HR: 10:50h
AN: B21G-03    [PDF]
TI: Vital Effects on Stable Isotopes and Trace Elements in Foraminifera and Corals in View of Their Biomineralization Mechanisms
AU: * Erez, J
EM: erez@vms.huji.ac.il
AF: Institute of Earth Sciences, The Hebrew University of Jerusalem, Givat Ram, Jerusalem, 91904 Israel
AU: Bentov, S
EM: bentov@vms.huji.ac.il
AF: Institute of Earth Sciences, The Hebrew University of Jerusalem, Givat Ram, Jerusalem, 91904 Israel
AU: Schneider, K
EM: kenneth@vms.huji.ac.il
AF: Institute of Earth Sciences, The Hebrew University of Jerusalem, Givat Ram, Jerusalem, 91904 Israel
AB: Based on microscopic observations, we propose that modified seawater is the solution from which biomineralization in formainifera and corals commences. Seawater vacuolization is clearly observed in foraminifera while seawater pumping between the skeleton and the chalicoblastic epithelium is proposed for corals. In both cases CaCO$_{3}$ precipitation proceeds within a semi-restricted biomineralization reservoir that the organism modifies mainly in term of its pH and in the case of foraminifera possibly its Mg also. The pH may be elevated by an active proton pump and/or by Ca-ATPase, which was demonstrated in corals. Internal cycling of metabolic CO$_{2}$ is an important source for the final carbonate ion in the site of calcification in both corals and foraminifera. A competition for CO$_{2}$ between the symbiotic algae and the basic biomineralization reservoir has been demonstrated in foraminifera and may also occur in corals. Bicarbonate uptake must be a common feature in both groups especially in endosymbiotic associations. Consequently the isotopic composition of the symbiotic algae in both groups is enriched in $^{13}$C. The source of inorganic carbon for symbiont photosynthesis is a mixture of respiratory CO$_{2}$ and seawater bicarbonate. The $\delta^{13}$C of respiratory CO$_{2}$ is determined by the proportion of external food carbon and symbiont photosynthesis and their respective $\delta^{13}$C values. The fractionation of the symbiotic algae is probably very low and may be in the range of less then 10 $\permil$. The proportion of seawater carbon to respiratory CO$_{2}$ and their respective $\delta^{13}$C in the biomineralization reservoir determine the skeletal $\delta^{13}$C. Complications arise because of the effect of the carbonate ion on $\delta^{18}$O, and kinetic fractionation of carbon that is associated with diffusive supply of CO$_{2}$ into the basic biomineralization reservoir (Baertschi effect). Trace elements are partitioned into the solid CaCO$_{3}$ according to the composition of the biomineralization reservoir. Because this reservoir is essentially slightly modified seawater, the divalent trace elements show a behavior that resembles precipitation from a partially closed reservoir. The main exception is Mg, which in the case of foraminifera may be lowered by the organism (perhaps by complexation with organic compounds associated with the organic matrix or some of its components).
DE: 0400 Biogeosciences
DE: 1050 Marine geochemistry (4835, 4850)
DE: 4267 Paleoceanography
DE: 4870 Stable isotopes
DE: 4875 Trace elements
SC: Biogeosciences [B]
MN: 2003 Fall Meeting