Possible Eukaryotic Magnetite in the Paleocene-Eocene Boundary Clay, Ancora, New Jersey

We report the discovery of new forms of magnetic particles in the Paleocene-Eocene (P-E) boundary clay from the borehole at Ancora, New Jersey (Ocean Drilling Program Leg 174AX). The P-E boundary clay shows anomalous magnetic properties suggesting enrichment in single-domain (SD) magnetite. Transmission electron microscopy of the magnetic separates shows that the majority of the SD particles are magnetofossils of known crystal shape and size [1]. There are, however, larger magnetite particles having unusual morphology, including: (1) elongated prismatic magnetite of 100 nm width and up to 1 micron length, (2) leaflike magnetite particles up to 2 microns in length, and (3) elongated, conelike particles with an aspect ratio of 4:1. Lattice-fringe images and X-ray microanalysis of these particles show single-crystal structure and stochiometric magnetite composition, similar to magnetite crystals produced by magnetotactic bacteria. Although the dimensions of some of the type 2 and type 3 particles are outside that expected for single-domain behaviour as typically calculated for parallelepipeds and ellipsoids [2], electron holographic analysis reveals a SD signature. It is likely that these magnetic particles are of biogenic origin, as we were unable to find significant amounts of obviously detrital magnetite in the sediments. As these unusual magnetic particles are confined to the narrow P-E boundary layer, environmental changes along the eastern Atlantic margin of North America during the Paleocene-Eocene Thermal Maximum (PETM) may have led to enhanced growth, and perhaps diversification, of magnetite-forming microorganisms. The dimensions of the observed magnetite particles, however, exceed the size of prokaryotes; if they were, indeed, biogenic, they were likely formed by an unknown eukaryotic organism. References: [1] R. E. Kopp et al., 2007. Paleoceanography (in press). [2] R. E. Kopp and J. L. Kirschvink, 2007. Earth Science Reviews. doi:10.1016/j.earscirev.2007.08.001.