Environmental Engineering Reference
In-Depth Information
are currently supersaturated (
W
> 1) for the two major forms used by marine
organisms, aragonite (corals, many mollusks) and calcite (coccolithophores,
foramaniferia, and some mollusks,). Because of pressure effects and higher
metabolic CO
2
from organic matter respiration,
W
decreases with depth
often becoming undersaturated (
W
< 1), at which point unprotected shells
and skeletons begin to dissolve.
The controls on seawater pH and saturation state vary with time-scale.
In the surface ocean, the pH of seawater varies substantially over annual
to interannual time scales due to the net biological formation of organic
matter (lowers CO
2
and raises pH and
W
) and CaCO
3
shells and skeletons
(the reverse). Upwelling of CO
2
-rich water from below and variations in
temperature, salinity, and alkalinity (a measure of the acid buffering ca-
pacity of seawater) also influence surface water carbonate chemistry. The
saturation state of polar waters is lower in large parts because of colder
temperatures. In coastal waters, pH and
W
exhibit large natural spatial and
temporal variations due to the interplay of river runoff, strong biological pro-
ductivity, and in, some locations, coastal upwelling (Salisbury et al., 2008).
Over decadal to century time scales, ocean carbon chemistry is modulated
by net CO
2
uptake from the atmosphere and trends in ocean circulation
and biological productivity, which tend to redistribute dissolved inorganic
carbon and alkalinity within the ocean water-column. On even longer time
scales of many centuries to millennia, the weathering of calcium carbonate
rocks on land adds alkalinity, a measure of the acid buffering capacity, in
the form of calcium ions (Ca
2+
) and carbonate ions (CO
3
2-
), and alkalinity
is removed by the burial, on continental shelves and margins, of biologi-
cally formed carbonate sediments made of the shells and skeletons of some
plankton, corals, and other calcifying organisms (Figure 4.26). Carbonate
sediment burial rates are sensitive to seawater chemistry, and on millennial
time scales longer, efficient damping feedbacks act to stabilize mean ocean
alkalinity and pH.
At the small number of available open-ocean time-series sites, signifi-
cant secular trends in surface ocean carbonate chemistry are well docu-
mented for the past two decades (Figure 4.27). The time-series records
document clearly an increase in surface water pCO
2
and DIC and a decline
in pH that is consistent with the rate of change in atmospheric CO
2
(Dore et
al., 2009). The WOCE/JGOFS Global CO
2
Survey completed in the 1990s
provided a global estimate of ocean anthropogenic CO
2
distributions and a
baseline for assessing changes in ocean chemistry with time (Sabine et al.,
2004). Decadal resurveys of a subset of the WOCE/JGOFS ocean transects
also exhibit decreasing pH through time over the upper thermocline and
