Magnetofossil dissolution in a palaeomagnetically unstable deep-sea sediment

Abstract

MAGNETOFOSSILS¹, the fossil remains of bacterial magneto-somes², are found in various deep-sea sediments, and have been linked to the preservation of stable natural remanent magnetization in many of them^1,3–5. They have also been extracted and identified from lithified carbonates of Jurassic⁵ and Precambrian⁶ age, showing that magnetotactic bacteria have been a sedimentary source of fine-grained magnetite for much of geological time. Some clay-rich deep-sea sediments, however, do not record a stable remanent magnetization, for unknown reasons. Here we report results from a high-resolution transmission-electron-microscope study on samples of this type collected by the Deep Sea Drilling Project (DSDP); the material contains a complex mixture of single-domain magnetic minerals. Well-preserved magnetofossils show the same crystal structures as those found in magnetosomes from recent bacteria^7–10, whereas others in these same preparations display a wide range of dissolution, corrosion and aggregation effects. Rock magnetic measurements are consistent with the presence of these alterations in many DSDP sediments, including those that preserve reliable palaeomagnetic directions. As we were unable to fina authigenic magnetic minerals in our sample, and as the magnetic fraction is dominated by well preserved magnetofossils, we suggest that the poor preservation of the magnetization is a result of diagenetic interactions between the magnetofossils and the clay minerals in the matrix.