Chemistry Reference
In-Depth Information
Figure 16.
Sub-equilibrium
18
O and
13
C
levels in photosynthetic and non-
photosynthetic corals.
HCO
3
−
must precipitate fairly rapidly to prevent oxygen isotopic equilibration with water,
implying that CO
2
HCO
3
−
reactions have somewhat one-way character with respect to
the slow process of isotopic equilibration. Kinetic rather than equilibrium isotope
fractionations on CO
2
→
HCO
3
−
reactions therefore dominate.
→
HCO
3
−
reactions produce DIC with low
18
O content partly because the H
2
O and
especially the OH
−
with which the CO
2
reacts contain relatively little
18
O (Fig. 17). Kinetic
discrimination against
18
O will further reduce the
18
O content on product HCO
3
−
. Without
complete isotopic equilibration, the skeleton will inherit some of this
18
O-depletion.
“Carbonate” scenarios
Spero et al. (1997) and Bijma et al. (1999) recently suggested that foraminiferal
18
O
levels depend on the abundance of the isotopically light CO
3
2−
ion in solution, as if
foraminifera essentially precipitate ambient dissolved inorganic carbonates (DIC) (Fig.
18a). This casts doubt on both the “kinetic” isotope model and CO
2
based calcification,
although the contradiction is less severe than it might seem. Spero's forams contained
slightly less
18
O than ambient DIC, as if DIC isotopically equilibrated with water at a pH
somewhat above ambient, or CO
2
contributed somewhat to the skeleton, as would be
appropriate for mild alkalinization of the calcification site according to Figure 15b. A
more serious problem for the “carbonate” explanation is that skeletal
CO
2
→
13
C varied even
δ
13
C ambient DIC remained constant. Hence some
process within the forams caused their
18
O (Fig. 18b), while the
more than
δ
δ
13
C variations. This same internal process
δ
18
O.
Qualitatively similar but even more extreme isotopic patterns also showed up in corals
that had probably never experienced much variation in ambient pH. For such reasons,
corals and forams probably acquire their
18
O, judging from the correlations between
13
C and
probably controlled their
δ
δ
δ
18
O ranges from internal processes, not by
precipitating isotopically equilibrated ambient DIC.
Adkins et al. (2003) and Rollion-Bard et al. (2003) suggested that the low
18
O
content of corals might result from
18
O equilibration between DIC and H
2
O at the
alkaline calcification site. Rollion-Bard also calculated pH at the calcification site from
δ
