OXYGEN ISOTOPES IN MARINE DIATOMS - A COMPARATIVE-STUDY OF ANALYTICALTECHNIQUES AND NEW RESULTS ON THE ISOTOPE COMPOSITION OF RECENT MARINE DIATOMS

Oxygen isotope analyses of marine diatoms were performed in two indepe ndent ways. Stepwise fluorination of hydrous opal-A results in plateau delta(18)O values representing the isotopic composition of the silica frame oxygen. A method of controlled isotope exchange before silica d ehydration also produces reliable results, although the exchangeabilit y of the silica was variable. Consequently, a calibration of the isoto pe exchange method using the results from stepwise fluorination experi ments is very useful (and sometimes essential) in order to select a wa ter vapor of an appropriate isotopic composition to be used for equili bration. Sediment diatom samples Ethmodiscus rex and Thalassiothrix lo ngissima from the Antarctic and the North Atlantic Ocean, respectively , show strong O-18 enrichments of 46.8 and 44.1 parts per thousand, wh ich are caused by large isotope fractionation occurring at the low tem perature prevailing during silica-water isotope exchange reactions. Ho wever, phytoplankton samples from surface waters of the Norwegian-Gree nland Sea and the Bellingshausen Sea (Antarctica) have delta(18)O valu es between 30.4 and 35.0 parts per thousand. Thus, the true silica-wat er isotopic fractionation is approximately 3 to 10 parts per thousand lower than the temperature-dependent silica-water equilibrium publishe d in the literature for sedimentary diatoms. Our results indicate that successive isotope exchange reactions of diatomaceous silica with amb ient seawater and/or pore water determine the isotope values of diatom s in sediments. Copyright (C) 1997 Elsevier Science Ltd