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

Abstract

The Paleocene-Eocene Thermal Maximum (PETM) was a geologically rapid global warming event that occurred ∼56 million years ago. The Earth warmed 5°C-8°C over several thousand years during the PETM. The magnitude and pace of warming during the PETM led to increased ocean temperatures (3°C-4°C in surface waters and 2°C-6°C in bottom waters), ocean acidification and bottom water deoxygenation (and subsequent benthic foraminifera extinctions), rapid radiation of land mammals, expansion of tropical forests at high latitudes, the appearance of giant insects and reptiles, and stronger, more intense seasonal weather events like flash floods, hurricanes, and frequent wildfires (e.g., McInerney & Wing, 2011). The PETM is marked by a −3‰ carbon isotope excursion (CIE) in marine bulk carbonate, and it is an analog, albeit an imperfect one, for anthropogenic global warming and environmental change (Gutjahr et al., 2017;Kennett

Key Signatures of Magnetofossils Elucidated by Mutant Magnetotactic Bacteria and Micromagnetic Calculations

“…Nevertheless, thermal activations are still sufficiently large to produce a measurable vertical offset of the central ridge ( Egli, 2013 , 2021 ), due to the intrinsic time asymmetry of the FORC measurement protocol ( Berndt et al, 2018 ). The central ridge offset of high-resolution FORC measurements of magnetofossil-rich sediments, which is of the order of 0.3–0.5 mT ( Egli et al, 2010 ; Ludwig et al, 2013 ; Wagner et al, 2021 ), can be explained by a Stoner-Wohlfarth model of thermally activated UNISD particles ( Berndt et al, 2018 ; Lanci & Kent, 2018 ) with β 0 ≈ 400–600.…”

“…In this case, a single magnetofossil endmember is expected from PCA analyses. More than one endmember is needed to describe magnetofossil signatures that change in time, reflecting, for instance, variable proportions of equidimensional and elongated crystals ( Hesse, 1994 ; Wagner et al, 2021 ; Yamazaki & Kawahata, 1998 ), or single-stranded and multi-stranded chains.…”

“…They represent some of the most ancient biomineralizing organisms, with a proposed origin of ~3 Ga ( Lin et al, 2017 ). They are markers of oxic-anoxic transition zones in aquatic and sedimentary environments ( Flies et al, 2005 ; Simmons et al, 2004 ), making them useful paleoenvironmental indicators ( Chang et al, 2018 ; Hesse, 1994 ; Roberts et al, 2011 ; Usui et al, 2017 ; Wagner et al, 2021 ). Magnetosomes are assembled in chains, and provide the bacteria with a permanent magnetic dipole for navigation purposes ( McCausland & Komeili, 2020 ).…”

“…The lack of direct in situ observations of magnetofossil structures, the mixed character of the magnetic signature of magnetofossil-rich sediments, and the sometimes-poor definition of characteristic coercivity components, leave margins for contrasting interpretations of some geological materials. Most notable examples include the controversial biogenic origin of magnetite nanoparticles in the Martian meteorite ALH84001 ( Golden et al, 2004 ; Thomas-Keprta et al, 2000 ; Weiss et al, 2004 ), and the origin of the sudden rise in SD magnetite concentration during the Paleocene-Eocene thermal maximum ( Kent et al, 2003 ; Kopp et al, 2007 ; Wagner et al, 2021 ; H. Wang et al, 2013 ).…”

Influence of Magnetofossils on Paleointensity Estimations Inferred From Principal Component Analyses of First‐Order Reversal Curve Diagrams for Sediments From the Western Equatorial Pacific

“…It is considered that the FORC-PCA method can reduce the ambiguity of magnetofossil proportion estimations compared with the k ARM /SIRM ratio. Furthermore, information on bending and/or collapse of biogenic magnetite chains in sediments and differences in the morphology of biogenic magnetite can also be obtained from FORC diagrams (Berndt et al, 2020;Chang et al, 2019;Usui & Yamazaki, 2021;Wagner et al, 2021;Yamazaki et al, 2020), which may also influence RPI estimations. In this study, we use three sediment cores taken from the western equatorial Pacific, in which large differences in the proportion of magnetofossils to terrigenous magnetic minerals are expected.…”