Tag Archives: K-Pg extinction

Better dating of Deccan Traps, and the K-Pg event

Predictably, the dialogue between the supporters of the Deccan Trap flood basalts and the Chicxulub impact as triggers that were responsible for the mass extinction at the end of the Mesozoic Era (the K-Pg event) continues. A recent issue of Science contains two new approaches focussing on the timing of flood basalt eruptions in western India relative to the age of the Chicxulub impact. One is based on dating the lavas using zircon U-Pb geochronology (Schoene, B. et al. 2019. U-Pb constraints on pulsed eruption of the Deccan Traps across the end-Cretaceous mass extinction. Science, v. 363, p. 862-866; DOI: 10.1126/science.aau2422), the other using 40Ar/39Ar dating of plagioclase feldspars (Sprain, C.G. et al. 2019. The eruptive tempo of Deccan volcanism in relation to the Cretaceous-Paleogene boundary. Science, v. 363, p. 866-870; DOI: 10.1126/science.aav1446). Both studies were initiated for the same reason: previous dating of the sequence of flows in the Deccan Traps was limited by inadequate sampling of the flow sequence and/or high analytical uncertainties. All that could be said with confidence was that the outpouring of more than a million cubic kilometres of plume-related basaltic magma lasted around a million years (65.5 to 66.5 Ma) that encompassed the sudden extinction event and the possibly implicated Chicxulub impact. The age of the impact, as recorded by its iridium-rich ejecta found in sediments of the Denver Basin in Colorado, has been estimated from zircon U-Pb data at 66.016 ± 0.050 Ma; i.e. with a precision of around 50 thousand years.

407458aa.2

The Deccan Traps in the Western Ghats of India (Credit: Wikipedia)

Because basalts rarely contain sufficient zircons to estimate a U-Pb age of their eruption, Blair Schoene and colleagues collected them from palaeosols or boles that commonly occur between flows and sometimes incorporate volcanic ash. Their data cover 23 boles and a single zircon-bearing basalt. Sprain et al. obtained 40Ar/39Ar ages from 19 flows, which they used to supplement 5 ages obtained by their team in previous studies that used the same analytical methods and 4 palaeosol ages from an earlier paper by Schoene’s group.

The zircon U-Pb data from palaeosols, combined with estimates of magma volumes that contributed to the lava sequence between each dated stratigraphic level, provide a record of the varying rates at which lavas accumulated. The results suggest four distinct periods of high-volume eruption separated by long. periods of relative quiescence. The second such pulse precedes the K-Pg event by up to 100 ka, the extinction and impact occurring in a period of quiescence. A few tens of thousand years after the event Deccan magmatism rose to its maximum intensity. Schoene’s group consider that this supports the notion that both magmatism and bolide impact drove environmental deterioration that culminated in mass extinction.

The Ar-Ar data derived from the basalt flows themselves, seem to tell a significantly different story. A plot of basalt accumulation, similarly derived from dating and stratigraphy, shows little if any sign of major magmatic pulses and periods of quiescence. Instead, Courtney Sprain’s team distinguish an average eruption rate of around 0.4 km3 per year before the K-Pg event and 0.6 km3 per year following it. Yet they observe from climate proxy data that there seems to have been only minor climatic change (about 2 to 3 °C warming) during the period around and after the K-Pg event when some 75% of the lavas flooded out. Yet during the pre-extinction period of slower effusion global temperature rose by 4°C then fell back to pre-eruption levels immediately before the K-Pg event. This odd mismatch between magma production and climate, based on their data, prompts Sprain et al. to speculate on possible shifts in the emission of climate-changing gases during the period Deccan volcanism: warming by carbon dioxide – either from the magma or older carbon-rich sediments heated by it; cooling induced by stratospheric sulfate aerosols formed by volcanogenic SO2 emissions. That would imply a complex scenario of changes in the composition of gas emissions of either type. They suggest that one conceivable trigger for the post-extinction climate shift may have been exhaustion of the magma source’s sulfur-rich volatile content before the Chicxulub impact added enough energy to the Earth system to generate the massive extrusions that followed it. But their view peters out in a demand for ‘better understanding of [the Deccan Traps’] volatile release’.

A curious case of empiricism seeming to resolve the K-Pg conundrum, on the one hand, yet pushing the resolution further off, on the other …

More discussion on the K-Pg event can be read here

The winter of dinosaurs’ discontent

Under the auspices of the International Ocean Discovery Program (IODP), during April and May 2016 a large team of scientists and engineers sank a 1.3 km deep drill hole into the offshore, central part of the Chicxulub impact crater, which coincided with the K-Pg mass extinction event. Over the last year work has been underway to analyse the core samples aimed at investigating every aspect of the impact and its effects. Most of the data is yet to emerge, but the team has published the results of advanced modelling of the amount of climate-affecting gases and dusts that may have been ejected (Artemieva, N. et al. 2017. Quantifying the release of climate-active gases by large meteorite imp-acts with a case study of Chicxulub. Geophysical Research Letters, v. 44; DOI: 10.1002/2017GL074879).  . From petroleum exploration in the Gulf of Mexico the impact site is known to have been underlain by about 2.5 to 3.5 km of Mesozoic sediments that include substantial amounts of limestones and evaporitic anhydrite (CaSO4) – thicknesses of each are of the order of a kilometre. The impact would inevitably have yielded huge volumes of carbon- and sulfur dioxide gases, as well as water vapour plus solid and molten ejecta. The first, of course, is a critical greenhouse gas, whereas SO2 would form sulfuric acid aerosols if it entered the stratosphere. They are known to block incoming solar radiation. So both warming and cooling influences would have been initiated by the impact. Dust-sized ejecta that lingered in the atmosphere would also have had climatic cooling effects. The questions that the study aimed to answer concerns the relative masses of each gas that would have reached more than 25 km above the Earth to have long-term, global climatic effects and whether the dominant effect on climate was warming or cooling. Both gases would have added the environmental effects of making seawater more acid.

Chicxulub2

3-D simulation of the Chicxulub crater based on gravity data (credit: Wikipedia)

Such estimates depend on a large number of factors beyond the potential mass of carbonate and sulfate source rocks. For instance: how big the asteroid was; how fast it was travelling and the angle at which it struck the Earth’s surface determine the kinetic energy involved and the impact mechanism. How that energy was distributed between atmosphere, seawater and the sedimentary sequence, together with the pressure-temperature conditions for the dissociation of calcite and anhydrite all need to be accounted for by modelling. Moreover, the computation itself becomes extremely long beyond estimates for the first second or so of the impact. Earlier estimates had been limited by computer speeds to only the first few seconds of the impact and could not allow for other than vertical impacts. The new study, by supercomputers and improved algorithms, used a likely 60° angle of impact, new data on mineral decomposition and simulated the first 15 to 30 seconds. The results suggested that 325 ± 130 Gt of sulfur and 425 ± 160 Gt CO2 were ejected, compared with earlier estimates of 40-560 Gt of sulfur and 350-3,500 Gt of CO2.  The greater proportion of sulfur release to the stratosphere pushes the model decisively towards global cooling, probably over a lengthy period – perhaps centuries. Taking dusts into account implies that visible sunlight would also have been blocked, devastating the photosynthetic base of the global food chain, in the sunlit parts of oceans as well as on land.

But we have to remember that these are the results of a theoretical model. In the same manner as this study has thrown earlier modeling into doubt, more data – and there will be a great many from the Chicxulub drill core itself – and more sophisticated computations may change the story significantly. Also, the other candidate for the mass extinction event, the flood basalt volcanism of the Deccan Traps, and its geochemical effects on the climate have yet to be factored in. The next few lines of Shakespeare’s soliloquy for  Richard III may well emerge from future work

… Made glorious summer by this sun of York;
And all the clouds that lour’d upon our house
In the deep bosom of the ocean buried …

See also: BBC News comment on 31 October 201