BIOMINERALIZATION MODEL FOR THE INCORPORATION OF TRACE-ELEMENTS INTO FORAMINIFERAL CALCIUM-CARBONATE

Measurements of Sr/Ca of benthic foraminifera show a linear decrease w ith water depth which is superimposed upon significant variability ide ntified by analyses of individual foraminifera. New data for Cd/Ca sup port previous work in defining a contrast between waters shallower and deeper than similar to 2500 m. Measured element partition coefficient s in foraminiferal calcium carbonate relative to sea water (D) have be en described by means of a one-box model in which elements are extract ed by Rayleigh distillation from a biomineralization reservoir that se rves for calcification with a constant fractionation factor (alpha), s uch that D = (1 - f(alpha))/(1 - f), where f is the proportion of Ca r emaining after precipitation. A modification to the model recognises d ifferences in element speciation. The model is consistent with differe nces between D-Sr, D-Ba and D-Cd in benthic but not planktonic foramin ifera. Depth variations in D for Sr and Ba are consistent with the mod el, as are differences in depth variation of D, in calcitic and aragon itic benthic foraminifera. The shallower depth variations may reflect increasing calcification rates with increasing water depth to an optim um of about 2500 m. Observations of unusually lower D-Cd for some deep waters, not accompanied by similar D-Sr or D-Ba, may be because of di ssolution or a calcification response to a lower carbonate saturation state.