Records of trace metals in sediments from the Oregon shelf and slope: Investigating the occurrence of hypoxia over the past several thousand years

Andrea M. Erhardt, Clare E. Reimers, David Kadko, Adina Paytan

Research output: Contribution to journalArticlepeer-review

8 Scopus citations


Hypoxic (<62μmol/kg or 1.43mL/L O2) to anoxic conditions have been repeatedly observed over the last 10years on the Oregon shelf, while similar conditions are absent in historical records from 1950 to 1999. This study seeks to identify whether similar instances of decadal length hypoxia/anoxia have occurred in the Oregon coastal zone prior to recorded history and to shed light on potential causes for these events. We have measured redox-sensitive metals, uranium, vanadium, and molybdenum concentration profiles in 7 cores across the coastal affected region and in 3 cores from deeper water sites. Results indicate regional variability in redox conditions through time. The northern sites show no metal enrichment throughout the cores, while the southern sites show strong metal enrichment at the base of the cores, indicative of previous hypoxic/anoxic conditions. The southern sites indicate progression in time toward less hypoxic/anoxic burial, in conflict with recent hydrographic trends. Analysis of offshore sediments representing sites beneath the California Undercurrent shows an opposite trend to that observed in the coastal sites. Excess Mo concentrations generally increase toward the present in cores collected within the upwelling source waters at ~300m water depth, suggesting a trend toward oxygen depletion. The Mo enrichment corresponds to increases in δ13C, total organic carbon (TOC), and declines in carbon:nitrogen (C:N) ratios which may indicate that a localized rise in marine productivity has contributed to oxygen drawdown. However, these metal and associated geochemical enrichments are not clearly seen in other cores collected in deeper water in the same region, suggesting that widespread changes in productivity or hypoxia may not have occurred. When the Mo enrichment records for two dated mid-depth sites are compared to climatic indicators such as the Pacific Decadal Oscillation no clear relationships are found on decadal time scales. These results are consistent with the hypothesis that modern hypoxic conditions observed on the continental shelf throughout the region are driven by modern climate changes, not observed in this region for the past hundreds to thousands of years.

Original languageEnglish
Pages (from-to)32-43
Number of pages12
JournalChemical Geology
StatePublished - Aug 29 2014

Bibliographical note

Funding Information:
Joseph Street, Chih-Ting Hsieh, Sora Kim, and Dyke Andreasen assisted with the weighing and measurement of carbon isotope values. Rob Franks provided support for the measurement of trace metal ratios. We would like to thank Thomas Algeo and an anonymous reviewer for their constructive suggestions that improved this manuscript—their efforts were instrumental in refining and clarifying our interpretation. The work was funded through an NSF CAREER grant OCE-0449732 to A.P. and funding for sample collection on the R/V Wecoma was through NSF Grant OCE 0726984 to C.R.


  • California current
  • Carbon isotopes
  • Coastal hypoxia
  • Oregon shelf
  • Trace metals

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology


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