In December 2004 EPN commented on what appears to be a serious challenge to claims of geochemical evidence that would support a major impact associated with the largest of all mass extinctions in the Phanerozoic, that at the close of the Permian Period and the Palaeozoic Era, around 251 Ma ago. Newly published analyses from two other well-constrained P-Tr boundary sites found no signs of the elements that would be expected from a major collision with a metal or silicate-rich asteroid (Koeberl, C. et al. 2004. Geochemistry of the end-Permian extinction event in Austria and Italy: No evidence for an extraterrestrial component. Geology, v. 32, p. 1053-1056). Koeberl of the University of Vienna and colleagues from the US and UK focussed on platinum-group elements (PGEs), and osmium and helium isotopes. Both sites are stratigraphically similar and dominated by carbonate sediments, with evidence from one site for deepening water that laid down organic-rich marls. Sure enough, there is a “spike” in iridium at the level of these marls, which had been documented at the Austrian site in 1989, and there is another 50 m higher in the sequence. The new work confirmed both, and also found the marl-related “spike” in Italy. But the reason why iridium has been used to suggest extraterrestrial impacts is because, of all the PGEs, it is the easiest to analyse at very low concentrations. That can give rise to “false positives”, for there are purely terrestrial processes that can concentrate PGEs. An unambiguous arbiter between these processes and impacts lies in the isotopic composition of the metal osmium. Rocks of the Earth’s crust have high rhenium (Re) and low osmium (Os) contents, whereas in meteorites the Re/Os ratio is very much smaller. The unstable isotope 187Re decays to produce a daughter 187Os that adds to the common 188Os isotope. Consequently, terrestrial rocks acquire high 187Os/!88Os rapidly after they crystallise from magmas and that “signature” is imparted to the entire surface environment through weathering and solution. On the other hand, meteorites have low 187Os/!88Os ratios, so the two influences on the geochemical record can be distinguished – if you have good enough analytical facilities. The two iridium spikes fail that test, as regards an impact origin. It seems likely that they originated through precipitation of PGEs from sea water under reducing conditions on the deep sea floor. The helium isotope data carry the same negative message; they are typically terrestrial.
Impact-induced extinctions, particularly ones that wipe out a sizeable proportion of all organisms, are likely to be unremittingly sudden – direct effects being felt within hours over the whole planet, and secondary effects such as “nuclear winter” and acid rainfall over a matter of a few years or decades. Radiometric dating is incapable of resolving such short periods, and at the age of the P-Tr boundary probably not even several hundred millennia. Faunal sequences can give a better indication of abruptness. To most intents the marine record at the time does look as if extinction was very sharp, but it does not indicate anything by way of clear evidence for an impact, such as glass spherules, shocked quart grains and other tell-tale signs. The continental record is pretty sparse, so has not figured much in the debate. However, the Karoo basin of South Africa contains thick continental sediments that span the boundary, and is famous for its primitive reptile fauna, some of which became extinct around the time of the P-Tr event. Incidentally, this die-off created the genetic conditions for the adaptive radiation in the Mesozoic that led not only to the dinosaurs but also the mammals and birds. Charting the timing of the Karoo extinctions has proved difficult, although it appears not to have been sudden in a stratigraphic sense. New age data has emerged from studies of palaeomagnetic field reversals in the sediments, together with variations in carbon isotopes, that allow timing to be better assessed through comparison with magnetic and carbon records from other sections (Ward, P.D. et al. 2005. Abrupt and gradual extinction among Late Permian land vertebrates in the Karoo Basin, South Africa. Science [soon to be published, currently available on Sciencexpress at www.sciencemag.org/sciencexpress/recent.shtml]). The signs are that the proto-reptiles died off over tens to hundreds of thousand years due to some protracted crisis, probably connected with the giant continental flood basalt eruptions that formed the Siberian Traps. Those lavas overlap the timing of the P-Tr boundary, and would certainly have added sufficient CO2 to give substantial global warming and also massive emissions of SO2 that would have created chemically hazardous conditions on a global scale.
New predators on the Mesozoic block
Most people have been led to believe that, although the earliest mammals appeared in the Triassic fossil record, throughout the Mesozoic they were tiny and meekly scurried and skulked while the dinosaurs reigned supreme over land, sea and air. They had to wait for the K-T extinction to develop their full ecological potential. That is now a myth, for Chinese strata (yet again) have revealed much larger mammals than ever thought possible, and some of them ate dinosaurs (Hu, Y. et al. 2005. Large Mesozoic mammals fed on young dinosaurs. Nature, v. 433, p. 149-152). One indisputable mammal skeleton contained the bones of young dinosaurs in its body cavity. In fact so many that one wonders if it met its end through greed.