Tag Archives: Mass extinctions

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


The Higgs, gravity waves and now: dark matter and the dinosaurs

The discovery around 50 years ago that in orbiting the centre of the Milky Way galaxy the solar system regularly wobbles to either side of its path. If the galaxy’s physical properties varied in a direction at right angles to the plane of the Milky Way then the Sun and its planets would experience that variation in a regular and predictable way (see Galactic controls https://earth-pages.co.uk/2011/12/15/galactic-controls/). Such oscillations might therefore show up as periodic changes in the geological record. There are loads of such cycles some not so regular, such as the accretion and disaggregation of supercontinents, and some involved in climatic change that have almost the predictability of a metronome.
One of these periodicities has thrilled geoscientists ever since it first began to emerge from improved dating of events in the geological record and more extensive knowledge of what it contained. Massive floods of basaltic magma blurt from the mantle every so often; more specifically approximately every 35 Ma. Intriguingly, there is a rough tally between the timing of such large igneous provinces and pulses in biological extinction. The wobbles in the solar system’s galactic passage are – wait for it – about every 35 Ma. A supposed link between LIPs, extinctions and galactic motions simply will not go away as a topic for speculation. Add to that some evidence that terrestrial impact cratering might have a 35 Ma period and you have ‘a story that will run and run’. The apparent periodicity of impacts, besides encouraging links with life and death and magmas, now seems to have spurred links with the dark side of cosmology.

English: Artist's conception of the spiral str...

Artist’s conception of the spiral structure of the Milky Way with two major stellar arms and a central bar (credit: Wikipedia)

It does indeed seem that the galactic magnetic field and dust concentrations vary across the plane of the Milky Way, but their affects during solar peregrinations have been raised long before now (Steiner, J. 1967. The sequence of geological events and the dynamics of the Milky Way Galaxy. Journal of the Geological Society of Australia, v. 14, p. 99–132.). The latest novelty concerns the possibility that galaxies might somehow collect the fabled but as yet undiscovered ‘dark matter’ in a flat disc within the galactic plane. Well, matter, ‘dark’ or not, should have mass, and mass must have a gravitational effect (thanks of course to the Higgs boson), even if it is hidden. Instead of some Nemesis or Death Star, as once was proposed to nudge comets from the outer reaches of the solar system, a gigantic dish of dark matter through which the Sun might pass on a regular basis might serve more plausibly (Randall, L. & Reece, MM. 2014. Dark matter as a trigger for periodic comet impacts. Physical Review Letters. arXiv:1403.0576 [astro-ph.GA]). Interestingly, Comments on the paper at the arXiv site read “Accepted by Physical Review Letters. 4 figures, no dinosaurs”

Solar System, in Perspective

Solar System, in Perspective (credit: NASA Goddard SFC)

Enhanced by Zemanta