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

Abstract

The Paleocene–Eocene boundary (∼55.8 million years ago) is marked by an abrupt negative carbon isotope excursion (CIE) that coincides with an oxygen isotope decrease interpreted as the Paleocene–Eocene thermal maximum. Biogenic magnetite (Fe3O4) in the form of giant (micron-sized) spearhead-like and spindle-like magnetofossils, as well as nano-sized magnetotactic bacteria magnetosome chains, have been reported in clay-rich sediments in the New Jersey Atlantic Coastal Plain and were thought to account for the distinctive single-domain magnetic properties of these sediments. Uncalibrated strong field magnet extraction techniques have been typically used to provide material for scanning and transmission electron microscopic imaging of these magnetic particles, whose concentration in the natural sediment is thus difficult to quantify. In this study, we use a recently developed ultrahigh-resolution, synchrotron-based, full-field transmission X-ray microscope to study the iron-rich minerals within the clay sediment in their bulk state. We are able to estimate the total magnetization concentration of the giant biogenic magnetofossils to be only ∼10% of whole sediment. Along with previous rock magnetic studies on the CIE clay, we suggest that most of the magnetite in the clay occurs as isolated, near-equidimensional nanoparticles, a suggestion that points to a nonbiogenic origin, such as comet impact plume condensates in what may be very rapidly deposited CIE clays.

Diversification of Iron‐Biomineralizing Organisms During the Paleocene‐Eocene Thermal Maximum: Evidence From Quantitative Unmixing of Magnetic Signatures of Conventional and Giant Magnetofossils

“…Giant magnetofossils are the scarcest of all the categories (<1% of the total magnetofossils in each extract), consistent with findings from a different site reported by Wang, Wang, et al. (2015). Results from our magnetofossil counts are summarized in Tables .…”

“…The distinct FORC signatures of uniaxial single-domain magnetite in marine sediments are often interpreted to indicate abundant magnetofossils, particularly intact chains of conventional magnetofossils (Heslop et al, 2014;Roberts, Chang, et al, 2012). Similar signatures have also been attributed to isolated single-domain magnetite particles preserved in volcanic glass, igneous rocks, and detrital magnetite (Carvallo et al, 2006;Chang, Roberts, Heslop, et al, 2016;Kent, Lanci, et al, 2017;Lin, Wang, et al, 2013;Muxworthy, Evans, et al, 2013;Wang, Wang, et al, 2015). The sedimentology of WL-A sediments and TEM images of magnetic extracts (which visually document the abundance of magnetite or maghemite particles with magnetofossil morphologies) do not support these alternative sources.…”

“…PETM sediments from the New Jersey margin contain abundant single‐domain magnetite that has been interpreted as the result of detrital inputs, impact melts, fires, magnetotactic bacteria (MTB), or a combination of these sources (Kent, Cramer, et al., 2003; Kent, Lanci, et al., 2017; Kopp, Raub, et al., 2007; Kopp, Schumann, et al., 2009; Lippert & Zachos, 2007; Schaller et al., 2016; Wang, Kent, & Jackson, 2013; Wang, Wang, et al., 2015). Electron microscopy and rock magnetic analyses of these and other PETM sections provide evidence for abundant magnetofossils (the fossilized remains of MTB) and reveal the occurrence of so‐called giant magnetofossils (Chang, Harrison, et al., 2018; Chang, Roberts, Williams, et al., 2012; Kopp, Raub, et al., 2007; Kopp, Schumann, et al., 2009; Lippert & Zachos, 2007; Schumann et al., 2008).…”

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

“…It thus appears that FMR and most other magnetic techniques like hysteresis properties and firstorder reversal curves (FORCs) (Egli et al, 2010) can indeed distinguish the presence of SD grains but not whether they are aligned in chains. Imaging of bulk samples of Marlboro Clay using ultrahigh-resolution, synchrotron-based, full-field transmission X-ray microscopy (Wang et al (2015) was only able to confirm the presence of giant biogenic magnetofossils Schumann et al, 2008) but whose estimated total magnetic contribution is only ∼10% of bulk sediment. In any case, recent work has shown that giant magnetofossils can be found before and after the CIE and may have a rather widespread geographic, environmental, and temporal distribution (Chang et al, 2012).…”

Classification of a Complexly Mixed Magnetic Mineral Assemblage in Pacific Ocean Surface Sediment by Electron Microscopy and Supervised Magnetic Unmixing

“…In contrast, relatively few studies have used TEM observations to study the mineralogy and chemistry of other magnetic mineral types in sediments (e.g., Gibbs-Eggar et al, 1999;Franke et al, 2007;Chang et al, 2016b;Zhang et al, 2018;Li et al, 2019). While magnetofossils are relatively easy to recognize in TEM observations due to their distinctive crystal morphologies and chain structures compared to other magnetic mineral types (e.g., Kopp and Kirschvink, 2008;Jimenez-Lopez et al, 2010;Li et al, 2013b), this can lead to bias in overestimating their magnetic contributions, and/or to ignoring contributions from, for example, weakly interacting or noninteracting single domain (SD) magnetite particles hosted by silicates (e.g., Wang et al, 2015;Chang et al, 2016b).…”