A biogenic origin for anomalous fine‐grained magnetic material at the Paleocene‐Eocene boundary at Wilson Lake, New Jersey

Abstract

[1] The Paleocene-Eocene Thermal Maximum, which occurred $55.5 Ma, was caused by a massive release of carbon, as indicated by an $3% negative carbon isotope excursion recorded in the marine, atmospheric, and terrestrial reservoirs. One suggested source for the carbon, a cometary impactor, is based on the sudden appearance and high concentration of single-domain (SD) magnetite in Paleocene-Eocene (P-E) boundary cores from the North Atlantic continental margin. We evaluate the potential sources of SD magnetite at the P-E boundary by presenting new magnetic hysteresis, low-temperature magnetic remanence, and transmission electron microscopy data from the North Atlantic coastal ocean. Our results show a similar increase in SD material but demonstrate that the magnetic material has a biogenic origin. These findings indicate that the high concentrations of SD magnetite immediately above the P-E boundary are the result of unusual accumulations and/or preservation of magnetotactic bacteria. Such bacteria typically occupy the oxic-anoxic transition zone near the sediment-water interface or in the water column. The high abundances of SD magnetite in sediments from across the shelf may be an artifact of nonsteady state redox conditions and exceptional preservation of SD magnetite. It may also indicate that the oxic-anoxic redox boundary shifted into the water column. The latter explanation implies transient eutrophy of the coastal ocean in this region, most likely due to seasonally enhanced runoff, and increased stratification and nutrient loading.

Quantified abundance of magnetofossils at the Paleocene–Eocene boundary from synchrotron-based transmission X-ray microscopy

“…Subsequent studies have confirmed the anomalously high concentrations of SD-like material from the CIE clay from the Atlantic Coastal Plain, making these CIE sections the thickest dominated by SD magnetite recognized thus far in the stratigraphic record (9,10). In these follow-up studies, scanning electron microscopic (SEM) and TEM observations on magnetically extracted materials revealed the presence of chains of magnetite crystals that strongly resembled magnetosomes of magnetotactic bacteria (MTB); this finding led to the conclusion that the SD-like magnetic properties of the CIE clays were predominantly of biogenic origin.…”

Magnetofossil spike during the Paleocene‐Eocene thermal maximum: Ferromagnetic resonance, rock magnetic, and electron microscopy evidence from Ancora, New Jersey, United States

“…The highest ratios are exhibited in the shallower cores: M r /M s during the PETM is nearly uniformly $0.40 at Clayton and $0.35 at Ancora (Figure 2), while at more distal Bass River, M r /M s varies within the clay between $0.10 and $0.30 [Kent et al, 2003]. Lippert and Zachos [2007] profile magnetic hysteresis parameters through the Wilson Lake drill core, proximal to Clayton.…”

“…If biological productivity and organic carbon preservation were constant, the $4 -5 fold increase in sedimentation rate should produce a comparable decrease in organic carbon content. At Wilson Lake, however, organic carbon content predominantly remains $0.3% to $0.4% before, during, and after the CIE [Lippert and Zachos, 2007;Zachos et al, 2006]. Moreover, because particles with high mineral surface area generally preserve organic carbon better than coarser particles [Hedges and Keil, 1995], clay sediments deposited during the CIE should more effectively preserve organic carbon than the underlying and overlying silts.…”

“…[5] As a test of the bolide impact and magnetofossil hypotheses, we present in this paper new ferromagnetic resonance (FMR), isothermal and anhysteretic remanent magnetization (IRM and ARM), first-reversal curve (FORC), and transmission electron microscopy (TEM) analyses of PETM shelf sediments from one of the drill cores Kent et al studied, Ancora. Elsewhere in this issue, Lippert and Zachos [2007] present an independent rock magnetic and TEM study of correlative sediments from the more shoreward Wilson Lake drill core. Recent rock magnetic analyses of continental margin sediments from Marlborough, South Island, New Zealand, suggest that the distinctive magnetic properties observed in New Jersey are not a global phenomenon [Villasante-Marcos et al, 2007].…”

Big discovery for biogenic magnetite

“…Two independent studies (7,8) recently demonstrated that the unusual abundance of Ϸ30-to 120-nm-sized magnetite (Fe 3 O 4 ) particles in PETMage sediments from several drill cores from New Jersey (9) was produced by magnetotactic bacteria. Rock magnetic data from similar studies on marine sediments from New Zealand (10) and Egypt* are consistent with the occurrence of magnetofossils at these locales as well.…”