GP33E-01
Constraints on the Acquisition of Magnetization in Fine-Grained Sediments Derived from Redeposition Experiments
Despite many attempts, the transfer function between the remanent magnetization of sediments measured in laboratory and the characteristics of the geomagnetic field coeval with the magnetization remains poorly documented. We performed redeposition experiments using carbonate-rich (ODP site 851) and clay-rich (ODP site 854) sediments. The originality of our approach was to use a dilute solution of gelatin which gels below 20°C, thereby allowing to measure the amount of magnetization acquired at different water concentrations which is linked to mechanical blocking of the magnetic grains. We observed two critical results: -1) the efficiency of DRM decreases with increasing sediment concentration (c) and is zero when it reaches 50%. -2) post-depositional remanent magnetization (pDRM) is important below a concentration of 50%. We compared the original NRM/ARM values of natural magnetization with those obtained during the redeposition experiments. The intersection of the NRM/ARM values measured in the original sediment with the value of redeposition was obtained for a concentration of 44 percent at site 851. We infer that the DRM was acquired at this site for 44 percent concentration of sediment and that pDRM was significant within the depth interval going from 44 to 56 percent of sediment. If the concentration profile for the uppermost sediment were known at site 851, we could define the filter function. In contrast, almost all magnetization was "locked-in" with a concentration of 50 percent at site 854 which is dominated by clay-rich sediment. Finally, the dependence of DRM efficiency on c suggests that changes in the thickness of the sediment layer which is homogenized could change DRM efficiency.
GP33E-02 INVITED
Influence of the Earth's Magnetic Field on Ellipsoidal Particle Alignment in Viscous Media
We present a model that describes the rotation of ellipsoidal magnetic particles in a viscous fluid under the influence of hydrodynamic and magnetic forces, with an aim to better understand how sediments acquire their remanent magnetizations. Analyses of the governing equations elucidate how magnetic particles rotate for different values of leading parameters including particle shape, remanent and induced magnetic intensity, magnetic field intensity and direction, strain rate, shear direction, and viscosity. Numerical solution of the governing equations makes it possible to visualize the rotation path and the magnetic direction of a particle through time. Thus, the model can discern the time scales and trajectories of magnetic particles rotating due to torque of the magnetic field couple while simultaneously entrained in a velocity gradient. When applied to laboratory experiments, the viscous model successfully matches the observed data, particularly after accounting for mechanical interaction and flocculation effects. Magnetic anisotropies calculated from multiparticle systems of hematite yield typical sedimentary fabrics with relatively low percentages of anisotropy (<5%) and maximum principal axes that lie in the sedimentation plane.
GP33E-03 INVITED
The effect of flocculation on DRM direction and intensity
Sedimentary rock records, being relatively more continuous than igneous records, are indispensable for studying past geomagnetic field variations. Sedimentary paleointensity studies rest upon the basic assumption that detrital remanent magnetization (DRM) is linearly related to the ambient field in which sediments acquired magnetization and paleosecular variation studies assume that the magnetization is parallel to the geomagnetic field. Yet, DRM is complicated. Factors other than the magnetic field such as particle size, shape, nature of sediments, hydrodynamic forces etc all affect how sediments get magnetized. In light of the many factors affecting DRM the key assumptions of linearity and parallelism are open to question. Inclination error has been known since the very early days of sedimentary paleomagnetism, yet the mechanisms that control it are poorly understood. Moreover, it has recently been suggested that even linearity may not always hold true. One of the most important control on DRM is now thought to be flocculation. Its role in controlling DRM magnitude has been explored but its influence on the directional properties of DRM is virtually unknown. In a series of laboratory experiments in different field and flocculation states we have confirmed the strong non-linearity of certain sedimentary systems. In addition, we have discovered a marked dependence of inclination error with the field strength and flocculation state. Our findings have serious implications for paleosecular variation studies using sediments, in particular from low salinity environments, such as fresh water lakes in which very small changes in salinity could result in very large changes in flocculation state and DRM acquisition.
GP33E-04
Synthetic clay-magnetite aggregates designed for controlled deposition experiments
The behavior of magnetic particles in fluid environments is key to the acquisition of detrital remanence magnetization and is essential to a multitude of industrial applications. This study introduces a series of synthetic clay-magnetite aggregates whose physical attributes can be tailored for controlled depositional experiments. We describe the mineralogical structure and magnetic behavior of montmorillonite platelets coated with nanometer-scale magnetite crystals using both electron microscopy and rock magnetism techniques. Selected area electron diffraction of the magnetite and the montmorillonite host shows no evidence of preferred orientation or oriented aggregation. Grain size distributions of magnetite in three different clay-magnetite assemblages were directly measured using conventional bright-field transmission electron microscopy. The spacing of the magnetite grains and their three-dimensional distribution around individual clay platelets was imaged using a tomographic reconstruction generated from high-angle annular dark-field (HAADF) images. The grain size distributions determined from the bright-field images and the tomographic reconstruction agree within error with estimates derived from magnetic granulometry techniques based on magnetic hysteresis and low-field susceptibility measurements. All three samples behave superparamagnetically at room temperature, and display increasing levels of single domain behavior as the samples are cooled to liquid nitrogen temperatures (- 195°C). Off-axis electron holography images show that superparamagnetic grains are also stabilized into flux closure structures at -195°C. The average spacing between adjacent magnetite crystals and the overall platelet shape of the aggregates creates an anisotropy of magnetic susceptibility that allows assemblages to align with external magnetic fields at room temperature. By adjusting the dimensions and concentrations of the magnetite grains in these aggregates, we can create well-characterized materials with known grain size distributions that are ideally suited for controlled depositional experiments.
GP33E-05
Sedimentary paleointensity records by the suspension method from Pacific deep-sea sediments
We applied the suspension method (Yoshida and Katsura, 1985) for grayish calcareous ooze collected from the equatorial western Pacific and obtained relative paleointensity records. Although sedimentary paleointensity data have been accumulated, the acquisition processes of detrital remanence are still not fully understood and intensity normalizers such as ARM or IRM have been empirically chosen for particular sediment cores. The suspension method can give a concentration of magnetic material that is free from interaction effects and called complete alignment magnetization (CAM), and the distribution of grain size is also obtained. The estimated grain size is about 70 nm, which is of the single-domain magnetite and consistent to the TEM observation. The normalized NRM intensities by CAM show quite similar patterns with those normalized by ARM and IRM, and concordant with other coeval records from the western Pacific. The suspension method can be served as a reliable and promising approach in sedimentary paleointensity studies.
GP33E-06
Possible overcompensation of non-interacting biogenic component by normalization with ARM in sedimentary paleointensity estimations
In relative paleointensity studies, it has sometimes been reported that a significant coherence between the normalized intensity and the normalizer occurs when ARM is used as the normalizer, whereas this does not occur when SIRM is used. I conducted a rock magnetic study of deep-sea sediments from the Pacific Ocean to clarify the cause of the coherence. Characterization of magnetic grains in the sediments was carried out using first- order reversal curve (FORC) diagrams and iso-thermal remanent magnetization (IRM) acquisition curves. The FORC diagrams revealed that the magnetic grains consist mainly of a non-interacting single-domain (SD) component and an interacting SD component, and additionally of a multi-domain (MD) component. Relative abundance of these components were estimated semi-quantitatively by curve fitting of cross-sections along a line parallel to the axis of local interaction fields (Hu) and passes through a peak in the coercivity (Hc) with three components assuming a Gaussian distribution. The IRM acquisition curves could be described by two dominant components assuming a log-Gaussian distribution. The mean coercivities of the two components are ~40 and ~100 mT, respectively. It is estimated that the former component is carried by biogenic magnetite, and roughly corresponds to the non-interacting SD component derived from the FORC diagrams, and the latter is carried by eolian maghemite, corresponding to the interacting SD component. The ratio of anhysteretic remanent magnetization (ARM) to saturation IRM (SIRM) decreases with an increase in the proportion of the interacting SD component. This implies that the ARM/SIRM ratio is significantly affected by magnetostatic interactions, and does not necessarily reflect magnetic grain size although it is often used as such a proxy. In relative paleointensity estimations, ARM is often used as a normalizer to correct for the difference in magnetizability of sediments. The effect of magnetostatic interaction on detrital remanent magnetization (DRM) acquisition is not well understood, but if it is similar to that on SIRM, normalization by ARM may overcompensate the non-interacting SD component, and cause a significant coherence between the normalized intensity and the normalizer.
GP33E-07
Observations on the Sediment Magnetic Acquisition Process from Multicores Collected From the SE Alaska Margin
Understanding of the magnetic acquisition processes of marine sediments has been hindered by a lack of fundamental observation from natural settings. Here we present new paleomagnetic observations from a variety of sedimentary environments that characterize the SE Alaska Margin. Twenty-five multicores collected during the R/V Ewing cruise EW0408 were studied. These sites range from biogenically dominated, organic carbon-rich sediments from coastal environments along the Alaska Panhandle to glacimarine and paraglacial coastal and open marine settings with high rates of terrigenous sediment accumulation. Paleomagnetic and environmental magnetic records were studied by progressive AF demagnetization of u-channel samples emplaced in split multicores that were sub-sampled around the u-channel to prevent deformation. The sediments were characterized using x-radiography, physical grain-size, density, organic carbon and biogenic silica analyses. Biogenic mixing rates and depths were determined by excess Th-234 (t1/2=24 d) verified with x-radiographs and accumulation rates were established by Pb-210 constrained by bomb-derived Cs-137 activity. Initial age models were developed and used to compare the component magnetizations with Jackson's historical field model locally constrained by the Sitka Geomagnetic Observatory. In general, there is a strong, significant correlation between the organic carbon and biogenic silica concentrations and biological mixing parameters, indicating that bioturbation intensity and depth is determined by the labile carbon flux. There is an inverse correlation between bioturbation intensity and the NRM efficiency, measured by the normalized remanence values, and quality, measured by comparisons with historical data and PCA analysis of the demagnetization data. Depth to lock-in, as determined by inclinations reaching historically consistent values of approximately 70° and stability of the normalized remanence values, varies widely and is also generally a function of the organic carbon content. In terrigenous-rich, low biogenic environments the magnetization lock-in zone is generally less than 10-cm and in some cases too thin to be detectable. In the biogenically dominated environments, lock-in occurs substantially later, implying that the thickness of the lock-in zone is a function of the thickness of the bioturbated layer.
GP33E-08
At what depth is a post-depositional remanent magnetization locked-in?
Despite the fact that sedimentary magnetizations have been widely analysed in paleomagnetic studies over the last 60 years, considerable debate remains concerning the mechanism(s) by which sediments become magnetized. Bioturbation disturbs detrital magnetic particles after deposition, so accurate recording of the ancient geomagnetic field in bioturbated sediments is widely attributed to acquisition of a post-depositional remanent magnetization (pDRM). A pDRM is acquired when the geomagnetic field exerts a torque on a magnetic particle and aligns it with the field after the final mixing event experienced by the particle. The relationship between the Matuyama-Brunhes (M-B) boundary and oxygen isotope age tie points has been widely used to determine the pDRM lock-in depth in marine sediments. However, such analyses can be badly affected by phase discrepancies among different paleoclimatic proxies and by varying isotopic compositions of seawater in different locations and from the presence of different water masses at different depths at the same location. It is therefore necessary to separately compare benthic and planktonic oxygen isotope records for sites from the same water mass to avoid inadvertently introducing age differences to the analysis. When the global data set is subjected to such a rigorous analysis, few reliable data remain for the M-B boundary. Correlation of two benthic oxygen isotope records from ODP sites 982 and 983 (northeast Atlantic), and two planktonic oxygen isotope records from sites V28-238 and V28-239 (western equatorial Pacific), provides lock-in depth estimations for these two regions of about 23 and 21 cm, respectively. The reliability of the V28 data has been questioned, which leaves a small and poorly constrained global data set. Systematic acquisition of high quality data is needed to resolve this problem. In addition to this empirical analysis, we have analytically evaluated a range of commonly used lock-in functions to better understand complexities associated with pDRM lock-in. This analysis indicates that often-used lock-in functions will not only serve as a filter that smoothes the input geomagnetic signal, but that these functions also complicate recording by downward shifting geomagnetic signals of different wavelengths to different depths, yielding an asymmetric recording. By comparing magnetic records from ODP sites 982 and 983, the downward displacement of the M-B boundary due to the lock-in process is estimated to be about 5 cm. Therefore, we conclude that the total displacement (about 23 cm) of the M-B boundary at these two sites is due to the summation of a thick surficial mixing layer and a 5 cm lock-in depth. Our work provides a practical means of decomposing the downward displacements of magnetic signals due to different processes.