Chemistry Reference
In-Depth Information
Respired CO
2
in the skeleton
Respired CO
2
is often considered a major source of skeletal carbon, based largely on
a double isotope labeling technique pioneered by Goreau (1963). Using this technique,
Goreau, Erez (1978) and Furla et al. (2000) suggested that corals build their skeletons
mainly out of respired CO
2
. Other observations minimize the skeletal contributions of
respired CO
2
however. Spero and Lea (1986) fed
13
C labeled foods to forams and
detected little in the skeletons. Griffin et al. (1989) and Adkins et al. (2003) showed that
CO
2
probably contributed 5
10% of skeletal carbon in deep-sea corals (Fig. 20a).
McConnaughey et al. (1997) compared the amount of CO
2
produced through respiration
with the amount of environmental CO
2
flushed through an animal's body during the
course of gas exchange with the environment, to obtain O
2
. He concluded that aquatic
invertebrates flush 10 times more CO
2
through their bodies than they produce by
respiration, and therefore incorporate only about 10% respired CO
2
into their skeletons
(Fig. 20b). Air breathers, on the other hand, dilute their respired CO
2
much less, because
air contains 30 times less CO
2
relative to O
2
.
13
C deficiencies, and
−
13
C correlations in corals
CO
2
contains about 8‰ less
13
C than HCO
3
−
, and CO
2
reactions with H
2
O and OH
−
discriminate against
13
C by about 7‰ and 27‰ respectively (Marlier and O'Leary 1984;
Siegenthaler and Münnich 1981). DIC produced in the calcification site from CO
2
reactions can therefore be quite depleted in
13
C. These factors apparently cause most
coral skeletons to contain less
13
C than aragonite precipitated in
13
C equilibrium with
seawater DIC (Fig. 16). CO
2
exchange across the basal epithelium erodes this
13
C
deficiency, bringing DIC in the calcification space back toward seawater
18
O -
δ
δ
13
C values.
This CO
2
exchange carries C and O atoms together, causing simultaneous equilibration of
both isotopes. Seawater input to the calcification site also adds DIC that is isotopically
equilibrated with respect to both C and O isotopes. Non-photosynthetic corals therefore
tend to display linear correlations between
18
O and
13
C, extending upward toward
isotopic equilibrium for both isotopes.
Photosynthetic corals often contain about 10‰ more
13
C than non-photosynthetic
corals, when one compares rapidly growing skeletal parts of the reef coral with materials
from the non-photosynthetic coral showing similar degrees of
18
O disequilibrium
δ
Figure 20.
Skeletal incorporation respired CO
2
. (a) Deep-sea corals of Griffin et al. (1989) and Adkins
et al. (2003) illustrate how skeletal CaCO
3
resembles ambient DIC, not coral foods or tissues, in
14
C
content. (b) McConnaughey et al.'s (1997) model for internal dilution of respired CO
2
by ambient CO
2
,
absorbed in the course of gas exchange with the environment.
