Authigenic magnetite formation in suboxic marine sediments

Abstract

The resolution and reliability of magnetostratigraphy and reconstructed time series of geomagnetic field behaviour depend critically on where remanence is acquired and fixed in the sediment column and whether the magnetization is altered chemically after deposition. If authigenic magnetic minerals are formed at depth or if the magnetic carriers are changed after deposition, then the nature and timing of magnetic events can be affected by depth offsets in remanence acquisition and mixing of detrital and authigenic signals. Using palaeo- and rock-magnetic and sediment geochemical analyses, we have studied how early diagenesis affects the magnetic properties of suboxic hemipelagic sediments. Here, we report evidence of the formation of fine-grained authigenic magnetites near the commonly observed brown–tan-green colour boundary, which marks the transition from Fe-oxidizing to Fe-reducing conditions. We propose that biogenic magnetite, produced by magnetotactic bacteria, forms as part of the microbially mediated sequence of reactions involved in the oxidation of organic matter. The magnetite is created as a metabolic by-product of the microorganisms' use of iron redox transitions as a source of energy. Active magnetite formation appears to be restricted to a zone between the levels of nitrate and iron reduction.