A magnetofossil spike at the Paleocene-Eocene Thermal Maximum

What causes the magnetically anomalous properties previously observed in the Paleocene-Eocene boundary clay of New Jersey? How wide-spread are such properties, spatially and temporally?

Fifty five million years ago, at the beginning of the Eocene epoch, the Earth experienced a large, rapid warming in association with the release of several thousand gigatons of light carbon into the global carbon cycle. The ~5ºC warming event, the Paleocene-Eocene Thermal Maximum (PETM), began in less than ~10,000 years but persisted for ~100 to ~200 thousand years. The PETM is of clear interest to those concerned about the modern rapid release of light carbon by industrial civilization. Yet its cause in certain; hypotheses include methane clathrate destabilization, massive wild fires, dessication of an epicontential seaway, the release of thermogenic methane associated with volcanic dikes penetrating through organic-rich sediments, and a comet impact.

Kent et al. (2003) proposed an extraterrestrial trigger for the PETM based in large part on the magnetically anomalous properties of a clay layer deposited during the event on the Atlantic Coastal Plain of New Jersey. This clay layer contains abundant single-domain magnetite; indeed, iut is the thickest sedimentary interval yet reported that is so consistently dominated by single-domain magnetite. Kent et al. hypothesized that the single-domain magnetite was produced either by magnetotactic bacteria or by a bolide impact, based on an analogy to magnetic nanoparticles produced by the Cretaceous-Tertiary impact. Their electron microscopy revealed only sparse, equidimensional particles, not the chains of particles and elongate particles expected from magnetotactic bacteria, so they concluded an impact origin was more likely.

Working with collaborators at Caltech and McGill University, I have re-examined the New Jersey PETM clay with the techniques I have been developing for searching for magnetofossils. (See the discussion below). Our results provide unambiguous evidence that the clay?s magnetically anomalous properties are produced not by extraterrestrial activity but by the unusual rich growth or preservation of magnetotactic bacteria. In order to determine whether these unusual conditions were local or reflective of global trends, we are now extending our search to other PETM localities, as well as to clay-rich intervals deposited at other times on the Atlantic Coastal Plain.

Related Publications

• R. E. Kopp, T. D. Raub, D. Schumann, H. Vali, A. V. Smirnov, and J. L. Kirschvink (2007). Magnetofossil Spike During the Paleocene-Eocene Thermal Maximum: Ferromagnetic Resonance, Rock Magnetic, and Electron Microscopy Evidence from Ancora, New Jersey, USA. Paleoceanography 22: PA4103. doi:10.1029/2007PA001473 [Journal link]