Figuring out what happened hundreds of millions of years ago requires some guesswork, and that lack of precision plagues controversial theories about whether complex animals arose before or after the last global ice age, and whether sea ice actually did once cover even the tropics. Now MIT geologists have introduced more method to the madness, and in doing so have exciting new insights regarding ancient climate and early animals, and the link between the two. Their findings, which are crucial to models of both evolution and climate change, appeared in the April 1, 2005, issue of Science.
Geologists believe that Earth experienced at least three global ice ages from 800 to 580 million years ago (mya) during the Neoproterozoic Era, at the end of the Precambrian Age. In most places, evidence of the glaciations has disappeared, but traces remain in several areas with "cap carbonates," thin layers of limestone containing distinctive ratios of carbon isotopes. "When the ice melts, the oceans rapidly precipitate carbonate, which appears in the sedimentary rock," explains one of the geologists, Daniel Condon, a postdoctoral fellow in geology/geochemistry. "Cap carbonates are very unusual and restricted to glacial periods in the Neoproterozoic."
Extreme fluctuations of this carbon signature in the rock allow geologists to correlate time periods from continent to continent. But it's also easy to make mistakes and align the wrong squiggles on the graph, Condon says, like mismatching buttons on a sweater.
Such mismatching may have happened in China regarding the age of tiny fossil animal embryos. The overlying rocks have a similar carbon spike to cap carbonates, so geologists assigned the rock to a 580 mya glaciation. Then, using relative comparisons, they assumed the embryos were 600 million years old.
That age made many scientists uncomfortable, however, because it left a gaping 45 million years between the age of the embryos and the appearance of larger, more complex animals in Russia, evidence of which was in rocks presumed to be 555 mya. "Evolution just doesn't stand still for so long," points out Condon's advisor, Samuel A. Bowring, especially when a global ice age intervenes, as in this case. Bowring is a professor of geology/geochemistry in MIT's Department of Earth, Atmospheric and Planetary Sciences. Condon and Bowring collaborated on their paper with scientists from the Nanjing Institute of Geology and Paleontology in China.
The geologists suspected that the embryo ages were off, so they applied the zircon dating technology they had been fine-tuning, essentially adding centimeters to a meter measuring stick. This method uses the rate of decay of uranium to lead in volcanic ash to calculate the absolute age of rocks containing such ash. Then, the team headed upstream of China's Three Rivers Dam to the Yangtze Gorge. There, the Doushantuo rock formation has the fortuitous and unusual combination of glacial deposits, volcanic ash, and the fossil embryos. "Here we can date the fossils directly," Condon says, "and for the first time, we can accurately calibrate the relationship between glaciations and biology."
Contrary to previous assumptions, the researchers found the carbon spike did not occur with the 580 glaciation. It was in fact much younger, around 551, the approximate age of fossil animals found in Russia. Moreover, the embryos in China were also younger, appearing not long before the larger animals. "Our data probably make a lot of people feel more comfortable because it compresses the gap in time between the first appearance of the embryos and occurrences of more complete animals," Bowring comments.
What then caused the unusual carbon spike in 551, if not a cap carbonate linked to the melting of a global glacier? According to Bowring and Condon, the new animals did--by virtue of their guts. "We find complex traces in rocks of this age, markings that show wormlike movements, as if searching for food," Bowring explains. They hypothesize that those grazing animals, and similar ones swimming in the ocean, would have had guts, and would have dropped fecal pellets to the ocean floor, causing a carbon signature similar to a cap carbonate. Since the carbon was no longer recycled in the shallow ocean, the evolution of these animals was probably also linked to changes in the global carbon cycle and in levels of oxygen, which set the stage for the evolutionary "explosion" of complex life in the Cambrian period.
"Now we can apply this calibration using zircon dating to other places," Condon says. For instance, they found that cap carbonates in China and Namibia in western Africa have virtually identical ages, within 200,000 years. "The snowball Earth theory had predicted that sea ice would melt almost instantaneously around the world, but no one has been able to prove it," explains Bowring. "The fact that we can date these cap carbonates on widely separated continents to the same time supports the model of rapid deglaciation on a global scale." Incidentally, the researchers also discovered that that deglaciation happened around 635 mya, not the previously estimated 600.
In addition to clarifying how and when events in the ancient world unfolded, this work bolsters the theory that atmospheric chemistry, climate and biology are strongly linked. Such synergy between geology and biology that will help solidify the framework for the current debate about global climate change in the modern world.