Prior posts have discussed the Cambrian explosion, and noted the evidence suggesting that it was a more gradual process than stereotypically described. However, there has been little solid evidence to explain why, following this alleged burst of complex life, a substantial extinction set in. As with other extinctions, many researchers had speculated that a low oxygen content in the oceans was a key factor. However, there was little solid evidence for the thesis. Researchers may have recently come forth with a viable explanation.
Researchers looked at a subset of the Cambrian extinctions, that which occurred around 499 million years ago and lasted 2-4 million years. They sought rock samples of the appropriate age from Nevada, Utah, Missouri, Australia, and Sweden. By analyzing sulfur and carbon isotopes the researchers were able to track various changes, such as the nature of the sediment being buried on the ocean bottom, a process that alters chemistry in the waters above.
The researchers concluded that the amount of carbon in the rocks, as compared to the amount of sulfur, could only have come about if the water was low in oxygen and high in the sulfide form of sulfur. Ironically, this is similar to the environment that can be found today in the oxygen-starved Black Sea. The researchers note that most of the toxic waters would have shoaled up along the coasts, where the majority of animals lived. Overall, their new data suggests that pulses of rising and falling oxygen and sulfur would have repeatedly devastated marine ecosystems.
While the researchers have identified evidence to support what was happening, they are unable at present to explain why these events transpired. They also note that it is unknown if such events were a recurrent theme in the world's oceans, or merely an occasional event.
The report can be found at http://www.nature.com/nature/journal/v469/n7328/full/469042a.html.