Geoscience Reference
In-Depth Information
pH (i.e. increased hydrogen ion activity) and the
atmosphere with higher
p
CO
2
. The carbon has been
distributed (or partitioned) between atmosphere
and ocean. The subsequent steps of fossil fuel neu-
tralization include dissolution of carbonate sedi-
ment in the deep sea and terrestrial weathering of
carbonate and silicate minerals. Near-complete
removal of fossil fuel carbon from the atmosphere
will take tens to hundreds of thousands of years
(e.g. Archer 2005 ; Uchikawa and Zeebe 2008 ; Zachos
et al.
2008 ).
Under natural steady-state conditions, the oceanic
inventories of
C
T
and
A
T
can be considered essen-
tially constant on a timescale of ~1000 years.
Exceptions to this are rapid carbon inputs from
otherwise long-term storage reservoirs such as com-
bustion of fossil fuel by humans. Further examples
include catastrophes from possible impact events
over carbonate platforms, or other abrupt carbon
releases from geological reservoirs [e.g. during the
Palaeocene-Eocene Thermal Maximum (PETM), see
below]. In the case of rapid CO
2
addition to the
ocean-atmosphere system, dissolution of carbonate
sediment may occur on timescales shorter than their
usual response time of >1000 yr (see Section 2.3.3).
2.3.2 Millennial timescale
On timescales of the order of ~1000 years, the deep
ocean reservoir becomes an important component
of the surface carbon cycle (modern whole ocean
inventory ~38 000 Pg C, Fig. 2.1A). In fact, most of
the anthropogenic carbon will eventually be
absorbed by the ocean and neutralized by reaction
with carbonate sediments (see below).
Once released to the atmosphere, it takes about a
year for CO
2
to mix throughout the atmosphere. The
very upper boundary of the ocean is nearly instanta-
neously in equilibrium with the overlying atmos-
phere; however, on average it takes somewhat less
than a year for the ocean mixed-layer to equilibrate
with the overlying atmosphere. Once in the mixed-
layer, this CO
2
is available for transport to the deep
ocean. Movement of cold seawater away from the
surface in regions of deep-water formation such as
the North Atlantic, as well as subduction associated
with frontal systems (and the formation of, for exam-
ple, Antarctic Intermediate Water) carries the excess
C
T
to the deep ocean. Replacement of these surface
waters from lower latitudes (and warmer, lower
C
T
environments) or nutrient-depleted (and hence
C
T
depleted) upwelled waters, allows the cycle of
C
T
uptake and transport to be repeated. By this
means, an anomaly in pH (and other carbonate
chemistry parameters) is gradually propagated into
the ocean interior. Once emissions have ceased and
the ocean has had time to fully mix on a ~1000 yr
timescale, a new equilibrium is established between
ocean and atmosphere, with the partitioning of CO
2
in a roughly 1:3 ratio between atmosphere and ocean
(Archer 2005). The greater the total release, the larger
the exhaustion of oceanic buffering, and hence the
greater i nal airborne fraction. It is thought that cli-
mate change will both warm the ocean surface and
increase net precipitation and ice melting at high
latitudes, with the result that vertical stratii cation in
the ocean will increase at both low and high lati-
tudes. This is expected to slow the propagation of
the
C
T
and carbonate chemistry anomaly into the
ocean interior. Furthermore, a warmer overall ocean
will result in a higher airborne CO
2
fraction because
of the effect of temperature on the solubility of CO
2
.
After ~1000 years, with no additional processes
operating, the ocean would be left with a reduced
2.3.3
Millennial to 100 000 yr timescale
On timescales of 1000 to 100 000 years, l uxes
between reactive carbonate sediments (~5000 Pg C)
and the oceans' inventories of
C
T
and
A
T
have to be
considered as well. Oceanic inventories may vary,
for instance, during glacial-interglacial cycles (see
so-called calcite compensation below). The magni-
tude of these changes is, however, limited and so
are the associated changes in ocean chemistry and
atmospheric CO
2
. The fate of anthropogenic CO
2
emissions on this timescale involves reaction of fos-
sil fuel carbon with deep-sea carbonate sediments
and terrestrial carbonates, which will ultimately
facilitate removal of carbon from the ocean-atmos-
phere system (so-called fossil fuel neutralization).
Another mechanism that affects ocean carbonate
chemistry on the millennial to 100 000 yr timescale
is associated with changes in ocean carbon pumps
(see below). This can lead to shifts in the vertical
distribution of ocean
C
T
and
A
T
, while not affecting
their inventories. This process is believed to be
