Earth experienced the highest rate of extinctions in its history at the end of the Permian period. Scientists have long explained the "Great Dying," as it is informally known, based on rapid climatic changes due to extreme volcanic activity. Patterns of extinction suggest that warming and acidification of the oceans lowered marine oxygen levels. But these stresses insufficiently explain the whole pattern of extinctions.
Now scientists at the University of Calgary have submitted a paper to the journal Geology suggesting that late Permian mercury anomalies contributed to the massive extinctions of the era. Steve Grasby, co-author of the paper, states:
No one had ever looked to see if mercury was a potential culprit. This was a time of the greatest volcanic activity in Earth’s history and we know today that the largest source of mercury comes from volcanic eruptions. We estimate that the mercury released then could have been up to 30 times greater than today’s volcanic activity, making the event truly catastrophic.
The scientists found a geological record of the mercury deposition in Buchanan Lake sedimentary records. Buchanan Lake, located in the Canadian High Arctic, once formed part of the ancient continent of Pangea downwind from the volcanic region known today as the Siberian Traps. The scientists describe naturally-occurring mercury levels similar to man-made contamination found today near smelters, where significant aquatic ecosystem damage has also been experienced.
If mercury contributed to extinctions, it is another example of earth's delicate balances being overwhelmed. Usually, marine algae scavenge mercury from seawater and precipitate it onto the sea floor, where it no longer threatens most marine species. “But in this case, the load was just so huge that it could not stop the damage,” says Hamed Sanei, another co-author.
The scientists interpret this finding partly as good news about the ecosystem's ability to restore itself after devastating disruptions. Earth eventually cleaned the mercury out of its system, stabilized its climate, and -- although most life was destroyed -- began to move onto the next phase of the evolution of life on this small blue pearl of a planet.