STRONTIUM AND FLUORINE MARKINGS IN SHELLS OF A MOLLUSK (PAPHIES-SUBTRIANGULATA) WITH POTENTIAL IMPORTANCE IN BIOLOGY AND ARCHAEOLOGY

In earlier work sections of the shells of tuatua (Paphies subtriangulata) were scanned longitudinally with a focused beam of 2.5 MeV protons, and the distributions of calcium and the minor element strontium determined by the PIXE method. In a few narrow bands in each shell the strontium concentration was higher by a factor of 2 or 3. These strontium ''markings'', which correspond to about one month of growth, appear to record the annual spawning period. A growth curve for the species at one site was determined in this way. We have now shown that the experimental procedure can be simplified: one scan across the shell near the umbo (hinge) will display every marking. The exact position and angle are not critical, though the relative heights and spacings will show variations. Markings are clearly seen in this and a related species (Paphies australe), but the distributions are more complex than anticipated. Distributions in different shells show the same general form but differences in detail. Unexpectedly, there is enough fluorine in the shells to reveal similar patterns with peaks which coincide exactly with those from strontium.