Scholz, Florian , Severmann, Silke, McManus, James, Noffke, Anna, Lomnitz, Ulrike and Hensen, Christian (2014) On the isotope composition of reactive iron in marine sediments: Redox shuttle versus early diagenesis. Chemical Geology, 389 . pp. 48-59. DOI 10.1016/j.chemgeo.2014.09.009.

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Abstract

The isotope composition of reactive iron (Fe) in marine sediments and sedimentary rocks is a promising tool for
identifying Fe sources and sinks across ocean basins. In addition to cross-basinal Fe redistribution, which can
modify Fe isotope signatures, Fe minerals also undergo diagenetic redistribution during burial. The isotope fractionation
associated with this redistribution does not affect the bulk isotope composition, but complicates the
identification of mineral-specific isotope signatures. Here, we present new Fe isotope data for Peru margin sediments
and revisit previously published data for sediments from the California margin to unravel the impact of
early diagenesis on Fe isotope compositions of individual Fe pools.
Sediments from oxic California margin sites are dominated by terrigenous Fe supply with Fe release from sediments
having a negligible influence on the solid phase Fe isotope composition. The highly reactive Fe pool
(sum of Fe bound to (oxyhydr)oxide, carbonate, monosulfide and pyrite) of these sediments has a light isotope
composition relative to the bulk crust, which is consistent with earlier studies showing that continental
weathering shifts the isotope composition of Fe (oxyhydr)oxides to lighter values. Ferruginous sedimentswithin
the Peruvian oxygen minimumzone are depleted in Fe relative to the lithogenic background, which we attribute
to extensive Fe release to the water column. The remaining highly reactive Fe pool has a heavier isotope composition
compared to California margin sediments. This observation is in agreement with the general notion of an
isotopically light benthic Fe efflux. Most of the reactive Fe delivered and retained in the sediment is transferred
into authigenic mineral phases within the topmost 10 to 20 cm of the sediments. We observe a first-order relationship
between the extent of pyritization of Fe monosulfide and the isotope composition of authigenic pyrite.
With increasing pyritization, the isotope composition of authigenic pyrite approaches the isotope composition of
the highly reactive Fe pool. We argue that the isotope composition of authigenic pyrite or other Fe minerals that
may undergo pyritization may only be used to trace water column sources or sinks if the extent of pyritization is
separately evaluated and either close to 100% or 0%. Alternatively, one may calculate the isotope composition of
the highly reactive Fe pool, thereby avoiding isotope effects due to internal diagenetic redistribution. In depositional
settings with high Fe but lowsulfide concentrations, source and sink signatures in the isotope composition
of the highly reactive Fe pool may be compromised by sequestration of Fe within authigenic silicate minerals.
Authigenic silicate minerals appear to be an important burial phase for reactive Fe below the Peruvian oxygen
minimum zone.

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