234 U/ 238 U ratios in quaternary planktonic foraminifera

Other

Scientific paper

Rate now

  [ 0.00 ] – not rated yet Voters 0   Comments 0

Details

4

Scientific paper

234 U/ 238 U measurements on foraminifera could potentially provide an indicator of foraminifera diagenesis, a proxy for past seawater 234 U/ 238 U 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 234 U/ 238 U 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 234 U/ 238 U 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 234 U/ 238 U 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 mol/mol, suggesting that they had been effectively cleaned of U hosted in the Mn-coating but, nevertheless, had 234 U/ 238 U which increased with age of sample, rather than decreasing as would be expected if they had behaved as closed systems. The high 234 U/ 238 U seen in old foraminifera appears to reflect both addition of U from, and exchange of U with, sediment porewaters with high 234 U/ 238 U. 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.

No associations

LandOfFree

Say what you really think

Search LandOfFree.com for scientists and scientific papers. Rate them and share your experience with other people.

Rating

234 U/ 238 U ratios in quaternary planktonic foraminifera does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.

If you have personal experience with 234 U/ 238 U ratios in quaternary planktonic foraminifera, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and 234 U/ 238 U ratios in quaternary planktonic foraminifera will most certainly appreciate the feedback.

Rate now

     

Profile ID: LFWR-SCP-O-1180362

  Search
All data on this website is collected from public sources. Our data reflects the most accurate information available at the time of publication.