Abstract
Banded iron formations (BIFs) are enigmatic chemical sedimentary rocks that chronicle the geochemical and microbial cycling of iron and carbon in the Precambrian. However, the formation pathways of Fe carbonate, namely siderite, remain disputed. Here, we provide photomicrographs, Fe, C and O isotope of siderite, and organic C isotope of the whole rock from the ~2.52 Ga Dagushan BIF in the Anshan area, China, to discuss the origin of siderite. There are small magnetite grains that occur as inclusions within siderite, suggesting a diagenetic origin of the siderite. Moreover, the siderites have a wide range of iron isotope compositions (δ56FeSd) from −0.180‰ to +0.463‰, and a relatively negative C isotope composition (δ13CSd = −6.20‰ to −1.57‰). These results are compatible with the reduction of an Fe(III)-oxyhydroxide precursor to dissolved Fe(II) through microbial dissimilatory iron reduction (DIR) during early diagenesis. Partial reduction of the precursor and possible mixing with seawater Fe(II) could explain the presence of siderite with negative δ56Fe, while sustained reaction of residual Fe(III)-oxyhydroxide could have produced siderite with positive δ56Fe values. Bicarbonate derived from both DIR and seawater may have provided a C source for siderite formation. Our results suggest that microbial respiration played an important role in the formation of siderite in the late Archean Dagushan BIF.
Highlights
Banded iron formations (BIFs) are chemical sedimentary rocks composed of iron-rich and silica-rich bands [1,2,3] that were commonly precipitated during the Archean and Paleoproterozoic
The relatively negative δ13 C and highly variable δ56 Fe values are inconsistent with the siderites forming in equilibrium with Archean seawater and negate the potential for the direct precipitation from the water column in our study area
Our siderite samples cannot be used as a direct proxy of paleomarine conditions, but instead they represent a valuable record of early diagenetic processes
Summary
BIFs reflect the massive extent of iron (Fe) cycling and deposition during the Precambrian, but they record critical information about ancient seawater geochemistry and the different (bio)chemical pathways through which the atmosphere–biosphere–hydrosphere systems have been linked throughout Earth’s history [4,5,6,7]. Sulfide facies is not generally used to study sedimentary environments, as it is composed mainly of pyritic carbonaceous shale or slate and does not represent a chemical precipitate [2,3,8]. The abundance and distribution of these facies have been used to infer environmental conditions during deposition and subsequent sedimentary–diagenetic–metamorphic evolutionary processes [1,2,3,8]
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