title: Paleomagnetic and paleoenvironmental implications of magnetofossil occurrences in late Miocene marine sediments from the Guadalquivir Basin, SW Spain.
abstract: Although recent studies have revealed more widespread occurrences of magnetofossils in pre-Quaternary sediments than have been previously reported, their significance for paleomagnetic and paleoenvironmental studies is not fully understood. We present a paleo- and rock-magnetic study of late Miocene marine sediments recovered from the Guadalquivir Basin (SW Spain). Well-defined paleomagnetic directions provide a robust magnetostratigraphic chronology for the two studied sediment cores. Rock magnetic results indicate the dominance of intact magnetosome chains throughout the studied sediments. These results provide a link between the highest-quality paleomagnetic directions and higher magnetofossil abundances. We interpret that bacterial magnetite formed in the surface sediment mixed layer and that these magnetic particles gave rise to a paleomagnetic signal in the same way as detrital grains. They, therefore, carry a magnetization that is essentially identical to a post-depositional remanent magnetization, which we term a bio-depositional remanent magnetization. Some studied polarity reversals record paleomagnetic directions with an apparent 60-70 kyr recording delay. Magnetofossils in these cases are interpreted to carry a biogeochemical remanent magnetization that is locked in at greater depth in the sediment column. A sharp decrease in magnetofossil abundance toward the middle of the studied boreholes coincides broadly with a major rise in sediment accumulation rates near the onset of the Messinian salinity crisis (MSC), an event caused by interruption of the connection between the Mediterranean Sea and the Atlantic Ocean. This correlation appears to have resulted from dilution of magnetofossils by enhanced terrigenous inputs that were driven, in turn, by sedimentary changes triggered in the basin at the onset of the MSC. Our results highlight the importance of magnetofossils as carriers of high-quality paleomagnetic and paleoenvironmental signals even in dominantly terrigenous sediments.
authors.affiliation: Instituto Geológico y Minero de España Madrid, Spain ; Institute of Earth Sciences Jaume Almera, CSIC Barcelona, Spain.
authors.forename: Juan C
authors.initials: JC
authors.lastname: Larrasoaña
authors.affiliation: State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences Beijing, China.
authors.forename: Qingsong
authors.initials: Q
authors.lastname: Liu
authors.affiliation: State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences Beijing, China.
authors.forename: Pengxiang
authors.initials: P
authors.lastname: Hu
authors.affiliation: Research School of Earth Sciences, The Australian National University Canberra, Australia.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Roberts
authors.affiliation: Instituto Geológico y Minero de España Madrid, Spain.
authors.forename: Pilar
authors.initials: P
authors.lastname: Mata
authors.affiliation: Instituto Geológico y Minero de España Madrid, Spain.
authors.forename: Jorge
authors.initials: J
authors.lastname: Civis
authors.affiliation: Departamento de Geologïa, Universidad de Salamanca Salamanca, Spain.
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: Sierro
authors.affiliation: Department of Earth Sciences, University of Geneva Geneva, Switzerland.
authors.forename: José N
authors.initials: JN
authors.lastname: Pérez-Asensio
keywords: Guadalquivir Basin; Messinian salinity crisis; late Miocene; magnetotactic bacteria; marine sediments; rock magnetism