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Chang, L.
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Chang, L.
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- PublicationRestrictedMagnetic properties of Pelagic Carbonates(2013-12)
; ; ; ; ; ; ;Roberts, A. P.; Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia ;Florindo, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Chang, L.; Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia ;Heslop, D.; Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia ;Jovane, L.; Departamento de Oceanografia Física, Instituto Oceanográfico, Universidade de São Paulo, Praça do Oceanográfico, 191, 05508-120 São Paulo, Brazil ;Larrasoaña, J. C.; Instituto Geológico y Minero de España, Unidad de Zaragoza, C/Manuel Lasala 44, 9B, Zaragoza 50006, Spain; ; ; ; ; Pelagic carbonates are deposited far from continents, usually at water depths of 3000–6000 m, at rates below 10 cm/kyr, and are a globally important sediment type. Recent advances, with recognition of widespread preservation of biogenic magnetite (the inorganic remains of magnetotactic bacteria), have fundamentally changed our understanding of the magnetic properties of pelagic carbonates. We review evidence for the magnetic minerals typically preserved in pelagic carbonates, the effects of magnetic mineral diagenesis on paleomagnetic and environmental magnetic records of pelagic carbonates, and what magnetic properties can tell us about the open-ocean environments in which pelagic carbonates are deposited. We also discuss briefly late diagenetic remagnetisations recorded by some carbonates. Despite recent advances in our knowledge of these phenomena, much remains undiscovered. We are only at early stages of understanding how biogenic magnetite gives rise to paleomagnetic signals in sediments and whether it carries a poorly understood biogeochemical remanent magnetisation. Recently developed techniques have potential for testing how different magnetotactic bacterial species, which produce different magnetite morphologies, respond to changing nutrient and oxygenation conditions. Future work needs to test whether it is possible to develop proxies for ancient nutrient conditions from well-calibrated modern magnetotactic bacterial occurrences. A tantalizing link between giant magnetofossils and Paleogene hyperthermal events needs to be tested; much remains to be learned about the relationship between climate and the organisms that biomineralised these large and novel magnetite morphologies. Rather than being a well-worn subject that has been studied for over 60 years, the magnetic properties of pelagic carbonates hold many secrets that await discovery.264 59 - PublicationRestrictedSearching for single domain magnetite in the “pseudo-single-domain” sedimentary haystack: Implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations(2012-08-22)
; ; ; ; ; ;Roberts, A. P.; National Oceanography Centre, University of Southampton, Southampton, UK. ;Chang, L.; National Oceanography Centre, University of Southampton, Southampton, UK. ;Heslop, D.; Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia. ;Florindo, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Larrasoaña, J. C.; Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia.; ; ; ; 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.267 27 - PublicationRestrictedMagnetotactic bacterial abundance in pelagic marine environments is limited by organic carbon flux and availability of dissolved iron(2011-10-15)
; ; ; ; ; ; ; ; ; ;Roberts, A. P.; National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK ;Florindo, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Villa, G.; Dipartimento Scienze della Terra, Università di Parma, Viale Usberti 157A, 43100 Parma, Italy ;Chang, L.; National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK ;Jovane, L.; National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK ;Bohaty, S. M.; National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK ;Larrasoaña, J. C.; Área de Cambio Global, IGME, Oficina de Proyectos de Zaragoza, Manuel Lasala 44 9B, Zaragoza 50006, Spain ;Heslop, D.; Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia ;Fitz Gerald, J. D.; Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia; ; ; ; ; ; ; ; Magnetotactic bacteria intracellularly biomineralize magnetite of an ideal grain size for recording palaeomagnetic signals. However, bacterial magnetite has only been reported in a few pre-Quaternary records because progressive burial into anoxic diagenetic environments causes its dissolution. Deep-sea carbonate sequences provide optimal environments for preserving bacterial magnetite due to low rates of organic carbon burial and expanded pore-water redox zonations. Such sequences often do not become anoxic for tens to hundreds of metres below the seafloor. Nevertheless, the biogeochemical factors that control magnetotactic bacterial populations in such settings are not well known. We document the preservation of bacterial magnetite, which dominates the palaeomagnetic signal throughout Eocene pelagic carbonates from the southern Kerguelen Plateau, Southern Ocean. We provide evidence that iron fertilization, associated with increased aeolian dust flux, resulted in surface water eutrophication in the late Eocene that controlled bacterial magnetite abundance via export of organic carbon to the seafloor. Increased flux of aeolian ironbearing phases also delivered iron to the seafloor, some of which became bioavailable through iron reduction. Our results suggest that magnetotactic bacterial populations in pelagic settings depend crucially on particulate iron and organic carbon delivery to the seafloor.211 27 - PublicationRestrictedLow-temperature magnetic properties of pelagic carbonates: Oxidation of biogenic magnetite and identification of magnetosome chains(2013-12)
; ; ; ; ; ; ; ; ; ;Chang, L.; Paleomagnetic Laboratory “Fort Hoofddijk,” Department of Earth Sciences, Utrecht University, Utrecht, Netherlands ;Winklhofer, M.; Department of Earth and Environmental Sciences, Ludwig-Maximilians University, Munich, Germany ;Roberts, A. P.; Research School of Earth Sciences, The Australian National University, Canberra, Australia ;Heslop, D.; Research School of Earth Sciences, The Australian National University, Canberra, Australia ;Florindo, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Dekkers, M. J.; Paleomagnetic Laboratory “Fort Hoofddijk,” Department of Earth Sciences, Utrecht University, Utrecht, Netherlands ;Krijgsman, W.; Paleomagnetic Laboratory “Fort Hoofddijk,” Department of Earth Sciences, Utrecht University, Utrecht, Netherlands ;Kodama, K.; Center for Advanced Marine Core Research, Kochi University, Nankoku, Kochi, Japan ;Yamamoto, Y.; Center for Advanced Marine Core Research, Kochi University, Nankoku, Kochi, Japan; ; ; ; ; ; ; ; 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.301 49