Closure for the K-Pg extinction event?

Anyone who has followed the saga concerning the mass extinction at the end of the Cretaceous Period (~66 Ma ago) , which famously wiped out all dinosaurs except for the birds, will know that its cause has been debated fiercely over four decades. On the one hand is the Chicxulub asteroid impact event, on the other the few million years when the Deccan flood basalts of western India belched out gases that would have induced major environmental change across the planet. Support has swung one way or the other, some authorities reckon the extinction was set in motion by volcanism and then ‘polished-off’ by the impact, and a very few have appealed to entirely different mechanism lumped under ‘multiple causes’. One factor behind the continuing disputes is that at the time of the Chicxulub impact the Deccan Traps were merrily pouring out Disentanglement hangs on issues such as what actual processes directly caused the mass killing. Could it have been starvation as dust or fumes shut down photosynthesis at the base of the food chain? What about toxic gases and acidification of ocean water, or being seared by an expanding impact fireball and re-entering incandescent ejecta? Since various lines of evidence show that the late-Cretaceous atmosphere had more oxygen that today’s the last two may even have set the continents’ vegetation ablaze: there is evidence for soots in the thin sediments that mark the K-Pg boundary. The other unresolved issue is timing: of volcanogenic outgassing; of the impact, and of the extinction itself. A new multi-author, paper may settle the whole issue (Hull, P.M and 35 others 2020. On impact and volcanism across the Cretaceous-Paleogene boundary. Science, v. 367, p. 266-272; DOI: 10.1126/science.aay5055).

K-Pg oxygen
Marine temperature record derived from δ18O and Mg/Ca ratios spanning 1.5 Ma that includes the K-Pg boundary: the bold brown line shows the general trend derived from the data points (Credit: Hull et al. 2020; Fig 1)

The multinational team approached the issue first by using oxygen isotopes and the proportion of magnesium relative to calcium (Mg/Ca ratio) in fossil marine shells (foraminifera and molluscs) in several ocean-floor sediment cores, through a short interval spanning the last 500 thousand years of the Cretaceous and the first  million years of the Palaeocene. The first measures are proxies for seawater temperature. The results show that close to the end of the Cretaceous temperature rose to about 2°C above the average for the youngest Cretaceous (the Maastrichtian Age; 72 to 66 Ma) and then declined. By the time of the mass extinction (66 Ma) sea temperature was back at the average and then rose slightly in the first 200 ka of Palaeocene to fall back to the average at 350 ka and then rose slowly again.

Changes in carbon isotopes (δ13C) of bulk carbonate samples from the sediment cores (points) and in deep-water foraminifera (shaded areas) across the K-Pg boundary. (Credit: Hull et al. 2020; Fig 2A)

The second approach was to look in detail at carbon isotopes (δ13C) – a measure of changes in the marine carbon cycle –  and oxygen isotopes (δ18O) in deep water foraminifera and bulk carbonate from the sediment cores, in comparison to the duration of Deccan volcanism (66.3 to 65.4 Ma). The δ13C measure from bulk carbonate stays roughly constant in the Maastrichtian, then falls sharply at 66 Ma.  The δ13C of the deep water forams rises to a peak at 66 Ma. The δ18O measure of temperature peaks and declines at the same times as it does for the mixed fossils. Also examined was the percentage of coarse sediment grains in the muds from the cores. That measure is low during the Maastrichtian and then rises sharply at the K-Pg boundary.

Since warming seems almost certainly to be a reflection of CO2 from the Deccan (50 % of total Deccan outgassing), the data suggest not only a break in emissions at the time of the mass extinction but also that by then the marine carbon system was drawing-down its level in air. The δ13C data clearly indicate that the ocean was able to absorb massive amounts of CO2 at the very time of the Chicxulub impact and the K-Pg boundary. Flood-basalt eruption may have contributed to the biotic aftermath of the extinction for as much as half a million years. The collapse in the marine fossil record seems most likely to have been due to the effects of the Chicxulub impact. A third study – of the marine fossil record in the cores – undertaken by, presumably, part of the research team found no sign of increased extinction rates in the latest Cretaceous, but considerable changes to the marine ecosystem after the impact. It therefore seems that the K-Pg boundary impact ‘had an outsized effect on the marine carbon cycle’. End of story? As with earlier ‘breaks through’; we shall see.

See also: Morris, A. 2020 Earth was stressed before dinosaur extinction (Northwestern University)

How far has geochemistry led geology?

 

Granite pmg ss 2006
Thin section of a typical granite: clear white and grey grains are quarts (silica); striped black and white is feldspar; coloured minerals are micas (credit: Wikipedia)

In the Solar System the Earth is unique in having a surface split into two distinct categories according to their relative elevation; one covered by water, the other not. More than 60% of its surface – the ocean basins – falls between 2 to 11 km below sea level with a mean around 4 to 5 km deep. A bit less than 40% – land and the continental shelves – stands higher than 1 km below sea level up to almost 9 km above, with a mean around 1 km high. Between 1 and 2 km below sea level is represented by only around 3 % of the surface area. This combined hypsography and wetness is reckoned to have had a massive bearing on the course of climate and biological evolution, as far as allowing our own emergence. The Earth’s bimodal elevation stems from the near-surface rock beneath each division having different densities: continental crust is less dense than its oceanic counterpart, and there is very little crustal rock with an intermediate density. Gravitational equilibrium ensures that continents rise higher than oceans. That continents were underpinned mainly by rocks of granitic composition and density, roughly speaking, was well known by geologists at the close of the 19th century. What lay beneath the oceans didn’t fully emerge until after the advent of plate tectonics and the notion of simple basaltic magmas pouring out as plates became detached.

In 1915 Canadian geologist Norman Levi Bowen resolved previously acquired knowledge of the field relations, mineralogy and, to a much lesser extent, the chemistry of igneous rocks, predominantly those on the continents in a theory to account for the origin of continents. This involved a process of distillation or fractionation in which the high-temperature crystallisation of mafic (magnesium- and iron-rich) minerals from basaltic magma left a residual melt with lower Mg and Fe, higher amounts of alkalis and alkaline earth elements and especially enriched in SiO2 (silica). A basalt with ~50% silica could give rise to rocks of roughly granitic composition (~60% SiO2) – the ‘light’ rocks that buoy-up the continental surface – through Bowen’s hypothetical fractional crystallisation. Later authors in the 1930s, including Bowen’s teacher Reginald Aldworth Daly, came up with the idea that granites may form by basalt magma digesting older SiO2-rich rocks or by partially melting older crustal rocks as suggested by British geologist Herbert Harold Read. But, of course, this merely shifted the formation of silica-rich crust further back in time

A great deal of field, microscope and, more recently, geochemical lab time has been spent since on to-ing and fro-ing between these hypotheses, as well as on the petrology of basaltic magmas since the arrival of plate theory and the discovery of the predominance of basalt beneath ocean floors. By the 1990s one of the main flaws seen in Bowen’s hypothesis was removed, seemingly at a stroke. Surely, if a basalt magma split into a dense Fe- Mg-rich cumulate in the lower crust and a less dense, SiO2-rich residual magma in the upper continental crust the bulk density of that crust ought to remain the same as the original basalt. But if the dense part somehow fell back into the mantle what remained would be more able to float proud. Although a neat idea, outside of proxy indications that such delamination had taken place, it could not be proved.

Since the 1960s geochemical analysis has became steadily easier, quicker and cheaper, using predominantly X-ray fluorescence and mass-spectrometric techniques. So geochemical data steadily caught up with traditional analysis of thin sections of rock using petrological microscopes. Beginning in the late 1960s igneous geochemistry became almost a cottage industry and millions of rocks have been analysed. Recently, about 850 thousand multi-element analyses of igneous rocks have been archived with US NSF funding in the EarthChem library. A group from the US universities of Princeton, California – Los Angeles and Wisconsin – Madison extracted 123 thousand plutonic and 172 thousand volcanic igneous rocks of continental affinities from EarthChem to ‘sledgehammer’ the issue of continent formation into a unified theory (Keller, C.B. et al. 2015. Volcanic-plutonic parity and the differentiation of the continental crust. Nature, v. 523, p. 301-307).

In a nutshell, the authors compared the two divisions in this vast data bank; the superficial volcanic with the deep-crustal plutonic kinds of continental igneous rock. The gist of their approach is a means of comparative igneous geochemistry with an even longer pedigree, which was devised in 1909 by British geologist Alfred Harker. The Harker Diagram plots all other elements against the proportionally most variable major component of igneous rocks, SiO2. If the dominant process involved mixing of basalt magma with or partial melting of older silica-rich rocks such simple plots should approximate straight lines. It turns out – and this is not news to most igneous geochemists with far smaller data sets – that the plots deviate considerably from straight lines. So it seems that old Bowen was right all along, the differing deviations from linearity stemming from subtleties in the process of initial melting of mantle to form basalt and then its fractionation at crustal depths. Keller and colleagues found an unexpected similarity between the plutonic rocks of subduction-related volcanic arcs and those in zones of continental rifting. Both record the influence of water in the process, which lowers the crystallisation temperature of granitic magma so that it freezes before the bulk can migrate to the surface and extrude as lava. Previously. rift-related magmas had been thought to be drier than those formed in arcs so that silica-rich magma should tend to be extruded.

But there is a snag, the EarthChem archive hosts only data from igneous rocks formed in the Phanerozoic, most being less than 100 Ma old. It has long been known that continental crust had formed as far back as 4 billion years ago, and many geologists believe that most of the continental crust was in place by the end of the Precambrian about half a billion years ago. Some even reckon that igneous process may have been fundamentally different before 3 billion years ago(see: Dhuime, B., Wuestefeld, A. & Hawkesworth, C. J. 2015. Emergence of modern continental crust about 3 billion years ago.  Nature Geoscience, v. 8, p.552–555). So big-science data mining may flatter to deceive and leave some novel questions unanswered .