U-234/U-238 RATIOS IN QUATERNARY PLANKTONIC-FORAMINIFERA
234U 238U measurements on foraminifera could potentially provide an indicator of foraminifera diagenesis, a proxy for past seawater 234U 238U and, significantly, a direct dating tool for foraminifera. Such measurements are complicated, however, by the very low concentration of U in foraminifera and...
| Published in: | Geochimica et Cosmochimica Acta |
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| Main Authors: | , |
| Format: | Article in Journal/Newspaper |
| Language: | unknown |
| Published: |
2016
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| Subjects: | |
| Online Access: | https://doi.org/10.1016/0016-7037(95)00327-4 https://ora.ox.ac.uk/objects/uuid:787db210-df18-4029-9995-d5cce196d7fd |
| Summary: | 234U 238U measurements on foraminifera could potentially provide an indicator of foraminifera diagenesis, a proxy for past seawater 234U 238U 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 234U 238U 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 234U 238U 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 234U 238U 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 μmol/mol, suggesting that they had been effectively cleaned of U hosted in the Mn-coating but, nevertheless, had 234U 238U which increased with age of sample, rather than decreasing as would be expected if they had behaved as closed systems. The high 234U 238U seen in old foraminifera appears to reflect both addition of U from, and exchange of U with, sediment porewaters with high 234U 238U. 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 ... |
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