Origin of stable remanent magnetization of siliceous sediments in the central equatorial Pacific

Abstract

Siliceous sediments distributed in the central equatorial Pacific have a stable remanent magnetization regardless of age or depth. This is in contrast with the unstable remanence of unfossiliferous red clay (defined as “pelagic clay” here) which accumulates in the middle latitudes of the Pacific. We have conducted a rock-magnetic study of the siliceous sediments to clarify the magnetic carriers of the stable remanence. For this purpose, we combined magnetic granulometry by the suspension method and observation of magnetic grains with a transmission electron microprobe (TEM). The advantage of the former is that the magnetic extraction procedure is not required for obtaining size distribution of the magnetic grains in sediments. The TEM observations revealed that most of the magnetic extracts are identical in size and shape to bacterial magnetosomes. The suspension method proved that the magnetic assemblages of the siliceous sediments of both Quaternary and early Miocene age have mean diameters of about 0.05 μm, which is within the single-domain range of magnetite, and have narrow size distributions compared with pelagic clay. These characteristics can be explained if the magnetofossils observed by TEM are the major constituent of the magnetic grains in the sediments, and have been preserved for a long period of time. The difference in the stability of the remanent magnetization between the siliceous sediments and the pelagic clay can be explained by the difference in the size distribution of magnetic grains, which would reflect differences in their sources, i.e., biogenic vs. detrital (eolian).