Topics: The EarthÂ´s geodynamo and its product, the geomagnetic field, are some of the most dynamic features of EarthÂ´s history. Geomagnetic pole positions, field intensities and polarities vary significantly at time scales between 100 and 100.000.000 years. Past magnetic field conditions have been recorded by cooling (mostly magmatic) rocks and consolidating sediments. This natural remanent magnetization forms the base of three paleomagnetic age dating methods - paleosecular variation, relative paleointensity and magnetostratigraphy - which enable us to date sediment sequences of continental margins as well as deep sea basins.
Geomagnetic field reversals in the history of human evolution
Eoceneâ€“Oligocene chronology of the SW Pacific Scientists:Edoardo Dallanave, Tilo von Dobeneck Projects: DFG Da 1757/2-1, IODP Exp 371 (Tasman Frontier Subduction Initiation and Paleogene Climate)
Summary: During the middle Eocene, the southwest Pacific was affected by widespread convergent deformation, reverse faulting and uplift, followed by major late Eoceneâ€“Oligocene subsidence. This event coincides with the onset of the Tonga-Kermadec subduction initiation, the developing of which may have significantly influenced the global climate trends. This is because a Pacific Plate westward-dominated subduction beneath oceanic crust, instead of subduction only occurring beneath the continental crust of the American cordillera, would result in a significant drop of CO2 volcanic degassing, modulating the shift from the early Eocene global greenhouse condition to the ensuing middleâ€“late Eocene cooling. The first objective of IODP Exp. 371 was constraining in time this large-scale tectonic event. Preliminary data from six sites from the Tasman area show that the drilled sediments can pin in time the complex tectonic evolution of the area. Integration of this magnetic dataset with magnetostratigraphic records from coeval records exposed in New Zealand and New Caledonia will give a complete frame of the paleogeographic evolution of northern Zealandia.
East African and Indian Ocean magnetostratigraphic studies Scientists: Janna Just Projects: IODP Exp 361 (Southern African Climates), DFG SFB 806
Summary: Paleomagnetic time-equivalent markers are of integral importance for the construction of age models for sedimentary archives. Reconstructions of the relative paleointensity of the EarthÂ´s Magnetic Field are widely used for synchronizing sediment cores, throughout the last 1.5 Ma. Longer records of the strength of the ancient EarthÂ´s Magnetic Field are rare. Sediment cores ranging back to the Pliocene have come available from IODP expedition 361 from the SE Indian Ocean. As the cores were retrieved from oligotrophic ocean basins, organic carbon content is very low, which is why magnetic minerals in the sediment cores have only marginally suffered from reductive early diagenesis, and the primary paleomagnetic signal is preserved. In this project the shipboard polarity magnetostratigraphies of IODP expedition 361 are being refined. Moreover, suitable marine and continental archives are utilized to produce (long) records of the relative paleointensity of the EarthÂ´s magnetic field through the past 4 Ma.
Summary: "Relative paleointensity" (RPI) is a relatively new high-resolution magnetostratigraphic technique tying calibrated sedimentary NRM intensity records to past fluctuations of geomagnetic dipole strength. The method assumes that the intensity of post depositional remanent magnetization (PDRM) depends exclusively on the geomagnetic field strength and the concentration of the magnetic carriers. Sedimentary remanence is regarded as an equilibrium state between aligning geomagnetic and randomizing interparticle forces. Just how strong these mechanical and electrostatic forces are, depends on many petrophysical factors related to mineralogy, particle size and shape of the matrix constituents.We have therefore systematically tested the reliability of the RPI method in different sediment facies and environments, finding that grain-size, matrix and diagenesis bias effects on RPI are sufficiently significant to warrant their consideration in the calibration procedure.
Summary: High geographic latitudes are highly interesting areas of paleomagnetic research under several aspects.
One of these is dating being an essential prerequisite of almost all paleoeceanographic or paleoclimatic research on marine sediments. Conventional dating tools that are usally applied to long sediment sequences like isotope stratigraphy are often unfeasable due to the lack of calcareous nannofossils or foraminifera in these areas. Thus there is large demand for alternative methods for age determination. Temporal variations of the geomagnetic field as a global signal may serve as an alternative or complementary tool.
Another aspect refers to the proximity of high latitude regions to the magnetic poles. It is still under debate whether the observation that geomagnetic excursions are apparently more frequently recorded in sediments deposited at high latitudes than in those from low latitudes is an artifact due to depositional conditions or a true feature of the Earth's magnetic field.