INTRASPECIFIC STABLE-ISOTOPE VARIABILITY IN THE PLANKTIC FORAMINIFERAGLOBIGERINOIDES-SACCULIFER - RESULTS FROM LABORATORY EXPERIMENTS

Laboratory experiments conducted on the multichambered planktic forami nifera Globigerinoides sacculifer are used to determine the effect of symbiont photosynthesis on shell carbon and oxygen isotopic compositio n. Stable isotope analyses of individual chambers secreted under contr olled temperature and light levels show that chamber deltaC-13 values increase with increasing light levels. The effect of ontogeny on chamb er deltaC-13 is minimal in the size range 350 to 850 mum. Shell size: deltaC-13 covariance in the fossil record is most probably due to the effect of symbiont photosynthesison both size and deltaC-13 as photosy nthetic rate varies with ambient light levels. Chamber & deltaO-18 val ues are not affected by ontogeny but decrease with increasing light le vels, presumably due to symbiont activity. Calculations of equilibrium deltaO-18 values for these experiments indicate that the chambers are enriched in O-18 relative to what would be expected by kinetic fracti onation models. We hypothesize that the secretion of a gametogenic cal cite layer on the surface of the shell under ambient conditions at the end of the life cycle may be the cause for this enrichment. The repor ted O-18 enrichment observed in fossil core top G. sacculifer relative to living specimens in the overlying water column might be explained by the addition of this isotopically-enriched layer.