About 540 million years ago, animal life on Earth suddenly boomed during an event known as the Cambrian explosion. In just tens of millions of years—a mere geological moment—life evolved rapidly, with all of the major groups of animals alive today making their first appearance. While researchers still aren't sure what could have triggered this burst of biodiversity, many suspect it had to do with a sudden rise in oxygen levels in the atmosphere, which would have allowed complex, multicellular organisms—and eventually higher organisms like ourselves—to flourish. Where this flood of oxygen came from is also a mystery, but whatever caused it, scientists know that it must have been a big event.
As one of the most important riddles in geology, this question has long interested Caltech geologists. In particular, professors John Grotzinger and Woody Fischer have been puzzling over a surprising discovery made in the early 1990s—a discovery that provided some clues as to what might have happened all those millions of years ago.
Digging in the oil-rich desert of Oman, geologists had looked at carbon isotopes in rocks. Measuring isotope ratios is a basic part of a geobiologist’s tool kit, allowing these scientists to piece together Earth's environmental history. The researchers found that sedimentary rocks from a time just before the Cambrian explosion—during the so-called Ediacaran period—were curiously short on a carbon isotope called carbon-13. In fact, the ratios of carbon-13 to carbon-12 that these researchers discovered were the lowest that have ever been seen.
The cause of this drop in carbon-13—named the Shuram excursion, after the rocks in which it was found—is a mystery that's still unsolved today. Now, Grotzinger, Fischer, and David Fike of Washington University in St. Louis have offered a new hypothesis in a review paper published in Nature on April 17. They still need to hunt for more data, but their idea is the latest attempt to reconcile seemingly contradictory evidence that has sparked a significant debate in the field.
When geologists first found this depletion in carbon-13, they thought it was just due to the usual set of processes that turn sediment into rock over time—rainwater can react with the rock, for instance, or high temperatures and pressures can induce chemical changes. Called diagenesis, this smorgasbord of reactions alters isotopic ratios. Diagenesis is typically a local process, depending on the environmental and geological conditions of a particular area—in this case, Oman.
In the last decade or so, however, geologists have found similar carbon-13 depletions widely distributed across other parts of the world, including southern China, southern Australia, and just a couple of hours away from Caltech, in Death Valley. In samples from each of these locations, geologists found a huge dip in the carbon isotope ratios at around the same depth—and therefore from around the same time in history. "We've never seen curves like this," Fischer says. "This is rare and unique in the rock record."
The fact that the Shuram excursion appears around the world seems to indicate that it's the chemical fingerprint of some global event that could be related to the rise of oxygen.
One of the most recent analyses came in February, when Caltech graduate student Charlie Verdel (now at the University of Michigan), geology professor Brian Wernicke, and Samuel Bowring at MIT published new data on the sediments in Death Valley. According to Grotzinger, this analysis provides some of the best evidence that the Shuram excursion was indeed a worldwide phenomenon. In another paper that's in press, graduate student Kristin Bergmann, along with Rebecca Zentmyer of Cerritos College and Fischer, have just published more observations of the Death Valley sediments that support this argument.
And yet, the data also support diagenesis. First, the depletion in carbon-13 only exists in inorganic carbon—carbonate minerals—and not in fossilized organic carbon deposited in the same sediment. If the Shuram excursion were caused by some global event, then it should also be recorded in the organic matter deposited in the same rocks.
The other piece of evidence that points to diagenesis is the oxygen-isotope ratios. In the rock samples that showed the Shuram excursion, researchers also found a relative lack of oxygen-18, which could be a sign that the sediment reacted with different kinds of fluids. The ratio of oxygen-18 to oxygen-16 dropped in much the same way as the ratio of carbon-13 to carbon-12 did, suggesting that whatever caused the carbon dip also caused an oxygen dip—a typical sign of diagenesis.
But at the same time, this data could be explained without invoking diagenesis. In 2006, Grotzinger and Fike, who was his graduate student at the time, along with Lisa Pratt at Indiana University and Roger Summons at MIT, published a paper in Nature suggesting that the data could be consistent with a global event related to the worldwide appearance of oxygen.
In this scenario, Earth's ocean during the Ediacaran was chock full of organic carbon, which lacks carbon-13. A sudden increase in oxygen would have oxidized this oceanic carbon, producing carbon dioxide that then formed carbonate rock—inorganic carbon that's deficient in carbon-13, which is exactly what characterizes the Shuram excursion.
Another characteristic of the Shuram data is that the organic carbon found in the sediment did not show a similar decrease in carbon-13—but the researchers could explain this, too. There was so much organic carbon in the oceans that its presence would have masked any dip that might be seen in the sediment's organic carbon.
Despite this evidence, some researchers still argue for a purely diagenetic origin of the Shuram excursion, according to Grotzinger. But no matter which side they're on, all scientists in the field are wrestling with this perplexing data.
"We were sort of sitting around the room, thinking of how we could explain this," Grotzinger recalls. He and his colleagues wondered whether it might be possible for the Shuram excursion to have been caused by diagenesis, but—in a departure from the way people have always thought about it—via a unique global version of the process.
The researchers propose that a rise in atmospheric oxygen would have altered the planet's chemistry, helping sediment get better at oxidizing organic carbon by enriching that sediment with such compounds as iron oxide and sulfate-bearing minerals. The sediment could then oxidize any organic carbon that flowed through it as a hydrocarbon-rich fluid, like oil. Oxidizing organic carbon would produce carbon dioxide that would then form carbonate. Again, since the organic carbon in water or oil has low amounts of carbon-13, this would result in similarly low levels in carbonate (inorganic carbon), the prime feature of the Shuram data.
So far, the evidence has seemed to point to both diagenesis and a global event, and previous ideas favored one or the other. "What we're trying to do is open the idea that maybe it's a combination of both," Grotzinger says. Fischer adds, "I'm sure it'll be controversial." The researchers are now starting to gather more data to see if their new explanation is correct. Bergmann and another graduate student, Maggie Osburn, are now collecting more samples in Oman.
How all this links to the Cambrian explosion remains unclear. All researchers know for sure is that the Shuram excursion represents an unprecedented signal in Earth's geological record, and it happened right before the sudden surge in biodiversity. The evidence is circumstantial, but it’s certainly intriguing. "It requires a remarkable mechanism regardless of how you cut it—it's not a business-as-usual scenario," Fischer notes. "I'm totally open to how this shakes out. I can see it both ways now."