Anoxic development of sapropel S1 in the Nile Fan inferred from redox sensitive proxies, Fe speciation, Fe and Mo isotopes

Alan Matthews, Irit Azrieli-Tal, Ayelet Benkovitz, Miryam Bar-Matthews, Derek Vance, Simon W. Poulton, Nadya Teutsch, Ahuva Almogi-Labin, Corey Archer

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23 Scopus citations


Redox conditions and the mechanisms of redox development are a critical aspect of Eastern Mediterranean sapropels, whose formation in oxygen-depleted waters is closely related to water column stratification at times of global sea level rise and insolation maxima. Sapropels in the Nile Fan formed at relatively shallow water depths under the influence of the monsoon-driven freshwater output from the River Nile. This work evaluates the redox evolution of Holocene sapropel S1 in VALPAMED cruise core MD9509, recovered at 880 mbsl in the NE Nile Fan, using a combination of geochemical element proxies, Fe speciation, Fe and Mo isotopes studies. The productivity and redox proxies (Ba/Al, Mo/Al, U/Al, V/Al, Sb/Al) show well-defined enrichments in the sapropel, but with a marked minimum at ca 8.2 ka indicative of reventilation corresponding to a well known global cooling event. Peak productivity and reducing signals occur close to the initiation of sapropel formation. The proxy signals in sapropel 9509 are stronger and of longer duration than those of a second sapropel S1, recovered at the same depth, but 380 km to the north (MD9501), supporting the notion (suggested in previous studies) of more reduced conditions in the Nile Fan. The MoEF vs. UEF enrichment factor variations in core 9509 infer a transition from open marine suboxic conditions in the enclosing non-sapropel sediments to anoxic non-sulphidic water column conditions in the sapropel. Correspondingly, the highly reactive Fe pool (FeHR) measured in Fe speciation studies is dominated by Fe(oxyhydr) oxide minerals in the background sediments, whereas pyrite (Fepy) becomes the dominant component of the FeHR pool in the sapropel. Maximum Fepy values in the sapropel coincide with peak productivity and reducing conditions, implying a clear link between trace element uptake, diagenetic bacterial sulphate reduction in anoxic porewater and Fe mobilization in the sapropel. Iron isotope compositions (δ56Fe) in the sapropel do not show any departure from primary (marine and detrital) source sediment values, and the absence of an Fe/Al vs. δ56Fe trend strongly argues against an Fe shuttle. Molybdenum isotopes, however, show marked non-conservative fractionation patterns. Background sediment δ98/95Mo values (0.2 to 0.7‰) are compatible with fractionation upon absorptive uptake by Fe (oxyhydr)oxides and pyrite. In contrast, minimum δ98/95Mo values exhibited at peak sapropel (reducing and pyrite producing) conditions are most closely modeled by Mo isotope fractionation during kinetically controlled conversion of aqueous molybdate to thiomolybdate species. The conservative Fe isotope behavior/Mo isotope fractionation minima in the sapropel may be a characteristic of organic-rich sediment diagenesis below an anoxic non-sulphidic water body, without the operation of a benthic Fe shuttle.

Original languageEnglish
Pages (from-to)24-39
Number of pages16
JournalChemical Geology
StatePublished - 25 Dec 2017
Externally publishedYes

Bibliographical note

Funding Information:
This research was supported by THE ISRAEL SCIENCE FOUNDATION GRANT 1140/12 to A.M. and M.B.M. and Swiss Science Foundation grant 200021_143262 to D.V. SWP acknowledges support from a Royal Society Wolfson Research Merit Award. We would like to thank Dr. Olga Yoffe of the GSI for help with the geochemical analyses of trace elements, Dr. Faina Gelman also of the GSI for TOC analyses, and to Ophir Tirosh from the Hebrew University for assisting with the clean laboratory work. Perceptive and very helpful critical comments of the reviewers and editor are very much appreciated by the authors.

Publisher Copyright:
© 2017 Elsevier B.V.


  • Anoxic non-sulphidic waters
  • Fe isotope fractionation
  • Fe speciation
  • Mo isotope fractionation
  • Nile Fan sapropel S1
  • Porewater pyrite diagenesis


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