Long before the non-avian dinosaurs died out the Earth suffered other mass extinctions, some even greater in the proportion of the life that was lost. Now another possible ancient catastrophe has been added to the timeline, a cataclysm thought to have occurred when the Earth was barely half its current age, identified through a change in the chemistry of the era’s rocks.
These are mere blinks of time, however, compared to the approximately 2.5 billion years that have passed since the Great Oxidation Event (GOE), which represented perhaps the greatest transformation in Earth’s history. Back then, however, all life on Earth was single-celled and ill-suited to leaving fossils, so the details of this event, or if it happened at all, have been hazy. Now, however, evidence from Canadian rocks suggest the ending of the GOE just over 2 billion years ago caused a phenomenal decline in the quantity of life in the oceans.
Stanford University PhD student Malcolm Hodgskiss examined barite, a mineral deposited in rocks on Canada’s Belcher Islands. In the Proceedings of the National Academy of Sciences he reports a dramatic change in the barite’s chemistry indicating a reduction in the amount of life on Earth 2.05 billion years ago.
"The fact that this geochemical signature was preserved was very surprising," Hodgskiss said in a statement. "What was especially unusual about these barites is that they clearly had a complex history."
The GOE marks the point where single-celled organisms started producing so much oxygen it could no longer all be mopped up bonding to elements like iron, and instead changed the composition of the atmosphere and oceans. The addition of such a reactive gas to the atmosphere killed off lifeforms adapted to an anoxic environment, just as new ones flourished. The nature of this change is hazy, some suspect it caused the first mass extinction, but the process was so slow it’s possible new life appeared as fast as the old died out.
Once it was over, however, we might expect the oxygen-adapted life to trend ever upwards, but Hodgskiss found otherwise. Instead, the oxygen, sulfur, and barium isotopes in the barite all point to a fall of more than 80 percent in primary productivity, a good measure of the amount of life on Earth. Even Thanos might have found that excessive. We don’t know what that meant for individual species, but it likely involved an even greater reduction in the diversity of life, outdoing every subsequent mass extinction on this measure, albeit at a much slower pace.
The paper argues the cause must have been a reduction in available nutrients, with phosphorus deficiency the most likely suspect.