The intensity of the Earth's magnetic field over the past 160 million years

Abstract

Nature © Macmillan Publishers Ltd 1998 8 letters to nature 878 NATURE | VOL 394 | 27 AUGUST 1998reflection does not vary significantly along-axis 9 , so we use a waveform inversion method that assumes that the layers are horizontally stratified. The inversion scheme is implemented in intercept time-slowness domain 23 . This transformation also allows a clear representation of the AMC and converted arrival (P melt S) to be seen (Fig. 3) without interference of slow-phase-velocity events such as the sea-floor reflection. The waveform inversion method determines the P-and S-wave velocities of the crust by improving the fit between observed data and synthetically calculated data. It is an automated method, and therefore reduces human bias and also provides error estimates on the final solution 6,23 . The large-scale initial P-wave velocity was taken from Tolstoy et al. 9 . The initial S-wave velocity was obtained from the P-wave velocity assuming a Poisson's ratio of 0.26 (ref. 24). We used a P-wave attenuation coefficient of 16 for the first 200 m, and 40-90 for the crust below 24 . The S-wave attenuation coefficient was half of the P-wave attenuation coefficient. The model consists of a stack of 8-m-thick layers, for which the P-wave and S-wave velocities, density and attenuation are defined. This sampling interval was based on the minimum thickness that can be resolved from the observed data, which has a frequency bandwidth of 5-30 Hz. As data from all the slownesses were inverted simultaneously, the inverted results contain a model which is consistent with the data from all slownesses, and is therefore less likely to be influenced by incoherent noise due to two-and three-dimensional effects.The turning rays above the sill and the P-wave reflection arrivals from the AMC constrain the P-wave and S-wave velocities above the AMC. Further constraint on S-wave velocity structure above the AMC comes from the arrival time of the P melt S phase. For this phase, the P-to-S conversion occurs at the solid-fluid interface of the AMC and at the sea bed such that the wave travels one leg between the sea bed and AMC as a P-wave, and the other as an S-wave. The waveforms of the AMC reflection and the P melt S phase constrain P-wave and S-wave velocities within the AMC and its neighbourhood. However, as with all seismic techniques, we have to guard against non-uniqueness of the final solution. To gain confidence in our results, we altered individual features within various candidate models while keeping the rest of the model fixed, and re-ran the inversion; the models that gave the best fit with the data are presented here. Stacking. To enhance the partial-stack images, we firstly applied a frequencywavenumber filter to the CMP gathers to remove sea-floor contamination of the AMC events, then performed a normal-moveout correction of 2.10 and 1.85 km s −1 for the P-wave and S-wave image, respectively. The partial stacks were obtained by stacking data between 2 and 3 km offsets, and all other processing parameters were as for the ϳ...

The Cretaceous superchron geodynamo: Observations near the tangent cylinder

“…However, the mean value is 2-3 times greater, with much less variation. For example, some estimates of the field strength over the last 5 million years suggest a SD of 30% to Ͼ40% of the mean (38,39), several times greater than results reported here.…”

“…The presence of a stable, efficient dynamo during the K-N Superchron is also supported by our analyses of intensity. The field intensity values we report are not outside the range of virtual dipole moments reported from 0-5 Ma basalt and basaltic glass (38,39). However, the mean value is 2-3 times greater, with much less variation.…”

The intensity of the time-averaged geomagnetic field: the last 5 Myr

“…[8]) generally have average values that are lower than the present geomagnetic ¢eld [9]. Finally, the average value of the Cenozoic and late Mesozoic has recently been shown to be half that of the present ¢eld [1]. These indirect clues hint that the average ¢eld during the so-called`Mesozoic dipole low' [10] was perhaps closer to the time-averaged value of the geomagnetic ¢eld than is the present dipole moment.…”

Paleointensity determination of Late Cretaceous basalts in northwest South Korea: implications for low and stable paleofield strength in the Late Cretaceous

“…The directional records of the geomagnetic field are well documented for that time, but the characteristics of paleointensity are not as well known, even though the paleointensity during the Late Cretaceous plays an important role in understanding the Earth's evolution in relation to geomagnetic field reversal and heat flow across the core-mantle boundary. It is notable that the PINT database (Biggin et al, 2010) presents only four paleointensity studies (Juárez et al, 1998;Smirnov and Tarduno, 2003;Tauxe, 2006;Shcherbakova et al, 2007) satisfying modern standards (i.e., the use of the doubleheating method with alteration checks) within the time interval 83-72 Ma. Of these, three studies used submarine basaltic glass (SBG) as an experimental material: Juárez et al (1998) reported a high dipole moment of 9.0 ± 0.7 × 10 22 Am 2 at ∼72 Ma; Tauxe (2006) suggested 11.3 ± 0.7 × 10 22 Am 2 at ∼72 Ma and 6.8 ± 5.1 × 10 22 Am 2 at ∼77.6 Ma; and Smirnov and Tarduno (2003) documented a low dipole moment of 2.1 ± 0.7 × 10 22 Am 2 at ∼76 Ma.…”

“…It is notable that the PINT database (Biggin et al, 2010) presents only four paleointensity studies (Juárez et al, 1998;Smirnov and Tarduno, 2003;Tauxe, 2006;Shcherbakova et al, 2007) satisfying modern standards (i.e., the use of the doubleheating method with alteration checks) within the time interval 83-72 Ma. Of these, three studies used submarine basaltic glass (SBG) as an experimental material: Juárez et al (1998) reported a high dipole moment of 9.0 ± 0.7 × 10 22 Am 2 at ∼72 Ma; Tauxe (2006) suggested 11.3 ± 0.7 × 10 22 Am 2 at ∼72 Ma and 6.8 ± 5.1 × 10 22 Am 2 at ∼77.6 Ma; and Smirnov and Tarduno (2003) documented a low dipole moment of 2.1 ± 0.7 × 10 22 Am 2 at ∼76 Ma. The only study to have obtained paleointensity data from basalts (Shcherbakova et al, 2007) showed a low dipole moment of 3.3 ± 0.1 × 10…”