Dynamos, Reversals, and Much More Posters
Presiding: L Brown, University of Massachusetts; K Schwehr, Scripps Institution of Oceanography
GP13B-01 1330h
Ensemble Numerical Dynamo Simulation on Independent clusters
Geodynamo simulation is very expensive computationally, a single numerical test could require several days of computing time on a mid-level PC clusters. More computing resource is required if higher resolutions are used in numerical modeling. In geomagnetic data assimilation, an ensemble of such tests is needed for our studies, thus is difficult to be managed by a single computing facility currently available at our disposal. In NASA GSFC, we have developed a paralleled model to simulate the geodynamo. We utilize very fast network and some available middlewares (e.g. Globus kit, XCAM) to allocate available but often heterogeneous resources for our model. The model can run with specific parameters independently on different clusters, ideal for the ensemble simulation. Our system has been tested successfully on a single cluster where the nodes have been configured to represent two independent systems. We are testing and improving our system with a PC cluster and a Mac cluster on different campus, aiming at managing modeling processes on heterogeneous environment.
GP13B-02 1330h
Spatial Resolutions of Numerical Dynamo Solutions: Implications for Model Error in Data Assimilation
We have implemented a 6th-order compact implicit finite difference algorithm in the radial direction in the MoSST core dynamics model (Kuang and Bloxham, 1999; Kuang and Chao, 2003) in order to improve the convergence of the numerical solution. The algorithm is tested on a prototype system similar to that of the geodynamo for various spherical harmonic degrees and parameters. With the improved model we simulate the geodynamo processes with different spatial resolutions, aiming at understanding the time-variation of the model error arising from different spatial resolutions. In particular, treating the high-resolution model solutions as the "true states", and the low-resolution solutions as the "numerical outputs", we seek to understand the effect of inserting knowledge of the "true state" on the model solutions. This analysis can then be used to get an estimate of error growth, which is needed for geomagnetic data assimilation.
GP13B-03 1330h
Ensemble Calculation of Error Covariances in the MoSST Core Dynamics Model
Data assimilation is the methodology by which observations are combined with a model output to get an improved estimate of the state of a system. An optimal estimate can potentially be obtained using Bayesian techniques, provided that good estimates of observation and model error statistics are available. Geomagnetic observations, through the geomagnetic field models, have well understood error characteristics. Geodynamo models, on the other hand, have not yet begun to develop the means to estimate error statistics. One approach to obtain these estimates, currently used in ocean and atmospheric data assimilation, is to carry out an ensemble of model runs that have initial conditions consisting of a converged geodynamo solution plus a random perturbation. The final state of these ensemble runs can then be used to estimate variance, spatial correlation and cross correlations between the different state variables (velocity, magnetic field and temperature). The correlations are especially important because magnetic field observations are made only at the Earth's surface and the depth to which observations need to correct the magnetic field needs to be well defined. Also, the cross correlations can be used to determine how much the observations should correct the velocity and temperature fields. We present the results of an ensemble calculation of background error covariances in a numerical geodynamo model. The covariances are presented in terms of spherical harmonic coefficients, as a function of radial position. Cross correlations are presented between magnetic, velocity and temperature fields. We discuss the implications of these correlations for carrying out geomagnetic data assimilation.
GP13B-04 1330h
A Two-dimensional Version of the Niblett-Bostick Transformation for Magnetotelluric Interpretations
An imaging technique for two-dimensional magnetotelluric interpretations is developed following the well known Niblett-Bostick transformation for one-dimensional profiles. The algorithm uses a Hopfield artificial neural network to process series and parallel magnetotelluric impedances along with their analytical influence functions. The adaptive, weighted average approximation preserves part of the nonlinearity of the original problem. No initial model in the usual sense is required for the recovery of a functional model. Rather, the built-in relationship between model and data considers automatically, all at the same time, many half spaces whose electrical conductivities vary according to the data. The use of series and parallel impedances, a self-contained pair of invariants of the impedance tensor, avoids the need to decide on best angles of rotation for TE and TM separations. Field data from a given profile can thus be fed directly into the algorithm without much processing. The solutions offered by the Hopfield neural network correspond to spatial averages computed through rectangular windows that can be chosen at will. Applications of the algorithm to simple synthetic models and to the COPROD2 data set illustrate the performance of the approximation.
GP13B-05 1330h
Effects of Geomagnetic Disturbances on Socio-economic Development in Nigeria.
The analyses from this work has significantly shown that geomagnetic disturbances adversely and negatively affect socioeconomic development in Nigeria as in other parts of the world. Most frequently discovered are the geomagnetic storms and substorms that commence most often after intense solar flares. It was discovered that these disturbances affect electrical systems which invariably result in catastrophic failure of electrical systems.These include; power outages, pipe line corrosion, telephone systems collapse and biological systems are also affected.It was also discovered that these disturbances affect weather and climate around their environment. This weather/climate disruption result in malfunctions of railway signals, problem of navigation, geological survey and radio communication. It is suggested that more work be carried out on the investigation and prediction of solar flare, hence that of geomagnetic disturbances. In so doing, some of the damages will be prevented, hence the negative adverse effects on socio-economic development will be drastically reduced.
GP13B-06 1330h
Paleomagnetism of Basalt Flows near McKenzie Bridge, Oregon
A section of twelve Quaternary basalt flows near McKenzie Bridge, Oregon was sampled in July 2004 for paleomagnetic analysis. The thick sequence was chosen for study because field surveys suggest it is the proper age to potentially record the Matuyama-Brunhes transition, and field measurements with a fluxgate magnetometer indicated the presence of both normal and reversely magnetized units (Conrey, pers. comm.). At each site, six to ten oriented cores were collected and analyzed to determine the mean magnetic direction and Virtual Geomagnetic Pole (VGP) position. Standard alternating field demagnetization to 999 oe and thermal demagnetization to 680° C were performed to remove secondary magnetization and get clues to magnetic mineralogy. Eleven of the twelve sites yielded VGPs that cluster in Eastern Siberia. The mean inclination and declination of these 11 sites are 59.5° and 328.3° with a95 = 2.95° . The remaining flow yielded radically different directions from the other sites (D = 278.8°, I = 8.7° ) and a VGP just north of Australia (lat. = 9.4° S long. = 145.0° ). Further investigation of this site needs to be done in order to ensure that the outcrop is in place and yielding a reliable magnetic direction. Changes in intensity during thermal demagnetizations suggest titano-magnetite as the main carrier of magnetic signal. SEM and optical examination will be used to confirm magnetic mineralogy. Assuming a Geocentric Axial Dipole (GAD) field, the expected mean direction for the McKenzie Bridge area is D = 0°, I = 63.4° . The difference in the expected and mean directions for the 11 sites can be explained in several ways. The area may have undergone counterclockwise rotation since these lavas were erupted, or the units sampled may not represent a long enough time period to adequately average paleosecular variation. The low-latitude VGP of the other site suggests an intriguing possibility; that the section does record a reversal. The VGP for this site and VGPs for the other 11 sites fall along one of the preferred longitudinal VGP paths for reversals suggested by several researchers (eg. Love, 1998) and in Hoffman's (1996) patches. Paleomagnetic analyses of 12 flows sitting atop the section studied here reveal similar Siberian VGP locations (LeValley and Valentine, this volume). Further study of flows lying below both of these should clarify the cause of the directional anomaly.
GP13B-07 1330h
Paleomagnetism in Central Oregon Cascade Basalts
Twelve Pleistocene-aged basalt flows near McKenzie Bridge in Central Oregon were sampled and the paleomagnetic stratigraphy recorded in the rocks analyzed. The purpose of the study is to examine tectonic crustal motions since eruption of these flows, paleosecular variation during their eruption, and/or a record of a field reversal. The lavas were chosen because they appear to be the right age to record the Matuyama-Brunhes reversal and record both normal and reverse polarity fields (Conrey, pers comm). Four to six samples from each flow were analyzed using stepwise alternating field demagnetization, effecting a neutralization of any secondary magnetism the rock had recorded since its formation. Two samples from each site were heated and demagnetized in a rock oven to give clues to the magnetic mineralogy of the basalt as well as giving another look at the primary thermal remanent magnetization. Thermal demagnetization data suggests the primary source of magnetism is from titanomagnetite. Samples will be analyzed by scanning electron microscope and reflected light microscopy to determine precise magnetic mineralogy. The mean site direction from all twelve flows shows a field with D = 327.1°, I = 59.8°, and a95 = 5.36°. The Virtual Geomagnetic Pole (VGP) from this data lies in Siberia (long. 151.8°, lat. 65.5° N. The expected mean direction for the McKenzie Bridge area, assuming that all measurements of the field over time will eventually result in a magnetic pole resting at the geographical north pole, would have a declination of 0° and an inclination of 63.4°. Several possibilities could account for the declination difference and low-latitude VGP. The first is that the flows sampled do not represent a long enough period of time for the true average geomagnetic field to be measured, and the paleopole was at the resultant VGP position during the eruption of the flows. The second explanation is that the McKenzie Bridge area basalts may have been rotated counter-clockwise since their formation. Finally, the section may record a portion of a reversal. The Siberian location of the VGPs lies along a suggested longitudinal preferred path for poles during reversals (eg. Love, 1998) and in one of Hoffman's (1996) patches. A concurrent study of underlying basalt flows produced similar results (White and Valentine, this volume). Further study of the rest of this section should help determine the cause of the directional anomaly.
GP13B-08 1330h
Evidence From Reversals and Excursions for a Role of Precession in the Geodynamo.
Three aspects of geomagnetic field behavior suggest that the geodynamo is powered at least in part by lunisolar precession. First, excursions, or events, with field intensity lows during the last 500 kyrs, correlate with minima in the obliquity signal. Second, in the past 5 Myrs reversals occur preferentially when the amplitude of the obliquity signal is low. Third, the occurrence of the last reversal is located near to the minimum of the obliquity cycle in which it occurs. Despite earlier rejection of precession powered dynamos, discussions of dynamos powered by lunisolar precession of the mantle and core have been revitalized by the demonstration that (1) the available energy is more than sufficient and (2) the nature of the induced flows are likely to give field regeneration. The first order flow shows the core rotating as a nearly rigid sphere lagging the mantle in precession by a small angle. This gives a westward drift with respect to the mantle, consistent with the westward drift observed in secular variation. The lag generates relative velocities and displacements over the core mantle boundary as second order flows, which can through the coalescence of small vortices give patterns similar to flow patterns calculated near to the core mantle boundary. A third order flow of nested cylinders concentric to the core axis was also seen. Alternate cylinders move slowly prograde, or retrograde and alternate northward and southward internal flow. These geometries generated by precession are reminiscent of convective cells in a rotating liquid core because like them, they are dominated by the Coriolis force. The observation that the third order flow in the nested cylinders can stop and restart with the same, or a reverse sense, suggests a possible field reversal mechanism. Since transitional VGPs have preferred paths that appear to be related to regions with colder mantle above the core and these same regions are sources of persistent main field features, there must also be some role for convection. Thus the dynamo is probably driven by precessional flows that are modulated by convection and Coriolis dominated.
GP13B-09 1330h
A Sedimentary Record of the Lower Jaramillo Transition Zone
Biostratigraphically dated Apsheronian sediments were sampled from two parallel sections near Adjidhere, NW Turkmenistan. Demagnetisation experiments reveal well defined normal and reversed magnetisations carried by magnetite and haematite. The lower/upper parts of the sections correspond to the Matuyama/Jaramillo chrons, and corresponding mean directions coincide with the local dipole field. The central part of the composite section reveals a remarkable and complex record of the transitional field, a significant feature of which is the clustering of VGPs in the S Atlantic, between the S Pole and southern S America. This feature predominates in the earlier phases of the transition and corresponds to the cluster patch previously identified from volcanic records of different transition zones. Another striking feature is the repetition of excursion patterns, with the VGP paths repeatedly moving along preferred paths, before finally reversing in the central part of the profile. Initialy, the field instability is predominantly reversed in character. After the reversal, however, the record shows instability from a predominantly normal condition. These data will be presented and discussed
GP13B-10 1330h
Early Miocene R-N Reversal Recorded in Lavas From Queensland, Australia: Evidence for Stationary Mantle-held Flux Over the Past 21 Ma
The contention of a long-lived transitional field VGP cluster patch near Australia suggests that mantle-held flux at the core surface below this region may be largely responsible. To shed more light on this claim we have been investigating transitional field records from Australasia, the region closest to such a flux concentration and, hence, likely to be most affected by it during field reversal. Here we present paleodirectional data associated with a late Miocene (about 21 Ma) reverse-to-normal polarity transition recorded in a continuous sequence of some 30 flows in southeastern Queensland. The lavas record magnetic remanence that is either remarkably stable to demagnetization by alternating field, or displays unidirectional behavior with little or no viscous overprint. FORC diagrams confirm the presence of a high coercivity component, most likely hematite, in all samples studied. There is strong microscopic evidence that the high coercivity component was produced by the oxidation of ilmenite. This oxidation is one that occurs at high temperature. Samples which contain a significant fraction of titanomagnetite render transitional vector directions wholly compatible with those found in the most stable samples, some of which display almost no titanomagnetite grains under microscopic examination. Hence, the natural remanence can be considered to have been entirely recorded during the initial cooling of the lavas. The R-N transitional VGP path is dominated by two clusters, the first, off the west coast of Australia; the second, within Siberia. Each cluster is associated with several sequential lavas with no recorded movement of the VGP between them. The geographical location of each of these clusters correlates with a vertical field concentration as seen at Earth's surface in the modern-day field after removal of the axial dipole term (i.e. the NAD-field). This finding provides evidence that the lower-most mantle has kept the associated flux concentrations essentially stationary for at least the past 21 Ma. In contrast, the published Liverpool Volcano Oligocene (about 34 Ma) R-N reversal, recorded in detail by lavas in nearby New South Wales some 13 Myr prior to the Queensland record, is dominated by two Southern Hemisphere VGP clusters, each located to the east of a modern-day NAD-field flux concentration. Such a comparison raises the question as to whether mantle-held flux features at the top of the outer core may intermittently drift relative to Earth's surface.
GP13B-11 1330h
Cretaceous Apparent Polar Wander of North America: Leisurely Stroll, but not Standing Still
It has long been suggested that the North American pole for the Cretaceous can be represented by a single "standstill" pole. Here we present new paleomagnetic data that can be used to test this model. The new data are from the Fort Hays Limestone Member of the Niobrara Formation sampled in Kansas and Colorado east of the Rocky Mountains. The Fort Hays Limestone is almost entirely Coniacian in age (85.9-89.3 Ma) and consists mainly of medium to thick beds of chalky limestone. Shive & Frerichs [1974] previously showed that this unit carried a weak but stable remanent magnetization. All specimens from the 38 reliable sites (8-10 samples per site) that we collected from eight localities were measured in a cryogenic magnetometer and subjected to either stepwise alternating field or thermal demagnetization followed by principal component analysis. Intensity of the natural remanent magnetizations from reliable sites typically exceeds 3 x 10-4 A/m. For specimens from these 38 sites, the demagnetization paths are generally linear and toward the origin above 20 mT or 240 degrees C. The characteristic remanent magnetization direction obtained from these differs significantly from the present field direction. The demagnetization data, along with IRM results, indicate that the characteristic remanent magnetization is carried by magnetite. Twenty flat-lying sites at three localities in the stable interior of North America in Colorado and Kansas were sampled to determine a pole position for stable North America. Eighteen tilted sites from five localities along the western edge of stable North America in Colorado were sampled and used to construct a fold test of the magnetic stability of the Fort Hays Limestone. A highly significant positive classical fold test is obtained from either the 38 combined site mean poles or the 5 locality mean poles, consistent with the characteristic magnetization having been acquired at or near the time of deposition. The mean of the 20 site mean poles from the flat-lying sites is 75.97N, 188.16E with a A95 of 1.9 degrees (K = 300.9), which lies only 3.6 great circle degrees from the Early Cretaceous (ca. 120 Ma) pole, but differs significantly from it. Thus, apparent polar wander was slow, but the pole has not stood still. Instead, the Early Cretaceous, Coniacian, Paleocene, and Eocene poles define a consistent age progressive track spanning 11 great circle degrees. The Campanian pole for North America, based on paleomagnetic results from the Adel Mountain Volcanics, Elkhorn Mountain Volcanics, and the Maudlow Formation lies off the track suggesting, but not proving, that their sampling regions have undergone a small clockwise vertical axis rotation during Laramide deformation. Implications for inter-hotspot motion and true polar wander will be discussed.
GP13B-12 1330h
Remagnetization Overprints in the Upper Ordovician Kope Fm., Kentucky
The Upper Ordovician (Edenian) Kope Formation was sampled at 61 stratigraphic levels in four overlapping sections in northern Kentucky. Although the objective was to evaluate the usefulness of remanence for magnetostratigraphic correlations between these sections, the samples, mainly silty limestones and siltstones, are pervasively remagnetized. The NRM directions are well-grouped, slightly steeper than the expected present field direction. However, AF and thermal demagnetization, up to 100 mT a.f. peak field and 380C respectively, reveal remagnetization trends in vector orthogonal and stereographic plots. Endpoints and break-points in these remagnetization trends are consistent with the expected field directions calculated for this locality from Ordovician and Permian (Kiaman) reference poles for North America. The predominance of the overprints obviates the use of remanence for stratigraphic correlations of the Kope sections sampled. Poles computed from the remagnetized directions are compared with Ordovician and Permian poles for North America.
GP13B-13 1330h
Magnetic Grain-size Proxies in Loessic Soils and Their Potential use in Paleorainfall Reconstruction
As part of our ongoing rock-magnetic study of loessic soil profiles we sampled over 70 in Nebraska, Iowa, Missouri and Illinois. Our sampling sites are located in stable upland positions and extend along a rainfall gradient which ranges from an average annual precipitation of less than 500 mm/year in southwestern Nebraska to almost 1000 mm/year in central Missouri. Soil cores were obtained with the aid of a hydraulic soil probe, described and subsampled into small plastic bags. Samples were air-dried in the laboratory and the < 2mm fraction was used for magnetic analyses. We measured magnetic susceptibility X and several remanence parameters (ARM, IRM) for all samples. Hysteresis measurements, IRM acquisition curves and time dependence of IRM acquisition were measured for a subset of samples. All samples show magnetically enhanced A- and B-horizons, which results in increased values of X, ARM and IRM. Changes in the ratio of ARM/IRM suggest an increase in the relative abundance of stable single domain (SSD) particles. VRM analyses show that the upper soil horizons are enhanced in ultrafine superparamagnetic (SP) ferrimagnets as well. Changes in the relative abundance of SP and SSD ferrimagnets along our transsect correlates well with the modern precipitation gradient, suggesting the use of grain-size dependent magnetic parameter as a potential paleorainfall proxy when analyzing paleosols.
GP13B-14 1330h
Slumps and sediment deformation: AMS of the Gaviota and Goleta Slides
During the last year, we have collected six gravity cores from the northern side of the Santa Barbara Basin for paleomagnetic deformation tests. The first two cores were collected on the smaller Gaviota Slide on the western side of the basin. These cores provide the slump end member, since one core comes from the evacuated zone while the other comes from the toe or accumulation zone. Two additional cores were collected on the toes of the much older Goleta Slides and were designed to provide an undeformed reference fabric; it was our hope that they would be mostly pelagic drape but also penetrate into the top of the deformed material. One core appears to be an excellent reference core with clear laminations, while the other has distorted laminations that show the sediment has undergone some type of deformation. In between the two slides, we collected two cores on either side of the crack that runs between the two slides to investigate creep and slope stability. All cores have been sampled at a 3 cm interval with 8cc cubes to measure anisotropy of magnetic susceptibility (AMS) and AF demagnetiziation. The Gaviota slide sediments show a dominantly triaxial fabric with some alignment of the maximum eigen values, while the drape materials from the Goleta Slide show a typical oblate undeformed fabric. There are hints of deformation at major clay layers, which we continue to investigate. Several of these cores appear to show a biogentic component to the AMS signal that fades with depth. This component has a tendency to mask the deformation signal in the top sections of the cores. The NRM inclinations of the deformed core regions tend to be shallow and erratic showing what look at first glance to be excursional features.
http://schwehr.org/Gaviota/bpsio-Aug04/