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ruber and Neogloboquadrina dutertrei, (a mixed layer and
thermocline dweller, respectively). There is a marked de-
crease in Li/Ca of both species during the MI that is con-
sistent with lower carbonate ion concentrations (Figure 4).
A precise transfer function for
Δ
Li/Ca to [CO
3
=
] is compli-
cated by temperature changes that would have occurred
during deglaciation [Hall and Chan, 2004; Lear and Ro-
senthal, 2006]. Nonetheless, the two lowest Li/Ca values
for G. ruber correspond in timing with excursions in
date the accumulation of excess volcanogenic CO
2
during
glaciations. The only reservoir where CO
2
and CO
2
-rich
fluids could be stored is in sediments that accumulate on
flanks of volcanic centers. Typically, these sediments have
porosities between 50% and 80% [Dadey and Klaus, 1992],
which implies a needed storage capacity of ~3000
5000 km
3
.
The thickest deposits of volcaniclastic sediments today are
in waters deeper than 1000 m where hydrate would have
remained stable on glacial/interglacial time scales. On the
other hand, liquid CO
2
and CO
2
-rich
-
14
C
documented by Marchittoet al. [2007] (Figure 4). The mag-
nitude of Li/Ca change is also larger than could be ac-
counted for by temperature change alone [Marriott et al.,
2004], particularly at the VM21-30 site where the glacial to
interglacial SST change was only on the order of 1.5°C
[Koutavas and Sachs, 2008]. In fact, virtually all of the
glacial to interglacial SST warming at this location occurred
after the decrease in Li/Ca (Figure 5). We therefore interpret
the decrease in Li/Ca at this location to a decrease in [CO
3
=
]
during the MI in response to a release of CO
2
to the atmo-
sphere and subsequent equilibration with the ocean.
Δ
fluids are mobile and
can migrate upward from deeper horizons through hydro-
thermal conduits [Fisher and Wheat, 2010]. The removal of a
hydrate cap at shallower depths during deglaciation would
open up additional conduits for upward migration CO
2
-rich
fluids. The volume of sediment required to accommodate
2200 Gt of CO
2
storage during glaciations may therefore
include sediments at deeper water depths. Further modeling
of thermodynamic constraints on subsurface
ow and the
storage capacity are also required to better constrain this
aspect of the hypothesis.
In addition to the large storage volume required by this
hypothesis, the production rate of CO
2
at subducting margins
must also accommodate the amount of CO
2
stored and re-
leased on the 100 kyr glacial/interglacial timescale. This
calculation implies a production of ~2400 Gt of excess CO
2
during a 100 kyr glacial cycle. Taking average spreading
rates and average carbonate content on subducting slabs
provides an approximate estimate of the CO
2
production
potential. Using this approach does not negate this hypothe-
sis. However, the production rate of CO
2
emitted at subduct-
ing margins is not necessarily in steady state on geologic
time scales. Indeed, there are several lines of evidence indi-
cating that CO
2
production at submarine arcs is not a steady
state function of the long-term spreading rates [Leeman and
Davidson, 2005; Nkrintra Singhrattna et al., 2005; Resing et
al., 2009; Smith and Price, 2006]. Resing et al. [2009] point
out that there is a wide range of CO
2
6. STORAGE AND PRODUCTION OF CO
2
DURING GLACIATIONS
The density of liquid CO
2
at intermediate water depths
(~1200
-
500 m) is close to that of seawater. Hence, approx-
imately 2200 km
3
of storage space is required to accommo-
fluxes between active
arcs in the western Pacific, and the large diversity of CO
2
ects a diverse history of magmatic development
and subsequent arc aging. These authors proposed a simple
model for magmatic development and evolution that would
account for a wide range of CO
2
uxes re
s recent
geologic past that involves progressive magmatic develop-
ment, followed by aging and geochemical evolution. Over
time, the distance between the magmatic source and the
surface of the arc volcano increases. During this aging pro-
cess, acid is ultimately consumed, and the circulating fluids
become increasingly enriched in CO
2
. As the magmatism
evolves further, the distance between the magmatic source
and the volcano
'
s surface becomes larger, and all of the mag-
matic SO
2
is consumed, causing pH to rise. The subsequent
uxes over Earth
'
mol mol
1
)ofG. ruber from VM21-30 (open
diamonds) and the UK
Figure 5. Li/Ca (
μ
37-based SST reconstruction for VM21-30
by Koutavas and Sachs [2008] (solid diamonds).
'
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