Calcium, strontium and fluorine patterns in shells of Pacific Oyster and Rock Oyster

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Coote, G.E. 1996 Calcium, strontium and fluorine patterns in shells of Pacific Oyster and Rock Oyster . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 96/31 9 p.

Abstract: The IGNS proton microprobe has been applied in a study of the distribution of Calcium, Strontium and Fluorine in the calcite shells of Pacific Oyster (Crassostrea gigas) and the native Rock oyster (Crassostrea glomerata). The ultimate aim is to derive information which could have application in such fields as archaeology, biology, palaeontology, aquaculture and environmental studies. Calcium and strontium were determined from their emissions of K X-rays under proton bombardment, and fluorine from the 19F(p,alpha gamma)16O nuclear reaction. The three elements were determined simultaneously at points no more than 20 micrometre apart. A total of 14 one and 17 two-dimensional scans were performed on sections of the shells embedded in epoxy resin. We studied shells from nine Pacific oysters (five transferred from the Marlborough Sounds to Wellington Harbour and four from the Bay of islands). The first (southern) group have a number of thin growth bands, from 0.1 to 0.5 mm wide, which contain F in concentrations up to 1600 ppmw and two to five times more Sr than in adjacent areas. The peaks in the F and Sr distributions usually coincide exactly, often with a corresponding reduction in Ca yield of about 5%. Since the heating by the proton beam carbonized the protein in the outermost layer the scans can be seen under the microscope as thin brown lines, which are darker where the beam traversed layers which contained more protein than usual. The F and Sr peaks coincide with visible bands which are slightly more opaque than the translucent nacre. Regions in the shells with a chalk-like appearance have a Ca content about 40% lower than the nacre, implying a much higher ratio of protein to mineral. Both species in the second (northern) group of shells show thin layers with higher F concentration but the maximum concentrations are about 20 times lower than in the Pacific oysters of the southern group ( the rock oyster in not found there). Peaks in Sr level are rarely seen. We deduce that when stress on the animal has reduced the rate of calcification , the growth layers have a higher ratio of protein to mineral than in the normal nacre, are more opaque and have higher F/Ca and Sr/Ca ratios. The environment in the Bay of Islands, with its milder winter temperatures, may explain why the F/Ca and Sr/Ca anomalies are much smaller than in shells from the southern region, though the oysters were young and may habe not begun to spawn. if specimens of suitable age from both sites were made available the effects of environment and spawning on shell markings from male and female oysters could be clarified.