The environmental and climatic impacts of volcanic ash deposition - Volcanism and Global Environmental Change - page 269

elevated carbon export to depth. Secondly, eruption volume, duration and depos-
itional environment will have a very large effect on any subsequent fertilization.
A large-scale silicic eruption that is largely deposited in the open ocean will have a
relatively limited immediate effect, given the speed of settling of ash through the
water column (Wiesner et al ., 1995 ), volcanogenic atmospheric effects during the
period of deposition (e.g. a H 2 SO 4 -induced volcanic winter; Self, 2006 ) and
uncertainties of residence times of insoluble nutrients such as Fe in the euphotic
zone in the absence of biological uptake. Marine ash deposition may, however,
have a delayed effect because benthic waters, enriched by nutrient release, may be
subsequently upwelled (Cather et al ., 2009 ). Sustained surface-water fertilization
that decreases atmospheric CO 2 is more likely for episodes of many smaller-scale
eruptions, or the remobilization of a large terrestrial ash blanket.

17.4.2 Silicate weathering
The abundance of easily weatherable, fine-grained material leads to the

consumption of CO 2 through the breakdown of primary silicates, such as this
reaction for plagioclase feldspar:

Ca 2 þ 2 HCO 3 - CaAl 2 Si 2 O 8 þ 2CO 2 þ 3H 2 O ! Al 2 Si 2 O 5 (OH) 4 þ þ (17.1)

The non-clay components are then transported to the oceans, where the vast
majority of the alkali earth metals form carbonates by:
Ca 2 þ þ 2 HCO 3 ! CaCO 3 þ CO 2 þ H 2 O

(17.2)

The net effect over geological timescales is that nearly one mole of CO 2 is
consumed for each mole of Ca 2 þ and Mg 2 þ weathered and released into seawater.
The reworking, weathering and erosion of volcanic deposits will therefore result
in a considerable drawdown of atmospheric CO 2 (Gislason et al ., 2009 ). Given that
even moderate-sized eruptions, such as from Mount St Helens in 1980, can emit
tephra with an estimated total speci 10 8 km 2 (equivalent to

the Earth ' s surface area; Ayris and Delmelle, 2012 ), the subsequent weathering
after a large eruption could markedly decrease atmospheric CO 2 concentrations
over millennial timescales.

c surface area of ~ 5

17.4.3 Carbon burial
A third factor affecting long-term climate impacts is the marine carbon export
to sediments, which results from ash deposition ( Figure 17.3M , N ) . Signi

cant
marine ash deposition instigates vertical density currents (Wiesner et al ., 1995 ;
Manville and Wilson, 2004 ), allowing the transport of organic material in affected