Enhanced magnetization of the Marlboro Clay as a product of impact-induced soil pyrogenesis at the Paleocene-Eocene boundary

The Marlboro Clay, a shallow water shelf deposit on the eastern margin of North America whose base corresponds to the onset of the carbon isotope excursion (CIE) at the Paleocene-Eocene boundary, is characterized by an anomalously high concentration of magnetic nanoparticles of enigmatic origin that give rise to a particularly intense bulk magnetization1-3. Recent rock magnetic results (TFT, high-resolution FORC, etc.) show that the magnetic assemblage is dominated by near-equant single-domain magnetic particles that are isolated and not arranged in chains characteristic of bacterial magnetosomes4. However, the 6 m average thickness of the widespread Marlboro, far in excess of known ejecta layers, makes a proposed origin of the nanoparticles as condensates of an impact plume problematical5. New data from a core at Wilson Lake reveals that the saturation remanence of the Marlboro is not only more intense but also more highly anisotropic compared to the immediately underlying Vincentown Formation, even though high field magnetic susceptibilities are not significantly different between the units. We suggest that the magnetization of the Marlboro originates from pyromagnetic soil enhancement by widespread wildfires on the adjoining drainage area (e.g.,6,7). The soils were soon washed down from the denuded landscape and rapidly deposited as mud-waves across the shelf to become the Marlboro. The pyromagnetic nanoparticles evidently nucleated within altered soil clays whose subsequent compaction on the shelf can account for the observed high magnetic anisotropy. A plausible trigger for the widespread wildfires needed to produce the unusual amount of pyrogenetic material in the Marlboro is a fireball from the impact of a modest-sized object at moderate range8, for example, a 10 km diameter carbon-rich comet coming down something like the distance to Bermuda away. The high concentration of magnetic nanoparticles in the Marlboro may thus be only a very indirect result of an extraterrestrial impact whose occurrence at the onset of the CIE is gaining powerful support from independent evidence. REF: 1Lanci+2002 JGR; 2Lippert+Zachos 2007 Paleocean.; 3Kopp+2009 Paleocean.; 4Wang+2013 PNAS; 5Kent+2003 EPSL; 6Le Borgne 1960 Annales Geophys.; 7Clement+2010 Earth Surface Proc.; 8Collins+2005 Meteor. Planet. Sci.