Stable isotope fractionation in benthic foraminifera from the Peruvian continental margin

Abstract

Stable isotope ratios of the carbonate tests of benthic foraminifera are now widely used to reconstruct past changes in ocean circulation and chemistry. Such studies depend on the accuracy with which changing environmental conditions are recorded and preserved in the tests of foraminifera found in deep-sea deposits. Assessment of this aspect of the method is best accomplished through rigorous study of modern core-tops collected from a wide range of depths and bottom water temperatures. Stable isotope analyses of eighteen taxa of benthic foraminifera from surface sediments of the Peru continental margin indicate that most foraminifera are slightly depleted in 18O relative to the benthic foram Uvigerina peregrina. For at least five taxa we observed subparallel δ 18 O-temperature relationships with slopes nearly equal to that predicted by Shackleton (1974) for Uvigerina. For seven taxa, isotopic analysis of six size fractions (in the range 210–>590 mm) reveals that, with the exception of U. peregrina, variation between different size classes for each species is small, <0.35‰) for δ 18 O and <0.5‰) for δ 13 C . A slight inverse correlation between foraminiferal 18O and 13C content implies a complex interplay of biological and environmental factors which control isotopic composition.