Biological Sources of Discrepancies in Modern Calibrations

Although biological or "vital" effects are broadly recognized as a source of uncertainty or discrepancy in coral paleo-climatic records relatively little attention has been given to the study of how corals form their skeletons and how, in detail, these biologically induced processes can affect the geochemical

Temperature °C

Figure 3: Diagram showing discrepancy between calibrations of SST vs. skeletal 818O for five nubbins coming from the same parent colony of Acropora sp. cultured by the method of Reynaud-Vaganay et al. (1999). Note that these five nubbins were growing in aquarium under constant condition, except temperature.

Temperature °C

Figure 3: Diagram showing discrepancy between calibrations of SST vs. skeletal 818O for five nubbins coming from the same parent colony of Acropora sp. cultured by the method of Reynaud-Vaganay et al. (1999). Note that these five nubbins were growing in aquarium under constant condition, except temperature.

composition of the skeletons. For example, the isotopic effect of seawater temperature on skeletal 818O has been tested in laboratory by using cultures of corals (Reynaud-Vaganay et al, 1999). Whereas all the nubbins were cultured in the same conditions, in the same aquarium, and originated from one parent colony, their isotopic measurements could be scattered over 1% for an identical temperature (Fig. 3). Although oxygen isotopic ratios as well as Sr/ Ca ratios often are in disequilibrium with respect to their ambient seawater, the observed elemental and isotopic fractionations are usually treated in the context of thermodynamic equilibrium, modified by kinetic effects associated with the precipitation of the skeleton (McConnaughey, 1989; Gagan et al., 1998; Ren et al., 2002; Adkins et al., 2003). Among geochemists, the widespread idea is that the skeleton is largely produced by simple physiochemical precipitation of calcium carbonate concentrated from seawater. However, from recent studies of coral skeletons, conducted at length scales of relevance to the biomineralization processes, it has become clear that this idea is too simplistic to be right. In the following we briefly discuss recent progress in the study of biologically driven isotopic and trace element variations at the millimeter- to nanometer-length scales. In general, it seems true that we need to understand the coral biomineralization process at the cellular scale, the fundamental-length scale of life, before we can reliably use these structures to draw conclusions about environmental changes on a global scale.

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