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LarrasoaƱa, Juan-Cruz
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LarrasoaƱa, Juan-Cruz
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LarrasoaƱa, Juan Cruz
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- PublicationRestrictedSearching for single domain magnetite in the āpseudo-single-domainā 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 ;LarrasoaƱa, J. C.; Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia.; ; ; ; Magnetic hysteresis measurements of sediments have resulted in widespread reporting of āpseudo-single-domainā-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 1ā15 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, UniversitĆ 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 ;LarrasoaƱa, J. C.; Ćrea 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 - PublicationRestrictedComplex polarity pattern at the former PlioāPleistocene global stratotype section at Vrica (Italy): Remagnetization by magnetic iron sulphides(2010)
; ; ; ; ; ;Roberts, A. P.; National Oceanography Centre, University of Southampton, European Way, Southampton SO14 3ZH, UK ;Florindo, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;LarrasoaƱa, J. C.; National Oceanography Centre, University of Southampton, European Way, Southampton SO14 3ZH, UK and Area de Cambio Global, Instituto GeolĆ³gico y Minero de EspaƱa, Oļ¬cinade Proyectos de Zaragoza, C/Manuel Lasala 44, 9B, Zaragoza 50006, Spain ;O'Regan, M. A.; National Oceanography Centre, University of Southampton, European Way, Southampton SO14 3ZH, UK and Department of Geology and Geochemistry, Stockholm University, SE-10691 Stockholm, Sweden ;Zhao, X.; National Oceanography Centre, University of Southampton, European Way, Southampton SO14 3ZH, UK; ; ; ; The Vrica section in Calabria, southern Italy, was the global stratotype for the PlioceneāPleistocene boundary until this boundary was redeļ¬ned in 2009. Several paleomagnetic investigations have been carried out at Vrica to determine the age of the formerly deļ¬ned PlioceneāPleistocene boundary, which was a key calibration point for the astronomical polarity timescale(APTS). Each study has documented a complex polarity pattern at and above the top of the Olduvai subchron and in relation to the existence of the so-called Vrica subchron. When constructing the APTS, two alternative interpretations for the Vrica section were proposed,neither of which could be conclusively supported. Authigenic growth of magnetic iron sulphide minerals was proposed to explain the complex magnetic polarity record. Availability of a fresh 50-m sediment core enabled us to test this possibility. Our magnetostratigraphic record is similar to that of previous studies, but it is also complex above the Olduvai subchron. We conļ¬rm abundant occurrences of authigenic greigite and pyrrhotite, along with detrital titanomagnetite. Authigenic monoclinic pyrrhotite indicates growth signiļ¬cantly later than deposition, and greigite can grow at any time during diagenesis, depending on the availability of dissolved iron and sulphide. The spatially variable magnetic polarity pattern at Vrica is therefore interpreted to have resulted from post-depositional magnetic iron sulphide formation at variable times. Tectonism along the Calabrian arc provides a plausible mechanism for forcing reducing ļ¬uids through the sediments, thereby supplying the dissolved ions needed to produce late diagenetic sulphide growth and remagnetization. The complex magnetostratigraphic record at Vrica was taken into account when the APTS was developed, and alternative interpretations result in a maximum age difference of 50 kyr for the upper Olduvai reversal. Our results therefore do not undermine the APTS. Rather, they explain the complex magnetic polarity pattern at this globally important location and highlight the importance of remagnetization processes in such sediments.171 27 - PublicationOpen AccessSignatures of Reductive Magnetic Mineral Diagenesis From Unmixing of First-Order Reversal CurvesDiagenetic alteration of magnetic minerals occurs in all sedimentary environments and tends to be severe in reducing environments. Magnetic minerals provide useful information about sedimentary diagenetic processes, which makes it valuable to use magnetic properties to identify the diagenetic environment in which the magnetic minerals occur and to inform interpretations of paleomagnetic recording or environmental processes. We use a newly developed first-order reversal curve unmixing method on well-studied samples to illustrate how magnetic properties can be used to assess diagenetic processes in reducing sedimentary environments. From our analysis of multiple data sets, consistent magnetic components are identified for each stage of reductive diagenesis. Relatively unaltered detrital and biogenic magnetic mineral assemblages in surficial oxic to manganous diagenetic environments undergo progressive dissolution with burial into ferruginous and sulfidic environments and largely disappear at the sulfate-methane transition. Below the sulfate-methane transition, a weak superparamagnetic to largely noninteracting stable single domain (SD) greigite component is observed in all studied data sets. Moderately interacting stable SD authigenic pyrrhotite and strongly interacting stable SD greigite are observed commonly in methanic environments. Recognition of these characteristic magnetic components enables identification of diagenetic processes and should help to constrain interpretation of magnetic mineral assemblages in future studies. A key question for future studies concerns whether stable SD greigite forms in the sulfidic or methanic zones, where formation in deeper methanic sediments will cause greater delays in paleomagnetic signal recording. Authigenic pyrrhotite forms in methanic environments, so it will usually record a delayed paleomagnetic signal.
64 44 - PublicationRestrictedThe Global Stratotype Section and Point (GSSP) for the base of the Lutetian Stage at the Gorrondatxe section, Spain(2011-06)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;Molina, E.; Departamento de Ciencias de la Tierra, Universidad de Zaragoza, E-50009 Zaragoza, Spain ;Alegret, L.; Departamento de Ciencias de la Tierra, Universidad de Zaragoza, E-50009 Zaragoza, Spain ;Apellaniz, E.; Departamento de EstratigrafĆa y PaleontologĆa, Universidad del PaĆs Vasco, E-48080 Bilbao, Spain ;Bernaola, G.; Departamento de EstratigrafĆa y PaleontologĆa, Universidad del PaĆs Vasco, E-48080 Bilbao, Spain ;Caballero, F.; Departamento de EstratigrafĆa y PaleontologĆa, Universidad del PaĆs Vasco, E-48080 Bilbao, Spain ;DinarĆØs-Turell, J.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Hardenbol, J.; Global Sequence, Chronostratigraphy Inc., 826, Plainwood Drive, Houston, Texas 77079-4227, USA ;Heilmann-Clausen, C.; Department of Earth Sciences, Aarhus University, Dk-8000, Aarhus C, Denmark ;LarrasoaƱa, J. C.; Instituto GeolĆ³gico y Minero de EspaƱa, Unidad de Zaragoza, E-50006, Zaragoza, Spain ;Luterbacher, H.; Museo GeolĆ³gico, Seminario Conciliar, E-08007, Barcelona, Spain ;Monechi, S.; Dipartimento di Scienze della Terra, UniversitĆ di Firenze, I-50121 Firenze, Italy ;Ortiz, S.; Departamento de Ciencias de la Tierra, Universidad de Zaragoza, E-50009 Zaragoza, Spain and Department of Earth Sciences, University College London, WC1E 6BT, London, UK ;Orue- Etxebarria, X.; Departamento de EstratigrafĆa y PaleontologĆa, Universidad del PaĆs Vasco, E-48080 Bilbao, Spain ;Payros, A.; Departamento de EstratigrafĆa y PaleontologĆa, Universidad del PaĆs Vasco, E-48080 Bilbao, Spain ;Pujalte, V.; Departamento de EstratigrafĆa y PaleontologĆa, Universidad del PaĆs Vasco, E-48080 Bilbao, Spain ;RodrĆguez-Tovar, F. J.; Departamento de EstratigrafĆa y PaleontologĆa, Universidad de Granada, E-18002 Granada, Spain ;Tori, F.; Dipartimento di Scienze della Terra, UniversitĆ di Firenze, I-50121 Firenze, Italy ;Tosquella, J.; Departamento de GeodinĆ”mica y PaleontologĆa, Universidad de Huelva, E-21071 Huelva, Spain ;Uchman, A.; Institute of Geological Sciences, Jagiellonian University, KrakĆ³w, Pl-30063, Poland; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; The GSSP for the base of the Lutetian Stage (early/ middle Eocene boundary) is defined at 167.85 metres in the Gorrondatxe sea-cliff section (NW of Bilbao city, Basque Country, northern Spain; 43Āŗ22'46.47" N, 3Āŗ 00' 51.61" W). This dark marly level coincides with the lowest occurrence of the calcareous nannofossil Blackites inflatus (CP12a/b boundary), is in the middle of polarity Chron C21r, and has been interpreted as the maximumflooding surface of a depositional sequence that may be global in extent. The GSSP age is approximately 800 kyr (39 precession cycles) younger than the beginning of polarity Chron C21r, or ~47.8 Ma in the GTS04 time scale. The proposal was approved by the International Subcommission on Paleogene Stratigraphy in February 2010, approved by the International Commission of Stratigraphy in January 2011, and ratified by the International Union of Geological Sciences in April 2011.227 32 - 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 FĆsica, Instituto OceanogrĆ”fico, Universidade de SĆ£o Paulo, PraƧa do OceanogrĆ”fico, 191, 05508-120 SĆ£o Paulo, Brazil ;LarrasoaƱa, J. C.; Instituto GeolĆ³gico y Minero de EspaƱa, Unidad de Zaragoza, C/Manuel Lasala 44, 9B, Zaragoza 50006, Spain; ; ; ; ; Pelagic carbonates are deposited far from continents, usually at water depths of 3000ā6000 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