Searching for single domain magnetite in the “pseudo-single-domain” sedimentary haystack: Implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations (original) (raw)
Related papers
Journal of Geophysical …, 2012
1] Magnetic hysteresis measurements of sediments have resulted in widespread reporting of "pseudo-single-domain"-like magnetic properties. In contrast, the ideal single domain (SD) properties that would be expected to be responsible for high quality paleomagnetic records are rare. Determining whether SD particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment. We apply a range of such techniques and find that SD particles are much more common than has been reported in the literature and that magnetite magnetofossils (the inorganic remains of magnetotactic bacteria) are widely preserved at depth in a range of sediment types, including biogenic pelagic carbonates, lacustrine and marine clays, and possibly even in glaci-marine sediments. Thus, instead of being rarely preserved in the geological record, we find that magnetofossils are widespread. This observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments. Magnetofossils also have grain sizes that are substantially smaller than the 1-15 mm size range for which there is reasonable empirical support for relative paleointensity studies. The different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies. This issue needs much closer attention. Despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years, much remains to be understood about how they become magnetized. (2012), Searching for single domain magnetite in the "pseudo-single-domain" sedimentary haystack: Implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations,
Geology, 1983
Many sedimentary carbonate rocks carry stable magnetizations that can be shown to reside in magnetite. When such magnetizations are observed, it is often argued or demonstrated that the magnetite was incorporated into the sediment during deposition. However, paleomagnetic and rock magnetic studies in conjunction with analyses of magnetic extracts from the Helderberg and Bonneterre carbonates (United States) indicate that the magnetite present in these rocks is most likely of diagenetic (i.e., postdepositional) origin.
Journal of Geophysical Research, 2000
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.
Depositional and postdepositional processes in the magnetization of sediments
Reviews of Geophysics, 1977
This paper surveys 40 years of research on the processes by which a sediment acquires a detrital remanent magnetization (DRM). These processes can be divided into two types: depositional processes, which arise from interactions between the magnetic carriers and the substrate at the sediment/water interface, and postdepositional processes, which arise from the mobility of magnetic carriers within fluidfilled voids in the sediment. Depositional DRM is well understood theoretically and experimentally, but its applicability to natural sediments may be quite limited. Postdepositional DRM, which has not been studied in sufficient detail, may well be the dominant process by which sediments acquire a magnetization. The nature and composition of both the magnetic carriers and the matrix as well as the pore water content of the sediment influence the balance between depositional and postdepositional DRM. The identification of the method of magnetization in a given sedimentary environment, such as glacial lakes, the deep sea, or lakes and marginal seas, can be a difficult problem. Since postdepositional DRM accurately records the ambient geomagnetic field, whereas depositional DRM does not, such an identification is necessary in order to use the detrital reinanent magnetism of sediments in studies of the fine scale behavior of the geomagnetic field, the paleomagnetic correlation between sedimentary environments, and the possible relation between paleomagnetic parameters and paleoclimatic indicators. CONTENTS
Rock magnetic criteria for the detection of biogenic magnetite
Earth and Planetary Science Letters, 1993
We report results on the magnetic properties of magnetites produced by magnetotactic and dissimilatory iron-reducing bacteria. Magnetotactic bacterial (MTB) strains MS1, MV1 and MV2 and dissimilatory iron-reducing bacterium strain GS-15, grown in pure cultures, were used in this study. Our results suggest that a combination of room temperature eoercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain (SD) particles of magnetite from MTBs. The most useful magnetic property measurements include: (1) acquisition and demagnetization of isothermal remanent magnetization (IRM) using static, pulse and alternating fields; (2) acquisition of anhysteretic remanent magnetization (ARM); and (3) thermal dependence of low temperature (20 K) saturation IRM after cooling in zero field (ZFC) or in a 2.5 T field (FC) from 300 K. However, potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference between low temperature zero-field and field cooled SIRMs on warming through the Verwey transition (T= 100 K). Intact chains of unoxidized magnetite magnetosomes have ratios of aFC/6ZF c greater than 2, where the parameter a is a measure of the amount of remanence lost by warming through the Verwey transition. Disruption of the chain structure or conversion of the magnetosomes to maghemite reduces the 6Fc/~zv c ratio to around 1, similar to values observed for some inorganic magnetite, maghemite, greigite and GS-15 particles. Numerical simulations of ~Fc/azvc ratios for simple binary mixtures of magnetosome chains and inorganic magnetic fractions suggest that the 6Fc/6zv c parameter can be a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomes and can be a useful magnetic technique for identifying them in whole-sediment samples. The strength of our approach lies in the comparative ease and rapidity with which magnetic measurements can be made, compared to techniques such as electron microscopy.
Geochemistry, Geophysics, Geosystems, 2014
1] The widespread occurrence of magnetotactic bacteria (MTB) in several types of marine and freshwater sediment, and the role of fossil magnetosomes (magnetofossils) as main remanent magnetization carriers therein, has important paleomagnetic and paleoenvironmental implications. Despite numerous studies on MTB biology and on magnetofossil preservation in geological records, no detailed information is yet available on how magnetotaxis (i.e., the ability to navigate along magnetic field lines) is performed in sedimentary environments, and on how magnetofossils possibly record the Earth magnetic field. We provide for the first time experimental evidence for these processes. MTB living in sediment are poorly aligned with the geomagnetic field, contrary to what is observed in water. This can explain the seemingly excessive magnetic moment of most MTB. The observed alignment is sufficient for supporting magnetotaxis across the typical thickness of chemical gradients. Experiments with magnetofossil-rich sediment suggest that a natural remanent magnetization (NRM) is acquired by magnetofossils in the so-called benthic mixed layer, where natural MTB populations usually occur. The acquired NRM is proportional to the applied field at least up to $160 mT, and its intensity is compatible with values observed in nature for same sediment types. Therefore, if fossil magnetosome chains are not subjected to further alteration by early diagenetic processes, they can provide a useful relative paleointensities. We propose a preliminary model to explain early stages of magnetofossil NRM acquisition as the result of a dynamic equilibrium between magnetic torques and randomizing forces due to sediment mixing.