Discussing what actually killed off around 95% of all species 251 Ma ago has become the perennial mass-extinction topic, now that the K-T boundary event is more or less done and dusted, bar a little murmuring over the Deccan Trap. Michael Benton of the University of Bristol has summarised the current state of play for the Permian-Triassic (P-Tr) event (Benton, M. 2003. Wipeout. New Scientist, 26 April 2003, p. 38-41). Despite many attempts to link an impact to the annihilation – such evidence as there is (see Buckyballs and the end-Palaeozoic extinction, EPN March 2001) has not been reproduced by independent analysis of the material. Weighty evidence comes instead for an Earth-induced event, from the coincidence of the monstrous Siberian Traps with the 100 thousand years or less that the extinction occupied, and from complete sequences across the P-Tr boundary in a Japanese ophiolite and a shallow marine section at Meishan in South China. As well as an intricate series of faunal changes, the Meishan sequence has now provided a complete record of oxygen and carbon isotopes that span the boundary event. The oxygen data suggest a 6ºC rise in global temperature at exactly the stratigraphic level of the extinction and of a massive lurch towards light carbon. Such a high proportion of 12C occurs at the boundary that it cannot have been induced by sterilisation of the oceans, which may well have happened as a result of the extinctions. Nor can even the huge belch of mantle CO2 emitted by Siberian continental flood basalts. The two combined only account for 40% of the carbon-isotope excursion. Release of methane from long-term storage as gas hydrate on the Permian sea floor is the only conceivable candidate. So it looks as if a runaway “greenhouse”, plus toxic gas and maybe acid rain put paid to most living things. Such a wiping out left lifeless oxygen-poor oceans – originally dubbed “Strangelove oceans” by Ken Hsu after the eponymous insane doctor. Triassic times did not see explosive reoccupation of abandoned niches, recovery taking up to 50 Ma from a tiny population of not very diverse organism. Benton has written a book on the P-Tr event (When Life Nearly Died. Thames & Hudson), and that is likely to be a rattling read. New Scientist maintains it’s irritating habit of never referring to sources in its articles, so to go further, you will have to buy the book.
Microbes showed no sign of change following a “Snowball Earth”
The “Snowball Earth” hypothesis has suffered quite a lot since its original promotion (see: Meltdown for Snowball Earth?, February 2002 EPN; Snowball Earth hypothesis challenged, again, December 2002 EPN). Whatever the eventual fate of the notion that the entire Earth was iced over from pole to pole, the fact that glaciers reached sea level at low latitudes at least twice in the Neoproterozoic seems to be an established fact. Such climate swings must surely have had an effect on life, either by driving up the rates of extinction and adaptive radiation because of stress, or perhaps providing nutrients to the oceans in vast amounts that allowed the phytoplankton base of the food chain to explode (see: The Malnourished Earth hypothesis – evolutionary stasis in the mid-Proterozoic, September 2002). One of the first discoveries of low-latitude glaciogenic deposits was around Death Valley, California by the late Preston Cloud, who worked there during the 1960s. So it is fitting that palaeobiologists associated with the Preston Cloud Research Laboratory at the University of California, Santa Barbara have dissected sediments within and immediately beneath the 750 Ma diamictites that Cloud interpreted as glacial in origin, to test for signs of evolutionary change (Corsetti, F.A., Awramik, S.M. & Pierce, D. 2003. A complex microbiota from snowball Earth times: Microfossils from the Neoproterozoic Kingston Peak Formation, Death Valley, USA. Proceedings of the National Academy of Science, v. 100, p. 4399-4404). In cherts within carbonate units they found a surprisingly diverse range of undoubted microfossils, that are probably auto- and heterotrophic Eucarya, but no difference between pre-glacial and glacial levels, in terms of their biota. Although this single piece of work does not prove that there was no biological change associated with a major cooling during the Neoproterozoic, it does cast doubt on the severity of its effects on life. Most important, the study shows that well-preserved cellular material is available for study in sediments that occur with glaciogenic diamictites, and should open up a new line of research bearing on the rise of the metazoan (multi-celled) Eucarya, which appeared in large numbers shortly after the last (~600 Ma) glacial epoch. Most if not all Neoproterozoic carbonates, whose universal presence in close stratigraphic proximity to glaciogenic strata first hinted at low-latitude frigidity, contain abundant chert nodules that are the best preserving medium for delicate and tiny cell structures.