Schmidt, Mark , Botz , R., Stoffers , P., Anders , T. and Bohrmann , Gerhard (1997) Oxygen isotopes in marine diatoms: a comparative study of analytical techniques and new results on the isotope fractionation during phytoplankton growth. Geochimica et Cosmochimica Acta, 61 . pp. 2275-2280. DOI 10.1016/S0016-7037(97)00081-1.

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Abstract

Oxygen isotope analyses of marine diatoms were performed in two independent ways. Stepwise fluorination of hydrous opal-A results in plateau δ180 values representing the isotopic composition of the silica frame oxygen. A method of controlled isotope exchange before silica dehydration also produces reliable results, although the exchangeability of the silica was variable. Consequently, a calibration of the isotope exchange method using the results from stepwise fluorination experiments is very useful (and sometimes essential) in order to select a water vapor of an appropriate isotopic composition to be used for equilibration.

Sediment diatom samples Ethmodiscus rex and Thalassiothrix longissima from the Antarctic and the North Atlantic Ocean, respectively, show strong 180 enrichments of 46.8 and 44.1‰, which are caused by large isotope fractionation occurring at the low temperature prevailing during silica-water isotope exchange reactions. However, phytoplankton samples from surface waters of the Norwegian-Greenland Sea and the Bellingshausen Sea (Antarctica) have δ180 values between 30.4 and 35.0‰. Thus, the true silica-water isotopic fractionation is approximately 3 to 10‰ lower than the temperature-dependent silica-water equilibrium published in the literature for sedimentary diatoms. Our results indicate that successive isotope exchange reactions of diatomaceous silica with ambient seawater and/or pore water determine the isotope values of diatoms in sediments.

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