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Pelletier, J.D., Paleointensity Variations of Earth's Magnetic Field

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Pelletier, J.D., Paleointensity Variations of Earth's Magnetic Field Physics of the Earth and Planetary Interiors 110Ž. 1999 115–128 Paleointensity variations of Earth’s magnetic field and their relationship with polarity reversals Jon D. Pelletier DiÕision of Geological and Planetary Sciences, California Institute of Technology, Mail Stop 150-21 Pasadena, CA 91125, USA Received 7 August 1998; accepted 9 October 1998 Abstract Power-spectral analyses of the intensity of Earth’s magnetic field inferred from ocean sediment cores and archeomagnetic data from time scales of 100 yr to 10 Myr have been carried out. The power spectrum is proportional to 1rf where f is the frequency. These analyses compliment previous work which has established a 1rf 2 spectrum for variations at time scales less than 100 yr. We test the hypothesis that reversals are the result of variations in field intensity with a 1rf spectrum which occasionally are large enough to cross the zero intensity value. Synthetic binormal time series with a 1rf power spectrum representing variations in Earth’s dipole moment are constructed. Synthetic reversals from these time series exhibit statistics in good agreement with the reversal record, suggesting that polarity reversals may be the end result of autocyclic intensity variations with a 1rf power spectrum. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Magnetic field; Frequency; Intensity 1. Introduction analysis of paleointensity using historical observa- tions and sediment cores. He identified a broad, Earth’s magnetic field has exhibited significant continuous power spectrum with a steep dependence variability over a wide range of time scales. On time on time scale for time scales less than a century and scales less than a couple of hundred years historical a flatter spectrum at longer time scales. data are available for variations in the intensity and The geomagnetic field also exhibits reversals with orientation of the geomagnetic field. Archeomagnetic a complex history including variations over a wide data can be used to infer the intensity of the field range of time scales. The reversal history can be from time scales of centuries to millenia. Sediment characterized by the polarity interval distribution and cores provide the widest range of time scales of the reversal rate. Polarity intervals vary from those variations in the geomagnetic field with internal short enough to be barely resolved in the magnetic origin: 1 kyr to 10 Myr. Techniques of time series anomalies of the seafloor to the 35 Myr Cretaceous analysis can be used to characterize this variability. superchron. Reversals are also clustered in time such The power spectrum is the square of the coefficients that short polarity intervals tend to be followed by of the Fourier transform of the time series. It quanti- short polarity intervals and long intervals by long fies the average variability of the series at different intervals. This clustering has been quantified with time scales. BartonŽ. 1982 has performed spectral the reversal rate which gradually decreases going 0031-9201r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0031-9201Ž. 98 00140-X 116 J.D. PelletierrPhysics of the Earth and Planetary Interiors 110() 1999 115–128 back to 100 Ma and then increases going back netic field from volcanic lavas for 0–10 Ma. They further in time before the Cretaceous superchron. concluded that the distribution of paleointensity is Due to the availability of many new time series well approximated by a symmetric binormal distribu- data sets for paleointensity of Earth’s magnetic field tion with mean 8.9=10 22 Am2 and standard devia- since the work of BartonŽ. 1982 , it would be useful tion 3.4=10 22 Am2. One normal distribution is to perform power-spectral analyses of some of these applicable to the field when it is in its normal recent data sets to further characterize the temporal polarity and the other when it is in its reversed variability of the geomagnetic field. In this paper, we polarity. perform power-spectral analyses of time series data We have utilized four datasets for computing the for the intensity of Earth’s magnetic field inferred power spectrum of the intensity of Earth’s magnetic from sediment core and archeomagnetic data. We field. They are archeomagnetic data from time scales find that the power spectrum of the intensity of the of 100 yr to 1.6 kyr from KovachevaŽ. 1980 , marine geomagnetic field from time scales of 100 yr to 10 sediment data from the Somali basin from time Myr is well approximated by a 1rf dependence, scales of 1.6 kyr to 25 kyr from Meynadier et al. where f is the frequency. Variations in the intensity Ž.1992 , marine sediment data from the Pacific and of the geomagnetic field in one polarity exhibit a Indian Oceans from 25 kyr to 4 Myr from Meynadier normal distribution. When a fluctuation crosses the et al.Ž. 1994 , and marine sediment data from 10 kyr zero intensity value a reversal occurs. We test the to 10 Myr from Tauxe and HartlŽ. 1997 . The data hypothesis that reversals are the result of intensity were published in table form in KovachevaŽ. 1980 , variations with a 1rf power spectrum which occa- obtained from L. MeynadierŽ. Meynadier, 1995 for sionally are large enough to cross the zero intensity the marine sediment data in Meynadier et al.Ž. 1992 value, driving the geodynamo into the opposite po- and Meynadier et al.Ž. 1994 , and obtained from L. larity state. Synthetic time series with a 1rf power TauxeŽ. Tauxe, 1998 for the marine sediment data in spectrum and a binormal distribution are used to Tauxe and HartlŽ. 1997 . The archeomagnetic data generate reversal statistics. These are found to be in from KovachevaŽ. 1980 is a master curve for south- good agreement with those of the real reversal his- eastern Europe. The intensity is expressed in terms tory. The reversal statistics are sensitive to the form of the Virtual Axial Dipole MomentŽ. VADM where of the power spectrum of intensity variations. We the effects of latitude of the measurement locations conclude that the agreement between the synthetic were taken into account in estimating the strength of reversal record produced with 1rf noise variations Earth’s dipole moment. This calibration is important in the intensity of Earth’s magnetic field is strong for comparison with marine sediment data on longer support for 1rf behavior over the length of the time scales which are calibrated in units of VADM reversal record. This suggests that processes internal by comparison with the average paleointensity esti- to the coreŽ. autocyclic may determine geomagnetic mated from volcanic lavas from the same time range variability up to very long time scales. This contrasts as that of each sediment core. Thus, although each with the hypothesis that internal core processes dom- core represents variations in the geomagnetic field at inate secular variations while changes in conditions different locations, they are calibrated with respect to at the core–mantle boundary determine variations on the estimated dipole intensity so that they may be longer time scalesŽ. McFadden and Merrill, 1995 . compared and compiled to obtain information of the variation of the geomagnetic field over a continuous range of frequencies. 2. Power-spectral analyses of the virtual axial Techniques for assessing the paleointensity of dipole moment Earth’s magnetic field are reviewed in TauxeŽ. 1993 . King et al.Ž. 1983 has established minimum quality Paleomagnetic studies clearly show that the polar- standards for establishing the reliability of a sedi- ity of the magnetic field has been subject to rever- ment core to determine accurate relative paleointen- sals. KonoŽ. 1971 and Tanaka et al. Ž. 1995 have sity measurements. All of the cores we analyze compiled paleointensity measurements of the mag- satisfy these criteriaŽ Meynadier et al., 1992; Mey- J.D. PelletierrPhysics of the Earth and Planetary Interiors 110() 1999 115–128 117 nadier et al., 1994; Tauxe and Hartl, 1997. The different frequencies is unchanged. This can also be cores were tested for the homogeneity of rock mag- seen by considering the units of power-spectral den- netic properties and for the stationarity of sedimenta- sity: field intensity squared multiplied by time. If the tion rate. Meynadier et al.Ž. 1992, 1994 noted that time scale changes by a constant factor, the power- except for the topmost sections, their cores had spectral density at all frequencies simply goes up or homogeneous magnetic properties within the stan- down by the same factor and the exponent is un- dards of King et al.Ž. 1983 . The cores of Tauxe and changed. HartlŽ. 1997 were described as ‘remarkably consis- Marine sediment data can be accurate measures of tent’ in terms of their magnetic properties. All cores relative paleointensity but give no information on feature steady sedimentation rates. The effect of absolute intensity. In order to calibrate marine sedi- variable sedimentation rate has been investigated by ment data, the data must be compared to absolute Guyodo and ValetŽ. 1996 . They found that variable paleointensity measurements from volcanic lavas sedimentation rate can act as a high-pass filter re- sampled from the same time period as the sediment moving some of the variance at the highest frequen- record. Meynadier et al.Ž. 1994 have done this for cies of the core. However, this is not expected to be the composite Pacific and Indian Ocean dataset. the case in these cores due to their uniform sedimen- They have calibrated the mean paleointensity in terms tation rates. The other potential uncertainty in the of the virtual axial dipole moment for 0–4 Ma as data we analyze is associated with the time scale 9=1022 Am 2 Ž.
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