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...

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Published in:Geochimica et Cosmochimica Acta
Main Authors: Henderson, G, Onions, R
Format: Article in Journal/Newspaper
Language:unknown
Published: 2016
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
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spelling ftuloxford:oai:ora.ox.ac.uk:uuid:787db210-df18-4029-9995-d5cce196d7fd 2024-09-30T14:41:33+00:00 U-234/U-238 RATIOS IN QUATERNARY PLANKTONIC-FORAMINIFERA Henderson, G Onions, R 2016-07-28 https://doi.org/10.1016/0016-7037(95)00327-4 https://ora.ox.ac.uk/objects/uuid:787db210-df18-4029-9995-d5cce196d7fd unknown doi:10.1016/0016-7037(95)00327-4 https://ora.ox.ac.uk/objects/uuid:787db210-df18-4029-9995-d5cce196d7fd https://doi.org/10.1016/0016-7037(95)00327-4 info:eu-repo/semantics/embargoedAccess Journal article 2016 ftuloxford https://doi.org/10.1016/0016-7037(95)00327-4 2024-09-06T07:47:37Z 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 ... Article in Journal/Newspaper Planktonic foraminifera ORA - Oxford University Research Archive Geochimica et Cosmochimica Acta 59 22 4685 4694
institution Open Polar
collection ORA - Oxford University Research Archive
op_collection_id ftuloxford
language unknown
description 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 ...
format Article in Journal/Newspaper
author Henderson, G
Onions, R
spellingShingle Henderson, G
Onions, R
U-234/U-238 RATIOS IN QUATERNARY PLANKTONIC-FORAMINIFERA
author_facet Henderson, G
Onions, R
author_sort Henderson, G
title U-234/U-238 RATIOS IN QUATERNARY PLANKTONIC-FORAMINIFERA
title_short U-234/U-238 RATIOS IN QUATERNARY PLANKTONIC-FORAMINIFERA
title_full U-234/U-238 RATIOS IN QUATERNARY PLANKTONIC-FORAMINIFERA
title_fullStr U-234/U-238 RATIOS IN QUATERNARY PLANKTONIC-FORAMINIFERA
title_full_unstemmed U-234/U-238 RATIOS IN QUATERNARY PLANKTONIC-FORAMINIFERA
title_sort u-234/u-238 ratios in quaternary planktonic-foraminifera
publishDate 2016
url https://doi.org/10.1016/0016-7037(95)00327-4
https://ora.ox.ac.uk/objects/uuid:787db210-df18-4029-9995-d5cce196d7fd
genre Planktonic foraminifera
genre_facet Planktonic foraminifera
op_relation doi:10.1016/0016-7037(95)00327-4
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container_title Geochimica et Cosmochimica Acta
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