The geomagnetic field intensity measured at the surface of the planet is a potential indicator of the dynamo activity in the conducting liquid core of the Earth. Rapid field variations must be generated by the rapid fluid motions within the core, whereas long-term changes could be associated with other processes such as changes in boundary conditions at the core–mantle interface and/or at the outer (liquid)–inner (solid) core boundary. For about 50 years, paleomagnetists gathered records of absolute paleointensity using the magnetization of lava flows distributed over the globe in order to reconstruct the field intensity changes during the past 3000Ma. Despite recent acquisition of paleointensity records, the temporal distribution of the results remains limited (except within a few time intervals) to extract coherent features of the time-averaged field intensity over periods of a few million years. However, significant informations can be gained from the recent updated paleointensity database by averaging the virtual dipole moment over very long time intervals. The results suggest the existence of a long-term evolution of the dipole field intensity during the past 3 billion years (from 3 × 1022Am2 at 1000–2000Ma to 8 × 1022Am2 at present times).
"Paleointensity estimates (ratio of NRM to a 50 mT laboratory TRM on 150 samples) follow similar trends with depth, decreasing from $600 mT at the surface to $37 mT at $2 m depth. The latter value is in general agreement with the paleointensity of the Earth's magnetic field at $2 Ga (Figure 8) [Macouin et al., 2004]. REM (NRM/IRM(0.9 "
[Show abstract][Hide abstract] ABSTRACT: The Vredefort impact crater in South Africa is one of the oldest and
largest craters on Earth, making it a unique analog for planetary
basins. Intense and randomly oriented remanent magnetization observed in
surface samples at Vredefort has been attributed to impact-generated
magnetic fields. This possibility has major implications for
extraterrestrial paleomagnetism since impact-generated fields have been
proposed as a key alternative to the dynamo hypothesis for magnetization
on the Moon and asteroids. Furthermore, the presence of single-domain
magnetite found along shock-generated planar deformation features in
Vredefort granites has been widely attributed to the 2.02 Ga impact
event. An alternative hypothesis is that the unusual magnetization
and/or rock magnetic properties of Vredefort rocks are the products of
recent lightning strikes. Lightning and impact-generated fields can be
distinguished by measuring samples collected from below the present
surface. Here we present a paleomagnetic and rock magnetic study of
samples from two 10 m deep vertical boreholes. We show that the
magnetization at depth is consistent with a thermoremanent magnetization
acquired in the local geomagnetic field following the impact, while
random, intense magnetization and some of the unusual rock magnetic
properties observed in surface rocks are superficial phenomena produced
by lightning. Because Vredefort is the only terrestrial crater that has
been proposed to contain records of impact-generated fields, this
removes a key piece of evidence in support of the hypothesis that
paleomagnetism of the Moon and other extraterrestrial bodies is the
product of impacts rather than past core dynamos.
Journal of Geophysical Research Atmospheres 10/2012; 117(E1):1007-. DOI:10.1029/2011JE003919 · 3.43 Impact Factor
"Average paleointensity estimates for this time span differ slightly in different publications: 1H 0 in (Bol'shakov and Solodovnikov, 1981; Heller et al., 2003) and 0.7H 0 in (Macouin et al., 2004), possibly, for the data scarcity and high variability of the Permian–Carboniferous field. The paleointensity databases and synthesis of results from (Heller et al., 2003; Macouin et al., 2004) indicate quite a high field about 1H o . "
[Show abstract][Hide abstract] ABSTRACT: Published and new data on the Earth’s past magnetic field have been interpreted in terms of its links with the frequency of magnetic polarity reversals and with tectonic events such as plume-related eruptions and rifting. The paleointensity and reversal frequency variations show an antiphase correlation between 0 and 160 Ma, and the same tendency likely holds for the past 400 Myr. The geomagnetic field intensity averaged over geological ages (stages) appears to evolve in a linearly increasing trend while its variations increase proportionally in amplitude and change in structure. Both paleointensity and reversal frequency patterns correlate with rifting and eruption events. In periods of high rifting activity, the geomagnetic field increases (15 to 30%) and the reversals become about 40% less frequent. Large eruption events between 0 and 150 Ma have been preceded by notable changes in magnetic intensity which decreases and then increases, the lead being most often within a few million years.
"The oldest (3.2 Ga BP) reliable palaeointensity record to date (Tarduno et al. 2007) reveals that the virtual dipole moment (VDM) was possibly as large as its present value of 8 × 10 22 A m 2 , although the consideration of the experimental cooling rate effect may lead to a twofold decrease in the determined VDM. As a result, the debate concerning the long-term evolution of the VDM remains lively (Dunlop & Yu 2004), though there are suggestions (Macouin et al. 2004) of a long-term average monotonous increase from 3 × 10 22 A m 2 at 1000–2000 Myr to 8 × 10 22 A m 2 at present times. In addition, although it has been proposed (Stevenson et al. 1983; Hale 1987) that the power increase subsequent to inner core nucleation and onset of chemical convection in the core could cause a sudden increase in the dipole moment, it appears that until now, the palaeomagnetic data scatter has prevented a proper resolution of this feature. "
[Show abstract][Hide abstract] ABSTRACT: Although it is known that the geodynamo has been operating for at least 3.2 Ga, it remains difficult to infer the intensity, dipolarity and stability (occurrence of reversals) of the Precam-brian magnetic field of the Earth. In order to assist the interpretation of palaeomagnetic data, we produce models for the long-term evolution of the geodynamo by combining core ther-modynamics with a systematic scaling analysis of numerical dynamo simulations. We update earlier dynamo scaling results by exploring a parameter space, which has been extended in order to account for core aspect ratios and buoyancy source distributions relevant to Earth in the Precambrian. Our analysis highlights the central role of the convective power, which is an output of core thermodynamics and the main input of our updated scalings. As the thermal evolution of the Earth's core is not well known, two end-member models of heat flow evolution at the core–mantle boundary (CMB) are used, respectively, terminating at present heat flows of 11 TW (high-power scenario) and 3 TW (low power scenario). The resulting models predict that until the appearance of the inner core, a thermal dynamo driven only by secular cooling, and without any need for radioactive heating, can produce a dipole moment of strength compa-rable to that of the present field, thus precluding an interpretation of the oldest palaeomagnetic records as evidence of the inner core presence. The observed lack of strong long-term trends in palaeointensity data throughout the Earth's history can be rationalized by the weakness of palaeointensity variations predicted by our models relatively to the data scatter. Specifically, the most significant internal magnetic field increase which we predict is associated to the sudden power increase resulting from inner core nucleation, but the dynamo becomes deeper-seated in the core, thus largely cancelling the increase at the core and Earth surface, and diminishing the prospect of observing this event in palaeointensity data. Our models additionally suggest that the geodynamo has lied close to the transition to polarity reversals throughout its history. In the Precambrian, we predict a dynamo with similar dipolarity and less frequent reversals than at present times, due to conditions of generally lower convective forcing. Quantifying the typical CMB heat flow variation needed for the geodynamo to cross the transition from a reversing to a non-reversing state, we find that it is unlikely that such a variation may have caused superchrons in the last 0.5 Ga without shutting down dynamo action altogether.
Geophysical Journal International 12/2009; 179:1414-1428. DOI:10.1111/j.1365-246X.2009.04361.x · 2.56 Impact Factor
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