The magnetic attraction of periodicities.

The magnetic attraction of periodicities

Scientific interest in the cyclic patterns of nature has passed through cycles of its own. The most recent upswing in the hunt for periodicities in the earth's history was sparked by the suggestions that mass extinctions have occurred regularly every 26 million years or so (SN: 10/1/83, p. 212) and that at least one mass extinction was caused either by meteorites that bombard the earth about every 30 million years (SN: 6/2/79, p. 356) or by episodes of volcanic eruptions.

While scientists go back and forth over the statistical validity of studies showing similar periods for events in a number of geologic records, they are becoming increasingly intrigued by the possibility that all these periodic processes are somehow linked. Two new papers, taking very different tacks, focus on the possible connection between extinctions and the rate at which the earth's magnetic field reverses its direction.

In the March 13 NATURE, Poorna C. Pal of the University of Ilorin in Nigeria and Kenneth M. Creer at the University of Edinburgh in Scotland resurrect a 30-million-year (Myr) pattern in the record of magnetic field reversals after the pattern had recently been questioned on statistical grounds (SN: 10/19/85, p. 245). Instead of analyzing the entire 165-Myr geomagnetic history, as other researchers have done, Pal and Creer focused on the last 83 Myr, in which the reversals have been relatively frequent. Pal and Creer reason that if impacts do disturb the geomagnetic field, their effect would be most evident during such periods of frequent reversals, when the field is the most unstable.

The researchers found that the reversal frequency increased sharply during three periods, each separated by 30-Myr intervals: at 8 to 12 Myr ago, 35 to 45 Myr ago and 65 to 75 Myr ago. They point out that the spurts coincide with globalscale catastrophic episodes signaled by mass extinctions, impact craters, geochemical anomalies and the production of small glass grains called tektites, which are thought to be created by impacts. Pal and Creer suggest that "the approximately periodic recurrences of catastrophic episodes caused reversal spurts during [times of frequent reversals].'

The researchers argue that during periods of frequent reversals, comets or asteroids bombarding the planet enhance the turbulence in the earth's fluid outer core. Many scientists believe that, somewhat like a giant dynamo, the motion of these electrically conductive core fluids produces the geomagnetic field (SN: 10/5/85, p. 218). Reversals may be triggered by changes in the fluid motion, such as increased core turbulence.

However, according to Richard Muller at the University of California at Berkeley, "the model that they [Pal and Creer] describe fails miserably when you try to work it out in detail.' Last December, Muller announced at the American Geophysical Union meeting in San Francisco that he and Donald Morris, also at UC Berkeley, have taken a more detailed look at how impacts might trigger field reversals. In their model, the dust thrown up into the atmosphere by impacts cools the planet, enhancing the growth of ice sheets at northerly latitudes, which changes the planet's moment of inertia by effectively bringing water closer to the earth's axis. This in turn speeds up the earth's rotation, disrupting the flow patterns of the liquid core and changing the geomagnetic reversal rate. Muller says he can't discuss the details of this model because the work has not yet been published.

David E. Loper and his co-workers at Florida State University in Tallahassee have also explored the relation between extinctions and the magnetic field. But unlike Pal, Creer and Muller, Loper does not think impacts play an important role in triggering either reversals or extinctions. Loper's group believes that the natural activity within the earth causes episodes of frequent reversals and bouts of vigorous volcanic eruptions, which, some researchers have argued, are responsible for mass extinctions (SN: 3/16/85, p. 172). Loper presented his ideas March 14 at a symposium on the environmental effects of volcanism, held at the University of Rhode Island in Kingston.

Loper's group suggests that a layer of hot mantle material close to the core periodically becomes unstable, releasing mobile plumes of hot mantle material that rise to the surface and feed volcanoes. The researchers argue that the release of the plumes also leads to field reversals. The loss of material, they say, thins the layer, allowing more energy to escape from the core. This enables the heat engine in the core to drive the geodynamo harder, and as the flux of energy to the geodynamo increases, the reversal rate increases as well. In a rough, preliminary calculation, Loper estimates that the layer becomes unstable about every 22 Myr.

Loper's group is conducting laboratory experiments to learn more about the behavior of the lower mantle layer, which seismologists have dubbed the D layer. The researchers place a layer of dyed water, representing the mobile D layer, under viscous corn syrup representing the colder overlying mantle. "We get what looks like episodic behavior' in the rising plumes of water, notes Loper.

Neither Loper nor Pal and Creer are the first to suggest and model links between these different periodic processes. But what is relatively new, say several observers, is that the findings of periodicity in a number of records are enjoying a resurgence of respectability. When all these hunts for periodicity resumed several years ago, says one scientist, "I thought it would be just a flash in the pan. But the idea has grown and periodicity is spreading waves throughout geologic thought.'