SR ISOTOPIC COMPOSITION OF HYDROXYAPATITE FROM RECENT AND FOSSIL SALMON - THE RECORD OF LIFETIME MIGRATION AND DIAGENESIS

By comparing the Sr isotopic composition of migratory fossil salmon, which lived in the ocean but died in continental regions, to the well established marine Sr isotopic record, the age of the continental deposit could be determined with high accuracy. This approach to marine-continental correlation and dating requires (1) that marine-resident salmon bear a marine Sr-87/Sr-86 value in their bones or teeth, and (2) that the original Sr-87/Sr-86 value of fossils is not overprinted by diagenesis. The vertebrae of modern, hatchery-reared salmon exhibit Sr isotopic variations indicative of freshwater to marine migration during bone growth. Modern marine Sr-87/Sr-86 values were preserved in growth layers formed later in life. Marine-phase growth layers in the bones and teeth of the late Miocene migratory salmon, Oncorhynchus rastrosus, were subjected to stepwise selective leaching to separate biogenic hydroxyapatite from diagenetic calcium carbonate and recrystallized hydroxyapatite. Although the procedure yielded leachates with Sr/Ca and Ca/P values characteristic of apatite, the leachates had Sr-87/Sr-86 values consistently less radiogenic than values for late Miocene seawater (approximately 0.7087). The fossils were substantially contaminated by Sr from the hosting clastic sediments. Specimens in continental deposits differed in Sr-87/Sr-86 value from host sediments by 0.0002 to 0.0200, supporting the conclusion that these salmon were migrants from marine waters. However, because the original Sr isotopic composition of fossil bones and teeth cannot be determined with confidence, archaeological, paleobiological and stratigraphic applications of this technique may be limited.