Temperature influence on the carbon isotopic composition of Globigerina bulloides and Orbulina universa (planktonic foraminifera)

Laboratory experiments with the planktonic foraminifera Globigerina bulloides (nonsymbiotic) and Orbulina universa (symbiotic) were carried out to examine the effects of temperature, irradiance (symbiont photosynthesis), [CO32-], and ontogeny on shell delta(13)C values. In ambient seawater ([CO32-] = 171 mu mol kg(-1)), the delta(13)C of G. bulloides shells decreases 0.11 parts per thousand degrees C-1, a pattern that likely results from the incorporation of more respired CO2 into shell carbon at higher metabolic rates. The delta(13)C of O. universa shells grown under low light (LL) levels is insensitive to temperature and records the delta(13)C value of seawater Sigma CO2, whereas the delta(13)C Of high light (I-IL) shells increases slightly with temperature (0.05 parts per thousand degrees C-1). HL O. universa grown in elevated [CO32-] seawater are isotopically depleted relative to those grown in ambient seawater, although it is uncertain from these experiments whether the [CO32-] influence on delta(13)C is affected by temperature. When applied to deep-sea core material, these results demonstrate that differences in sea surface temperature and [CO32-] can bias how we interpret downcore shifts in foraminiferal delta(13)C. (C) 2000 Elsevier Science B.V. All rights reserved.