Benthic iron cycling in a high-oxygen environment: Implications for interpreting the Archean sedimentary iron isotope record.

The most notable trend in the sedimentary iron isotope record is a shift at the end of the Archean from highly variable δ56 Fe values with large negative excursions to less variable δ56 Fe values with more limited negative values. The mechanistic explanation behind this trend has been extensively debated, with two main competing hypotheses: (i) a shift in marine redox conditions and the transition to quantitative iron oxidation; and (ii) a decrease in the signature of microbial iron reduction in the sedimentary record because of increased bacterial sulfate reduction (BSR). Here, we provide new insights into this debate and attempt to assess these two hypotheses by analyzing the iron isotope composition of siderite concretions from the Carboniferous Mazon Creek fossil site. These concretions precipitated in an environment with water column oxygenation, extensive sediment pile dissimilatory iron reduction (DIR) but limited bacterial sulfate reduction (BSR). Most of the concretions have slightly positive iron isotope values, with a mean of 0.15‰ and limited iron isotope variability compared to the Archean sedimentary record. This limited variability in an environment with high DIR and low BSR suggests that these conditions alone are insufficient to explain Archean iron isotope compositions. Therefore, these results support the idea that the unusually variable and negative iron isotope values in the Archean are due to dissimilatory iron reduction (DIR) coupled with extensive water column iron cycling.

A new model of the formation of Pennsylvanian iron carbonate concretions hosting exceptional soft-bodied fossils in Mazon Creek, Illinois.

Preservation of Pennsylvanian-aged (307 Ma) soft-bodied fossils from Mazon Creek, Illinois, USA, is attributed to the formation of siderite concretions, which encapsulate the remains of terrestrial, freshwater, and marine flora and fauna. The narrow range of positive δ34 S values from pyrite in individual concretions suggests microenvironmentally limited ambient sulfate, which may have been rapidly exhausted by sulfate-reducing bacteria. Tissue of the decaying carcass was rapidly encased by early diagenetic pyrite and siderite produced within the sulfate reduction and methanogenic zones of the sediment, with continuation of the latter resulting in concretion cementation. Cross-sectional isotopic analyses (δ13 C and δ18 O) and mineralogical characterization of the concretions point to initiation of preservation in high porosity proto-concretions during the early phases of microbially induced decay. The proto-concretion was cemented prior to compaction of the sediments by siderite as a result of methanogenic production of 13 C-rich bicarbonate-which varies both between Essex and Braidwood concretions and between fossiliferous and unfossiliferous concretions. This work provides the first detailed geochemical study of the Mazon Creek siderite concretions and identifies the range of conditions allowing for exceptional soft-tissue fossil formation as seen at Mazon Creek.