A novel authigenic magnetite source for sedimentary magnetization

Zhiyong Lin,Benjamin Brunner,Yang Lu,Junli Gong,Harald Strauss,Guanhua Li,Andrew P Roberts,Xin Yang,Xiaoming Sun,Jörn Peckmann

doi:10.1130/g48069.1

Abstract

AbstractWe 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.

Highlights

Magnetic signals preserved in sediments provide fundamental information for ancient tectonic, geomagnetic field, and environmental reconstructions
Sedimentary magnetic signals have traditionally been thought to be dominated by detrital magnetic iron-oxide particles, while biogenic magnetite with magnetically ideal stable single-domain (SD) properties has proven more recently to be a significant recorder of strong and stable sedimentary remanences over geological time scales (Chang and Kirschvink, 1989; Kopp and Kirschvink, 2008; Roberts et al, 2012)
There are two main pathways for biomineralization of ultrafine biogenic magnetite in sediments, one of which is used by magnetotactic bacteria

Summary

INTRODUCTION

Magnetic signals preserved in sediments provide fundamental information for ancient tectonic, geomagnetic field, and environmental reconstructions. Sedimentary magnetic signals have traditionally been thought to be dominated by detrital magnetic iron-oxide particles, while biogenic magnetite with magnetically ideal stable single-domain (SD) properties has proven more recently to be a significant recorder of strong and stable sedimentary remanences over geological time scales (Chang and Kirschvink, 1989; Kopp and Kirschvink, 2008; Roberts et al, 2012). Sulfidic conditions cause magnetite dissolution, which limits its geological preservation (Riedinger et al, 2005; Roberts, 2015). Surface magnetizations are depleted rapidly at the sulfate-methane transition zone (SMTZ) where hydrogen sulfide is released by sulfate-driven anaerobic oxidation of methane (Boetius et al, 2000; Jørgensen et al, 2004), which promotes magnetite dissolution (Riedinger et al, 2005; Roberts, 2015). Mineralogical, and geochemical analyses are presented to reveal the nature of these particles

MAGNETIC SIGNALS RECORDED IN METHANIC SEDIMENTS

NATURE OF THE MAGNETITE NANOPARTICLES