Enhanced magnetization of the Marlboro Clay as a product of soil pyrogenesis at the Paleocene–Eocene boundary?

Abstract

The kaolinite-rich Marlboro Clay was deposited on the inner shelf in the Salisbury Embayment of the U.S. Atlantic margin at the onset of the carbon isotope excursion marking the 56 Ma Paleocene-Eocene boundary and is characterized by an anomalously high concentration of magnetic nanoparticles of enigmatic origin that give rise to notably intense bulk magnetization. Recent studies point to a magnetic assemblage that is dominated by single-domain magnetite particles that tend to be isolated rather than arranged in chains, the most distinguishing feature of magnetotactic bacteria fossils. On the other hand, it is very unlikely that the nanoparticles can be condensates of an impact plume given the meter-scale thickness of the Marlboro Clay. We obtained new data from a landward proximal site at Wilson Lake on the New Jersey Coastal Plain and find that the abrupt increase in magnetite nanoparticles is virtually coincident stratigraphically with the recently reported impact spherule layer at the base of the Marlboro Clay in the same core. Yet the high field magnetic susceptibility, a measure of total iron concentration, and strontium isotope values on bulk sediment, an indicator of sediment weathering provenance, are not different in the Marlboro Clay from the immediately underlying Vincentown Formation. We suggest that the distinctive magnetic properties of the Marlboro Clay originated from pyromagnetic soil enhancement by widespread wildfires on the adjoining drainage area. The pyrogenetic products were soon washed from the denuded landscape and rapidly deposited as mud-waves across the shelf, becoming the Marlboro Clay. A few percent of incinerated biomass ends up as calcite known as wood ash stone and can inherit its light carbon isotope composition. Disseminated wood ash stone entrained in the Marlboro Clay could contribute to the landward increase in amplitude of the carbon isotope excursion in bulk carbonate data. A plausible trigger for the initial conflagration is a fireball from the impact of a sizable extraterrestrial object at moderate range.

Coupled microbial bloom and oxygenation decline recorded by magnetofossils during the Palaeocene–Eocene Thermal Maximum

“…The observed crystal morphologies, size distributions, and microstructures contrast significantly with those expected from detrital magnetic minerals that often do not have well-defined crystal shapes, small size distributions, and a lack of chain structures. Therefore, our TEM analyses, magnetic measurements, and simulations indicate consistently that magnetic nanoparticles within the PETM samples from Site 1263 are magnetofossils, and that other origins, such as comet-induced debris 27 and pyrogenesis 49 , are improbable. Magnetofossils are also evident in high-resolution SEM observations (Supplementary Fig.…”

Evidence for Shelf Acidification During the Onset of the Paleocene‐Eocene Thermal Maximum

“…In all records, grain size decreases below the base of the LCI and there is not a distinct grain size anomaly in this interval that might be expected for a pulse of terrigenous materials (Figure 7); the silty grain size at CD and HT compared to surrounding intervals is likely a result of the near-complete absence of coccoliths. Moreover, biostratigraphy suggests that the LCI represents a substantial amount of time (Figure 3), in contrast to some interpretations of the onset of the CIE (Kent et al, 2017;Wright & Schaller, 2013).…”

Early Eocene Thermal Maximum 3: Biotic Response at Walvis Ridge (SE Atlantic Ocean)

“…The main source (s) of the carbon emitted into the ocean-atmosphere system is under ongoing debate (e.g., McInerney & Wing, 2011;Meissner et al, 2014), with proposed sources including dissociation of oceanic methane hydrates (Dickens et al, 1995(Dickens et al, , 1997Dickens, 2011;Dunkley Jones et al, 2013;Frieling et al, 2016), magmatic exhalation, possibly combined with interaction of volcanic intrusives with organic-rich sediment (Eldholm & Thomas, 1993;Gutjahr et al, 2017;Svensen et al, 2010Svensen et al, , 2004Storey et al, 2007), burning of peat deposits (Kurtz et al, 2003), an impacting comet (Cramer & Kent, 2005;Kent et al, 2003Kent et al, , 2017Schaller et al, 2016), and periodically thawing Antarctic permafrost (DeConto et al, 2012). How and why the PETM ended is also debated, with most commonly named carbon sinks being weathering of silicates and deposition of organic matter on land or in the oceans (Bowen & Zachos, 2010;Giusberti et al, 2016;Gutjahr et al, 2017;Kelly et al, 2010;Zachos et al, 2005).…”