<i>The Magnetic Field of the Earth: Paleomagnetism, the Core, and the Deep Mantle</i>

Merrill, Ronald T.; McElhinny, M. W.; McFadden, P. L.; Banerjee, Subir K.

doi:10.1063/1.881919

Cited by 236 publications

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“…The fact that geomagnetic field measurements at the Earth's surface have changed over time (secular variation) was first noted by the Chinese by the thirteenth Century AD and later by Henry Gellibrand in Europe in 1634 (Merrill et al, 1998). We now have a good sense of historical secular variation (HSV) of the geomagnetic field for the last 400 years or so (e.g., Bullard et al, 1950;Yukutake, 1967;Yukutake and Tachinaka, 1968;Bloxham and Gubbins, 1985;Jackson et al, 2000).…”

Section: Introductionmentioning

confidence: 99%

“…However, paleomagnetic studies suggest that the average field direction at a locality during stable polarity (the period between global polarity reversals) is generally that of a dipole located at the Earth's center and aligned with the rotation axis; this hypothesis is termed the geocentric axial dipole (GAD) hypothesis (see Merrill et al, 1998 for overview). Merrill et al (1998) noted that "the interpretation of paleomagnetic results has always depended upon the fundamental hypothesis that the time-averaged geomagnetic field is that of a geocentric axial dipole." But, a large number of statistical PSV studies have made it clear that there are systematic (but usually subtle) differences in local field directions from the GAD when averaged over millions of years (e.g., Johnson and Constable, 1996;McElhinny et al, 1996;Merrill et al, 1998;Johnson et al, 2008) or shorter intervals (e.g., Lund et al, 1988Lund et al, , 2016a.…”

Section: Introductionmentioning

confidence: 99%

“…Merrill et al (1998) noted that "the interpretation of paleomagnetic results has always depended upon the fundamental hypothesis that the time-averaged geomagnetic field is that of a geocentric axial dipole." But, a large number of statistical PSV studies have made it clear that there are systematic (but usually subtle) differences in local field directions from the GAD when averaged over millions of years (e.g., Johnson and Constable, 1996;McElhinny et al, 1996;Merrill et al, 1998;Johnson et al, 2008) or shorter intervals (e.g., Lund et al, 1988Lund et al, , 2016a. The unresolved problem is what causes these non-GAD field directions and how do they vary over time-scales less than a million years.…”

Section: Introductionmentioning

confidence: 99%

“…Magnetic compass readings have been associated with the Earth as their source since perhaps the thirteenth Century AD; experiments by, first, Petrus Peregrinus (1269) and then Alexander Needham (1600) put that theory on firm ground (see historical summary in Merrill et al, 1998). The fact that geomagnetic field measurements at the Earth's surface have changed over time (secular variation) was first noted by the Chinese by the thirteenth Century AD and later by Henry Gellibrand in Europe in 1634 (Merrill et al, 1998).…”

Section: Introductionmentioning

confidence: 99%

“…The actual amount of time necessary to average the full range of local secular variation is still not well established (e.g., Merrill et al, 1998). However, paleomagnetic studies suggest that the average field direction at a locality during stable polarity (the period between global polarity reversals) is generally that of a dipole located at the Earth's center and aligned with the rotation axis; this hypothesis is termed the geocentric axial dipole (GAD) hypothesis (see Merrill et al, 1998 for overview). Merrill et al (1998) noted that "the interpretation of paleomagnetic results has always depended upon the fundamental hypothesis that the time-averaged geomagnetic field is that of a geocentric axial dipole."…”

Section: Introductionmentioning

confidence: 99%

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A New View of Long-Term Geomagnetic Field Secular Variation

Lund

2018

Front. Earth Sci.

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This study carries out a statistical analysis of high-resolution PSV records for the last ∼70 ka from three different regions of the Earth. We consider directional and intensity variability in each region on time scales of 10 3 -10 5 years in order to evaluate long-term PSV. We then compare those results with more traditional long-term PSV statistical studies averaged over ∼10 6 years. Three replicate PSV records from one region (subtropical North Atlantic Ocean) were averaged at overlapping 3 and 9 ka intervals. Variability in both scalar inclination and declination variability and vector angular dispersion are significant and coherent among the three records. The vector dispersion is relatively low for most of the time but contains two relatively narrow intervals (∼30-42 and 60-65 ka) of high dispersion. (Vector dispersion in all records was calculated after removing directions with true excursional VGPs, VGPs < 45 • N). We have carried out a comparable statistical analysis on two other PSV records from other parts of the Earth (Chile margin; Philippines/Indonesia). The results for these three regions are comparable in their overall style of variability. The scalar directional variability from the Philippines/Indonesia is quite different in detail from the other two regions, as might be expected, but the scalar directional variability between the Western Hemisphere regions is remarkably consistent considering their distance from one another. This may be associated with them being on the same longitude swath and having some coherent dynamo activity occurring along that path. Three magnetic field excursions occur in the study interval. All three excursions are associated with the two highest vector dispersion intervals. Paleointensity records from the three regions were subjected to the same statistical analysis as the directions. These records are all coherent in their pattern of variability. The similarity in paleointensity variability on a global scale is expected even though the detailed scalar directional variability is not coherent on a global scale. The pattern of intensity variability is strongly correlated with the pattern of vector dispersion and excursions on a global scale-high (low) intensity is associated with low (high plus excursions) vector dispersion. The fact that regional directional variability is always larger than "normal" during low intensity/excursional intervals, even though the effect of true excursional directions was removed, suggests that we need to reevaluate what field variability was like during low intensity/excursional intervals on a global scale and how/why it was different from today's field (last 10 4 years).

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

A New View of Long-Term Geomagnetic Field Secular Variation

Lund

2018

Front. Earth Sci.

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Apparent timing and duration of the Matuyama‐Brunhes geomagnetic reversal in Chinese loess

Zhao

Qiang

Sun

2014

Geochem Geophys Geosyst

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The Matuyama-Brunhes (MB) geomagnetic reversal in Chinese loess has been studied extensively as an important boundary for land-ocean stratigraphic and paleoclimatic correlations. However, the apparent timing and duration of the MB boundary remain controversial in Chinese loess deposits due to its inconsistent stratigraphic position and the uncertain chronologies. Here we synthesized high-resolution paleomagnetic records from four loess sequences in the central Chinese Loess Plateau and synchronized the loess-paleosol chronology by matching the grain-size variations to orbitally tuned grain-size time series. The synthesized paleomagnetic results reveal consistent features of the MB transition in Chinese loess, including the stratigraphic position (L 8 /S 8 transition), timing (808-826 ka), duration (14-16 ka), and rapid directional oscillations. Compared with the MB transition in marine records (770-775 ka), the timing of the MB transition is relatively older and longer in Chinese loess, due to a complex interplay between different remanence acquisition mechanisms which occurred during the course of postdepositional physical and chemical processes.

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Paleointensity determinations from the Etendeka province, Namibia, support a low‐magnetic field strength leading up to the Cretaceous normal superchron

Dodd

Muxworthy

Niocaill

2015

Geochem. Geophys. Geosyst.

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Paleointensity estimates provide much needed information on field generation within Earth's core and upon the convective processes at work within the mantle. We present new paleointensity estimates from the early Cretaceous Etendeka large igneous province in Namibia (135Ma), which add to the sparse Southern Hemisphere data set. The Early Cretaceous marks an important change in the Earth's magnetic field from a state of rapid polarity reversals, to one of long-term stability associated with the onset of the Cretaceous Normal Superchron at 121 Ma. Paleointensity determinations, using the IZZI protocol, were carried out on a total of 172 specimens from 14 sites encompassing the exposed stratigraphy of the Etendeka province. Numerous checks of data reliability were considered before results were accepted, including partial thermoremanent magnetization (pTRM) checks and pTRM tail checks, hysteresis properties, thermomagnetic analyses, observations under reflected light, and changes to room-temperature susceptibility during the experiments. Following these checks, a total of 64 individual samples from five sites were considered to provide reliable paleointensity determinations. These results were combined to provide site mean data with an overall average virtual dipole moment (VDM) for the study of 2.5 6 1.0 3 10 22 Am 2 . This value equates to approximately 30% of present Earth's field and, when considered alongside existing studies, suggests that Earth's field strength was low in the time leading up to the Cretaceous normal superchron.

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The Magnetic Field of the Earth: Paleomagnetism, the Core, and the Deep Mantle

Cited by 236 publications

References 0 publications

A New View of Long-Term Geomagnetic Field Secular Variation

A New View of Long-Term Geomagnetic Field Secular Variation

Apparent timing and duration of the Matuyama‐Brunhes geomagnetic reversal in Chinese loess

Paleointensity determinations from the Etendeka province, Namibia, support a low‐magnetic field strength leading up to the Cretaceous normal superchron

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