Ocean acidification

1
Ocean acidification
Focus on bicarbonate
2
A simplified carbonate buffer system placed in
the context of ocean acidification.
This second dissociation happens only at very
high pH so basically ignore, The vast majority
of DIC is as bicarbonate.
CO2 ? H2CO3 ? H HCO3-
H CO3-

CaCO3
Ca HCO3-
Dissolution of CaCO3 seds removes H. It
titrates the acidity added from the
dissolution of CO2
CO2 adds acidity. Dissolution of carbonate seds
balances that acidity. Higher pCO2 in the
atmosphere is faster than the rate of dissolution
of carbonate seds. Net addition of
acidity..lower pH.
3
Decade scale pH records
ESTOC Europe HOTS Hawaii BATS Bermuda
4
Regional pH depends on temperature (gas
solubility), pCO2 (e.g. upwelling), net
biological production (high production makes pH
go up), proximity to carbonate saturation. So
there will be regional responses to overall ocean
acidification.
5
Two forms of calcium carbonate Calcite and
aragonite Organisms form shells/tests from one
or the other. Regardless of the mineral form,
the water must be oversaturated in order for
shell formation to occur. Undersaturation occurs
when there is insufficient calcium carbonate in
solution to prevent dissolution of solid phase
calcium carbonate. For example if lots of the
calcium carbonate has been converted into Ca and
HCO3- due to the addition of acid. Calcite is
more stable and less soluble. Aragonite is less
stable and more soluble. The surface ocean is
oversaturated in both. The deep ocean is
undersaturated in both. The breakpoint is
called the saturation horizon. Because aragonite
is less oversaturated in the shallow water, its
saturation horizon is shallower. Problems arise
when the saturation horizon shoals into the
photic zone. Therefore organisms that use
aragonite will be more susceptible to
ocean acidification.
6
Projections Largest change in CO3 concentration
at low latitudes, but biggest shift of the
saturation horizon at higher latitudes.
Linking CO2, pH, and Saturation Horizon for
Aragonite.