In situ magnetic identification of giant, needle-shaped magnetofossils in Paleocene–Eocene Thermal Maximum sediments

Abstract

Near-shore marine sediments deposited during the Paleocene–Eocene Thermal Maximum at Wilson Lake, NJ, contain abundant conventional and giant magnetofossils. We find that giant, needle-shaped magnetofossils from Wilson Lake produce distinct magnetic signatures in low-noise, high-resolution first-order reversal curve (FORC) measurements. These magnetic measurements on bulk sediment samples identify the presence of giant, needle-shaped magnetofossils. Our results are supported by micromagnetic simulations of giant needle morphologies measured from transmission electron micrographs of magnetic extracts from Wilson Lake sediments. These simulations underscore the single-domain characteristics and the large magnetic coercivity associated with the extreme crystal elongation of giant needles. Giant magnetofossils have so far only been identified in sediments deposited during global hyperthermal events and therefore may serve as magnetic biomarkers of environmental disturbances. Our results show that FORC measurements are a nondestructive method for identifying giant magnetofossil assemblages in bulk sediments, which will help test their ecology and significance with respect to environmental change.

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

“…For a low‐noise FORC diagram for WL35950 see Wagner et al. (2021), where we stack and average multiple measurements from the same sample.…”

“…The subset of extended FORCs measured at higher fields show that the central ridge is mostly comprised within B c ≈ 200 mT (Figure 6), but continues to exist at higher fields. The minimum amplitude at ∼300–400 mT, close to the maximum theoretical coercivity of magnetite, marks the transition to small magnetic contributions from high‐coercivity minerals, such as goethite (Wagner et al., 2021). Other features of the FORC diagrams in Figures 6 and are much smaller than the central ridge in amplitude, but they contribute significantly to the bulk saturation remanence, due to their extension in both dimensions of the FORC space.…”

“…Given the dominance of the biogenic soft component in EM2, we conclude that the largest contribution of this coercivity component among the signature of conventional magnetosomes occurs within this interval. Furthermore, high‐resolution FORC diagrams (Wagner et al., 2021) indicate that sediments from the onset interval have three clearly recognizable magnetofossil components: biogenic soft (∼10–50 mT), biogenic hard (∼50–120 mT), and biogenic needle (∼120–300 mT), as discussed in more detail below. Some of the FORC coercivity spectra from the CIE core interval also indicate that biogenic soft and hard components are distinguished in these intervals (e.g., Figures 6b and 6d–6f).…”

“…We also observed teardrop-shaded grains in our extracts and, although these are not considered magnetofossils, we include them for comparison. Figure includes data presented in Wagner et al (2021). from the CIE core interval have larger amplitudes than those of specimens from the CIE onset interval.…”

“…The unique morphology, crystallography, and magnetic properties of giant magnetofossils suggests that they have a biogenic origin (Schumann et al., 2008). Micromagnetic modeling of these and similar giant magnetofossils suggests that some giant magnetofossils may contribute to the single‐domain magnetic signature typical of conventional magnetofossils (Chang, Harrison, et al., 2018; Chang, Roberts, Williams, et al., 2012; Wagner et al., 2021). Although most evidence is consistent with a biogenic origin for the single‐domain magnetite, some researchers argue that impact ejecta or pyrogenic particles contribute to the single‐domain enhancement of these sediments (Kent, Cramer, et al., 2003; Kent, Lanci, et al., 2017).…”