ENHANCED MAGNETIZATION OF THE MARLBORO CLAY AS A PRODUCT OF IMPACT-INDUCED SOIL PYROGENESIS AT THE PALEOCENE-EOCENE BOUNDARY

The kaolinite-rich Marlboro Clay is a shallow-water marine shelf deposit in the Salisbury Embayment on the eastern margin of North America. The sediment unit, whose base corresponds to the onset of the Paleocene-Eocene thermal maximum (PETM) and carbon isotope excursion (CIE), is characterized by an anomalously high concentration of magnetic nanoparticles of enigmatic origin that give rise to a particularly intense bulk magnetization. Recent studies show that the magnetic assemblage is dominated by single-domain magnetite particles that are isolated and not arranged in chains and thus unlikely to be bacterial magnetosomes. However, the significant (5–15 m) thickness of the Marlboro Clay, far in excess of most known ejecta layers, makes a proposed origin of the nanoparticles as condensates of an impact plume problematical. New data from a proximal site at Wilson Lake reveals that the saturation remanent magnetization of the Marlboro is not only uniformly more inetnse but also more highly anisotropic compared to the immediately underlying Vincentown Formation. Paradoxically, high field magnetic susceptibilities that include contributions from paramagnetic phases vary by only within a factor of two between the units.

We suggest that the magnetization of the Marlboro kaolinitic clay originates from pyromagnetic soil enhancement by widespread wildfires on the adjoining drainage area. The soils and pyrogenetic products were soon washed down from the denuded landscape and rapidly deposited as mud-waves across the shelf. The pyromagnetic nanoparticles were evidently embedded in altered iron-rich 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, perhaps even including some of the kaolinite, is a fireball from the impact of a modest-sized object at moderate range, for example, a ~10 km diameter comet coming down something like the distance to Bermuda away. Ironically, the high concentration of magnetic nanoparticles in the Marlboro may be only a very indirect result of an extraterrestrial impact whose occurrence at the onset of the PETM-CIE is now gaining powerful support from completely independent evidence.