My UNC colleague Justin Ries and his collaborators just published a paper in Geology that offers an important new clue about the cause of the Cambrian explosion, the rapid radiation and appearance of new life forms on earth just over half a billion years ago. One theory has been that suddenly increased oxygen levels made this rapid diversification of animal life possible. But many geologists dismissed this argument because they thought that oxygen concentration had already increased in the earth’s atmosphere by then. Justin and his team traveled to the desert of namibia to sample the Nama group carbonate rocks from which they measured the sulfur isotopic signature. Sulfur is used as a proxy for oxygen concentration. The teams findings indicate that oxygen concentration in shallow seas was indeed very low just before the Cambrian explosion.
Justin is a relatively new faculty in my department and UNC. He is a carbonate geochemist and also is doing some cutting edge work on the effects of ocean acidification on calcifying organisms.
“This period was a game-changer in terms of the evolutionary structure of life,” Ries says. “Our findings are consistent with the idea that it occurred because of major changes in the composition of the ocean and atmosphere at that time.”
Scientists have maintained that relatively high oxygen levels existed on the planet long before the Cambrian period, Ries says, but if that was the case and oxygen was key to the evolutionary event, why did it take until then for the few initial stems of animal life to expand into the thousands of lineages that emerged?
The new research appears to answer that puzzle. The team examined the chemical signature of limestone rocks in southern Namibia, Africa, that were deposited in the oceans between 553 million and 543 million years ago, just before the Cambrian Explosion and found that at that time, sulfate levels in the ancient ocean—and by implication, oxygen levels in the atmosphere—were much lower than previously thought.
Scientists are able to use sulfate—a molecule that is dissolved in seawater—as a proxy for the amount of oxygen that existed, because their respective levels vary in proportion with one another (marine sulfate is primarily derived from the oxidation of terrestrial sulfide).
“This implies that the subsequent alleviation of these low sulfate and low oxygen conditions may have led to the intense diversification of animals in early Cambrian time,” Ries concludes.