STABLE ISOTOPIC VARIATION IN PTEROPODS AND ATLANTIDS FROM NORTH PACIFIC SEDIMENT TRAPS
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To determine their usefulness in paleoenvironmental studies and to better understand isotopic equilibrium in biogenic aragonite, carbon and oxygen isotopic analyses were performed on six species of pteropod and two species of atlantid collected with free drifting sediment traps from the North Pacific Ocean. Where the depth habitat of a species is constrained enough to permit estimation of the ambient temperature and water 18O, the isotopic compositions of the shells are close to aragonite-water 18O equilibrium with the ambient water. None of the fauna are in 13C equilibrium with the dissolved bicarbonate. Assuming aragonite-water equilibrium fractionation, the 18O values of the shells provide reasonable average depths of shell growth. The distribution is as follows: (1) "shallow-water" species (< 75 m) - Diacria quadridentata, Cuvierina columnella (32N), and Atlanta (100 m trap); (2) "intermediate-water" species (75-125 m) - Diacria trispinosa, Styliola subula, Limacina bulimoides, C. columnella (26N), Peraclis reticulata, and Atlanta (400 m trap); and (3) "deep-water" species (> 125 m) - L. bulimoides (small). The 18O variation within specimens of C. columnella, a large, easily sectioned species, is complex with no clear record of seasonality. However, some minor seasonal effects are observed. The 13C values within specimens consistently show a 1-2 decrease from the caudal septum to the aperture, perhaps reflecting increased incorporation of metabolic CO2 with age. 1986.
