Uranium-234 anomalies in authigenic uranium as a new oxygenation proxy in the Southern Ocean
Abstract
Authigenic uranium (aU) is a sensitive indicator for suboxic conditions in marine pore waters that has been used to reconstruct past oxygenation conditions or organic matter export. aU suffers, however, from possible post-depositional remobilization or "burn-down" when the depth of the oxygenation front in the sediments undergoes rapid changes. In terms of isotope composition, the 234U/238U activity ratio, or δ234U in per mil notation, of authigenic U will reflect the isotope ratio in seawater (147‰) which has been relatively stable (within 15‰) for at least one ocean residence time of U (about 400 kyr). The δ234U ratio in bulk marine sediments should then reflect the mixture of the seawater ratio and the ratio of detrital U (0‰ or somewhat negative). In careful analysis of bulk δ234U over a peak in aU from Southern Ocean core ODP-1094, I found ratios higher than seawater (up to 250‰), not explainable by isotope mixing of known sources. I propose a new diagenetic effect in which a partial reoxidation of an aU emplacement can cause 234U that has been alpha-recoiled from in-situ 238U decay to diffuse into the aU emplacement. This means that with aU records that may be slightly altered by reoxidation, careful tracking of δ234U will allow proper identification of the depth/size of the original aU emplacement. Therefore, δ234U of aU is a more robust redox tracer than elemental proxies alone. In this presentation, I will recount the evidence for this assertion and lay out future research targets.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMPP13D..04H
- Keywords:
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- 1039 Alteration and weathering processes;
- GEOCHEMISTRYDE: 1050 Marine geochemistry;
- GEOCHEMISTRYDE: 4805 Biogeochemical cycles;
- processes;
- and modeling;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICALDE: 4924 Geochemical tracers;
- PALEOCEANOGRAPHY