Uranium isotopes as a proxy for primary depositional redox conditions in organic-rich marine systems

Michelle L. Abshire, Stephen J. Romaniello, Amy M. Kuzminov, Jessica Cofrancesco, Silke Severmann, Natascha Riedinger

Research output: Contribution to journalArticle

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 languageEnglish (US)
Article number115878
JournalEarth and Planetary Science Letters
Volume529
DOIs
StatePublished - Jan 1 2020

Fingerprint

uranium isotopes
uranium isotope
Uranium
redox conditions
Organic carbon
Isotopes
total organic carbon
Sediments
sediments
shelves
carbon
slopes
shelf break
sediment
continental shelves
continental margin
Water
nepheloid layer
oxidation
oxygen

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

Abshire, Michelle L. ; Romaniello, Stephen J. ; Kuzminov, Amy M. ; Cofrancesco, Jessica ; Severmann, Silke ; Riedinger, Natascha. / Uranium isotopes as a proxy for primary depositional redox conditions in organic-rich marine systems. In: Earth and Planetary Science Letters. 2020 ; Vol. 529.
@article{723917063ab047b7a0bd718f8aefe27d,
title = "Uranium isotopes as a proxy for primary depositional redox conditions in organic-rich marine systems",
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.",
keywords = "paleoceanography, redeposition, redox, upwelling, uranium, uranium isotopes",
author = "Abshire, {Michelle L.} and Romaniello, {Stephen J.} and Kuzminov, {Amy M.} and Jessica Cofrancesco and Silke Severmann and Natascha Riedinger",
year = "2020",
month = "1",
day = "1",
doi = "10.1016/j.epsl.2019.115878",
language = "English (US)",
volume = "529",
journal = "Earth and Planetary Science Letters",
issn = "0012-821X",
publisher = "Elsevier",

}

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 journalArticle

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 -