About 530 million years ago, the planet teemed with life after
Cambrian explosion, when most large animal groups we see today began to appear in the fossil record. Most of this abundance was concentrated in the oceans, which were rich in fantastic creatures such as armadillos trilobitesa three foot long shrimp and worms bristling with vicious spikes.
But the fossil record shows that just 20 years later, something wiped out 45% of all the animals in the ocean. This great extinction event caused destruction on an unprecedented scale.
For years, the prevailing opinion was that this sudden disappearance was mainly caused by the rapid spread of
low-oxygen or “anoxic” conditions.. One theory is that it was caused by the sudden explosion of Cambrian life, which covered the seafloor with decaying organic matter as plants and animals died, and in turn siphoned vast amounts of oxygen from the water column.
But research published in October in the journal
Geophysical Research Letters suggests that this wave of death may have been fueled by something else: a surge of a nasty chemical gas called hydrogen sulfide that suffocated life in the sea. “This chemical is lethal to all marine animals,” co-author of the study Bye Chang, a geochemist at Northwestern University in Xi’an, China, told Live Science. “Basically, no animal can survive in such an environment for long.” Connected: The 5 mass extinctions that shaped the Earth
Chang’s team found clues to this die-off in the Cambrian geological record of the Yangtze Platform, a vast plateau in southern China that once lay underwater. The team went there in search of
molybdenum, a chemical element that is carried from Earth’s rocks through rivers into the ocean. Molybdenum is long-lived, and in the ocean its concentrations in sediments vary with the chemical composition of the surrounding water. Both factors make it a good proxy for ocean conditions of past millennia, Chang said.
Samples from the mass extinction period contain high levels of molybdenum. This suggests that there must have been hydrogen sulfide in the water, because molybdenum “can combine with sulfur to form insoluble compounds” that are then deposited in sediments, Chang said. This occurs at much higher levels in sulfide waters than in ordinary waters, “especially when the concentration of hydrogen sulfide is at a high level,” he said. In other words, higher molybdenum concentrations may be associated with higher amounts of hydrogen sulfide in the sea.
But what caused this toxic spread? “At this stage, no one can say for sure what caused the expansion of the sulfide waters,” Chang said. However, the lack of oxygen may have been caused by an explosion of organic matter that subsequently sank to the sea floor and rotted, providing a banquet for trillions of microbes, he added.
As these microbes feasted on the decaying material, they would also eat sulfate that occurs naturally in seawater. But in the process, the growing population of microbes would have turned the sulfates into the byproduct, hydrogen sulfide—and pumped the water full of this gas, causing the toxic wave.
Although the researchers’ samples only came from modern China, they believe this toxic spread occurred on a global scale because molybdenum has a residence time of hundreds of thousands of years in the ocean. “Basically, this means that before the molybdenum is removed from the ocean, the seawater in the ocean will have already mixed completely many, many times,” Chang said. This long life cycle means that molybdenum isotope levels recorded in a single piece of ocean sediment will reflect the sea-wide average.
Follow-up research will aim to determine what caused the anoxic conditions and the sulfide wave that soon followed, Chang said. This type of work helps scientists sketch a picture of the limits of Earth’s habitability, he added.