Stable iron isotope signals indicate a “pseudo-abiotic" process driving deep iron release in methanic sediments

Abstract. The low δ⁵⁶Fe values of dissolved iron liberated by microbial iron reduction are characteristic for shallow subsurface sediments and benthic Fe fluxes into the water column. Here, we decipher whether stable Fe isotope signatures in pore water and the respective solid-phase sediment samples are also useful to unravel the processes driving Fe liberation in deeper, methanic sediments. We investigated the fine-grained deposits of the Helgoland mud area, North Sea, where Fe reduction in the methanic subsurface sediments was previously suggested to be coupled to methanogenic fermentation of organic matter and anaerobic methane oxidation. In the evaluated subsurface sediments, a combination of iron isotope geochemistry with reactive transport modelling for the deeper, methanic sediments hints, unsurprisingly, towards a combination of processes affecting the stable isotope composition of dissolved iron. However, the dominant process releasing Fe at depth does not seem to lead to notable iron isotope fraction. Under the assumption that iron reducing microbes generally prefer isotopically light iron, the deep Fe reduction in this setting therefore appears to be “pseudo-abiotic”: If fermentation is the main reason for Fe release at depth, the fermenting bacteria transfer electrons directly or indirectly to Fe(III), but our data does not indicate notable related isotopic fractionation. Our findings strongly contribute to the debate on the pathway for deep Fe²⁺ release by showing that the main underlying process is mechanistically different to the microbial Fe reduction dominating in the shallow sediments and encourages future studies to focus on the fermentative degradation of organic matter as a source of iron in methanic sediments.