Rock Magnetism
Analytics and petrology of rock magnetism
Scientists: Thomas Frederichs, Hendrik Müller, Janna Just, Christine Franke, Tilo von Dobeneck
Methods. Magnetic Remanence, Hysteresis and Susceptibility, Thermomagnetics
Projects: DFG Projekte BL 154/24, Do 705/2, IODP, RCOM C1, EUROPROX

Topics: All magnetic methods in geophysics rely on less than ten natural iron minerals, which all have complex and characteristic ferromagnetic properties. We command over a range of highly sensitive automated instruments to determine composition, concentration, grain-size and oxidation state of magnetic mineral assemblages in sediments and rocks.

Coupled compass needles are a simplified analogy of atomic exchange interaction

Magnetic petrology of the Equatorial Atlantic Ocean
Scientists: Christine Franke, Melanie Dillon, Tilo von Dobeneck, Ulrich Bleil
Projects: RCOM project C1, EUROPROX project 7

Summary: The magnetic micro- and nanoparticle inventories of oxic sediments from the equatorial Atlantic were investigated by electron microscopy to classify all present detrital and authigenic magnetic mineral species and to investigate their regional distribution, origin, transport, and preservation with the objective to establish source-to-sink relations and to constrain environmental magnetic proxy interpretations. Crystallographic, elemental, morphological, and granulometric data of more than 2000 magnetic particles were collected enabling us to define nine particle classes: (1) detrital magnetite, (2) titanomagnetite (fragmental and euhedral), (3) titanomagnetite-hemoilmentite intergrowths, (4) silicates with magnetic inclusions, (5) microcrystalline hematite, (6) magnetite spherules, (7) bacterial magnetite, (8) goethite needles, and (9) nanoparticle clusters. Each class was associated with fluvial, eolian, subaeric, and submarine volcanic, biogenic, or chemogenic sources. Large-scale sedimentation patterns were also delineated as well: Detrital magnetite is typical of Amazon discharge, fragmental titanomagnetite is a submarine weathering product of mid-ocean ridge basalts, and titanomagnetite-hemoilmenite intergrowths are common magnetic particles in West African dust. This clear regionalization underlines that magnetic petrology is an excellent indicator of source-to-sink relations. Hematite encrustations, magnetic spherules, and nanoparticle clusters were found at all investigated sites, while bacterial magnetite and authigenic hematite were only detected at the more oxic western site. At the eastern site, surface pits and crevices were seen on the crystal faces indicating subtle early diagenetic reductive dissolution. It was observed that paleoclimatic signatures of magnetogranulometric parameters such as the ratio of anhysteretic and isothermal remanent magnetizations can be formed either by mixing of multiple sources with separate, relatively narrow grain size ranges (western site) or by variable sorting of a single source with a broad grain size distribution (eastern site). Hematite, goethite, and possibly ferrihydrite nanoparticles occur in all sediment cores studied and have either high-coercive or superparamagnetic properties depending on their partly ultrafine grain sizes.
Selected Publications:
Franke C., von Dobeneck T., Drury M., Meeldijk J., Dekkers M. (2007)
Magnetic petrology of equatorial Atlantic sediments: Electron microscopy results and their implications for environmental magnetic interpretation
Paleoceanography, 22, PA4207
doi: 10.1029/2007PA001442

Franke C., Pennock G., Drury M., Engelmann R., Lattard D., Garming J., von Dobeneck T., Dekkers M. (2007)
Identification of magnetic Fe–Ti oxides in marine sediments by electron backscatter diffraction in scanning electron microscopy
Geophysical Journal International, 170, 545 - 555
doi: 10.1111/j.1365-246X.2007.03410.x

Funk J., von Dobeneck T., Wagner T., Kasten S. (2004)
Late Quaternary Sedimentation and Early Diagenesis in the Equatorial Atlantic Ocean: Patterns, Trends and Processes Deduced from Rock Magnetic and Geochemical Records
In: The South Atlantic in the Late Quaternary, Edited by Wefer G., Mulitza S., Ratmeyer V., Springer Berlin Heidelberg, 461-497
doi: 10.1007/978-3-642-18917-3_21

Magnetic minerals with low ordering temperatures
Scientists: Thomas Frederichs, Andrei Kosterov, Tilo von Dobeneck
Projects: DFG project BL 154/24-1, 24-2, AvH stipend A. Kosterov

Summary: in preparation (Tilo + Thomas)
Selected Publications:
Frederichs T., von Dobeneck T., Bleil U., Dekkers M. (2003)
Towards the identification of siderite, rhodochrosite, and vivianite in sediments by their low-temperature magnetic properties
Physics and Chemistry of the Earth, Parts A/B/C, 28, 669 - 679
doi: 10.1016/S1474-7065(03)00121-9

Kosterov A., Frederichs T., von Dobeneck T. (2006)
Low-temperature magnetic properties of rhodochrosite (MnCO3)
Physics of the Earth and Planetary Interiors, 154, 234 - 242
doi: 10.1016/j.pepi.2005.09.011

Formation of greigite layers in hemipelagic sediments
Scientists: Yanzhe Fu, Christine Franke, Tilo von Dobeneck
Projects: IODP Leg 308, DFG project Do 705/2-1

Summary: in preparation (Tilo)
Selected Publications:
Fu Y., von Dobeneck T., Franke C., Heslop D., Kasten S. (2008)
Rock magnetic identification and geochemical process models of greigite formation in Quaternary marine sediments from the Gulf of Mexico (IODP Hole U1319A)
Earth and Planetary Science Letters, 275, 233 - 245
doi: 10.1016/j.epsl.2008.07.034

Vasiliev I., Franke C., Meeldijk J., Dekkers M., Langereis C., Krijgsman W. (2008)
Putative greigite magnetofossils from the Pliocene epoch
Nature Geoscience, 1, 782 - 786
doi: 10.1038/ngeo335

Marum

Intercoast

Intercoast

Intercoast

IODP

MarTech

AWI

British Antarctic

British Antarctic

British Antarctic

British Antarctic

British Antarctic

British Antarctic

Stud.IP

University of Bremen Research group Marine Geophysics
Faculty of Geosciences | FB5