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

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doi: 10.1016/j.chemgeo.2017.10.028
Author(s): Matthews, Alan; Azrieli-Tal, Irit; Benkovitz, Ayelet; Bar-Matthews, Miryam; Vance, Derek; Poulton, Simon W.; Teutsch, Nadya; Almogi-Labin, Ahuva; Archer, Corey
Author Affiliation(s): Primary:
Hebrew University of Jerusalem, Institute of Earth Sciences, Jerusalem, Israel
Israel Geological Survey, Israel
Eidgenössische Technische Hochschule Zürich, Switzerland
University of Leeds, United Kingdom
Volume Title: Chemical Geology
Source: Chemical Geology, Vol.475, p.24-39. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541 CODEN: CHGEAD
Note: In English. Includes appendices. 121 refs.; illus., incl. 2 tables, sketch map
Summary: 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 ppm) 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.
Year of Publication: 2017
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 06 Petrology, Sedimentary; Anaerobic environment; Cenozoic; Depth; Diagenesis; East Mediterranean; Eh; Electron microscopy data; Fe-56/Fe-54; Foraminifera; Globigerinacea; Globigerinidae; Globigerinoides; Globigerinoides ruber; Holocene; ICP mass spectra; Invertebrates; Iron; Isotope fractionation; Isotope ratios; Isotopes; Kinetics; Leg 160; Marine sediments; Mass spectra; Mediterranean Sea; Metals; Microfossils; Mo-95; Mo-98; Mo-98/Mo-95; Molybdenum; Nile Fan; ODP Site 967; Ocean Drilling Program; Organic compounds; Pore water; Protists; Pyrite; Quaternary; Rotaliina; SEM data; Sapropel; Sea water; Sediments; Spectra; Stable isotopes; Submarine fans; Sulfides
Coordinates: N340411 N340411 E0324331 E0324331
Record ID: 2018020103
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands