef
ciency of organic material recycling in the water column, these effects could be
non-trivial. Ash undergoes relatively rapid diagenesis on the seafloor, leading to a
fast depletion of oxygen in pore waters through the oxidation of silicate-bound
Fe
2
รพ
(Haeckel
et al
.,
2001
; Hembury
et al
.,
2012
). This reduces the breakdown
of buried organic material, strongly increasing the preservation of C
org
below the
ash-layer surface.
In terrestrial environments, a large ash blanket can permanently decouple a soil
from the atmosphere, leading to the formation of a new soil over the coming
years to millennia and the storage of C in the palaeosol. An extreme example of
this comes from New Zealand, where an ignimbrite eruption from Taupo covered
20 000 km
2
and engulfed
1km
3
of mature podocarp forest (Hudspith
et al
.,
2010
). An albedo-related drop in soil temperature would slow microbial activity
and, therefore, the rate of organic-matter decomposition (Ayris and Delmelle,
2012
). New soil formation is often sluggish, given the dearth of N and C
org
in freshly deposited ash, but over millennial timescales even fragile ecosystems
can completely recover (Kilian
et al
.,
2006
).
~
17.4.4 Long-term global cooling?
The high correlation between volcanism and millennial-timescale cooling in ice-
cores (Bay
et al
.,
2004
), and between ignimbrite
flare-ups and onsets of glaciations
(Cather
et al
.,
2009
), has led several authors to conclude that Fe fertilization
of ocean surface waters could have accelerated global cooling. While each process
is poorly constrained, primary productivity, silicate weathering and carbon burial
all act as a sink for atmospheric CO
2
. This suggests that a combination of
these processes could lead to an extended global cooling by reducing
atmospheric CO
2
after large-scale explosive volcanism, although further research
is needed to constrain each process.
17.5 Summary
The deposition of volcanic ash can lead to numerous contemporaneous climate
forcings, which in turn will vary with location, volume, chemistry, climate, season
and depositional environment. Therefore, the environmental response will be
unique for each eruption. Ash deposition on land leads to changes in vegetation
coverage and surface albedo, which interplay with atmospheric and oceanic circu-
lation. Acid and metal release can poison ecosystems, while nutrient addition can
sustain or replenish affected ecosystems over longer periods. In marine settings,
