MAGNETOFOSSIL SPIKE DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM: FERROMAGNETIC RESONANCE, ROCK MAGNETIC, AND ELECTRON MICROSCOPY EVIDENCE FROM THE ATLANTIC COASTAL PLAIN OF NEW JERSEY

The discovery of a magnetically anomalous, kaolinitic clay layer in sediments deposited on the Atlantic Coastal Plain of New Jersey during the Paleocene-Eocene Thermal Maximum (PETM) led Kent et al. [1] to speculate that the PETM was triggered by a cometary impact. In this hypothesis, the magnetic properties indicating an unusual abundance of fine-grained, single-domain magnetite are produced by a phase that condensed from an impact ejecta plume. A more oxidized iron-rich nanophase has been found at several Cretaceous-Paleogene boundary sites.

Alternatively, these magnetic properties could be produced by an abundance of magnetite formed by magnetotactic bacteria. Whereas an impact condensate would likely produce roughly equidimensional iron-rich particles, either isolated or in clumps, magnetotactic bacteria typically produce chains of particles, often elongate. Ferromagnetic resonance (FMR) spectroscopy, which is uniquely sensitive to chain arrangement and particle elongation, is ideal for distinguishing between these two hypotheses [2].

FMR analysis, coupled with more conventional rock magnetic analyses and transmission electron microscopy, reveals that biology, not a bolide, is responsible for the magnetic properties of the New Jersey PETM kaolinite. Changes in ocean chemistry during the extreme global warming of the PETM led to enhanced production and/or preservation of bacterial magnetite, likely through an expansion of the suboxic zone of the sediments or water column.

[1] D. V. Kent et al. (2003), Earth Planet. Sci. Lett. 211, 13-26. [2] R. E. Kopp et al. (2006), Earth Planet. Sci. Lett. 247, 10-25.