A Review of Recent Results Obtained Using the Microwave Palaeointensity Method
Reliable records of geomagnetic field intensity are fundamentally important for fully understanding the Earth's magnetic field however, palaeointensity research is experimentally challenging and analysing results can be subjective. Two approaches have been taken to advance palaeointensity methodology; firstly, the addition of more and different reliability checks, and secondly, attempts to minimise known sources of non ideal behaviour. Research at the University of Liverpool over the last decade has concentrated on the latter approach resulting in the development of the microwave palaeointensity technique. The method which uses high frequency microwaves instead of heating in a conventional oven minimises the major problem of thermo chemical alteration of the sample during the experiment. As with conventional methods, it is still a requirement that the remanence is an original thermal remanence which is held by non interacting single domain behaving grains. Experiments are carried out a sample at a time and the laboratory field can be applied in any specified direction making it easy to use many different methodological protocols and tailor experiments to individual sample characteristics. As small (5mm diameter) cores are used the microwave method is ideal where limited material is available such as archaeological specimens. A key area of current research is archaeointensity determination and in particular the secular variation record held by SW Pacific Island ceramics and burnt archaeological samples from the UK. Going back further in the geological record a number of studies investigating excursions and reversals of the geomagnetic field have recently been carried out as well as investigations of field behaviour during the Cretaceous Normal Superchron. These studies include comparisons to results obtained using other methods and where alteration during the experiment occurs, using the microwave method improves the success. Intriguing differences are sometimes seen in comparative studies however different methodological protocols are often used so it is often not easy to determine whether it is the protocol or the microwave demagnetisation that causes the differences.
Fluctuation of Absolute Paleointensity During The Cretaceous Normal Superchron: New Observations From Volcanic Rocks in China
The Phanerozoic geomagnetic field is characterized by frequent reversals interrupted by three polarity superchrons (including the Cretaceous normal superchron, CNS, the Kiaman reverse superchron, and the Moyero reverse superchron), which are long intervals of tens of millions of years without reversals. Therefore, the determination of geomagnetic field strength during the special polarity superchrons is essential for understanding the working state of the geodynamo. We recently carried out Thellier paleointensity studies integrated with precise Ar-Ar dating analyses on the early Cretaceous volcanic-rock sequences in Inner Mongolia, Shandon, Zhejiang and Fujian provinces in eastern China. Here we will focus on several time-series paleointensity records for the early interval of the CNS (100-119 Ma). Lava-mean paleointensity fluctuated significantly, about one-third to one-and-half times in magnitude of the present field. Possible influences of magnetic mineral alterations and secular variation effects will be discussed. Finally, our data will be compared with available Thellier paleointensity dataset. Taken together, we infer that the field strength during the superchron was relatively high and unstable. On the other hand, lower paleointensity values observed just prior to (and after) the CNS support the prediction of a positive relationship between the length of the polarity interval and the dipole intensity. These observations provide new constraints for understanding the geodynamo during the superchron and for testing numerical models.
Exploring the Magnetic Sands of Time: Using Zircons and Other Sedimentary Detritus to Understand the Early Geodynamo
The onset and nature of the geomagnetic field is important for understanding the evolution of the core, atmosphere and, potentially, life. Ubiquitous metamorphism, however, imposes a series of restrictions on the materials that can be used to retrieve paleo-Archean and older magnetic records. These challenges are arguably best addressed through the study of single silicate crystals that host magnetic inclusions. Results have previously been reported from igneous rocks; here we examine the potential of crystals eroded from primary igneous rocks that are now found in sedimentary units. The benefit of this approach is that it might allow us to sample time intervals that are not otherwise available because the original igneous rocks have been lost to erosion. This requires dating of the silicate crystal itself, something that is commonly done with some sedimentary components (e.g. zircons). We present several examples of this approach from the Kaapvaal Craton of southern Africa. Zircons from the Ancient Gneiss Complex have suitable rock magnetic characteristics, but even with their small size (∼100 μm) care must be taken to avoid crystals with large inclusions that may have multidomain characteristics. A recently-reported positive conglomerate test on ca 3.45 Ga rocks (Usui et al., 2008) suggest that adjacent sandstones may preserve a magnetic history in their zircon populations. A preliminary characterization of these zircons will be presented.
Angular Standard Deviation of Virtual Geomagnetic Pole Positions as a Function of Observation Latitude and Sources Necessary to Produce the Observations .
A new analysis of secular variation over the past five million years has been done. This analysis includes data from lava flows which give low latitude VGPs, data which have been omitted from most but not all of the previous attempts to produce a model of VGP scatter. A total of 3579 lava flows with 3 or more samples per flow, a demagnetization code of 2 or more, a Fisher precision parameter greater than 100 and a confidence interval around the mean less than 10° was available. Lower values of precision were permitted if there were also smaller values of confidence interval. In order to accommodate the low latitude VGP data a new model distribution had to be used. This consisted of a Fisher distribution with a small scatter to explain the high latitude VGPs plus a uniform distribution to explain the low latitude VGPs. The proportion of model results in each distribution could be changed. It was found that this proportion showed no consistent variation as a function of observation latitude, but that the Fisher distribution increased scatter as a function of observation latitude, from about an angular standard deviation (ASD) of 10° at low observation latitudes to 20° at high latitudes. Instead of the uniform distribution a very scattered Fisher distribution for the low latitude VGPs (Shibuya et al., 1995) gave almost the same results. Even when the data selection was winnowed further (N≥4, DC≥3) the low observation latitude ASD remained at about 10° for the tight distribution. The reason for the variation of ASD as a function of observation latitude is that the non-dipole field sources that cause the SV have to be stronger at high latitudes than at low latitudes. Observations of the present day field show that the values of the Gauss coefficients that represent the core field (spherical harmonic degree less than 13) vary as a function of order, with lower orders being stronger than higher orders. This also shows that the non-dipole field has to have stronger high latitude components than low latitude components, because high latitude non-dipole sources are mainly represented by strong low order Gauss coefficients whereas low latitude sources are represented mainly by strong high order coefficients.
Bias Corrections for Regional Estimates of the Time-averaged Geomagnetic Field
We assess two sources of bias in the time-averaged geomagnetic field (TAF) and paleosecular variation (PSV): inadequate temporal sampling, and the use of unit vectors in deriving temporal averages of the regional geomagnetic field. For the first temporal sampling question we use statistical resampling of existing data sets to minimize and correct for bias arising from uneven temporal sampling in studies of the time- averaged geomagnetic field (TAF) and its paleosecular variation (PSV). The techniques are illustrated using data derived from Hawaiian lava flows for 0-5~Ma: directional observations are an updated version of a previously published compilation of paleomagnetic directional data centered on ± 20° latitude by Lawrence et al./(2006); intensity data are drawn from Tauxe & Yamazaki, (2007). We conclude that poor temporal sampling can produce biased estimates of TAF and PSV, and resampling to appropriate statistical distribution of ages reduces this bias. We suggest that similar resampling should be attempted as a bias correction for all regional paleomagnetic data to be used in TAF and PSV modeling. The second potential source of bias is the use of directional data in place of full vector data to estimate the average field. This is investigated for the full vector subset of the updated Hawaiian data set. Lawrence, K.P., C.G. Constable, and C.L. Johnson, 2006, Geochem. Geophys. Geosyst., 7, Q07007, DOI 10.1029/2005GC001181. Tauxe, L., & Yamazkai, 2007, Treatise on Geophysics,5, Geomagnetism, Elsevier, Amsterdam, Chapter 13,p509
Pliocene to Present Paleosecular Variation at Low Latitudes: Contribution to the Time- Averaged Field Global Database
In this study, we present results from 82 paleomagnetic sites (about 500 standard cores) from Trans-Mexican Volcanic Belt (including the Michoacan Guanajuato Volcanic Field) and Socorro Island (Pacific Ocean). All these sites were recently dated by means of K-Ar and/or 40Ar-39Ar systematics and span from 5 Ma to present. In addition, many burned and unburned archeological materials were used to estimate the variation of Earth's magnetic field during the last two millennia. The mean paleodirection obtained in this study, discarding intermediate polarity sites, is I = 36.3°, D = 2.1 °, k = 62, α95 = 5.1°. These directions are practically undistinguishable from the expected Plio-Pleistocene paleodirections, as derived from reference poles for the North American polar wander curve and in agreement with previously reported directions from nearby lavas of Trans-Mexican Volcanic Belt. This suggests that no major tectonic deformation occurred in studied area. The paleosecular variation is estimated trough the study of the scatter of virtual geomagnetic poles giving SF = 16.4 with SU = 20.6 and SL = 12.2 (upper and lower limits respectively). These values are slightly higher with respect to the predicted by the latitude-dependent variation models for the last 5 Ma.