Low‐temperature magnetic properties of pelagic carbonates: Oxidation of biogenic magnetite and identification of magnetosome chains

Abstract

[1] Pelagic marine carbonates provide important records of past environmental change. We carried out detailed low-temperature magnetic measurements on biogenic magnetite-bearing sediments from the Southern Ocean (Ocean Drilling Program (ODP) Holes 738B, 738C, 689D, and 690C) and on samples containing whole magnetotactic bacteria cells. We document a range of low-temperature magnetic properties, including reversible humped low-temperature cycling (LTC) curves. Different degrees of magnetite oxidation are considered to be responsible for the observed variable shapes of LTC curves. A dipole spring mechanism in magnetosome chains is introduced to explain reversible LTC curves. This dipole spring mechanism is proposed to result from the uniaxial anisotropy that originates from the chain arrangement of biogenic magnetite, similar to published results for uniaxial stable single domain (SD) particles. The dipole spring mechanism reversibly restores the remanence during warming in LTC measurements. This supports a previous idea that remanence of magnetosome chains is completely reversible during LTC experiments. We suggest that this magnetic fingerprint is a diagnostic indicator for intact magnetosome chains, although the presence of isolated uniaxial stable SD particles and magnetically interacting particles can complicate this test. Magnetic measurements through the Eocene section of ODP Hole 738B reveal an interval with distinct magnetic properties that we interpret to originate from less oxidized biogenic magnetite and enrichment of a biogenic "hard" component. Co-occurrence of these two magnetic fingerprints during the late Eocene in the Southern Ocean indicates less oxic conditions, probably due to increased oceanic primary productivity and organic carbon burial.

The Low‐Temperature Besnus Magnetic Transition: Signals Due to Monoclinic and Hexagonal Pyrrhotite

“…LTC-RT-SIRM curves for goethite increase systematically from 300 to 5 K (Liu et al, 2006). Maghemitized magnetite produces humped curves (Chang et al, 2013;Özdemir & Dunlop, 2010), but such curves are different in detail to those observed here, so a contribution from maghemite can also be excluded. Maghemitized magnetite produces humped curves (Chang et al, 2013;Özdemir & Dunlop, 2010), but such curves are different in detail to those observed here, so a contribution from maghemite can also be excluded.…”

Discrimination of biogenic and detrital magnetite through a double Verwey transition temperature

“…However, surface oxidation cannot explain some experimental observations. For example, most reported T v values for fresh MTB cells that should contain nonoxidized magnetite are still less than 110 K [e.g., Moskowitz et al ., ; Weiss et al ., ; Pan et al ., ; Kopp et al ., ; Prozorov et al ., ; Moskowitz et al ., ; Li et al ., , ; Li and Pan , ; Chang et al ., ]. These values are significantly lower than for stoichiometric inorganic magnetite [e.g., Walz , ].…”

“…Some effects, i.e., nonstoichiometry, lattice defects, oxidation, and ambient pressure, can decrease T v [e.g., Aragón et al ., ; Özdemir et al ., ; Moskowitz et al ., ; Kosterov , ; Rozenberg et al ., ; Özdemir and Dunlop , ]. Lower T v temperatures (often near 100 K) have been reported commonly for whole cell MTB samples [ Moskowitz et al ., ; Weiss et al ., ; Pan et al ., ; Kopp et al ., ; Prozorov et al ., ; Moskowitz et al ., ; Li et al ., , ; Chang et al ., ] and magnetofossils [e.g., Smirnov and Tarduno , ; Pan et al ., ; Roberts et al ., ; Chang et al ., , ]. In contrast, natural inorganic and synthetic stoichiometric magnetite samples often have T v close to 120 K [e.g., Özdemir et al ., ; Moskowitz et al ., ; Muxworthy and McClelland , ; Prozorov et al ., ].…”

“…Distinct changes in magnetic properties at T v for magnetite‐bearing samples have been extensively investigated in rock magnetic and environmental magnetic studies. For example, T v is used to detect magnetite, including biogenic magnetite [ Moskowitz et al ., ; Carter‐Stiglitz et al ., ; Weiss et al ., ; Pan et al ., ; Li et al ., ; Chang et al ., ], to analyze oxidation effects [e.g., Özdemir et al ., ; Torii , ; Kosterov , ; Özdemir and Dunlop , ; Chang et al ., ] and to analyze domain stability and dynamics at low temperatures [e.g., Özdemir et al ., ; Smirnov and Tarduno , ; Smirnov , ; Kasama et al ., ].…”

Particle Size-Specific Magnetic Measurements as a Tool for Enhancing Our Understanding of the Bulk Magnetic Properties of Sediments

“…Development of model functions that could incorporate non-Gaussian variation have shown to better fit and describe commonly occurring low-coercivity magnetic components [49][50][51] and performed better in identification of components with overlapping coercivity spectra [50,51]. These components can be summarized and discriminated using simple rock magnetic parameters (e.g., the median destructive field of the ARM (MDFARM) and volume normalized susceptibility of ARM to IRM (κARM/IRM), Figure 2) providing the basis for discrimination of bacterial magnetosomes, extracellular and pedogenic ferrimagnets, detrital, eolian, and loessic components, and those signatures related to urban pollution from a range of environmental samples ( Figure 2; [50][51][52][107][108][109][110]). Similar high resolution modelling approaches have been employed to unmix different shapes within hysteresis loops that relate to different magnetic components [55,111] and quantitative analysis of FORCs [84][85][86][112][113][114] are a useful tool to assess the nature, degree of magnetostatic interaction, and the abundance of different magnetic grain sizes within bulk samples [40,114].…”