EXPERIMENTAL-DETERMINATION OF CADMIUM UPTAKE IN SHELLS OF THE PLANKTONIC-FORAMINIFERA ORBULINA-UNIVERSA AND GLOBIGERINA-BULLOIDES - IMPLICATIONS FOR SURFACE-WATER PALEORECONSTRUCTIONS

Laboratory culturing is a direct means of determining Cd uptake in she lls of planktonic foraminifera. We employed a new stable isotope techn ique using both Cd-110 and Cd-111 to assess uptake in the symbiont bea ring species, Orbulina universa, and the nonsymbiont bearing species, Globigerina bulloides. In certain experiments with G. bulloides the th ree endmember isotope dilution method was combined with a recently des cribed Ca-48 labeling technique. Shells of Orbulina universa cultured under a 12 h high light:12 h dark cycle are found to incorporate very little Cd. Foraminifera can be induced to take up slightly more Cd by growth under 24 h darkness or under a 12 h high light:12 h dark cycle with exposure to the photosynthesis inhibitor, DCMU. These results dem onstrate that O. universa under-represents the Cd concentration of sea water in which the shell is precipitated. Additionally, the results su ggest a previously unknown influence of symbiotic dinoflagellates on f oraminiferal shell chemistry. There are two different mechanisms by wh ich symbionts might play a role in influencing Cd uptake in O. univers a: (1) algal sequestration and removal of Cd from the foraminiferal ca lcification microenvironment or (2) photosynthetic enhancement of calc ification rate, leading to Cd exclusion. If these results apply genera lly to bioactive trace metal uptake by dinoflagellate-bearing plankton ic foraminifera, they suggest that shells of species such as O. univer sa only record qualitative changes in surface water metal concentratio ns. The response for Cd uptake in nonsymbiont bearing G. bulloides (cu ltured under a 12 h high light:12 h dark cycle) appears linear within and slightly above the range of Cd concentrations found in the modern ocean, with an effective partition coefficient equal to 1.9 +/- 0.2 (9 5% C.I.). The Cd partition coefficient determined for G. bulloides fal ls within the range of D-Cd previously found for fossil benthic forami nifera but is twenty times higher than that for O. universa (D-Cd = 0. 095 +/- 0.021 (95% C.I.)) grown under identical culture conditions. Be cause G. bulloides appears to reliably record seawater Cd concentratio ns, it should be suitable for accurate paleoreconstructions of surface water Cd and PO43- concentrations. Copyright (C) 1997 Elsevier Scienc e Ltd.