Reductive diagenesis,magnetite dissolution,greigite growth and paleomagnetic smoothing in marine sediments: A new view |
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| Authors: | Christopher J Rowan Andrew P Roberts Thomas Broadbent |
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| Institution: | 1. North-East Interdisciplinary Scientific Research Institute, named after N.A. Shilo, Far Eastern Branch of the Russian Academy of Sciences, ul. Portovaya 16, Magadan, 685000, Russia;2. A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia;3. Limnological Institute, Siberian Branch of the Russian Academy of Sciences, ul. Ulan-Batorskaya 3, Irkutsk, 664033, Russia;1. Institute for Rock Magnetism, N. H. Winchell School of Earth Sciences, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN, 55455 USA;2. Limnological Research Center, N. H. Winchell School of Earth Sciences, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN, 55455 USA;1. Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China;2. Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China;3. Key Laboratory of Marine Sedimentology and Environmental Geology, First Institute of Oceanography, Ministry of Natural Resources, Qingdao 266061, China;4. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China;5. Marine and Coastal Resources Research and Development Institute, Department of Marine and Coastal Resources, Bangkok 10210, Thailand |
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| Abstract: | In many anoxic sedimentary environments, the onset of sulfate reduction, and pyritization of detrital iron-bearing minerals, leads to a precipitous decline in magnetic mineral concentration during early diagenesis. The usefulness of the surviving paleomagnetic record in such environments is usually argued to depend on how much of the primary detrital magnetic assemblage survives diagenetic dissolution. Detailed rock magnetic and electron microscope analyses of rapidly deposited (~ 7 cm/kyr) latest Pleistocene–Holocene sediments from the continental margins of Oman (22°22.4′N, 60°08.0′E) and northern California (38°24.8′N, 123°58.2′W) demonstrate that pyritization during early diagenesis also leads to the progressive down-core growth of the ferrimagnetic iron sulfide greigite. Greigite growth begins with nucleation of large concentrations of superparamagnetic (SP) nanoparticles at the inferred position of the sulfate–methane transition, which can explain the apparently paradoxical suggestion that diagenetically reduced sediments contain enhanced concentrations of SP particles. Looping of hysteresis parameters on a “Day” plot records the dissolution of single domain (SD) (titano-)magnetite and the formation of SP greigite, which then slowly and progressively grows through its SD blocking volume and acquires a stable paleomagnetic signal. This looping trend is also evident in data from several published records (Oregon margin, Korea Strait, Japan Sea, Niger Fan, Argentine margin, and the Ontong–Java Plateau), indicating that these processes may be widespread in reducing environments. Our observations have profound implications for paleomagnetic records from sulfate-reducing environments. The paleomagnetic signal recorded by greigite is offset from the age of the surrounding sediments by 10's of kyr, and ongoing growth of greigite at depth results in smoothing of the recorded signal over intervals of 10's to 100's of kyr. We therefore expect the presence of greigite to compromise paleomagnetic records in a wide range of settings that have undergone reductive diagenesis. |
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