URANIUM IN FORAMINIFERAL CALCITE AS A RECORDER OF SEAWATER URANIUM CONCENTRATIONS

We present results of an investigation of uranium/calcium ratios in cleaned foraminiferal calcite as a recorder of seawater uranium concentrations. For accurate reconstruction of past seawater uranium content, shell calcite must incorporate uranium in proportion to seawater concentration and must preserve its original uranium composition over time. Laboratory culture experiments with live benthic (Amphistegina lobifera) and live planktonic (Globigerinella calida) foraminifera show that the U/Ca ratio of cleaned calcite tests is proportional to the concentration of uranium in solution. After correcting results for the presence of initial calcite, the apparent distribution coefficient D = (U/Ca)(calcite)/(U/Ca)(solution) 10.6 +/- 0.3 (X 10(-3)) for A. lobifera and D = 7.9 +/- 0.1 (X 10(-3)) for G. calida. U/Ca ratios in planktonic foraminifera from core tops collected above 3900 m in the equatorial Atlantic and above 2100 m in the Pacific Ocean show no significant difference among the species analyzed. D estimated from core top samples range 7.6 +/- 0.4 (X 10(-3)) for O. universa to 8.4 +/- 0.5 (X 10(-3). U/Ca and Mg/Ca in G. tumida and G. sacculifer from core tops taken near and below the regional lysocline decrease with water depth. Smaller decreases in U/Ca and Mg/Ca with depth were observed in C. wuellerstorfi. In the planktonic species, we believe that U/Ca and Mg/Ca are lower in the more dissolution-resistant fraction of calcite, leading to lower U/Ca in more highly dissolved samples.