A stable isotope study of multiple species of planktonic foraminifera across sapropels of the Tyrrhenian Sea, ODP Site 974

Abstract

The planktonic foraminifera Globigerinoides ruber (white variety), Globigerinoides elongatus, Orbulina universa, Globigerina bulloides, Globorotalia inflata, and the fine fraction <38 μm (considered to represent the calcareous nannoplankton association) were sampled for stable isotope analyses across Pleistocene sapropels from Hole 974B in the Tyrrhenian Sea. The aim of this study was to establish if changes in salinity and water column stratification could be observed across the sapropel layers that were recovered at Site 974 in the western Mediterranean during ODP Leg 161. The differences in absolute isotopic compositions of the foraminifera reflect the preferred depth habitats and seasonal distribution of the different species. The magnitudes of the isotope shifts across sapropels are highly variable, but in general more pronounced in the shallow dwelling forms. G. ruber shows an average isotopic shift of 1.3‰ δ 18O and 0.44‰ δ 13C, whereas the isotopic shifts for G. bulloides are −0.85‰ for oxygen and −0.5‰ for carbon. The difference in δ 18O composition between G. bulloides and G. ruber in sapropel layers averages 2.0‰, whereas in non-sapropel times it is 1.8‰. The difference in carbon isotope composition between these two species, on the other hand, is constant at −2.2‰. The larger oxygen isotope shifts observed in the shallow dwelling species G. ruber during sapropel times indicate a more pronounced freshwater influence on the surface waters and/or larger temperature gradients between summer and winter. These data suggest that possibly a stronger water column stratification was present during times of sapropel deposition in the western Mediterranean.