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

The planktonic foraminifera Globigerinoides ruber (white variety), Globiger inoides elongatus, Orbulina universa, Globigerina bulloides, Globorotalia i nflata, and the fine fraction <38 mu m (considered to represent the calcare ous nannoplankton association) were sampled for stable isotope analyses acr oss Pleistocene sapropels from Hole 974B in the Tyrrhenian Sea. The aim of this study was to establish if changes in salinity and water column stratif ication 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 mag nitudes of the isotope shifts across sapropels are highly variable, but in general more pronounced in the shallow dwelling forms. G. ruber shows an av erage isotopic shift of 1.3 parts per thousand delta(18)O and 0.44 parts pe r thousand delta(13)C, whereas the isotopic shifts for G. bulloides are -0. 85 parts per thousand for oxygen and -0.5 parts per thousand for carbon. Th e difference in delta(18)O composition between G. bulloides and G. ruber in sapropel layers averages 2.0 parts per thousand, whereas in non-sapropel t imes it is 1.8 parts per thousand. The difference in carbon isotope composi tion between these two species, on the other hand, is constant at -2.2 part s per thousand. The larger oxygen isotope shifts observed in the shallow dw elling species G. ruber during sapropel times indicate a more pronounced fr eshwater influence on the surface waters and/or larger temperature gradient s between summer and winter. These data suggest that possibly a stronger wa ter column stratification was present during times of sapropel deposition i n the western Mediterranean. (C) 2000 Elsevier Science B.V. All rights rese rved.