A novel authigenic magnetite source for sedimentary magnetization

(MTB) and the other by dissimilatory iron-reducing bacteria (DIRB) (Moskowitz, 1995; Roberts, 2015). Intracellular magnetite produced by MTB has well defined sizes, morphologies, chain arrangements, and stoichiometries (Devouard et al., 1998; Kopp and Kirschvink, 2008). The magnetic nanoparticulate remains of MTB are preserved post-mortem as magnetofossils and are found in diverse sedimentary environments We report a novel authigenic nanoscale magnetite source in marine methane seep sediments. The magnetite occurs in large concentrations in multiple horizons in a 230 m sediment core with gas hydrate?bearing intervals. In contrast to typical biogenic magnetite produced by magnetotactic bacteria and dissimilatory iron-reducing bacteria, most particles have sizes of 200?800 nm and many are aligned in distinctive structures that resemble microbial precipitates. The magnetite is interpreted to be a byproduct of microbial iron reduction within methanic sediments with rapidly changing redox conditions. Iron sulfides that accumulated at a shallow sulfate-methane transition zone were oxidized after methane seepage intensity decreased. The alteration process produced secondary iron (oxyhydr)oxides that then became a reactive iron source for magnetite authigenesis when methane seepage increased again. This interpretation is consistent with 13C depletion in coexisting carbonate nodules. The authigenic magnetite will record younger paleomagnetic signals than surrounding sediments, which is important for paleomagnetic interpretations in seep systems. The microbial and possibly abiotic processes that caused these magnetic minerals to form at moderate burial depths remain to be determined.

Automated summary

The study reports a novel authigenic nanoscale magnetite source in marine methane seep sediments, with characteristics of larger particles and distinctive structures, mainly as a byproduct of microbial iron reduction within methanic sediments with rapidly changing redox conditions. This magnetite may have formed due to alterations in redox conditions within methane sediments.