GP41A-01 INVITED
Secular Variation of the Geomagnetic Dipole during the past two Thousand Years
We have constructed a very simple model of a time varying geocentric dipole based on the archeomagnetic records obtained at four widely separated sites on the globe for the past 2 ka. The predictions of the model in terms of directional variations have been tested against actual archeomagnetic data from 12 sites distributed over the globe, being aware of the uneven distribution of the sites for this period. The directions compiled at each site are compared with those derived from the time-varying spherical harmonic models produced by Hongre et al (1998) and recently by Korte et al (2005) who developed CALS7K-2 to predict both the field and secular variation with harmonics up to degree 10. We find that the misfits between our simple dipole and the actual records is equivalent to the performance of the spherical harmonic models for the European sites and not strikingly larger for the rest of the world. Many discrepancies can be accounted for by uncertainties inherent to the archeomagnetic records, which, along with the small number and poor geographical distribution of sites, leads us to conclude that the present state of the database does not allow to extract secular variations described by terms going beyond degree 2 and maybe even degree 1. It appears also that dipole tilt could be responsible for the main part of the secular variation associated with time constants exceeding 102 years. As a second step, we used the paleointensity records contained in the same database to construct the curve depicting the variations of the true dipole moment. The present decrease of the dipole did not begin prior to one thousand years ago and the dipole was actually increasing from 0 until AD 500. The dipole moment of CALS7K is moslty and sometimes significantly lower than the present estimate. The tilt and strength of the dipole give the dipole field at any site. The curve can thus be used to predict dipole field intensity at any location and offers potential for studies related to the evolution of the magnetosphere. As a direct application, we calculated the variations of dipole field intensity in Paris and compared them with the archeomagnetic records of total field intensity. The large amplitude of the dipolar changes is reflected by the records of total field and the archeomagnetic jerks coïncide with periods of either strong dipolar or non dipolar fields.
GP41A-02
A new selection of archeomagnetic data for high resolution geomagnetic field modeling
To understand the content and the causes of the changes in the Earth's magnetic field beyond the observatory record one has to rely on archeomagnetic and lake sediment paleomagnetic data. Regional archeomagnetic intensity and direction curves are often of variable quality and completeness, hampering the ability to uncover non-dipole field contributions in global data analysis and modeling. Poor age control in many records further limits the temporal resolution attainable. Millennial scale models like CALS7K.2 are built using all the available data, but an alternative strategy is to use specific selection criteria and (inevitably) smaller numbers data in an attempt to produce higher resolution models. High resolution variation of the geomagnetic field during the last 3000 years is investigated using selected archeointensity and directional data, drawn from the current GEOMAGIA50 database, and a collection of directional data initially based on the CALS7K compilation but now updated to include some missing data and recently published results. The maximum standard age error for each datum was set to 100 years, the maximum standard VADM error to 2 VADM units, and the maximum α95 confidence cone to 5 degrees. The constrained dataset is used to create new high resolution global geomagnetic field models for the past 3000 years. The modeling strategy is generally similar to that used for CALS7K. Minor modifications to the algorithm allow us to use the GUFM historical field model as an additional constraint and recover a higher resolution field model fro 1600AD to the present. This addresses a weakness of the CALS7K model which suffers from diminishing numbers of paleomagnetic records for the most recent times, and is unreliable near both the beginning and end of the time interval spanned. We will present results illustrating the effects of our data selection and modeling efforts for the 0-3~ka geomagnetic field.
GP41A-03
Preliminary paleointensity investigation of Iron Age pottery shards from southern Africa
The intensity of Earth's magnetic dipole magnetic field has been rapidly decaying over the last 150 years. This decline is well documented by magnetic observatory data, and if it were to continue, the dipole field would be nonexistent in a few millennia. Recent studies have suggested that the decay commenced at approximately 1840, whereas the dipole field was of nearly constant strength from 1590 to 1840. These suggestions rely on an analysis of global archeomagnetic results. However, there are few results for Southern Hemisphere sites. To obtain a better spatial view of the history of the geodynamo during this interval, we are studying pottery shards from several southern Africa sites (including the Mapungubwe cultures of the Limpopo area). Preliminary magnetic hysteresis data (and first-order reversal curves) measured using a Princeton Measurements Alternating Gradient Force Magnetometer and P2 probes, Curie temperature determinations measured using a Kappabridge K4S Susceptibility Bridge, and XRD data indicate that some of the shards contain magnetic minerals suitable for paleointensity analyses. We will compare several methods of paleointensity determination, including the Thellier-Coe method (in an Argon atmosphere) and multiple specimen approaches, and examine these values versus model paleointensity predictions for the field area.
GP41A-04 INVITED
Resampling the 0-5 Ma Geomagnetic Field Recorded by Lavas
Paleomagnetic models of both the time-averaged field and paleosecular variation rely on directions and absolute intensities recorded by igneous rocks. Such modeling implicitly supposes that in a statistical sense the available data provide representative sampling for the time span under consideration. In practice, the vagaries of eruptive processes leave large spatial gaps, and at any specific location often lead to multiple samples that are closely spaced in time followed by a long hiatus. The resulting data set is rarely consistent with the assumption of random uniform sampling of a specific time interval. Such problems have led to extensive discussions of the significance of inclination anomalies and low paleosecular variation in Hawaii, where there are numerous directional data that are known to be densely clustered in some parts of the 0-5~Ma time interval and sparse elsewhere. Various ad hoc methods have been proposed for dealing with the temporal sampling problem. Most are intended to remove the influence of serial correlation in directional data with known stratigraphic ordering and to acquire independent estimates of the field. These methods have included sparse temporal sampling, or averaging of successive directions that are not significantly different from one another. We investigate alternative strategies that use all available directions and age information and ascribe a probability density function (pdf) to the probable ages associated with paleomagnetic directions. The data are then resampled in time to produce an essentially uniform distribution and assess the influence on VGP dispersion, inclination anomaly, and other paleomagnetic statistics of interest. In its crudest form the temporal pdf for a specific datum might be uniform over the Brunhes polarity interval for example, while for well-dated flows one might use a gaussian with rather tight constraints on its variance based on the standard error in the age. Upper and lower bounds on the age can also be readily accommodated. This provides a realistic mechanism for capitalizing on substantial recent efforts in radiometric dating of lava flows and for improving estimates of field variability on million year time scales.
GP41A-05
PreliminaryEquatorial Paleomagnetic results from Mt Kenya lavas. Neil D Opdyke, 1, Dennis V Kent, 2, Kainian Huang ,1, J.P. Patel , 3
Field work on this study was carried out in August of 2006 by field parties from the University of Florida and Rutgers University. Mt Kenya is believed to be Plio-Pleistocene in age and an Argon dating survey is underway Ten samples were taken at each site consisting of one exposure in individual lava Flows. These exposures are usually in road cuts, streambeds and in some cases roadbeds. We sampled 100 sites distributed around the Mt Kenya Massif and to the northeast along the Nyambini range. The equator bisex's Mt Kenya and all sites were sampled within 40" north or south of the equator . The samples were returned to the US and processed at the University of Florida paleomagnetic laboratory. Many sites were severely affected by lightning however after demagnetization 68 sites yielded directions with alpha 95's equal to or less than 10°. Normal magnetized sites dominate, with N=58 (Dec=1°,Inc -0.1°,α95=2.6°) whereas only 10 reverse sites(Dec. =181.9,Inc. .6°α 95=8°) were identified. The combined site mean direction is Dec=1.1°, Inc..= -0.2° and α 95=3.2°. This result is not significantly different from what is expected from the geocentric axial dipole. VGP's were calculated from each site and the dispersion is low with the ASD = 11° which is in agreement with model "G" of MacFadden and McElhinny .No transitional directions were identified . Quadrupole components are not resolved. 1 Department of geological Sciences, the University of Florida , 2 Dept of Geology, Rutgers University,3,dept of Physics ,The University of Nairobi
GP41A-06
Secular Variation and Paleomagnetic Studies of Southern Patagonian Plateau Lavas, 46S to 52S, Argentina
Regional studies of paleosecular variation of the Earth's magnetic field can provide us with information beyond that available from one location. Southern Patagonia, Argentina (46S to 52S latitude and 68W to 72W longitude) is a place where numerous Plio-Pleistocene lava flows are available for such a study. Volcanic activity in this area is related to back arc volcanism due to slab window activity as the South Chile Ridge is subducted beneath western South America, producing Neogene volcanic centers capping Mesozoic basement extending far to the east of the active plate boundary. Published studies on young lavas from both the northern (Meseta del Lago Buenos Aires, Brown et al, 2004) and southern (Pali Aike Volcanic Field, Mejia et al, 2004) portions provide stable paleomagnetic data on nearly 70 lava flows. Paleosecular variation values for the two studies differ, with 17.1 degrees obtained from the Pali Aike field and 20.0 degrees from the Lago Buenos Aires field. Recent fieldwork in the plateau lavas between these two locations has provided some 80 new sites allowing us to better investigate secular variation and the time-averaged field over this entire region during the past 5 myr. Rock magnetic studies on selected new samples (isothermal remanent magnetization and hysteresis measurements) as well as optical observations indicate low titanium magnetite as the primary carrier of remanence. Hysteresis properties range from 0.1 to 0.4 for Mr/Ms and 1.4 to 3.0 for Hcr/Hc indicating psuedo-single domain behavior. Mean destructive fields for AF demagnetization average 40 to 60 mT. Thirty-three new sites, mostly from Gran Meseta Central (48°S), yield a mean direction of inclination –61.8, declination of 356.6 with an alpha-95 of 5.7 degrees. These directions, with additional sites recently collected from Meseta de la Muerte south to Rio Santa Cruz, will allow us to further investigate paleosecular variation over this wide region.
GP41A-07
Statistical Model of Secular Variation and Excursions and Drilling to the Moho
Almost all models for Earth's magnetic field secular variation derived from paleomagnetic results disregard results that give low latitude Virtual Geomagnetic Poles (VGPs). These results are regarded as "unusual" or "not part of the regular secular variation process". These low latitude VGPs are regarded as being produced at a time when the field is undergoing an excursion or a reversal and that therefore they should not be included in a secular variation study. However, it is almost impossible to determine logically the latitude of VGP below which results should be dismissed from study of the secular variation. And for some studies, it is important to include all results to arrive at a valid conclusion. This is the case with the paleomagnetic evidence that will be collected during the drilling through the oceanic crust to the Mohorovicic discontinuity, marking the top of the mantle. Accordingly, the results from the data set originally used by McElhinny and McFadden (1997) updated by addition of new results was expanded to include all the results giving low latitude VGPs that were left out of the updated data set. These expanded results were subjected to statistical analysis based on different latitude bands of the observation sites. This showed that for all latitude bands, the Fisher distribution gave a very bad fit to the data. However the data from every observational latitude band could be fitted by a Fisher distribution for about 90% of the data, plus a uniform distribution in latitude for the other 10% of the data. In addition, the Fisher distribution angular standard deviation showed a linear increase with observation latitude, rising from 11.5º at 10º latitude to 20º at 66º latitude. These results can be used to define a simple model which allows us to estimate the scatter to be found at any latitude, and in particular to determine the scatter in inclination that would be obtained from unoriented samples of basalt recovered during the Integrated Ocean Drilling Program (IODP). Since the magnetization of the lower crust is believed to contribute significantly to sea floor spreading magnetic anomalies it is important to be able to distinguish between normally and reversely magnetized rocks in the lower crust in order to estimate their contribution to the sea floor spreading magnetic anomalies. In the absence of magnetic orientation this can only be done by studying their magnetic inclination. For very low latitudes, there is only a small likelihood of being able to use inclination to determine polarity, whereas at higher latitudes, the probability of being correct rises. In order to achieve an accuracy of 95% the paleolatitude of the chosen hole has to be at an absolute latitude greater than 19º. Improvement of this result by moving to even higher latitudes is slow. 97% accuracy is only achieved at an absolute latitude greater than 28º. The current plan calls for a major hole to be drilled on a site that was formed at the equator, thus rendering it impossible to tell anything about polarity unless the samples can be oriented.
GP41A-08
Testing paleosecular variation field models in the Pre-Cambrian
One of the most useful assumptions in paleomagnetism is that the geomagnetic field is on average close to that of a geocentric axial dipole (GAD). The assumption that the time-averaged geomagnetic field closely approximates that of a geocentric axial dipole (GAD) is valid for at least the last 5 million years and most paleomagnetic studies make this implicit assumption. Following the seminal work of Constable and Parker (1988), Tauxe and Kent (2004) developed a simple statistical model for the geomagnetic field, which allows prediction of the distribution of directions (and intensities) for any given latitude. They found a simple relationship between the North-South elongation of directions and the inclination. If the field model is approximately valid in the past, comparison of observed directions with predicted distributions allows paleomagnetists to assess the reliability of the observed directions in a novel way and in some cases allows data to be corrected for anomalous inclination shallowing, for example. While the model fits the published data for the last 5 million years reasonably well, it is unknown how far back in time the model is even approximately valid. The key objectives of this study were 1) to sample a sequence of lava flows that are some 1.2 Ga, exposed on the western side of Lake Superior and 2) to obtain paleomagnetic directions from them as a test of the statistical field model in the past. We will present results from 80 ancient lava flows sampled from the North Shore Volcanics.