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Curie isotherm depth from aeromagnetic data constraining shallow heat source depths in the central Aeolian Ridge (Southern Tyrrhenian Sea, Italy)

Tue, 19 Mar 2013 23:00:00 GMT

Title: Curie isotherm depth from aeromagnetic data constraining shallow heat source depths in the central Aeolian Ridge (Southern Tyrrhenian Sea, Italy) Authors: De Ritis, R.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Ravat, D.; Department of Earth and Environmental Sciences, University of Kentucky; Ventura, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; Chiappini, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia Abstract: The Salina, Lipari, and Vulcano volcanic ridge and the surrounding sea sectors (Aeolian Archipelago, Southern Tyrrhenian Sea, Italy) are characterized by vents responsible for a recent (<40 ka—1889/1890 AD) effusive and explosive subareal activity and repeated, 56 to 7 ka in age, submarine explosive eruptions from source areas located between Lipari and Vulcano. A spectral depth estimation of the magnetic bottom using a fractal method on aeromagnetic data from Vulcano, Lipari, and Salina volcanic ridge allows us to constrain the Curie isotherm depth. The elevated portion of the isotherm is between 2 and 3 km below Salina and Vulcano and about 1 km below Lipari. The Curie depth results in the context of other geological and geophysical evidence suggest that the rise of the Curie isotherm is mainly due to the occurrence of shallow heat sources such as magma ponds and associated hydrothermal systems. The short-wavelength magnetic anomaly field reflects magnetic contrasts from highly magnetized volcanic bodies, low-magnetization sediments, and hydrothermally altered rocks. Borehole temperature data verify the Curie temperature derived from the magnetic methods on the island of Vulcano.We conclude that the whole Vulcano, Lipari, and Salina volcanic ridge is active and should be monitored.

Searching for single domain magnetite in the “pseudo-single-domain” sedimentary haystack: Implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations

Tue, 21 Aug 2012 22:00:00 GMT

Title: Searching for single domain magnetite in the “pseudo-single-domain” sedimentary haystack: Implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations Authors: 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. Abstract: 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.

Inconsistent magnetic polarities in magnetite-and greigite-bearing sediments: Understanding complex magnetizations in the late Messinian in the Adana Basin (southern Turkey)

Thu, 04 Oct 2012 22:00:00 GMT

Title: Inconsistent magnetic polarities in magnetite-and greigite-bearing sediments: Understanding complex magnetizations in the late Messinian in the Adana Basin (southern Turkey) Authors: Lucifora, S.; Dipartimento di Scienze Geologiche, Università Roma Tre, Largo San Leonardo Murialdo 1, IT-00146 Rome, Italy; Cifelli, F.; Dipartimento di Scienze Geologiche, Università Roma Tre, Largo San Leonardo Murialdo 1, IT-00146 Rome, Italy; Mattei, M.; Dipartimento di Scienze Geologiche, Università Roma Tre, Largo San Leonardo Murialdo 1, IT-00146 Rome, Italy; Sagnotti, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Cosentino, D.; Dipartimento di Scienze Geologiche, Università Roma Tre, Largo San Leonardo Murialdo 1, IT-00146 Rome, Italy; Roberts, A. P.; Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Abstract: We present paleomagnetic, rock magnetic and scanning electron microscope data from three upper Messinian stratigraphic sections from the Adana Basin (southern Turkey). The collected samples are from fine-grained units, which were deposited during the Messinian Salinity Crisis (within subchron C3r). Paleomagnetic results reveal an inconsistent polarity record, related to a mixture of magnetite and greigite that hinders determination of a reliable magnetostratigraphy. Three classes of samples are recognized on the basis of paleomagnetic results. The first is characterized by a single magnetization component, with normal polarity, that is stable up to 530–580 C and is carried by magnetite. The second is characterized by a single magnetization component, with reversed polarity, that is stable up to 330–420 C. This magnetization is due to greigite, which developed after formation of slumps and before tectonic tilting of the studied successions. The third is characterized by reversed polarity, which is stable up to 530–580 C. We interpret this component as a primary magnetization carried by fine-grained and magnetically stable detrital magnetite. Results indicate that in the Adana Basin the assumption that a primary magnetization is carried by magnetite, and a magnetic overprint carried by greigite, does not hold because a late magnetic overprint has also been found for magnetite-bearing samples. Our data illustrate the complexity of magnetostratigraphic reconstructions in successions characterized by variable mixtures of magnetic minerals with different magnetic stability that formed at different stages. We demonstrate the need to perform detailed magnetic mineralogy analyses when conducting magnetostratigraphic studies of clay-rich sediments from marine or lacustrine environments.

Correlation of welded ignimbrites on Pantelleria (Strait of Sicily) using paleomagnetism

Wed, 29 Feb 2012 23:00:00 GMT

Title: Correlation of welded ignimbrites on Pantelleria (Strait of Sicily) using paleomagnetism Authors: Speranza, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Di Chiara, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Rotolo, S. G.; Dipartimento di Scienze della Terra e del Mare (DISTeM), Università di Palermo, Via Archirafi 36, 90123 Palermo, Italy Abstract: Although the oldest volcanic rocks exposed at Pantelleria (Strait of Sicily) are older than 300 ka, most of the island is covered by the 45–50 ka Green Tuff ignimbrite, thought to be related to the Cinque Denti caldera, and younger lavas and scoria cones. Pre-50 ka rocks (predominantly rheomorphic ignimbrites) are exposed at isolated sea cliffs, and their stratigraphy and chronology are not completely resolved. Based on volcanic stratigraphy and K/Ar dating, it has been proposed that the older La Vecchia caldera is related to ignimbrite Q (114 ka), and that ignimbrites F, D, and Z (106, 94, and 79 ka, respectively) were erupted after caldera formation. We report here the paleomagnetic directions obtained from 23 sites in ignimbrite P (133 ka) and four younger ignimbrites, and from an uncorrelated (and loosely dated) welded lithic breccia thought to record a caldera-forming eruption. The paleosecular variation of the geomagnetic field recorded by ignimbrites is used as correlative tool, with an estimated time resolution in the order of 100 years. We find that ignimbrites D and Z correspond, in good agreement with recent Ar/Ar ages constraining the D/Z eruption to 87 ka. The welded lithic breccia correlates with a thinner breccia lying just below ignimbrite P at another locality, implying that collapse of the La Vecchia caldera took place at ~130–160 ka. This caldera was subsequently buried by ignimbrites P, Q, F, and D/Z. Paleomagnetic data also show that the northern caldera margin underwent a ~10° west–northwest (outwards) tilting after emplacement of ignimbrite P, possibly recording magma resurgence in the crust.

Stiffnites. Part II

Fri, 31 Dec 2010 23:00:00 GMT

Title: Stiffnites. Part II Authors: Pareschi, M. T.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia Abstract: The dynamics of a stiffnite are here inferred. A stiffnite is a sheet-shaped, gravity-driven submarine sediment flow, with a fabric made up of marine ooze. To infer stiffnite dynamics, order of magnitude estimations are used. Field deposits and experiments on materials taken from the literature are also used. Stiffnites can be tens or hundreds of kilometers wide, and a few centimeters/ meters thick. They move on the sea slopes over hundreds of kilometers, reaching submarine velocities as high as 100 m/s. Hard grain friction favors grain fragmentation and formation of triboelectrically electrified particles and triboplasma (i.e., ions + electrons). Marine lipids favor isolation of electrical charges. At first, two basic assumptions are introduced, and checked a posteriori: (a) in a flowing stiffnite, magnetic dipole moments develop, with the magnetization proportional to the shear rate. I have named those dipoles as Ambigua. (b) Ambigua are ‘vertically frozen’ along stiffnite streamlines. From (a) and (b), it follows that: (i) Ambigua create a magnetic field (at peak, >1 T). (ii) Lorentz forces sort stiffnite particles into two superimposed sheets. The lower sheet, L+, has a sandy granulometry and a net positive electrical charge density. The upper sheet, L–, has a silty muddy granulometry and a net negative electrical charge density; the grains of sheet L– become finer upwards. (iii) Faraday forces push ferromagnetic grains towards the base of a stiffnite, so that a peak of magnetic susceptibility characterizes a stiffnite deposit. (iv) Stiffnites harden considerably during their motion, due to magnetic confinement. Stiffnite deposits and inferred stiffnite characteristics are compatible with a stable flow behavior against bending, pinch, or other macro instabilities. In the present report, a consistent hypothesis about the nature of Ambigua is provided.

Stiffnites. Part I

Fri, 31 Dec 2010 23:00:00 GMT

Title: Stiffnites. Part I Authors: Pareschi, M. T.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia Abstract: I identify the early phases of a particular kind of gravity-driven submarine sediment flow, that I have named immature stiffnite. The mature flow dynamics is originally presented in an accompanying report, referred to here as Pareschi [2011]. An immature stiffnite is constituted by a liquefied flowing mixture of muddy to sandy particles (sea floor ooze) in contact or in close proximity to each other, with inter-granular pores saturated in water. Sliding hard grains, including microshells, fragment during its motion. To infer the dynamics of an immature stiffnite, I consider deposits from the literature. In the literature, however, those deposits have not been well defined and they have often been confused with turbidites. Turbidites are water currents with suspended fine sediments that progressively settle-out down an incline. Stiffnites are triggered by events that create overpressure in intergrain pore water of the sea floor over wide areas. A peak of magnetic susceptibility can occur at the base of an immature stiffnite deposit.

Sequence stratigraphy of the ANDRILL AND-2A drillcore, Antarctica: A long-term, ice-proximal record of Early to Mid-Miocene climate, sea-level and glacial dynamism

Sat, 14 May 2011 22:00:00 GMT

Title: Sequence stratigraphy of the ANDRILL AND-2A drillcore, Antarctica: A long-term, ice-proximal record of Early to Mid-Miocene climate, sea-level and glacial dynamism Authors: Fielding, C. R.; Department of Earth & Atmospheric Sciences, 214 Bessey Hall, University of Nebraska-Lincoln, NE 68588-0340, USA; Browne, G. H.; GNS Science, P.O. Box 30368, Lower Hutt, New Zealand; Field, B.; GNS Science, P.O. Box 30368, Lower Hutt, New Zealand; Florindo, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Harwood, D. M.; Department of Earth & Atmospheric Sciences, 214 Bessey Hall, University of Nebraska-Lincoln, NE 68588-0340, USA; Krissek, L. A.; School of Earth Sciences, Ohio State University, 125 South Oval Mall, Columbus, OH 43210, USA; Levy, R. H.; GNS Science, P.O. Box 30368, Lower Hutt, New Zealand; Panter, K. S.; Department of Geology, Bowling Green State University, Bowling Green, OH 43403, USA; Passchier, S.; Department of Earth & Environmental Sciences, Montclair State University, 252 Mallory Hall, 1 Normal Avenue, Montclair, NJ 07043, USA; Pekar, S. F.; School of Earth & Environmental Sciences, Queen's College, 65-30 Kissena Blvd., Flushing, NY 11367, USA Abstract: Present understanding of Antarctic climate change during the Early to Mid-Miocene, including major cycles of glacial expansion and contraction, relies in large part on stable isotope proxies from deep sea core drilling. Here, we summarize the lithostratigraphy of the ANDRILL Southern McMurdo Sound Project drillcore AND- 2A. This core offers a hitherto unavailable ice-proximal stratigraphic archive from a high-accommodation continental margin setting, and provides clear evidence of repeated fluctuations in climate, ice expansion/ contraction and attendant sea-level change over the period c. 20.2–14.2 Ma, with a more fragmentary record of Late Miocene and Pliocene time. The core is divided into seventy-four high-frequency (fourth- or fifthorder) glacimarine sequences recording repeated advances and retreats of glaciers into and out of the Victoria Land Basin. The section can be resolved into thirteen longer-term, composite (third-order) sequences, which comprise packages of higher frequency sequences that show a consistent stratigraphic stacking pattern (Stratigraphic Motif). The distribution of the six recognized motifs indicates intervals of less and more iceproximal, and temperate to subpolar/polar climate, through the Miocene period. The core demonstrates a dynamic climate regime throughout the Early to Mid-Miocene that may be correlated to some previouslyrecognized events such as the Mid-Miocene Climatic Optimum, and provides a detailed reference point from which to evaluate stable isotope proxy records from the deep-sea.

Magnetotactic bacterial abundance in pelagic marine environments is limited by organic carbon flux and availability of dissolved iron

Fri, 14 Oct 2011 22:00:00 GMT

Title: Magnetotactic bacterial abundance in pelagic marine environments is limited by organic carbon flux and availability of dissolved iron Authors: 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 Abstract: 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.

Magnetic Anisotropy

Fri, 31 Dec 2010 23:00:00 GMT

Title: Magnetic Anisotropy Authors: Sagnotti, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia Editors: Gupta, H. H.; National Geophysical Research Institute Council of Scientific and Industrial Research Hyderabad, India Abstract: Pioneering works on the magnetic anisotropy of rocks were carried out during the 1940s and 1950s (Ising, 1942; Graham, 1954). These authors first realized that magnetic methods may be used to characterize the preferred orientation of minerals within the rock samples. Ising studied varved clays in Sweden and noticed that the magnetic susceptibility was higher on the bedding plane than orthogonally to it. Graham recognized that the anisotropy of magnetic susceptibility (AMS) may be regarded as a petrofabric element; he later extended the analysis to various sedimentary rocks of the Appalachian Mountains and pointed out the existence of distinct and systematic relationships of the magnetic properties with structural setting (Graham, 1966). The studies progressively developed in the following decades and a first comprehensive review on magnetic anisotropy and its application in geology and geophysics was published by Hrouda (1982). Over the past 20–30 years, researches on magnetic anisotropy gained widespread use and were extended to examine the fabric in a variety of sedimentary, igneous, and metamorphic rocks (e.g., see reviews by Jackson, 1991; Jackson and Tauxe, 1991; Rochette et al., 1992; Tarling and Hrouda, 1993; Borradaile and Henry, 1997; Borradaile, 2001; Borradaile and Jackson, 2004; Tauxe, 2005; Lanza and Meloni, 2006; Hrouda, 2007). Presently, the study of the magnetic anisotropy of rocks is still one of the most promising research issues in the field of rock magnetism.

Magnetic properties of sedimentary greigite (Fe3S4): an update

Fri, 28 Jan 2011 23:00:00 GMT

Title: Magnetic properties of sedimentary greigite (Fe3S4): an update Authors: Roberts, A. P.; National Oceanography Centre, Southampton, University of Southampton, Southampton, UK and Now at Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia; Chang, L.; National Oceanography Centre, Southampton, University of Southampton, Southampton, UK; Rowan, C. J.; National Oceanography Centre, Southampton, University of Southampton, Southampton, UK and Now at School of GeoSciences, University of Edinburgh, Edinburgh, UK; Horng, C.‐S.; Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan; Florindo, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia Abstract: Greigite (Fe3S4) is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction. It can also grow at any time when dissolved iron and sulfide are available during diagenesis. Greigite is important in paleomagnetic, environmental, biological, biogeochemical, tectonic, and industrial processes. Much recent progress has been made in understanding its magnetic properties. Greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites. The crystallographic c axis is the easy axis of magnetization, with magnetic properties dominated by magnetocrystalline anisotropy. Robust empirical estimates of the saturation magnetization, anisotropy constant, and exchange constant for greigite have been obtained recently for the first time, and the first robust estimate of the low‐field magnetic susceptibility is reported here. The Curie temperature of greigite remains unknown but must exceed 350°C. Greigite lacks a low‐temperature magnetic transition. On the basis of preliminary micromagnetic modeling, the size range for stable single domain behavior is 17–200 nm for cubic crystals and 17–500 nm for octahedral crystals. Gradual variation in magnetic properties is observed through the pseudo‐single‐domain size range. We systematically document the known magnetic properties of greigite (at high, ambient, and low temperatures and with alternating and direct fields) and illustrate how grain size variations affect magnetic properties. Recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite, which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records, including widespread remagnetizations.