Register to reply

Geomagnetic Reversal 41k BP 240 years Paradox? Mechanism?

by betzalel
Tags: geomagnetic, mechanism, paradox, reversal
Share this thread:
Oct25-12, 09:38 PM
P: 40
There has been a series of papers recently published that provide observational data that indicates that something is cyclically abruptly changing the geomagnetic field.

The rapidity of the observed geomagnetic field changes cannot be explained by a core based mechanism. There is no geological mechanism that can change the liquid core cyclically and quickly enough to cause the observed surface geomagnetic field changes.

As the mantel is conductive and liquid core is conductive, counter emfs are generated that resit rapid core based changes. For that reason geomagnetic field reversals and excursions where believed based on the assumed toy model to take roughly 2000 to 5000 years.

These rapid paradoxical geomagnetic excursions correlate with abrupt climate changes and with super volcano eruptions.

The geomagnetic field changes come in a small, medium, and large. The small and medium geomagnetic field changes have been shown to correlate with very large solar magnetic cycle changes.

Some 41,000 years ago, a complete and rapid reversal of the geomagnetic field occured. Magnetic studies of the GFZ German Research Centre for Geosciences on sediment cores from the Black Sea show that during this period, during the last ice age, a compass at the Black Sea would have pointed to the south instead of north.

What is remarkable is the speed of the reversal: "The field geometry of reversed polarity, with field lines pointing into the opposite direction when compared to today's configuration, lasted for only about 440 years, and it was associated with a field strength that was only one quarter of today's field," explains Norbert Nowaczyk. "The actual polarity changes lasted only 250 years. In terms of geological time scales, that is very fast." During this period, the field was even weaker, with only 5% of today's field strength. As a consequence, Earth nearly completely lost its protection shield against hard cosmic rays,
The largest volcanic eruption on the Northern hemisphere in the past 100,000 years, namely the eruption of the super volcano 39,400 years ago in the area of today's Phlegraean Fields near Naples, Italy, is also documented within the studied sediments from the Black Sea. The ashes of this eruption, during which about 350 cubic kilometers of rock and lava were ejected, were distributed over the entire eastern Mediterranean and up to central Russia.

Is the geodynamo process intrinsically unstable?

Recent palaeomagnetic studies suggest that excursions of the geomagnetic field, during which the intensity drops suddenly by a factor of 5 to 10 and the local direction changes dramatically, are more common than previously expected. The `normal' state of the geomagnetic field, dominated by an axial dipole, seems to be interrupted every 30 to 100 kyr; it may not therefore be as stable as we thought. We have investigated a possible mechanism for the instability of the geodynamo by calculating the critical Rayleigh number (Rc) for the onset of convection in a rotating spherical shell permeated by an imposed magnetic field with both toroidal and poloidal components.

Recent studies suggest that the Earth's magnetic field has fallen dramatically in magnitude and changed direction repeatedly since the last reversal 700 kyr ago (Langereis et al. 1997; Lund et al. 1998). These important results paint a rather different picture of the long-term behaviour of the ¢eld from the conventional one of a steady dipole reversing at random intervals: instead, the field appears to spend up to 20 per cent of its time in a weak, non-dipole state (Lund et al. 1998). One of us (Gubbins 1999) has suggested that this is evidence of a rapid natural timescale (500 yr) in the outer core, and that the magnetic field is usually prevented from reversing completely by the longer diffusion time of the inner core (2 to 5 kyr). This raises a number of important but difficult questions for geodynamo theory. How can the geomagnetic field change so rapidly and dramatically? Can slight variations of the geomagnetic field affect the dynamics of core convection significantly? If so, is the geodynamo process intrinsically unstable?
Phys.Org News Partner Earth sciences news on
Oct25-12, 09:52 PM
P: 40
The following are additional papers that support the above comment.

Geomagnetic moment variation and paleomagnetic excursions since 400 kyr BP: a stacked record from sedimentary sequences of the Portuguese margin

A paleomagnetic study was performed in clayey-carbonate sedimentary sequences deposited during the last 400 kyr on the Portuguese margin (Northeast Atlantic Ocean). Declination and inclination of the stable remanent magnetization present recurrent deviations from the mean geomagnetic field direction. The normalized intensity documents a series of relative paleointensity (RPI) lows recognized in other reference records. Three directional anomalies occurring during RPI lows chronologically correspond to the Laschamp excursion (42 kyr BP),the Blake event (115-122 kyr BP) and the Icelandic basin excursion (190 kyr BP). A fourth directional anomaly recorded at 290 kyr BP during another RPI low defines the ‘Portuguese margin excursion’. Four non-excursional RPI lows are recorded at the ages of the Jamaica/Pringle Falls, Mamaku, Calabrian Ridge 1, and Levantine excursions. The RPI record is characterized by a periodicity of V100 kyr, paleointensity lows often coinciding with the end of interglacial stages. This record sets the basis of the construction of an authigenic 10Be/9Be record from the same sedimentary sequences [Carcaillet et al.,this issue].

Spectral analyses of the RPI record reveal a dominant periodicity at 100 kyr,already reported by other studies (e.g. [41,59,60]). The RPI and N18O records also present a phase shift of 18 kyr: RPI lows often coincide with the end of interglacial or interstadial stages. The geomagnetic moment loss (greater than 30%) over the last two millennia deduced from archeomagnetic results (e.g. [38,39]) might foreshadow the next excursion for the end of our present interglacial, even though this loss started 2200 years ago from an exceptionally high geomagnetic moment value.

Geomagnetic excursion captured by multiple volcanoes in a monogenetic field

Five monogenetic volcanoes within the Quaternary Auckland volcanic field are shown to have recorded a virtually identical but anomalous paleomagnetic direction (mean inclination and declination of 61.7° and 351.0°, respectively), consistent with the capture of a geomagnetic excursion. Based on documented rates of change of paleomagnetic field direction during excursions this implies that the volcanoes may have all formed within a period of only 50–100 years or less. These temporally linked volcanoes are widespread throughout the field and appear not to be structurally related. However, the general paradigm for the reawakening of monogenetic fields is that only a single new volcano or group of closely spaced vents is created, typically at intervals of several hundred years or more. Therefore, the results presented show that for any monogenetic field the impact of renewed eruptive activity may be significantly under-estimated, especially for potentially affected population centres and the siting of sensitive facilities.
Oct26-12, 05:06 PM
PF Gold
Andre's Avatar
P: 5,458
Quote Quote by betzalel View Post
The following are additional papers that support the above comment.
That's very, intruegingly, interesting, thanks for posting.

Even more interesting that the Lachamps event can't really be related to any other event. Maybe vaguely the Australian mammal extinction, but then again why not elsewhere.

Will we ever understand what's going on?

Register to reply

Related Discussions
What is the evidence for geomagnetic reversal ? Earth 49
Geomagnetic reversal and pole shift Earth 12
Geomagnetic propulsion General Physics 6
Geomagnetic Field Reversal Earth 5
Entropy, event reversal, and a paradox Classical Physics 4