Isotopic evidence for Fe cycling and repartitioning in ancient oxygen-deficient settings: Examples from black shales of the mid-to-late Devonian Appalachian basin

We have measured iron (Fe) isotope compositions of bulk samples and chemically extracted pyrite in two black shale units: the Oatka Creek Formation (OCF) and the younger Geneseo Formation (GF) deposited in the Appalachian basin during the mid-to-late Devonian. The depositional redox conditions for these units are well established through multiple paleoproxies, including degree of pyritization (DOP) and ratios of total Fe to Al (Fe_T/Al), suggesting that both deposits reflect oxygen-deficient environments, but that euxinia (anoxia with hydrogen sulfide in the bottom waters) was more frequent and persistent during deposition of the OCF. Iron isotopes show systematic variations that are consistent with the inferred water column redox conditions. Samples from the OCF yield low and variable bulk Fe isotope compositions (- 0.44‰ to 0.03‰ in δ⁵⁶Fe relative to average igneous rocks) that are inversely correlated with Fe_T/Al, whereas bulk δ⁵⁶Fe values (δ⁵⁶Fe_T) of the GF fall in a narrower range (- 0.09‰ to 0.12‰). δ⁵⁶Fe values of pyrite (δ⁵⁶Fe_Py) display good correlation with δ⁵⁶Fe_T in the OCF, but no such correlation is observed in the GF. The Fe isotope data and other paleo-redox indicators, when viewed collectively, point to a benthic Fe source on the shelf and shelf-to-basin transfer that operated during deposition of the OCF, similar to what has been observed in the modern Black Sea. For the first time this study confirms the strength of Fe isotopes in delineating this Fe enrichment mechanism in the ancient geological record and emphasizes the utility of the Fe isotope proxy for fingerprinting and quantifying ancient biogeochemical cycling of Fe.