Paleo secular variation dating


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Magnetic Geochronology




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Google Scholar Holcomb, R.

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Champion, and M. McWilliams,Dating recent Hawaiian lava flows using paleomagnetic secular variations, Geol. America Bull. Google Scholar King, J. Google APleo Kovacheva, M. Creer, P. Tucholka, and C. Barton, eds. Elsevier, — Zagniy,Archeomagnetic results from sdcular prehistoric sites in Bulgaria, Archeometry 27, — Google Scholar Lee, S. Google Scholar Liddicoat, J. Coe,Mono Lake geomagnetic excursion, Jour. Research 84, Union Trans. Google Sedular Lund, S. Research Letters 12, — Banerjee,Late Quaternary palcomagnetic field secular variation from two Minnesota Lakes, Jour.

Research 90, — Olson,Historic and paleomagnetic secular variation and the Earth's core dynamo process, Rev. Liddicoat, K. Lajoie, T. Henyey, and S. Robinson,Paleomagnetic evidence for longterm year memory and periodic behavior in the Earth's core dynamo process, Geophys. Research Letters 15, — Daying Abstract [1] A paleosecular variation PSV curve for western North America is presented on the basis of 94 virtual geomagnetic poles VGPs from dated volcanic rocks sampled at sites. The PSV record from lava PPaleo PSVLhowever, is perhaps more reliable in its general shape and chronology datting of the higher fidelity of volcanic rocks as magnetic field recorders and because of the greater density of 14C dates.

The new PSVL record provides a partial Holocene master curve for western North America and will be of particular value in dating geological and archeological materials using paleomagnetic directions. Sevular [2] Variation of the surface geomagnetic field seculsr to changes in th Earth's internal field, over periods of a year to millions of years, is called secular variation SV aecular Paleo secular variation dating variation PSV for prehistoric times. Such changes in the historical geomagnetic record have been primarily due to westward drift [ Bullard et al.

Virtual geomagnetic poles VGPs can be calculated from the directional data using the dipole Paleo secular variation dating but have only regional variarion due to the nondipolar contribution to the total field direction. In general, detailed geomagnetic field records have been used in investigations of hydrodynamic dynamo processes occurring within the Earth's outer core [ Bloxham and Gubbins, ; Constable et al. Study of thermoremanent magnetizations TRMs retained by baked archeological materials e. Such records exist for the last years in Europe [ Kovacheva, ], the last years in Japan [ Hirooka, ], and the last years in western North America [ Sternberg and McGuire, ].

The general locations of the volcanoes and their deposits are shown in Figure 1. Our curve compares well with that derived from cored lake sediments at Fish Lake, Oregon, and provides a partial Holocene master curve based on volcanic rocks for western North America. Kent et al. Dating sedimentary sequences like the Mono Lake is difficult, but so is dating very young lava flows like the Laschamp volcanics because of the low abundance of radioactive potassium. Zimmerman et al. Cassata et al. The question as to whether there are in fact two independent excursions is unresolved by these data.

Resolution of the Mono Lake-Laschamp mystery therefore lies in records with stratigraphic age control. The advantage of ice cores is that not only is the relative chronology straight-forward, layer counting in the ice gives ages that are accurate to within 60 years. The isotopes 36Cl and 10Be are produced in the atmosphere by cosmic ray bombardment which is modulated by the geomagnetic field strength and the strength of the solar wind. Therefore changes in production rate of these isotopes reflects to a large extent reflect changes in intensity of the field. The isotopic data from the Greenland Summit cores were summarized by Muscheler et al.

The 10Be flux data do not show two peaks, but a single peak centered at approximately 39 ka. The 36Cl data, however are less straight-forward. The data differ in two papers published in the same year on the same core by the same group Wagner et al. Another way of addressing the Mono Lake-Laschamp dilemma would be to look at the sedimentary relative paleointensity database. Tauxe and Yamazaki compiled nearly all of the published relative paleointensity records. While a few records appear to have both, the evidence is not overwhelming for two globally recorded features. Finally, we have the directional records from sedimentary sequences long enough to record both excursions. Inclinations and declinations expected from a normal and reverse GAD field are shown as dashed lines.

Laj and Channell reserve the term for what are essentially local phenomena that do not reach fully antipodal directions. Because this was first well documented as being a directional feature distinct from the Lashamp in the Irminger Basin Channell,perhaps it should be named the Irminger Basin excursion. Each has its own history and many may turn out to be as interesting and difficult to pin down as the Mono Lake-Laschamp feature s. The red triangles are from double heating experiments with pTRM checks see Chapter No selection criteria were applied. There is a fast cold ring around the Pacific, presumably from the influence of subducted slabs. When the polarity is the same as the present polarity it is said to be normal.

When it is in the opposite state, it is said to be reverse. The duration of the reversal process also appears to be a function of latitude Clement, The details of what happens during a polarity reversal are still rather unclear because they occur so quickly, geologically speaking. There are a few conclusions we can draw however: These swaths are seen in many data sets, but can be made to disappear when certain criteria are applied e. Whether or not the swaths exist has been debated ever since they were first observed. Line traces the reversal frequency number of reversals in a four million year interval estimated by Constable How the time scale is calibrated is discussed in the next chapter.

For now we will just take it as a given.

The details of the history variatiom reversals for times older than the oldest sea datinf magnetic anomaly record about Ma are sketchy, but will eventually be documented Paleo secular variation dating sedimentary records of the magnetic field see e. Furthermore, the frequency of reversals appears to change see for example, Constable, The reversal frequency is relatively high in the interval Ma, but appears to drop gradually to zero at the beginning of the so-called Cretaceous Normal Superchron CNSa period of some 38 Myr in which no or very few reversals occurred. Since the end of the CNS at about 83 Ma, the frequency of reversals has increased to the present average rate of about four per million years.

Time averaged intensity of the geomagnetic field. Beyond a certain age limit, however, there simply are not enough data with sufficient age control and spatial density to constrain a spherical harmonic Paleo secular variation dating. The variatino for longer time scales has been to varitaion at the sscular magnetic field or the statistical characterization of paleosecular variation data. We consider here the time averaged field. The last five million years has been a focus for time average field vzriation because the effects of plate motion are small and there are hundreds of studies Palleo draw from.

Data from lava flows from all over the world have been compiled into various databases and analyzed from variwtion variety of view points. It was recently realized that the data had been compiled using less than optimum criteria and that many more data of higher overall quality may be required for a robust TAF model to be produced. Data from the new TAF project are only just becoming available e. Although the field is not perfectly GAD, the flux patches seen in the historical field are nearly erased. One of the primary assumptions in many paleomagnetic studies is that the magnetic field, when averaged over sufficient time, averages to that of a GAD field.

This means that if VGPs are averaged from units spanning enough time to average out secular variation, the mean pole is coincident with the spin axis. Such a pole is called a paleomagnetic pole. As continents move, they carry with them rock units that retain a record of the spin axis in the continental reference frame, so these poles tend to form swaths called apparent polar wander paths or APWPs. It is worth mentioning here that it is not very well known exactly how much time is required to average out secular variation; the consensus is that it is more than years but less than 5 million. Most text books claim that — years is sufficient. Conventional wisdom suggests at least ten, while Tauxe et al.

The vertical dashed line is the surface expression of the edge of the tangent cylinder. Southern hemisphere data have been flipped to the Northern hemisphere. The tangent cylinder is denoted by the blue cylinder tangential to the red sphere inner core. Tauxe and Yamazaki updated the PINT03 database of Perrin and Schnepp to include all published paleointensity data through We also include the new data from Antarctica of Lawrence et al. Southern hemisphere data are combined with the Northern hemisphere to decrease latitudinal gaps. An axial dipole field would have polar intensities twice those expected at the equator, and although for the present field non-axial dipole contributions reduce this gain somewhat as shown by the solid black lineall conventional wisdom suggests that we would expect the average field strength to increase double with latitude.

Blue dots are submarine basaltic glass data. Red diamonds are single crystal results. Or, it is possible that the influence of the inner core manifests itself in lower average field strengths at and above the cylinder tangent to it the tangent cylinder. The geodynamo results from a complex combination of physical processes in the fluid outer core see, e. The influence of the Coriolis force, combined with the presence of the inner core results in a separation of the flow regimes into two distinct regions bounded by a cylinder tangent to the inner core, parallel to the spin axis. The spin of the Earth tends to generate columnar convection in the region outside the tangent cylinder, while inside, the convection tends to be more 3-dimensional Busse, It is possible that the flow regime inside the tangent cylinder results in a depressed field strength observed at high latitude.

Antipodes of reverse directions are used. The expected direction is at the star at the center of the equal area projection.

Variation dating secular Paleo

Directions in the upper lower half are above below those expected and those to the datnig left variatuon right-handed left-handed. The red ellipse illustrates the elongation E of the directional data where E datijg the ratio of the eigenvalues along the maximum and minimum axes here vertical and E-W respectively. At Palei particular instant in time, however, there secuar be significant deviations owing to the non-axial dipole contributions. This, combined with distortions in the recording process some of which were discussed in Chapter 5 and datig preservation of rocks datlng increasing age makes evaluating the GAD hypothesis increasingly difficult as we go back in time.

There has been considerable sscular in collecting the data relevant to describing Paleo secular variation dating statistical character of the geomagnetic field over time. First, it appears that the equatorial data are more elongate than those from higher latitudes something we mentioned in Chapter Secondly, the scatter in the directional data seems to go down with increasing latitude. Before we begin a quick tour of PSV models, we must introduce the concept of VGP scatter and briefly explain how it has been calculated. VGP scatter is quantified by the parameter Sp e. Ideally, one would use all the paleomagnetic data available, but we encounter two problems with this approach.

First, some directions are better determined while others have significant within site scatter resulting from sampling or experimental errors or lightning strikes! Secondly, the interest of the paleomagnetic community in unusual field states reversals and excursions has resulted in their over-representation in the published literature. McElhinny and McFadden defined a parameter Sf: To address the over-representation of unusual field states in the data base, some data compilations have used a fixed cutoff for VGP latitude. The latitudinal dependence of Sp means that a fixed cutoff biases against the more scattered data collected at higher latitude.

In an attempt to compensate for this problem, Vandamme proposed a variable VGP cutoff. Most early modeling efforts by the paleomagnetic community focussed on explaining the variable scatter in directions and VGPs with latitude see review by Tauxe et al. This has become known as Model B Irving and Ward Dipole wobble simulated by random variations in the three dipole terms of the spherical harmonic expansion of the geomagnetic field produces Fisher distributed sets of virtual geomagnetic poles VGPs.

These are centered around the spin axis. Because of the non-linear transformation from VGPs to directions see Chapters 2 and 12the directions associated with a circularly symmetric set of VGPs are not generally circular. A different PSV model, Model A of Irving and Wardstarts from Fisher distributed directional data modeled by adding directional perturbations drawn from a uniform distribution to the expected dipole direction. The VGP distribution resulting from such a process would be oval at the equator and become more circular toward the poles.


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