Researcher Joseph Kirschvink theorizes that the explosion of myriad life forms during the Cambrian was caused by a shift of the magnetic poles to the equator. Kirschvink and his associates believe that the whole outer layer of the Earth moved and not just the continents.
Full Text :COPYRIGHT 1997 Economist Newspaper Ltd.
THE fossil record is full of mass extinctions-the disappearances of vast numbers of species in what is (in geological terms) an instant. These are widely believed to be the results of catastrophes such as asteroid impacts and huge volcanic eruptions. But a study published in this week's Science, by Joseph Kirschvink of the California Institute of Technology and his colleagues, suggests that catastrophes can be creative as well as destructive. Dr Kirschvink proposes that the so-called "Cambrian explosion"-which saw the appearance in the fossil record, apparently from nowhere, of the ancestors of all the main animal groups that are alive today-was the result of a slowish, but literally earth-moving, catastrophe.
Dr Kirschvink's startling theory is that, at the beginning of the Cambrian period (about 540m years ago), the outer layers of the earth migrated over the planet's core until the places that had been at the poles ended up near the equator. Since all animal species at the time lived in the oceans, the resulting shifts of the oceanic circulation could, he suggests, have been the stimulus for the huge radiation of new species that is seen in the period's rocks.
The evidence that Dr Kirschvink uses to support this idea comes from the weak magnetic fields that are preserved in rocks when they form. These fields are induced by the earth's own magnetic field in the same way that iron filings can be aligned on a piece of paper by a magnet underneath it. This makes it possible to work out the direction of magnetic north at the time the rock was forming.
Geologists use such "paleomagnetism" to show how rocks (and thus the continents that carry them) move about over time. Such data are among the most persuasive pieces of evidence that the continents do, indeed, drift around. But they do so slowly. Today, even the nimblest are travelling at a mere 5cm a year. In the early Cambrian, if the paleomagnetic data are to be believed, they were skidding about at well over 30cm a year, and also twisting in most peculiar ways. Dr Kirschvink and his colleagues have looked again at the results from the early Cambrian (some of which they collected themselves), and have come up with a new interpretation: it was not the individual continents that were moving, but the whole of the earth's outer layer.
The earth's magnetic field is generated in its core (the exterior of which is made of liquid iron). Outside this is a thick, rocky zone known as the mantle and on top of that is the crust, including the continents. What the researchers are suggesting is not the slow movement of the continents across the top of the mantle (ie, normal continental drift), but the relatively rapid movement of crust and mantle as a whole with respect to the core.
What caused this, they believe, was an accumulation of mass in one place on the earth's surface. This could have been the result of normal continental drift, and there is evidence in the rocks to suggest a coming together of land-masses, just before the beginning of the Cambrian, that might have created such a build-up. That would, in turn, have made the planet asymmetrical and, as would be the case for any spinning globe with a lump on its surface (Dr Kirschvink uses the analogy of a basketball with lead weights glued on to it), centrifugal force would have tugged the lump towards the equator and changed the globe's spin accordingly.
Evidence to support this theory comes from two of the earth's neighbours-the moon and Mars. The lunar maria (the dark "seas" on its surface, which are made of heavier rock than their surroundings) are arranged symmetrically about the moon's equator as the laws of physics demand, and in the case of Mars, a huge (and heavy) volcanic region known as Tharsis also straddles the equator in an extremely suspicious manner.
In the case of the earth, according to Dr Kirschvink's theory, the liquid nature of the outer core meant that the core's axis of spin stayed pointing in the same direction while the mantle that floats on it migrated around to bring the concentration of mass to rest on the equator-a process that took about 15m years. One consequence of this migration would have been to redirect the flow of the Cambrian's oceanic currents at frequent intervals.
The argument that this, in turn, triggered the Cambrian explosion depends on a theory that having small ecosystems whose inhabitants are allowed to mix every so often is a good way to generate lots of species quickly. The idea is that frequent shifts in ocean currents would break up any large, stable ecosystems, and thus churn the evolutionary pot. Though this theory is by no means proven, there is evidence from more modern times to support it. The biodiversity of the South American rainforests is thought by some researchers to have evolved in this way as the forests alternately retreated into small patches during dry periods, and expanded and mingled during wet ones. And there are data which suggest that ten or more current shifts may have taken place as the poles moved.
The evidence for these shifts comes from the ratio between two isotopes of carbon in ancient rocks. Normally, the ratio of light carbon (!"C) to heavy carbon (!#C, which has an extra neutron in its nucleus) remains constant in the oceans. In the early Cambrian, though, the rocks indicate that it fluctuated sharply at least ten times. These fluctuations often coincide with the appearance of new groups of animals, and were thought by previous researchers to have been caused by them (biological processes have a slight preference for !"C). Dr Kirschvink and his team think it more likely that such sudden, large anomalies are caused by new currents sweeping material from the deep seabed and putting it (together with extra !"C) into circulation. The appearance of the new animals coincides with the carbon anomalies because the shifts in currents also stimulated new rounds of speciation. The diversification of the animals was, in other words, the result of the biggest rock-slide in history.
"The earth moves: the Cambrian explosion." The Economist [US] 26 July 1997: 70+. Academic OneFile. Web. 5 Dec. 2009.
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