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)

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