Isotopic reconstruction of iron oxidation-reduction process based on an Archean Ocean analogue.

The chemical composition and redox conditions of the Precambrian ocean are key factors for reconstructing the temporal evolution of atmospheric oxygen through time. In particular, the isotopic composition of iron are useful proxies for reconstructing paleo-ocean environments. Yet, respective processes and related signatures are poorly constrained, hindering the reconstruction of iron redox mechanisms in the Archean ocean. This study centers on Sihailongwan Lake, a stratified water body with a euxinic lower water column considered as an Archean ocean analogue. Results show that the anaerobic oxidation layer is so different from other similar lakes in which dissolved Fe oxidation is present in redoxcline layer. And the fractionation factor between ferrous Fe and iron hydroxide observed in nature water body of Sihailongwan Lake reaches to 2.6‰, which would benefit the production of the oxidations of BIF in sediment. By the spatial distribution of Fe isotope, the benthic water in autumn and the hypolimnetic anoxic water in spring has been identified as iron sulfide zone, where iron isotopic fractionation factor during iron sulfide formation is 1.16‰, accounting for partial scavenging of dissolved Fe(II) with an associated isotopic fractionation. However, pyrite in the sediment records the iron isotopic signal from the redoxcline but not in the iron sulfide or oxide zones of the water column. Above findings indicate that neither the iron isotope fractionation during partial transfer of ferrous iron to iron sulfide nor the partial oxidation of ferrous iron are recorded as pyrite in sedimentary rock. Importantly, the signal of Fe isotopic fractionation in water was archived in the suspended particulate matter and transferred into the sediment, rather than via ferrous iron directly deposited in the sediment. This study reveals that Fe isotopes from modern natural environments are useful proxies for reconstructing iron oxidation-reduction process during Earth's early history.

Impact of Aeolian Dry Deposition of Reactive Iron Minerals on Sulfur Cycling in Sediments of the Gulf of Aqaba.

The Gulf of Aqaba is an oligotrophic marine system with oxygen-rich water column and organic carbon-poor sediments (≤0.6% at sites that are not influenced by anthropogenic impact). Aeolian dust deposition from the Arabian, Sinai, and Sahara Deserts is an important source of sediment, especially at the deep-water sites of the Gulf, which are less affected by sediment transport from the Arava Desert during seasonal flash floods. Microbial sulfate reduction in sediments is inferred from the presence of pyrite (although at relatively low concentrations), the presence of sulfide oxidation intermediates, and by the sulfur isotopic composition of sulfate and solid-phase sulfides. Saharan dust is characterized by high amounts of iron minerals such as hematite and goethite. We demonstrated, that the resulting high sedimentary content of reactive iron(III) (hydr)oxides, originating from this aeolian dry deposition of desert dust, leads to fast re-oxidation of hydrogen sulfide produced during microbial sulfate reduction and limits preservation of reduced sulfur in the form of pyrite. We conclude that at these sites the sedimentary sulfur cycle may be defined as cryptic.