Chemistry Reference
In-Depth Information
work described earlier (Fig. 6) which demonstrated alternations of thin Mg and S rich
layers with thick layers of low Mg and S may represent alternations of primary and
secondary calcite, in good agreement with this scheme.
Relations between Ca
2+
and CO
3
2
−
during CaCO
3
precipitation
If indeed seawater is the major component from which the minerals are formed there
is a large discrepancy between the concentration of Ca
2+
and CO
3
2−
in seawater. Ca
2+
concentration is between 10 and 11 mM while CO
3
2−
is only 100
M, a factor of
~50 lower then Ca
2+
. Under these conditions the CO
3
2−
, and not Ca
2+
, is the limiting
compound for the precipitation of CaCO
3
. This situation is further accentuated because
during CaCO
3
precipitation the alkalinity drops at a rate double that of the C
T
causing a
drop in the pH, which further lowers the CO
3
2−
. Under these conditions the best strategy
for the organism is to raise the pH. This will immediately increase the CO
3
2−
concentration even if the C
T
continues to drop. Furthermore, if the pH is higher than the
bulk solution, CO
2(aq)
will diffuse into the basic compartment and provide C
T
for the
continued calcification process. This may also be the strategy preferred by corals, as
indeed reported by Al Horani et al. (2003). Using a pH microelectrode they showed that
the pH below the chalicoblastic epithelium is around 9. Under these conditions the CO
3
2−
concentration is half of the C
T
(which, if close to that of seawater, will be around 1 mM).
In the case of the radial foraminifera the pH increase occurs in the seawater vacuoles as
they mature (on the track from the endoplasm to the calcification space). These basic
vacuoles (again pH around 9) may serve as a sink for CO
2(aq)
either from respiratory
(metabolic) origin or from seawater. If the vacuoles are inside the endoplasm they may
accumulate very high C
T
concentrations because the pH in the cytosol is around 7.2
(based on normal marine cells and on preliminary microelectrode measurements on
ameboids). These high C
T
containing vacuoles may compete for CO
2(aq)
with the
symbiotic algae as indeed observed by ter Kuile et al. (1989a). Furthermore, the internal
carbon pool, which we discovered and described above (ter Kuile and Erez 1987, 1988; ter
Kule et al. 1989), is most probably these basic seawater vacuoles. The pulse-chase
experiments with
14
C (Fig. 4) showed that the internal carbon pool serves only for
calcification and that inorganic carbon uptake into the pool occurs during daytime only. As
showed above the concentration of C
T
in the pool of
A. lobifera
is 190 mM (2 orders of
magnitude higher than seawater concentration). However, the volume of the organism must
include also the pseudopodial network that may extend and increase the volume of the
organism considerably. For example if the thickness of the network is 400
−
300
µ
µ
m then the C
T
would be roughly 40 mM. C
T
concentrations of 20
40 mM at pH 8.5 to 9 would provide
enough CO
3
2−
to match the amount of Ca
2+
in the seawater vacuoles. This requirement is
brought forward because our Rayleigh distillation model for distribution coefficients of Ba,
Sr and Cd in foraminifera suggests that most of the Ca
2+
in the calcification reservoir is
precipitated (Elderfield et al. 1996). We suggest that these independent lines of reasoning
support each other. In fact, in the old models for foraminifera (ter Kuile et al. 1989b) Ca
2+
is pumped into the cells from seawater. If, however, modified seawater is the solution from
which CaCO
3
precipitates, there is no need to concentrate Ca
2+
from a strict stoichiometry
point of view. However, in the case of foraminifera the presence of Mg may impose
another role for Ca
2+
concentration (see below).
Global CO
2
considerations
As discussed in the introduction, the contribution of foraminifera to the global
carbon budget is significant. The possible negative feedback of CO
2
increase on the
calcification of these organisms has already been pointed out (Barker et al. 2003).
Changes in the CO
2
concentration between glacials and interglacials were used to
calculate the carbonate ion concentrations and a response in shell weights where
−
