Stable isotopes in modern benthic foraminifera; a study of vital effect

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Abstract
Fundamental to the application of stable isotopes in paleoenvironmental studies is an understanding of the processes influencing the isotopic composition of biogenic carbonate. This paper examines carbon and oxygen isotopic equilibrium in foraminiferal calcite and aragonite, and the characteristics and causes of disequilibria (i.e., vital effect). The major cause of vital effect appears to be incorporation of metabolic carbon-oxygen compounds into the test. Furthermore, a relationship is observed between isotopic "behavior" and environment. Taxa that tolerate low oxygen conditions, such as buliminaceans and cassidulinaceans, tend to precipitate their tests in oxygen isotopic equilibrium with the water. Discorbaceans, which generally occur in oxygenated environments, tend to exhibit vital effect. The .delta.13C of Uvigerina, Cassidulina spp., and Planulina wuellerstorfi reflects the isotopic composition of the ambient dissolved inorganic carbon (DIC) of bottom waters. Carbon isotopic compositions of certain benthic foraminifera, such as Globobulimina spp., record the chemistry of pore waters rather than bottom waters. Isotopic fractionation between the aragonitic Hoeglundina and ambient DIC shows a temperature dependence of -0.11.permill. per .degree.C. No temperature dependence is observed in the carbon isotopic fractionation of Uvigerina spp.