Non‐Chained, Non‐Interacting, Stable Single‐Domain Magnetite Octahedra in Deep‐Sea Red Clay: A New Type of Magnetofossil?. (12th July 2021)

• Record Type: Journal Article
• Authors: Usui, Yoichi; Yamazaki, Toshitsugu
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 22:Number 7(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-07-12
• Subjects: biogenic magnetite -- Ocean Drilling Program (ODP) -- Site 777 -- environmental magnetism -- paleoceanography -- eolian dust; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2021GC009770
• Languages: English
• ISSNs: 1525-2027

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. Plain Language Summary: A group of bacteria called "magnetic bacteria" produce chains of magnetite crystals inside their cells. Magnetic bacteria navigate along the geomagnetic field using these magnetite crystals. Magnetic bacteria have been found in a variety of environments in lake and marine sediments. Recently, bacterial magnetite has also been found to be abundant in deep‐sea sediment, where supply of organic matter is very low. However, we do not know what kind of magnetic bacteria produces the magnetite in such an environment. To investigate the diversity of the deep‐sea bacterial magnetite, we analyzed sediment from the northern Mariana basin using magnetic measurements, microscopic observations, and computer simulations. We found that non‐chained bacterial magnetite account for tens of percent of magnetite in the sediment. Although magnetic bacteria that produce non‐chained magnetite are known, this is the first time such magnetite has been found in natural sediment in a large quantity. Our results indicate that in the deep‐sea there is a greater diversity of magnetic bacteria than previously thought. Key Points: We identified five magnetic components in sediment from Ocean Drilling Program Site 777 (the northern Mariana Basin) Magnetic and microscopy data revealed that non‐chained biogenic magnetite carries a component with low coercivity (<10 mT) Non‐chained magnetofossils may have been overlooked in geological records