Did a Huge Impact Lead to the Cambrian Explosion?
About 570 million years ago, our Earth underwent a series of puzzling events: a drastic shift in climatic zones, a mysterious change in the carbon cycle, and a sudden burst of animal life that led to the Cambrian explosion.
Young has studied Precambrian rocks around the world for over fifty years, and he thinks these riddles could all be linked to a single catastrophic event. In a paper published this month in a journal of the Geological Society of America, he posits that a large marine impact, which may have occurred about 570 million years ago, could partly account for these phenomena.
According to his theory, the spin axis (or "tilt") of our planet would have shifted drastically upon impact, modifying climate patterns. A large volume of water would have also been blown into the atmosphere, accounting for the mysterious oxidation of the carbon found in oceans. The ensuing environmental re-organization would have then set the stage for the emergence of complex life.
"I'm not saying that the impact theory is one hundred percent certain," Young says. "But so many unusual changes seem to have taken place at the same time -- the most suggestive being the change in distribution of glacial deposits from the tropics to the poles."
In the 1960s, a geologist named Brian Harland made a startling discovery. He found that, in the Precambrian, glaciers had formed at sea-level near the equator, suggesting that tropical regions must have been much cooler back then.
Researchers came up with several theories to resolve this conundrum. One of them, the famous "Snowball Earth" hypothesis, held that glaciations in these ancient times were so severe that the entire planet was covered in ice. But a problem with that theory, according to Young, is that early glaciations were only found at low latitudes, and not in polar regions where they occur today.
In 1975, George Williams of the University of Adelaide proposed that in the Precambrian our Earth may have had a much higher tilt (about 54 degrees) than its present configuration (23.5 degrees). That high tilt could have been produced by the major impact that created our Moon 4 billion years ago.
According to Dr. Young, another large impact 570 million years ago could account for the drastic reduction in tilt to today's position. It would also explain why glaciers developed near the tropics throughout most of the Precambrian, whereas today they are found near the poles.
The Shuram Anomaly
In the early 1990s, geologists found a curious anomaly in the carbon isotopic composition of rocks in Shuram, Oman. In these rocks, the ratio of carbon-13 to carbon-12 was the lowest that had ever been seen. That drop in carbon ratio was later found in other formations all over the globe, suggesting a dramatic change in the composition of world oceans.
Such drops had occurred earlier, but they were usually associated with glaciations. The Shuram event was not. "Some researchers have proposed that an oxidizing event must have occurred in the deep ocean at that time," says Young, "but they couldn't explain why that happened."
What's more, several deep canyons were formed around that time, including in South Australia, California, and Uruguay -- all around the time of the Shuram anomaly. "The impact could have temporarily flooded continents and contributed to the formation of these deep canyons, as huge volumes of water invaded the land and returned to the oceans," Young says.
"The previously unexplained change in the chemistry of world oceans, together with the excavation of deep subaerial canyons, are considered to be the “smoking gun” for a cryptic, large oceanic impact," he adds.
Animal Life Emerges
These events marked the beginning of another drastic event known as the Cambrian explosion. Animal life on Earth suddenly blossomed, with all of the major groups of animals alive today making their first appearance.
"The important biological innovations of that period, commonly attributed to the “snowball Earth,” may be allied to massive environmental perturbations accompanying the proposed oceanic impact," Young writes. "If the impact origin for the Shuram anomaly is correct, its widespread effects and “instantaneous” nature would provide one of the most useful and precise time markers in the geological record."