Abstract
In marine sediments, authigenic uranium (U) enrichments and U isotope compositions are important tools for interpreting changes in redox conditions, however, their use as paleoproxies requires a comprehensive understanding of the dominant processes that contribute to sediments becoming enriched or depleted. This study focuses on the U content and 238U/235U ratio of organic-rich surface sediments from the Namibian continental margin, where high productivity results in an expanded oxygen minimum zone (OMZ). The investigated core sample sites are located on the shelf, shelf break, and slope where bottom water redox conditions vary from anoxic to suboxic to oxic, respectively. While all cores have relatively high total organic carbon (TOC) contents (up to 12 wt.%), each location displays a unique U to TOC relationship. Shelf sediment exhibit a fair correlation between U and TOC, while the shelf break and slope sediments show a pronounced decoupling of U and TOC. On the Namibia continental margin, particle-rich nepheloid layers transport organic-rich deposits from within the OMZ, through oxic water, to be redeposited on the slope. Due to the sensitivity of U to changes in redox conditions, this lateral movement results in the release of the reduced U phases back into the water column through oxidation while transporting the partially remineralized organic carbon to the slope. Oxidation of U during transport does not alter the average primary 238U/235U isotopic signature in redeposited sediment, and the combination of high TOC, low U content and high δ238U values may become a useful tool for the identification of the boundaries of ancient OMZs.
| Original language | English (US) |
|---|---|
| Article number | 115878 |
| Journal | Earth and Planetary Science Letters |
| Volume | 529 |
| DOIs | |
| State | Published - Jan 1 2020 |
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All Science Journal Classification (ASJC) codes
- Geophysics
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science
Keywords
- paleoceanography
- redeposition
- redox
- upwelling
- uranium
- uranium isotopes
Cite this
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Uranium isotopes as a proxy for primary depositional redox conditions in organic-rich marine systems. / Abshire, Michelle L.; Romaniello, Stephen J.; Kuzminov, Amy M.; Cofrancesco, Jessica; Severmann, Silke; Riedinger, Natascha.
In: Earth and Planetary Science Letters, Vol. 529, 115878, 01.01.2020.Research output: Contribution to journal › Article
TY - JOUR
T1 - Uranium isotopes as a proxy for primary depositional redox conditions in organic-rich marine systems
AU - Abshire, Michelle L.
AU - Romaniello, Stephen J.
AU - Kuzminov, Amy M.
AU - Cofrancesco, Jessica
AU - Severmann, Silke
AU - Riedinger, Natascha
PY - 2020/1/1
Y1 - 2020/1/1
N2 - In marine sediments, authigenic uranium (U) enrichments and U isotope compositions are important tools for interpreting changes in redox conditions, however, their use as paleoproxies requires a comprehensive understanding of the dominant processes that contribute to sediments becoming enriched or depleted. This study focuses on the U content and 238U/235U ratio of organic-rich surface sediments from the Namibian continental margin, where high productivity results in an expanded oxygen minimum zone (OMZ). The investigated core sample sites are located on the shelf, shelf break, and slope where bottom water redox conditions vary from anoxic to suboxic to oxic, respectively. While all cores have relatively high total organic carbon (TOC) contents (up to 12 wt.%), each location displays a unique U to TOC relationship. Shelf sediment exhibit a fair correlation between U and TOC, while the shelf break and slope sediments show a pronounced decoupling of U and TOC. On the Namibia continental margin, particle-rich nepheloid layers transport organic-rich deposits from within the OMZ, through oxic water, to be redeposited on the slope. Due to the sensitivity of U to changes in redox conditions, this lateral movement results in the release of the reduced U phases back into the water column through oxidation while transporting the partially remineralized organic carbon to the slope. Oxidation of U during transport does not alter the average primary 238U/235U isotopic signature in redeposited sediment, and the combination of high TOC, low U content and high δ238U values may become a useful tool for the identification of the boundaries of ancient OMZs.
AB - In marine sediments, authigenic uranium (U) enrichments and U isotope compositions are important tools for interpreting changes in redox conditions, however, their use as paleoproxies requires a comprehensive understanding of the dominant processes that contribute to sediments becoming enriched or depleted. This study focuses on the U content and 238U/235U ratio of organic-rich surface sediments from the Namibian continental margin, where high productivity results in an expanded oxygen minimum zone (OMZ). The investigated core sample sites are located on the shelf, shelf break, and slope where bottom water redox conditions vary from anoxic to suboxic to oxic, respectively. While all cores have relatively high total organic carbon (TOC) contents (up to 12 wt.%), each location displays a unique U to TOC relationship. Shelf sediment exhibit a fair correlation between U and TOC, while the shelf break and slope sediments show a pronounced decoupling of U and TOC. On the Namibia continental margin, particle-rich nepheloid layers transport organic-rich deposits from within the OMZ, through oxic water, to be redeposited on the slope. Due to the sensitivity of U to changes in redox conditions, this lateral movement results in the release of the reduced U phases back into the water column through oxidation while transporting the partially remineralized organic carbon to the slope. Oxidation of U during transport does not alter the average primary 238U/235U isotopic signature in redeposited sediment, and the combination of high TOC, low U content and high δ238U values may become a useful tool for the identification of the boundaries of ancient OMZs.
KW - paleoceanography
KW - redeposition
KW - redox
KW - upwelling
KW - uranium
KW - uranium isotopes
UR - http://www.scopus.com/inward/record.url?scp=85073005452&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85073005452&partnerID=8YFLogxK
U2 - 10.1016/j.epsl.2019.115878
DO - 10.1016/j.epsl.2019.115878
M3 - Article
AN - SCOPUS:85073005452
VL - 529
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
SN - 0012-821X
M1 - 115878
ER -