Saturday, December 05, 2009

Expanding Earth Model and Pre-Cambrian Era: Part IV

I have divided the discussion of Expanding Earth model for pre-Cambrian period in short postings: 5 altogether. In the first posting I discussedd the notions of super-continents, super-oceans, and glaciations during Neoproterozoic period with short coments inspired by R/2 scenario. In second posting I summarized the dominating Snowball model for the climate during Neoproterozoic period in the same spirit. In the third posting I dicussed TGD Expanding Earth view about super-continents, super-oceans and glaciations and compares it with Snowball Earth scenario. This posting is devoted to paleomagnetic tests for R/2 scenario. Fifth posting will discuss the Expanding Earth scenario about how life escaped to the underground lakes and seas created during the expansion and returned back in the beginning of Cambrian era.

Paleo-magnetic data and Expanding Earth model

Paleomagnetic data from pre-Cambrian period might allow to test R/2 hypothesis. This data could in principle help to trace out the time development R(t) from R/2 to R if the non-dipole contribution to magnetic field depends on R(t).

1. About paleo-magnetism

Paleomagnetism (see this) provides quantitative methods to determine the latitude at which the sample of sedimentary rock was originally. Magnetic longitude cannot be determined because of rotational symmetry so that other information sources must be used. There are several methods allowing to deduce the direction and also the magnitude of the local magnetic field and from this the position of the sample during the time the sample was formed.

  1. Below the Curie point thermal remanent magnetization is preserved in basalts of the ocean crust and not affected by the later magnetic fields unless they are too strong. This allows to deduced detail maps from continental drifting and polar wander maps after 250 Myr (Pangea period). During pre-Cambrian period the ocean floors of hypothetical oceans would have disappeared by subduction. In R/2 model there are no oceans: only inland seas.

  2. In the second process magnetic grains in sediments may align with the magnetic field during or soon after deposition; this is known as detrital remnant magnetization (DRM). If the magnetization is acquired as the grains are deposited, the result is a depositional detrital remnant magnetization (dDRM); if it is acquired soon after deposition, it is a post-depositional detrital remnant magnetization (pDRM).

  3. In the third process magnetic grains may be deposited from a circulating solution, or be formed during chemical reactions, and may record the direction of the magnetic field at the time of mineral formation. The field is said to be recorded by chemical remnant magnetization (CRM). The mineral recording the field commonly is hematite, another iron oxide. Red-beds, clastic sedimentary rocks (such as sandstones) that are red primarily because of hematite formation during or after sedimentary diagenesis, may have useful CRM signatures, and magnetostratigraphy (see this) can be based on such signatures. Snowball model predicts that nothing came to the bottoms of big oceans! How can we know that they existed at all!

During pre-Cambrian era the application of paleomagnetic methods (see this) is much more difficult.

  1. Reliable paleomagnetic data range up to 250 My, the period of Pangaea, and magnetization direction serves as a reliable information carrier allowing detailed polar wander maps. During pre-Cambrian era one cannot use polar wander maps and the polarity of the magnetic field is unknown. Therefore theoretical assumptions are needed including hypothetical super-continents, hypothetical oceans, and continental drift and plate tectonics. All this is on shaky grounds since no direct information about super-continents and ancient oceans exists. R/2 model suggests that continental drift and plate tectonics have not been significant factors before the expansion period when only inland seas and polar ice caps were present. Measurements have been however carried out about magnetization for pre-Cambrian sediments at continents recently and gives information about the strength of the magnetic field (see this): the overall magnitude of the magnetic field is same as nowadays.

  2. At Precambrian period the orientation of iron rich materials can serve as a record. The original records can be destroyed by various mechanisms (diagenesis). Also the orientations of the sediments can change in geological time scales.

  3. Tens of thousands of reversals of the magnetic polarity (see this) have occurred during Earth's history. There have been long periods of stability and periods with a high frequency of reversals. The average duration of glaciation is around one Myr. The determination of the polarity of B possible by using samples from different points.

  4. Mountain building orogeny (see this) releases hot water as a byproduct. This water can circulate in rocks thousands of kilometers and can reset the magnetic signature. The formation of fractures during the expansion of Earth could have released hot water having the same effect.

2. Could paleomagnetic data kill or prove R/2 model?

The first question is how one might kill R/2 model using data from pre-Cambrian era. Paleomagnetic data could do the job.

  1. Remanent magnetization is proportional to the value of magnetic field causing it in weak magnetic fields. Therefore the magnetization in principle gives information about the magnetic fields that prevailed in early times.

  2. Suppose that the currents generating the magnetic field can be idealized to conserved surface currents K around cylindrical surfaces of radius r and height h scaled down to to r/2 and h/2 and that the value of K is not affected in the process. With this assumptions the magnetic moment behaves μ ~ I r2h→ μ/8. A continuous current vortices with j = k/r, which is ir-rotational outside the symmetry axis, produce a similar result if the radius of the vortices scales as r→ r/2. Since dipole magnetic field scales as 1/r3 and is scaled up by a factor 8 in R→ R/2, the scalings compensate and the dipole magnetic fields at surface do not allow to distinguish between the two options. Non-dipole contributions might allow to make the distinction.

  3. The group led by Lauri J. Pesonen in Helsinki University (see this) has studied paleomagnetic fields at pre-Cambrian era. The summary of results is a curve at the home page of the group and shows that the scale of the magnetic during pre-Cambrian era is same as nowadays. On the other hand, the recent thesis by Johanna Salminen- one of the group members- reports abnormally high values of magnetization in Pre-Cambrian intrusions and impact structures in both Fennoscandia and South Africa (see this). No explanation for these values has been found but it is probably not the large value of primary magnetization.

Another manner to do test the R/2 model is by comparing the signs of the magnetizations at magnetic equator and poles. They should be of opposite sign for dipole field. The polarity of magnetic field varies and there are no pre-Cambrian polar wander maps. One can deduce from the condition Br/rBθ = 2cot(θ) holding true for dipole field the azimutal distance Δθ along the direction of the measured magnetic field to the pole along geodesic circle in the direction of the tangential component of B. One cannot however tell the sign of Δθ, in other words whether a given pre-Cambrian sample belongs to Norther or Southern magnetic hemisphere. There are however statistical methods allowing to estimate the actual pole position using samples from several positions (for an excellent summary see (see this).

For instance, if the magnetic field is in North-South direction during Rodinian period (see this), standard model would predict that the sign at the Equator is opposite to that at South Pole. In R/2 model the sample would be actually near North Pole and polarizations would have same sign. The sign of magnetization at apparent southern latitude around 45 degrees would have been opposite to that at South pole which is in conflict with dipole field character. Maybe the global study of magnetization directions when magnetic field was approximately in North-South direction could allow to find which option is correct. Also the dependence of the strength of the magnetic field as function of θ could reveal whether R/2 model works or not. The testing requires precise dating and position determination of the samples and a detailed model for the TGD counterpart of Rodinia and its construction requires a specialist.

If the expansion continued after 250 Myr with an accelerating rate and Earth radius was still considerably below its recent value, the comparison of pole wandering charts deduced from ocean floor paleomagnetic data at faraway locations might allow to show that the hypothesis about dipole field is not globally consistent for R option. Even information about the time evolution of the radius could be deduced from the requirement of global consistency.

For details see the new chapter Expanding Earth Model and Pre-Cambrian Evolution of Continents, Climate, and Life of "Genes and Memes".


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