Geoscience Reference
In-Depth Information
is remarkably constant (0.44 for the Black Sea
and 0.42 for both the Mediterranean Sea and the
global-ocean mean), except for the Baltic Sea where
the proportion is larger (0.53). Therefore over the
range of alkalinities found in the open ocean and
most marginal seas, the saturation factor S -1 (Eq.
3.10) does not vary substantially with total alkalin-
ity for surface waters in equilibrium with atmos-
pheric CO 2 .
Present-day carbonate ion concentrations in these
marginal seas may already be affecting the abun-
dance of marine calcifying organisms. In the Baltic
Sea, very low [CO 3 2- ] appears to be the factor that
prohibits growth of the calcareous phytoplankton
Emiliana huxleyi ; conversely, in the Black Sea, where
[CO 3 2- ] is high, large blooms of the same organism
are visible from space (Tyrrell et al. 2008 ). Well
before the end of the century, surface waters of the
Baltic Sea will become corrosive to all forms of cal-
cium carbonate. In the Black Sea and Mediterranean
Sea, there is no danger of surface waters becoming
corrosive to CaCO 3 before 2100, but they will suffer
sharp reductions in [CO 3 2- ] (-37% in the
Mediterranean Sea and -45% in the Black Sea under
the A2 scenario). These rapid chemical changes are
an added pressure on marine calcii ers and ecosys-
tems of marginal seas already inl uenced by other
anthropogenic factors.
Like the rest of the ocean, coastal waters are
affected by acidii cation from increasing concentra-
tions of anthropogenic CO 2 . But they are also affected
by acidii cation from other sources, including: (1)
freshwater input (Salisbury et al. 2008 ), (2) atmos-
pheric deposition of anthropogenic nitrogen and
sulphur (Doney et al. 2007), and (3) delivery of ter-
restrial organic matter and nutrients that enhance
coastal remineralization and redox cycling in adja-
cent coastal sediments. Freshwater typically has
higher [CO 2 ] and lower pH than does seawater.
Surface freshwater dilution also reduces [CO 3 2- ]
since it reduces A T while surface C T is partially com-
pensated by input of CO 2 from the atmosphere (see
Eq. 3.2). For example, low-salinity surface waters
within the Canada Basin have been observed to
have Ω a < 1 (Yamamoto-Kawai et al. 2009). In an
estuary, Puget Sound in Washington State, Feely
et al. (2010) observed larger than expected reduc-
tions in subsurface pH and estimated that up to half
of that is due to increases in anthropogenic CO 2 , the
remainder being due to degradation of organic mat-
ter produced from both natural and anthropogenic
inputs. Unfortunately, it will require centuries for
dissolution of coastal CaCO 3 sediments to have a
signii cant impact on coastal acidii cation (see
Chapter 7 ).
Observations in coastal waters off the west coast
of the USA illustrate naturally lower pH and [CO 3 2- ]
and large variability due to seasonal upwelling of
CO 2 -rich subsurface water (Feely et al. 2008 ). Natural
seasonality due to this upwelling is now exacer-
bated by increasing concentrations of anthropo-
genic CO 2 in subsurface waters. Thus seasonal
upwelling now brings with it undersaturated
waters (Ω a < 1) that reach the surface over the
Oregon Shelf during spring.
Similar upwelling occurs in other coastal areas,
especially along eastern margins (Hauri et al. 2009 ).
Yet modelling acidii cation in coastal zones requires
sufi cient horizontal resolution to adequately
resolve the small-scale coastal features and physical
processes such as bottom topography, eddies, and
convection that help set local circulation i elds and
affect carbonate chemistry. Coastal models must
also account for river l uxes and closer proximity to
the seal oor. New high-resolution regional model
coni gurations have been developed to study the
North Sea (Blackford and Gilbert 2007) and the
California Coastal Current system (Hauri et al.
2009), and these are being extended to include larger
coastal areas and other regions, particularly eastern
boundary upwelling systems.
3.7 Conclusions
As industrialization continues to drive atmospheric
CO 2 concentrations upward, the surface ocean is
responding by taking up more of this gas, which
reacts with water, reducing surface-ocean pH and
carbonate ion concentrations. The basic chemistry is
well understood, and the magnitude of these
changes is not debated by the scientii c community.
Indeed, these chemical changes are already measur-
able. Surface measurements of changes at three sub-
tropical time-series stations agree with what is
expected from the atmospheric CO 2 increase, assum-
ing air-sea CO 2 equilibrium.
 
 
Search WWH ::




Custom Search