Past changes in ocean carbonate chemistry - Ocean Acidification - page 23

seawater conditions and shows that such an ocean
would have had surface waters with a favourable
carbonate mineral saturation state, despite high
p CO 2 . For simplicity, the above i gures are based on
modern calcium concentrations; for variable cal-
cium see Tyrrell and Zeebe (2004). The bottom line
is that comparisons of seawater chemistry between
the Cretaceous, for instance, and the near future
cannot be based on one carbonate system parameter
alone (see Section 2.4.5).

merely leads to shifts in the vertical distribution of
ocean C T and A T , rather than changing their invento-
ries. This process can be important in changing
surface-ocean chemistry and reducing atmospheric
CO 2 on timescales shorter than ~10 000 years (see
Section 2.3.2). On a million year timescale, on the
other hand, the burial of CaCO 3 in marine sedi-
ments represents one major pathway for removing
carbon from the ocean-atmosphere system (see
Section 2.3.7).

2.2.3 Effect of CaCO 3 production and
dissolution on carbonate chemistry

2.2.4 Temperature and salinity

The abiotic environment plays a signii cant role in
setting the carbonate chemistry state, particularly at
the surface. For instance, CO 2 is less soluble at
higher temperatures, leading to out-gassing to the
atmosphere and hence locally reduced C T .
Conversely, CO 2 uptake takes place preferentially in
colder waters and C T is higher. Hence warm regions
tend to have higher [CO 3 2- ] and be more saturated
with respect to carbonate minerals than colder
regions. As surface-ocean temperatures have varied
in the past, both globally as well as regionally
(latitudinally), so has carbonate chemistry. Also
related to changes in climate is the importance of
salinity, as adding (or subtracting) freshwater will
reduce (increase) the concentration of C T and A T in a
1:1 ratio (they are conservative quantities). For
instance, the larger ice volume at the time of the
Last Glacial Maximum, equivalent to the removal
of around 3% of the water from the ocean (and stor-
age primarily in the great ice sheets of the Northern
Hemisphere), would have acted to increase [CO 2 ]
and hence atmospheric p CO 2 , and just at a time
when ice core records of p CO 2 show it was at a
record low. A multitude of other factors must then
come into play to counter the salinity effect and fur-
ther drive p CO 2 down to glacial concentrations (see
Kohfeld and Ridgwell 2009 ).

CaCO 3 precipitation decreases C T and A T in a ratio
of 1:2, and, counterintuitively, increases [CO 2 ]
although the inorganic carbon concentration has
decreased. Dissolution has the reverse effect. For a
qualitative understanding, consider the reaction

Ca

2 +

+ 2HCO

-

®+ CaCO CO + H O (2.2)

3 3 2 2

which indicates that CO 2 is liberated during CaCO 3
precipitation. Quantitatively, however, the conclu-
sion that [CO 2 ] in solution is increasing by one mole
per mole of CaCO 3 precipitated is incorrect because
of buffering. The correct analysis takes into account
the decrease of C T and A T in a ratio 1:2 and the buffer
capacity of seawater. That is, the medium becomes
more acidic because the decrease in total alkalinity
outweighs that of total inorganic carbon and hence
[CO 2 ] increases. For instance, at surface-seawater
conditions ( C T = 2000 μmol kg - 1 , pH T = 8.2, T = 15°C,
S = 35), [CO 2 ] increases by only ~0.03 μmol per μmol
CaCO 3 precipitated (for more details, see Zeebe and
Wolf-Gladrow 2001 ).
As a result, production of CaCO 3 in the surface
ocean and its transport to depth tends to increase
atmospheric CO 2 . This process represents one com-
ponent of the ocean's biological carbon pump and
has been dubbed the 'CaCO 3 counter pump' because
of its reverse effect relative to the organic carbon
pump, which tends to reduce atmospheric CO 2 .
One ironic consequence of this is that if marine cal-
cii ers were to disappear, it would constitute a small
negative feedback on rising atmospheric CO 2 levels
in the short term. It is also important that the ocean
carbonate pump on the timescale discussed above

2.3 Controls on ocean carbonate
chemistry

Under most natural conditions, the ocean invento-
ries of C T and A T determine the whole-ocean carbonate
chemistry. Changes in the C T and A T inventories
over time therefore constitute the major control on