February 19, 2016 | David F. Coppedge

Are Paleomagnetic Measurements Reliable?

Dating of past geophysical history depends on magnetic measurements that may be in error.

Geophysicists often speak confidently about when magnetic reversals occured, as in this press release from Carnegie Science:

Earth’s magnetic field is generated by the motion of liquid iron in the planet’s core. This “geodynamo” occasionally reverses its polarity—the magnetic north and south poles swap places. The switch occurs over a few thousand years, and the time between reversals can vary from some tens of thousands to tens of millions of years.

When magnetic polarity remains stable in one orientation for more than 10 million years the interval is dubbed a “superchron.” Within the last 540 million years—the time when animals have roamed the Earth’s land and seas—there are three known superchron periods, occurring about once every 200 million years.

It’s not clear, though, why field orientation should go through long stable periods separated by reversals of varying time frames orders of magnitude faster than the superchrons. Is the behavior of the Earth’s magnetic field that irregular? Or, is it possible the data they base this on is unreliable?

A recent paper in Nature Communications may give pause to geophysicists who assume they can infer past magnetic field orientations reliably. In “Microbially assisted recording of the Earth’s magnetic field in sediment,” European geophysicists examined the possibility that microbial bioturbation rotates, translates or randomizes magnetic field records in sediments.

Sediments continuously record variations of the Earth’s magnetic field and thus provide an important archive for studying the geodynamo. The recording process occurs as magnetic grains partially align with the geomagnetic field during and after sediment deposition, generating a depositional remanent magnetization (DRM) or post-DRM (PDRM). (P)DRM acquisition mechanisms have been investigated for over 50 years, yet many aspects remain unclear. A key issue concerns the controversial role of bioturbation, that is, the mechanical disturbance of sediment by benthic organisms, during PDRM acquisition. A recent theory on bioturbation-driven PDRM appears to solve many inconsistencies between laboratory experiments and palaeomagnetic records, yet it lacks experimental proof. Here we fill this gap by documenting the important role of bioturbation-induced rotational diffusion for (P)DRM acquisition, including the control exerted on the recorded inclination and intensity, as determined by the equilibrium between aligning and perturbing torques acting on magnetic particles.

Scientists may assume, in other words, that they are seeing a DRM (depositional remanent magnetism) signature—i.e., the record of the magnetic field at the time the rock formed—when they in fact are seeing a PDRM (post-DRM) alteration. That alteration can come about through microbes and other bioturbating animals who put a spin on the remanent magnetism or move it about similar to random Brownian motion.

Regardless of the acquisition mechanism, most PDRM models assume that lock-in of magnetization only begins once substantial surface mixing has ceased, that is, below the mixed layer, so that DRM and PDRM are mutually exclusive or almost so. A different viewpoint arises from a statistical model of PDRM acquisition in the surface-mixed layer. This model considers bioturbation as a rotational diffusion process similar to that of Brownian motion, which occurs in the presence of random inter-particle forces.

Sediments are assumed to take on the orientation of the magnetic field when they settle into rock layers. A figure caption in the paper, though, lists 7 “natural processes affecting sedimentary magnetizations“: “1: Marine snow, 2: flocculation, 3: settling, 4: sediment resuspension, 5: non-local mixing, for example, by polychaete worms, 6: local (diffusive) sediment mixing leading to particle reorientation and 7: burial in the consolidating layer.” The team’s experiments with sediments settling at the bottoms of vials seems to show significant “particle realignment after deposition,” giving rise to a PDRM instead of a DRM.

The authors are not clear about the degree of randomization possible in the rock record, or how post-depositional alterations might render interpretations of past magnetism questionable. It appears clear, though, that geophysicists should not assume that remanent orientations and intensities they measure in sediments correspond to the true magnetic field at the time.

We invite creation geologists to look at this paper carefully and consider the implications. A friend in this field has assured me that geologists are able to tell the difference between DRM and PDRM. But if this “known unknown” needed clarification, what “unknown unknowns” remain to come to light?

For more on problems with the secular geodynamo theory, see “Earth’s Geodynamo: An Energy Crisis” (1/25/16), “Stuff Geologists Think They Know Till Tomorrow” (12/22/15), “Magnetic News” (8/03/15), “What You’re Not Being Told About Earth’s Magnetic Field” (4/17/15), “Earth’s Magnetic Field Less Sustainable than Thought” (5/17/12), and “Findings that Comport with Genesis” (10/27/13).

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  • John C says:

    A new posting at SciDa, ‘Fossil analysis pushes back human split from other primates by two million years’ was interesting in light of this article. They used what they called magnetic reversals to date several teeth of a new fossil find at 8 Ma, supposedly pushing back human-gorilla split by 2 Ma. They also used argon-dating, apparently because it gave them the desired range of dates. They said nothing about U-Pb dates, or 14-C checks; apparently those would have been risky to their theory. And treated Ardi and Kedabba like they actually were confirmed as human ancestors. Would make an excellent follow up study to this article.

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