Late Pliocene variations in the carbon isotope values of North Atlantic benthic Foraminifera: Biotic control of the isotopic record?

Abstract

Analyses of benthic Foraminifera carbon isotope values were made in 172 samples from the Late Pliocene at DSDP Site 548 which is located in the Northeast Atlantic on the continental slope southwest of Ireland (1253 m water depth). The carbon isotope record is quite stable from about 3.4 to 2.5 Ma, and contrasts with the record at Site 552A in the North Atlantic (2311 m water depth) which is much more variable for this time period. The carbon record at Site 548 is also consistently more negative than that of Site 552A. After 2.5 Ma the benthic carbon isotope record of Uvigerina at Site 548 becomes extremely variable with large negative deviations. These isotopic variations might be associated with paleoceanic circulation changes caused by glaciation between 2.5 and 2 Ma. However, paired analyses of Uvigerina and Cibicidoides species for 38 samples within the glacial interval, yield carbon isotopic records which are quite different from one another. The Uvigerina record contains larger negative deviations than does the Cibicidoides record. Also, the isotopic difference between Uvigerina and Cibicidoides increases during glaciations and in a long term trend from 2.5 to 2.0 Ma. The lack of covariance between the Uvigerina and Cibicidoides carbon isotope records indicates that, after 2.5 Ma at Site 548, biotic factors influenced the benthic isotopic record.