The greatest mass extinction of the Phanerozoic closed the Palaeozoic Era at the end of the Permian, with the loss of perhaps as much as 90% of eukaryote diversity on land and at sea. It was also over very quickly by geological standards, taking a mere 20 thousand years from about 252.18 Ma ago. There is no plausible evidence for an extraterrestrial cause, unlike that for the mass extinction that closed the Mesozoic Era and the age of dinosaurs. Almost all researchers blame one of the largest-ever magmatic events that spilled out the Siberian Traps either through direct means, such as climate change related to CO2, sulfur oxides or atmospheric ash clouds produced by the flood volcanism or indirectly through combustion of coal in strata beneath the thick basalt pile. So far, no proposal has received universal acclaim. The latest proposal relies on two vital and apparently related geochemical observations in rocks around the age of the extinctions (Rothman, D.H. et al. 2014. Methanogenic burst in the end-Permian carbon cycle. Proceedings of the National Academy of the United States, v. 111, p. 5462-5467).
In the run-up to the extinction carbon isotopes in marine Permian sediments from Meishan, China suggest a runaway growth in the amount of inorganic carbon (in carbonate) in the oceans. The C-isotope record from Meishan shows episodes of sudden major change (over ~20 ka) in both the inorganic and organic carbon parts of the oceanic carbon cycle. The timing of both ‘excursions’ from the long-term trend immediately follows a ‘spike’ in the concentration of the element nickel in the Meishan sediments. The Ni almost certainly was contributed by the massive outflow of basalt lavas in Siberia. So, what is the connection?
Some modern members of the prokaryote Archaea that decompose organic matter to produce methane have a metabolism that depends on Ni, one genus being Methanosarcina that converts acetate to methane by a process known as acetoclastic methanogenesis. Methanosarcina acquired this highly efficient metabolic pathway probably though a sideways gene transfer from Bacteria of the class Clostridia; a process now acknowledged as playing a major role in the evolution of many aspects of prokaryote biology, including resistance to drugs among pathogens. Molecular-clock studies of the Methanosarcina genome are consistent with this Archaea appearing at about the time of the Late Permian. A burst of nickel ‘fertilisation’ of the oceans may have resulted in huge production of atmospheric methane. Being a greenhouse gas much more powerful than CO2, methane in such volumes would very rapidly have led to global warming. Before the Siberian Traps began to be erupted nickel would only have been sufficiently abundant to support this kind of methanogen around ocean-floor hydrothermal springs. Spread globally by eruption plumes, nickel throughout the oceans would have allowed Methanosarcina or its like to thrive everywhere with disastrous consequences. Other geochemical processes, such as the oxidation of methane in seawater, would have spread the influence of the biosphere-lithosphere ‘conspiracy’. Methane oxidation would have removed oxygen from the oceans to create anoxia that, in turn, would have encouraged other microorganisms that reduce sulfate ions to sulfide and thereby produce toxic hydrogen sulfide. That gas once in the atmosphere would have parlayed an oceanic ‘kill mechanism’’ into one fatal for land animals.
There is one aspect that puzzles me: the Siberian Traps probably involved many huge lava outpourings every 10 to 100 ka while the magma lasted, as did all other flood basalt events. Why then is the nickel from only such eruption preserved in the Meishan sediments, and if others are known from marine sediments is there evidence for other such methanogen ‘blooms’ in the oceans?