A diffusion-reaction model of carbon isotope fractionation in foraminifera

Fossil foraminiferal shells are utilized in paleoceanography to extract inf ormation about environmental conditions of the past ocean. Based on several assumptions, the ratio of C-13 and C-12 preserved in their shells is used to reconstruct, for example, the paleoproductivity or the oceanic pCO(2). M etabolism of the living organism and the sea water chemistry, however, can influence the incorporation of carbon isotopes during calcification such th at the signal of the shells differ from the signal of the sea water. These effects occur because the chemical microenvironment of the foraminifer (bou ndary layer thickness similar to 500 mu m) differs from the bulk sea water. Here, we present a numerical model that calculates the delta(13)C of the f oraminiferal shell as a function of the sea water chemistry and the magnitu de of vital effects. Concentration profiles of the chemical species of the carbonate system within the microenvironment of foraminifera are obtained b y solving diffusion-reaction equations. The compounds of dissolved forms of carbon dioxide containing either the stable carbon isotope C-13 or C-12 ar e considered separately. Spherical symmetry of the foraminifer is assumed. The model outcome is compared to results from culture experiments with the planktonic foraminifer Orbulina universa. Model results indicate that the i nteraction between vital effects of the foraminifer and the sea water chemi stry can account for changes in the delta(13)C of foraminiferal calcite of 0.3-0.4 parts per thousand when glacial and interglacial sea water conditio ns are compared. These effects occur even though the delta(13)C of the tota l dissolved inorganic carbon is kept constant. Thus, changes in sea water c hemistry should be distinguished from events which changed the delta(13)C O f the inorganic carbon of the sea water. (C) 1999 Elsevier Science B.V. All rights reserved.