HR: 16:15h
AN: B14B-02 INVITED     [Abstracts]
TI: Biological Effects in Coral Biomineralization: The Ion-Microprobe Revolution
AU: * Meibom, A
EM: meibom@pangea.stanford.edu
AF: Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AB: Scleractinian corals are among the most prolific biomineralizing organisms on Earth and massive, reef-building corals are used extensively as proxies for past variations in the global climate. It is therefore of wide interest to understand the degree to which biological versus inorganic processes control the chemistry of the coral skeleton. Early workers considered aragonitic coral skeleton formation to be a purely physiochemical process. More recent studies have increasingly emphasized the role of a skeletal organic matrix, or intercalated organic macro-molecules that control the macroscopic shape and size of the growing crystals. It is now well established that organic compounds play a key role in controlling the morphology of crystals in a wide variety of calcium carbonate biomineralization processes by binding to specific sites, thereby causing direction-specific binding energies on the crystal surfaces. Macro-molecules, such as aspartic acid-rich or glutamic proteins and sulfated polysaccharides, are known to be embedded within the aragonitic skeletal components of coral. In addition, endosymbiotic algae and the layer of cells adjacent to the mineralizing surface, the calicoblastic ectoderm, are believed to play important roles in driving and controlling hermatypic coral skeletogenesis. However, until recently, further progress has been somewhat limited because it was not possible to obtain chemical analyses of the coral skeleton with sufficiently high spatial resolution and sensitivity to correlate chemical variations with the micrometer scale organization of its different structural components. The recent emergence of new ion microprobe technology is changing this situation radically. Conventional ion microprobe and laser ablation techniques have already contributed substantially to our knowledge about the micro-distribution of key trace elements such as B, Mg, Sr, Ba and U. However, with the development of the NanoSIMS, a newly designed ion microprobe capable of trace-element and isotopic analysis with a spatial resolution down to $50-100$ nanometers, it has become possible to study the intimate relationship between the chemistry and the ultra-structure of the coral skeleton. Individual structural elements, such as centers of calcification and bundles of fibrous aragonite, can be clearly resolved and their chemical and isotopic composition mapped. In this talk preliminary results of a NanoSIMS imaging study of the aragonite skeleton of {\it Pavona clavus} will be shown. {\it Pavona clavus} is a massive reef-building coral frequently used for paleo-climate reconstructions. We find that Mg and Sr are distributed very differently in this coral. In contrast to Sr, the distribution of Mg is strongly correlated with the fine-scale structure of the skeleton and corresponds to the layered organization of aragonite fibers surrounding the centers of calcification, which have up to ten times higher Mg concentration. This could indicate a strong biological control over the Mg composition of all structural components within the skeleton. Magnesium may be used by the coral to actively control the growth of the different skeletal crystal components. Sub-micrometer scale chemical analysis will greatly advance our knowledge of the mechanisms that control the formation of the coral skeleton. However, in an effort to advance our understanding of biomineralization processes in general, the analytical capabilities of the NanoSIMS will be applied to a broad variety of mineralizing organisms. A consortium of researchers from Stanford University, the National Museum of Natural History in Paris, University of Paris XI-Orsay, LSCE in Gif sur Yvette, Centre Scientifique in Monaco, and Cameca are directly involved in these efforts.
DE: 4825 Geochemistry
DE: 4830 Higher marine organisms
DE: 4875 Trace elements
DE: 4267 Paleoceanography
DE: 4294 Instruments and techniques
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting