Geochemical Heterogeneities at the Ultra Structural Level

Fig. 4 illustrates the general morphology of the coral skeleton at the scale of one millimeter. However, in all coral skeletons there is an additional level of structure at the micrometer and nanometer-length scales, which is directly related to the mechanism of skeletal formation - we refer to this as the coral "ultrastructure". Fig. 6 illustrates the ultrastructure of a Porites skeleton, but we emphasize that the ultrastructure of all coral skeletons, although species dependent, share the same components and overall architecture. The coral skeleton consists of EMZs, which are small aggregations of nano-phase calcium carbonate, embedded in sulfated polysaccharides and other organic molecules. EMZs were previously called as centers of calcification (COC), but "EMZ" has been proposed as better description in the context of the overall growth process (Cuif et al., 2003; Cuif and Dauphin, 2005). EMZs are arranged by the coral in a pattern that reflects the overall morphology of the skeleton. Indeed, one could say that it is the organization of the EMZ (which is completely controlled by the coral) that defines the overall morphology of the skeleton. The EMZs are overgrown by subsequent layers of

(a) 6 months of growth

Figure 6: Microstructure of coral skeleton of Porites showing different sizes and shapes of the early mineralization zones (EMZ) and fibres. (a) Calice morphology, (b) Radiating fibres, (c) The stepping growth mode of fibres: each biomineralization growth layer is the elemental environment recording unit.

Figure 6: Microstructure of coral skeleton of Porites showing different sizes and shapes of the early mineralization zones (EMZ) and fibres. (a) Calice morphology, (b) Radiating fibres, (c) The stepping growth mode of fibres: each biomineralization growth layer is the elemental environment recording unit.

fibrous aragonite that serve to give the skeleton bulk mass and mechanical strength (Fig. 4). Of direct relevance to the utilization of coral skeletons to paleo-climatic reconstructions is the discovery of dramatic geochemical variations at the ultrastructure level.

Using conventional ion microprobe, Cohen et al. (2001) analyzed the Sr/ Ca ratio in both EMZ and fibres of Porites and found that the Sr/Ca vs. SST relationship differed dramatically for EMZ and aragonite fibres. Cohen et al. (2001) concluded that the Sr/Ca ratio of the aragonite fibres was strongly influenced by biological activity of symbiotic algae during daytime precipitation. In another study, Cohen et al. (2002) compared the Sr/Ca vs. SST relationship between symbiotic and asymbiotic coral colonies of Astrangia poculata and found again the Sr/Ca ratio in symbiotic corals to be strongly perturbed from the thermodynamic equilibrium values.

Cuif and Dauphin (1998) observed chemical heterogeneity (strontium, magnesium, and sulfur) between fibres and EMZ in the septa of 15 different coral species. Using synchrotron-based XANES analyses, Cuif et al. (2003) further established that sulfur in the coral skeleton is almost exclusively associated with sulfated polysaccharides, which are highly concentrated in the EMZ, but also present in the fibrous aragonite part of the skeleton, where they display a layered distribution corresponding to the layered organization of the fibres (Fig. 6).

These findings were subsequently extended to other trace elements by Meibom et al. (2004), who reported a distinct zonation of Mg in the coral Pavona clavus, again corresponding closely to the layered structure of aragonite fibres (Fig. 7). Meibom et al. (2004) furthermore found the EMZ to be greatly enriched in Mg. Strontium, on the other hand did not show similar banded distributions and it seems that different trace elements have different (metabolic) pathways from the seawater to the coral skeleton.

Figure 7: The distribution of Mg in different parts of the Pavona clavus skeleton from Meibom et al. (2004). Dark blue colors correspond to relatively low Mg concentrations; green, yellow and red colors correspond to increasingly high Mg concentrations. EMZ have the highest concentration of Mg. Arrow indicates direction of growth. Scale bars are 10 mm (For colour version, see Colour Plate Section).

Figure 7: The distribution of Mg in different parts of the Pavona clavus skeleton from Meibom et al. (2004). Dark blue colors correspond to relatively low Mg concentrations; green, yellow and red colors correspond to increasingly high Mg concentrations. EMZ have the highest concentration of Mg. Arrow indicates direction of growth. Scale bars are 10 mm (For colour version, see Colour Plate Section).

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