Posts Tagged ‘earth history’
a few reflections on the great dying…
This is a self education piece. Or maybe that applies to all my writing. Anyway, it’s about the great dying at the end of the Permian period, a couple of hundred million years ago. I’m trying, not too successfully, to get my head around eras and periods and epochs and such, so… the Permian-Triassic extinction event, aka ‘the great dying’, was the biggest event of its kind in the planet’s history, resulting in the wiping out of 90% to 96% of marine species and 70% of land vertebrates, not to mention lots of insects. It also, apparently, marked the boundary between the Palaeozoic and Mesozoic eras.
So why these different names? Apparently because the Palaeo- and Mesozoic eras span hundreds of millions of years (the Palaeozoic lasted from 541 to 252 million years ago), while the Permian period started about 299 million years ago and ended tens of millions of years later, 252 million years ago, bringing in the Triassic, which ended 201 million years ago, with the start of the Jurassic. Anyway, I’ll have to make a wall chart. So, hundreds of millions versus tens of millions. But surely there are other reasons? What do these terms actually mean?
Oh, but first, the great dying. It was caused by a relatively sudden global warming. Our Earth was very different then, with much or all of the land forming one super-continent, Pangaea. In the north, a sudden eruption of volcanic activity filled the atmosphere with methane, sulphur dioxide (SO2) and carbon dioxide – the precise levels and rates of change are still under dispute, and the Wikipedia article on the event is I think the longest piece I’ve ever seen from them.
No more trilobites after that, sadly – what pets they would’ve made….
So we’re now in the Phanerozoic eon? Aren’t periods and eras enough? It’s horrible, but apparently – I have to get this straight – there are four established ‘hierarchies’ of geologic time, at least for now. Well, actually, one hierarchy of four. Eons, sub-divided into Eras, subdivided into Periods, sub-divided into Epochs, which could then be subdivided into my birthdays, but no that’s probably too personal…
Sorry, the great dying. Methane (CH4) and CO2, the two greenhouse gases we’re most concerned about today (nitrous oxide, N2O, comes in third), due to abundance combined with individual effects… okay, here’s Wikipedia:
The scientific consensus is that the main cause of the extinction was the flood basalt eruptions that created the Siberian Traps, which released sulphur dioxide and carbon dioxide, resulting in euxinia (oxygen-starved, sulphurous oceans) elevated global temperatures, and acidified oceans. The level of atmospheric carbon dioxide rose from around 400 ppm to 2,500 ppm with approximately 3,900 to 12,000 gigatonnes of carbon being added to the ocean-atmosphere system during this period. (References removed).
It’s certainly very thought-provoking that this massive life-annihilating event, which lasted only a few tens of thousands of years, occurred a mere 252 million years ago on our approximately 4.6 billion-year-old planet – pretty late in the day, really. Has everything under the surface of our planet ‘stabilised’ since then? I wouldn’t bet on it. As to that measure of atmospheric CO2, shifting from 400 to 2,500 ppm, it’s worth comparing to the ‘recent’ levels, before the industrial revolution – about 280 ppm – and currently (about 430 ppm, and gradually rising, of course).
The whole extinction event lasted about 100,000 years, and involved an overall rise in temperature of about 8 degrees celsius, and an increase in ultraviolet radiation, which significantly affected plant growth. Much of the detail is necessarily speculative, but there seem to have been more intensive El Nino events, and, in amongst it, a massive extra-terrestrial impact, which created the Araguainha crater in what is now Brazil, and which caused a massive release of methane. Ozone layer destruction was also a factor. Increased ultraviolet radiation led to plant mutation and death, which, it’s speculated, led to increased fungal growth. Uncertainty abounds, however – land masses were completely different from what they are today, and different studies in different parts support everything, it seems to me, from overall gradualism to pulses of extinction followed by relative stability – but the last (million!) years of the Permian seem to have been the worst.
Examinations of marine versus terrestrial extinctions haven’t arrived at much consistency either. It has to be admitted that arriving at ‘the truth’ from examining such altered landscapes and seascapes is a near-impossible task. In any case, some believe that the two extinction types were asynchronous, others disagree, and both camps of course have evidence to back up their positions. It may be that both camps are correct due to global differences in temperature and atmospheric conditions. In any case, marine organisms suffered most, especially those with calcium carbonate skeletons. Ocean acidification due to increased CO2 made producing those skeletons more difficult. Another strong risk factor was endemism – being found only in specific regions.
Wikipedia goes into extreme detail on all the types affected – conodonts, ammonoids, bryozoans and foraminifera, among many others. I’ve looked up images of them, or of their fossilised remains, in memoriam. I think the key to this is to expect the unexpected – which is precisely something we cannot do, but we can, through knowledge, through a better understanding of our planet and the dangers to it, from within and without, and how we might avert those dangers – anthropogenic global warming being the most obvious – we can reduce the ‘degree of unexpectedness’. That’s one of the many things science is good for.
References
https://en.wikipedia.org/wiki/Permian–Triassic_extinction_event