At the end of the Triassic Period, around 200 Ma ago, life underwent a major crisis that so far has not been believably connected to either extraterrestrial or geological causes. Previous studies have shown that the mass extinction was accompanied by an decrease in 13C in sediments that suggests a short-lived global warming of between 2-4 °C at the Tr-J boundary. That CO2 levels rose is suggested by a decrease in the density of pores (stomata) on fossil leaves. It has been suspected for some time that the largest known continental igneous event, which accompanied early rifting of the modern Atlantic Ocean basin may have been responsible, but so far the dating of this Central Atlantic magmatic province (CAMP) has not been tied to the boundary conclusively. A large consortium of Italian, French, US, Moroccan and Swiss has addressed the sedimentary and igneous record around Tr-J times in the High Atlas of Morocco (Marzoli, A and 14 others 2004. Synchrony of the Central Atlantic magmatic province and the Triassic-Jurassic boundary climatic and biotic crisis. Geology, v. 32, p. 973-976). There, one of the few uneroded continental flood basalt sequences of CAMP (most preserved CAMP magmas are in the form of sills and dykes in offshore basins) occurs among Triassic and Jurassic sediments. Their base deforms the underlying sediments, suggesting that eruption was onto unlithified sediments, shortly after their deposition. Fossils from the sediments are of little help in tying down the age of eruption, however, Ar-Ar ages of the lavas are all within error of 200 Ma, and tally with magnetic stratigraphy from the Tr-J boundary elsewhere. Both age and geochemistry of the flows are remarkably similar to those of flood basalts from the other side of the Atlantic. Magmatic duration, like that in other large igneous provinces was of short duration, no more than a couple of million years. So it now seems that three of the “big five” mass extinctions (the others are end-Permian, connected with the Siberian Traps, and the K-T boundary and associated Deccan Traps) have at least a partial cause from CO2 release by massive volcanism.
Iron isotopes enter the Archaean life debate
Some years ago geochemists obtained carbon-isotope data from 3.8 Ga rocks in Greenland that seemed at the time to be persuasive evidence for the emergence of life during or shortly after Earth’s most traumatic period. Up to 3.8 Ga the Moon was bombarded by huge projectiles, and its companion Earth would have received at least 13 times the flux of destruction. The carbon was within sturdy apatite grains from supposed iron-rich metasediments, and may have been preserved from later high-grade metamorphism. Doubt has been cast on that hypothesis, either because of the unlikelihood of any carbon remaining unfractionated by heating, or because some aspects of the rocks’ geochemistry suggested that they we of igneous origin rather than sediments. Readers will have seen in previous years’ EPN that a controversy rages over even tangible signs that suggest cellular material from rocks half a billion years younger. Geochemists from France and the US have taken a different tack with the ancient Greenlandic rocks that ought to at least resolve the igneous versus sedimentary origin of the banded iron-rich rocks (Dauphas, N. et al. 2004. Clues from Fe isotope variations on the origin of Early Archean BIFs from Greenland. Science, v. 306, p. 2077-2080). They found that the heavy iron isotope 57Fe is more enriched in the ironstones than in any igneous rocks, with little chance that the difference was induced by thermal fractionation. They are metasediments. But therein lies a surprise. The heavy-iron signatures are greater than in less aged banded ironstones. One way in which that could have arisen is from biogenic precipitation of soluble reduced Fe-2, perhaps involving anoxygenic photosynthesisers – because of the strong capacity of photosynthesis for setting electrons in motion, all such organic reactions create local oxidising conditions, whether or not oxygen itself is produced.