Geoscience Reference
In-Depth Information
CHAPTER 2
carbonate chemistry
Richard E. Zeebe and Andy Ridgwell
2.1 Introduction
Over the period from 1750 to 2000, the oceans have
absorbed about one-third of the carbon dioxide
(CO
2
) emitted by humans. As the CO
2
dissolves in
seawater, the oceans become more acidic and
between 1750 and 2000, anthropogenic CO
2
emis-
sions have led to a decrease of surface-ocean total
pH (pH
T
) by ~0.1 units from ~8.2 to ~8.1 (see
Chapters 1 and 3). Surface-ocean pH
T
has probably
not been below ~8.1 during the past 2 million years
( Hönisch
et al.
2009 ). If CO
2
emissions continue una-
bated, surface-ocean pH
T
could decline by about 0.7
units by 2300 (Zeebe
et al.
2008 ). With increasing CO
2
and decreasing pH, carbonate ion (CO
3
2-
) concentra-
tions decrease and those of bicarbonate (HCO
3
-
) rise.
With declining CO
3
2-
concentration ([CO
3
2-
]), the sta-
bility of the calcium carbonate (CaCO
3
) mineral
structure, used extensively by marine organisms to
build shells and skeletons, is reduced. Other geo-
chemical consequences include changes in trace
metal speciation (Millero
et al.
2009) and even sound
absorption (Hester
et al.
2008 ; Ilyina
et al.
2010 ).
Do marine organisms and ecosystems really 'care'
about these chemical changes? We know from a
large number of laboratory, shipboard, and meso-
cosm experiments, that many marine organisms
react in some way to changes in their geochemical
environment like those that might occur by the end
of this century (see Chapters 6 and 7). Generally
(but not always), calcifying organisms produce less
CaCO
3
, while some may put on more biomass.
Extrapolating such experiments would lead us to
expect potentially signii cant changes in ecosystem
structure and nutrient cycling. But can one really
extrapolate an instantaneous environmental change
to one occurring on a timescale of a century? What
capability, if any, do organisms have to adapt to
future ocean acidii cation which is occurring on a
slower timescale than can be replicated in the labo-
ratory? Simultaneous changes in ocean temperature
and nutrient supply as well as in organisms' preda-
tion environment may create further stresses or
work to ameliorate the effect of changes in ocean
chemistry. Either way, the impact in the future ocean
may further diverge from projections based on sim-
ple manipulation experiments.
We know that the chemistry of the ocean has var-
ied in the past (see below). We also know something
about the past composition of marine ecosystems and
sometimes we can even infer changes in individuals'
physiological state, such as growth rate. In this
regard, the geological record may provide clues
about what the future will hold for changes in ocean
chemistry and their effects on marine life. When
studying the geological record, however, the critical
task is to identify an appropriate analogue for the
future. Among other things, this requires a basic
understanding of ocean chemistry controls during
long-term steady states versus transient events,
because carbonate chemistry parameters do not
have to vary with the same relationship if either the
rate of change or the initial chemistry is very differ-
ent. Future versus past comparisons conducted
without sufi cient appreciation about how the car-
bon cycle and ocean chemistry is regulated on geo-
logical timescales may ultimately lead to invalid
conclusions. In case of aberrations, knowledge of
the magnitude and timescale of the acidii cation
event is necessary, otherwise geological periods or
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