Maybe William Blake was right.
At TED and in his newest book, Peter Ward makes the case for Lee Kump’s hypothesis that many of Earth’s past mass extinctions (except for the now relatively well-understood meteor impact near the Yucatan that did in the dinosaurs) were caused by a complicated climactic shift, which resulted in changes in oceanic circulation, a reduced level of dissolved oxygen, the rise of the chemocline to the ocean surface, and sulfate-reducing bacteria churning out plumes of noxious hydrogen sulfide gas into the atmosphere. Hydrogen sulfide, he thinks, may be the poison that killed off huge swaths of land plants and animals in the “Great Dying” 251 million years ago, and at other times as well.
In a recent issue of Doklady Earth Sciences, a German and Russian team reported on the microbial production of chloroform, methylchloroform, trichloroethylene, and perchloroethylene — collectively, halogenated hydrocarbons (HHCs) — in the sediment of hypersalty lakes. These lakes typically have no outflows to the ocean, and are more or less huge puddles of standing water. Today, hypersaline lakes are found in only a few places, like near the Caspian Sea or South Africa. But in ancient times, for example, 251 Mya during the Great Dying, conditions may have been ripe for hypersaline lake formation, and total hypersaline lake surface area may have been up to 30 times greater than today. Extrapolating from current emissions levels, huge amounts of HHCs could have entered the atmosphere — enough to poison land plants and animals alike. Here is more on the hypothesis.
What did cause the Great Dying? Was it chloroform or hydrogen sulfide? Well, these two ideas aren’t the only ideas on the list, but these two are notable because they both posit that metabolic wastes from exotic bacteria (or archaea) poisoned the ancient atmosphere. We know little about the sulfate-reducing bacteria and next to nothing on the HHC producers. In the future, could these microbial pollution factories ride forth again, on a wave of human-driven climate change? If ocean currents are disrupted, or if changes in climate lead to the formation of more salt lakes, we should remember William Blake. Expect poison from the standing water.