Promise and progress with in situ visualization of magnetofossils in marine sediment chips
Abstract
Magnetofossils are the fossil remains of iron-biomineralizing organisms. They are classified into two general categories: conventional and giant magnetofossils. Conventional magnetofossils comprise the ~20-100 nm-sized equant, prismatic, and bullet-shaped particles made by magnetotactic bacteria (MTB). In contrast, giant magnetofossils are ~1 µm (or larger) needle, bullet, spindle, and spearhead-shaped particles made by as yet unidentified organisms. The stable single-domain magnetic state of most magnetofossils, and thus their correspondingly distinct magnetic coercivity components, makes them distinguishable in bulk sediments using non-destructive component-specific magnetic measurements. Some of the organisms responsible for these components flourish in specific environmental conditions and thus may be proxies for environmental change throughout the geologic record. This proxy development is challenged, however, by the current ambiguity surrounding in situ magnetofossil particle arrangement and the specific magnetic responses those arrangements produce. For example, micromagnetic simulations of collapsed magnetofossil chains (in contrast to intact chains) and single-domain particle aggregates produce coercivity components that, in some cases, cannot be distinguished from each other. These observations underscore the need for visual examination of magnetofossil taphonomy in situ within the sediment to better link magnetic measurements to magnetofossil interpretations. Here we image and reconstruct purported magnetofossils from clay-rich sediments using focused ion beam scanning electron microscopy slice-and-view nano-tomography (FIB-nT). We show the first application of FIB-nT to image putative magnetofossils and, potentially, the first in situ images of conventional magnetofossils. Our results document particle arrangement and preservation in their lithostratigraphic context. We also demonstrate how surface meshes of these imaged particles are used to generate 3D volumetric meshes for better-informed micromagnetic modeling.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFMGP33A..03W