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interactive carbon cycle model, a weakening of the westerly
winds and reduced equatorward Ekman transport during gla-
cials lowers the concentration of dissolved carbon in surface
waters. A concomitant reduction in the upwelling of bioactive
nutrients further reduces carbon export production. Com-
bined, these processes would produce only small decreases
in atmospheric CO
2
during glacials. Model simulations of
CO
2
cycling in the Southern Ocean under glacial conditions
must also simulate the net meridional transport or
overturning itself would not explain such a large decrease in
atmospheric
Δ
14
C, and thus, there must be another cause.
Two recent discoveries shed additional light on the glacial
CO
2
mystery and prompt us to consider a new hypothesis to
explain events during MI. First, Marchitto et al. [2007] pre-
sented evidence for a two-part excursion in the
14
Cat
intermediate water depths (705 m) within the northeastern
Pacific during last glacial termination. By measuring the
14
C
ages of benthic foraminiferal calcite taken from a sediment
core, these authors documented a ~200
Δ
“
residual
14
C from
~17 to 15 kyr B.P. and a second equivalently large negative
excursion between ~14 and 11 kyr B.P. (Figure 1). Marchitto
et al. [2007] argued that these
circulation,
which is the sum of both the Ekman transport
and the opposing eddy transport [Augustin et al., 2004;
Karsten and Marshall, 2002]. The development of realistic
simulations of these dynamical processes and obtaining val-
idations from proxies remain an important challenge to hy-
potheses that call upon Southern Ocean mechanisms to
explain glacial/interglacial CO
2
cycles [Fischer et al., 2010].
”
‰
drop in
Δ
14
C excursions resulted from
renewed ventilation of a deepwater mass that had been iso-
lated from the atmosphere during the last glacial. They spec-
ulated that retreating sea ice in the Southern Ocean led to
enhanced ventilation of CO
2
from the deep sea and to the
transport of a low-
Δ
14
C RECORD AND THE TIMING
OF ATMOSPHERIC CO
2
CHANGE
14
C signal through the ocean
3. THE
Δ
s interior via
Antarctic Intermediate Water (AAIW)/Subantarctic Mode
Water (SAMW). In another study, Stott et al. [2009] reported
a much larger
Δ
'
14
C change during the last
glacial termination provides an important constraint to any
hypothesis that attempts to explain glacial/interglacial
atmospheric CO
2
variability via an ocean-only mechanism.
The rise in atmospheric CO
2
during the last deglaciation
coincided with a long-term decrease in atmospheric radio-
carbon (
14
C excursion during the MI from a marine
core collected in the eastern tropical Paci
The record of surface ocean
Δ
Δ
c (VM21-30, 607
m) from the Galapagos margin. In the Galapagos core, the
benthic to planktonic
14
C age differences increased by more
than 6000 years during the MI (Figure 2). With no indication
of diagenesis and with several replicated
14
C ages for different
benthic and planktonic species from the same samples, it
appears that this
14
C) [Beck et al., 2001; Chiu et al., 2007; Fair-
banks et al., 2005; Hughen et al., 2004; Muscheler et al.,
2005; Voelker, 2000] (Figure 1). The magnitude and duration
of the atmospheric
Δ
14
C excursion in the eastern equatorial
Pacific is the same event documented by Marchitto et al. from
the Baja margin. However, the planktonic-benthic
14
C age
differences at the Galapagos site are 5000 to 6000 years. If the
source of
14
C-depleted carbon was a formerly isolated water
mass, it would have contained excess metabolic CO
2
that
accumulated over thousands of years and therefore would
have distinctly lower
13
C/
12
C composition as well. The met-
abolic CO
2
would have made that water highly corrosive to
carbonate and highly depleted in dissolved oxygen. In the
VM21-30 core, there is neither evidence of increased carbon-
ate dissolution through the deglacial section [Stott et al.,
2009], nor is there any evidence that the benthic fauna were
affected by a decrease in oxygen availability that should have
resulted from such an episode, and there is no large
Δ
14
C change during the last deglaciation
implies either a change in production of
14
C in the atmo-
sphere or large redistribution of carbon between the surface
ocean and a
14
C-depleted reservoir. Reconstructions of sur-
face ocean
Δ
14
C reveal several shorter-term excursions dur-
ing the past 30 kyr that were not associated with cosmogenic
isotope events, and thus, the excursions cannot be explained
by changes in the production rate of
14
C[Finkel and Nishii-
zumi, 1997; Muscheler et al., 2005].
The largest
Δ
14
C excursion was a 190
decrease between
17.5 and 14.5 kyr B.P. (Figure 1) at the beginning of the last
glacial termination. This excursion accompanied a 40 ppm
rise in atmospheric CO
2
documented in ice core records from
Antarctica [Monnin et al., 2001]. Denton et al. [2006] and
Broecker and Barker [2007] referred to the interval between
17.5 and 14.5 kyr B.P. as a
Δ
‰
13
C
δ
anomaly in the benthic foraminifera.
Additional
14
C results have now been obtained from other
intermediate depth sites, including one site located on the
southern Peru margin, which is bathed by the Subantarctic
Intermediate Water. At this site, De Pol-Holz et al. [2010]
found no anomalously old benthic foraminiferal
14
C ages
during the last deglaciation. In the northeastern Paci
(MI) because
of several widespread ocean and atmospheric changes that
coincided with the enigmatic
Δ
“
Mystery Interval
”
14
C excursion. The MI was a
time of massive iceberg discharge into the North Atlantic
[Bond et al., 1993; Bond and Lotti, 1995; Maslin et al., 1995]
and weakened North Atlantic Deep Water overturning circu-
lation [McManus et al., 2004]. A reduction in North Atlantic
c, the
Santa Barbara Basin should record the same deglacial
Δ
14
C
excursion as documented on the Baja margin by Marchitto
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