U-234/U-238 RATIOS IN QUATERNARY PLANKTONIC-FORAMINIFERA

U-234/U-238 measurements on foraminifera could potentially provide an indicator of foraminifera diagenesis, a proxy for past seawater U-234/U-238 and, significantly, a direct dating tool for foraminifera. Such measurements are complicated, however, by the very low concentration of U in foraminifera and by the much higher U content of the Mn-rich diagenetic layer which forms on them within the sediment column. Here, we demonstrate that U-234/U-238 measurements can be made successfully on U samples of 1 ng or smaller (equivalent to <50 mg of foraminifera sample) and we report uranium isotope and concentration measurements on a variety of foraminifera samples. Core-top foraminifera subjected to reducing and oxidizing washes before analysis generally yielded U-234/U-238 within error of modern seawater. Two million year-old foraminifera could not be successfully cleaned of their Mn-rich coating, however, and yielded U concentrations and U-234/U-238 ratios significantly higher than the equivalent core-top samples. A suite of downcore foraminifera ranging in age from 6-162 ka yielded roughly constant U concentrations, and Mn/Ca ratios of <60 mu mol/mol, suggesting that they had been effectively cleaned of U hosted in the Mn-coating but, nevertheless, had U-234/U-238 which increased with age of sample, rather than decreasing as would be expected if they had behaved as closed systems. The high U-234/U-238 Seen in old foraminifera appears to reflect both addition of U from and exchange of U with, sediment porewaters with high U-234/U-238. This result effectively prevents the use of foraminifera as a tracer of past seawater U concentration and isotopic composition, and removes the possibility of using U-series dating on foraminifera. The behaviour of U differs from that of other trace elements, notably Cd, which appear to be robust to diagenesis in foraminifera, implying that U is not incorporated directly into the calcite structure and instead resides interstitially, or in organic material within the calcite shell. The susceptibility to diagenesis seen for U may prove typical for other trace elements which are not incorporated directly within the calcite structure in foraminifera.