Non‐Chained, Non‐Interacting, Stable Single‐Domain Magnetite Octahedra in Deep‐Sea Red Clay: A New Type of Magnetofossil?

Abstract

Abstract Magnetic detection and classification of magnetofossils have been proposed as potential tools for paleoenvironmental studies. Magnetosomes in bacterial species living in different environmental conditions exhibit different grain morphologies and chain configurations, which determine their magnetic properties. Recently, abundant magnetofossils have been reported from unfossiliferous pelagic red clay. However, little is known about their geometry and magnetic properties. Here we report very low coercivity biogenic magnetite in red clay from Ocean Drilling Program (ODP) Site 777 in the northern Mariana Basin. Analyzed sediment showed non‐interacting, stable single‐domain‐like magnetic behaviors. Acquisition of isothermal remanence was decomposed into five components, and a component with mean coercivity below 10 mT accounted for around 25 % of the remanence in some samples. Based on comparisons with semi‐quantitative transmission electron microscopy observations of magnetic extracts, this component appears to be carried by octahedral grains with size and shape very similar to biogenic magnetite in red clay from other sites. Micromagnetic calculations indicated that isolated maghemite octahedra may be responsible for the observed low coercivity component. Based on these results, we conclude that the low coercivity component represents non‐chained biogenic magnetite that has been oxidized to maghemite. The crystal morphology, geological setting, and lithology do not suggest unusual environmental conditions for ODP Site 777, so the relative amount of chained versus non‐chained grains may represent subtle environmental differences. We suggest that non‐chained magnetofossils may be widespread in deep‐sea sediments. Some methods could overlook the presence of non‐chained magnetite, affecting the identification and quantification of magnetofossils.

Keywords

• biogenic magnetite
• Ocean Drilling Program (ODP)
• Site 777
• environmental magnetism
• paleoceanography
• eolian dust