STABLE-ISOTOPE BEHAVIOR IN PALEOCEANOGRAPHICALLY IMPORTANT BENTHIC FORAMINIFERA - RESULTS FROM MICROCOSM CULTURE EXPERIMENTS

The benthic: foraminifera, Cibicidoides pachyderma, Uvigerina peregrin a, Bulimina marginata and Discorbinella sp. were cultured for six mont hs in replicate sedimentary microcosms to examine shell isotopic signa tures within a controlled laboratory setting. Porewater delta(13)C and bottom water delta(13)C and delta(18)O were sampled weekly, All first generation (F-1) foraminiferal offspring were collected by depth from each microcosm and analyzed for shell delta(13)C and delta(18)O value s. Bottom water delta(18)O remained relatively stable over time, while delta(13)C varied, with a trend toward more positive values about thr ee months into the experiments, C. pachyderma shell delta(13)C tracked the delta(13)C(DIC) signal, with smaller (younger) individuals 2.56 p arts per thousand more positive than older (larger) individuals genera ted from earlier reproduction in the first three months of the experim ents. This offset is most likely due to fluctuating bottom water chemi stry, not ontogenetic isotope effects. U. peregrina delta(18)O was con sistently enriched relative to delta(18)O(eq), mostly inconsistent wit h field studies. B, marginata and Discorbinella sp. shell values clust ered together regardless of changes in bottom water chemistry over tim e, reflecting either an averaging of the signal within the shell or an inability to closely record changes in bottom water chemistry. Shell delta(13)C values of foraminifera collected from depth in the sediment s showed more positive values than surface-collected individuals, indi cating a small porewater influence in these microcosms.