Wildfires and climate at the K-Pg boundary

It is now certain that the Cretaceous-Palaeogene boundary 66 Ma ago coincided with the impact of a ~10 km diameter asteroid that produced the infamous Chicxulub crater north of Mexico’s Yucatán peninsula. Whether or not this was the trigger for the mass extinction of marine and terrestrial fauna and flora – the flood basalts of the Deccan Traps are still very much in the frame – the worldwide ejecta layer from Chicxulub coincides exactly with the boundary that separates the Mesozoic and Cenozoic Eras. As well as shocked quartz grains, anomalously high iridium concentrations and glass spherules the boundary layer contains abundant elemental carbon, which has been widely ascribed to soot released by vegetation that went up in flames on a massive scale. Atmospheric oxygen levels in the late Cretaceous were a little lower than those at present, or so recent estimates from carbon isotopes in Mesozoic to Recent ambers suggest (Tappert, R. et al. 2013. Stable carbon isotopes of C3 plant resins and ambers record changes in atmospheric oxygen since the Triassic. Geochimica et Cosmochimica Acta, v. 121, p. 240-262,) – other estimates put the level substantially above that in modern air. Whatever, global wildfires occurred within the time taken for the Chicxulub ejecta to settle from the atmosphere; probably a few years. It has been estimated that about 700 billion tonnes of soot were laid down, suggesting that most of the Cretaceous terrestrial biomass and even a high proportion of that in soils literally went up in smoke.

Charles Bardeen and colleagues at the University of Colorado, Boulder, have modelled the climatic and chemical effects of this aspect of the catastrophe (Bardeen, C.G. et al. 2017. On transient climate change at the Cretaceous−Paleogene boundary due to atmospheric soot injections. Proceedings of the National Academy of Sciences; doi:10.1073/pnas.1708980114). Despite the associated release of massive amounts of CO2 and water vapour by both the burning and the impact into seawater, giving increased impetus to the greenhouse effect, the study suggests that fine-grained soot would have lingered as an all enveloping pall in the stratosphere. Sunlight would have been blocked for over a year so that no photosynthesis would have been possible on land or in the upper ocean, the temperatures of the continent and ocean surfaces would have dropped by as much as 28 and 11 °C respectively to cause freezing temperatures at mid-latitudes. Moreover, absorption of solar radiation by the stratospheric soot layer would have increased the temperature of the upper atmosphere by several hundred degrees to destroy the ozone layer. Consequently, once the soot cleared the surface would have had a high ultraviolet irradiation for around a year.

The main implication of the modelling is a collapse in both green terrestrial vegetation and oceanic phytoplankton; most of the food chain would have been absent for long enough to wipe out those animals that depended on it entirely. While an enhanced greenhouse effect and increased acidification of the upper ocean through CO2 emissions by the Deccan flood volcanism would have placed gradually increasing and perhaps episodic stresses on the biosphere, the outcome of the Chicxulub impact would have been immediate and terrible.

More on mass extinctions and impacts here and here

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2 responses to “Wildfires and climate at the K-Pg boundary

  1. “the outcome of the Chicxulub impact would have been immediate and terrible”

    But (Dear Steve), don’t we also have to demonstrate that these effects of the impact were worldwide, or nearly so? For example, how geographically extensive are all these soot deposits? Otherwise, the devastation would only have been regional and so probably not enough to cause global extinctions on its own account.

    And are not some of the said outcomes of an impact indistinguishable from those caused by Deccan Traps eruptions? For example, both these ever-popular and well-known rival causes of end-Cretaceous extinctions would likely have affected bio-productivity, and even ultimately survival, of photosynthetic organisms. Certainly in the case of my own specialist subject, coral reefs and reef corals, I found that extinctions were greater amongst the latter, these being photosymbiotic and photosynthetic (cf. zooxanthellate corals) than amongst azooxanthellate corals. I argued that this result on its own does not tease out the ‘impact vs volcanism’ alternatives, and presumably much the same applies to all other photosynthetic organisms that went ‘preferentially’ extinct at the end of the Cretaceous.

    Best wishes

    Brian

    • Hi Brian, and thank you for your comment. As far as I know, the soot particles occur in all the occurrences of the K-T(Pg) event layer along with the other indicators of a major impact – shocked grains, melt spherules, high iridium etc. Interestingly though, it now seems that late-Cretaceous atmospheric oxygen levels were below those at present rather than far higher as once believed by the originators of the ‘global wildfire’ hypothesis back in the lat 80s.

      I’m sure you are right that impact- and volcanic-related extinctions cannot be distinguished. But I’m pretty sure that outcome of the Chicxulub impact would have been immediate and terrible – the equivalent of a small mountain falling out of the sky at about 15 km per second would have delivered well over a thousand years’ worth of global solar energy input in less than a second…

      The trouble with Earth-Pages is that I am limited to 700 words max for each item, and as they say, ‘I could go on…’!

      Coincidentally I have just received Michael Rampino’s slim volume to review for New Scientist and had just sat down to bash out a few opening comments when I saw the alert from Earth-Pages that you had made a comment.

      I really appreciate comments from all and sundry and wish that many more people would have a shot – nothing in the geosciences is cut and dry except perhaps the Law of Superposition, and a former graduate student of mine pointed out that even Steno’ initial contribution to stratigraphy doesn’t apply in palaeosols!

      Cheers
      Steve

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