Friday, 28 March 2014
Not a problem for tomorrow, but for yesterday
The story of our planet is written in its rocks. The study of rock layers, called stratigraphy, can tell us a lot about the nature of our world in times past. Deeper layers of sedimentary rocks correspond to time periods further and further away from the present, and they can be distinguished from each other by their internally consistent characteristics. The presence of certain fossils can also help us mark the boundary between one time period and another: the appearance of shelled organisms, for example, marks the start of the supereon called Phanerozoic.
"Phanerozoic", you'll say. Is that something like the Jurassic? Yes, yes indeed it is, in that it describes a certain division of time in the story of the Earth. These divisions, just like geographical divisions, include large sections that are subdivided into smaller ones. Just as we have continents subdivided in countries subdivided in regions when it comes to define space, we have supereons subdivided in eras, then periods, then epochs, then ages for our geologic history. The Jurassic, for example, is one of three periods that form the Mesozoic era. It was a time of dinosaurs, though not the time of the tyrannosaurus (no matter what the founder of Jurassic Park thought); the tyrannosaurus would live millions of years later, during the third period of the Mesozoic, called the Cretaceous.
Digging vertically and looking at the remains of past forms of life (in the form of fossils, mostly), we are essentially traveling through time. The deeper we are, the more long ago we are. This kind of study allows us to see how life changes over time. Some forms of life, recognized by particular body types, size and other characteristics, change gradually over millions of years (or remain pretty much the same, depending on the circumstances). Sometimes, certain types beget two or more slightly different subforms that, in turn, accrue more and more differences until they look very different from their ancestor. And sometimes, certain lines just... stop. A particular form that might have been present, albeit in more or less modified form, in layers going from 150 millions to 70 millions of years ago is no longer found in the layers corresponding to that moment until now. That's how we understand that this particular lineage went extinct.
In five particularly spectacular instances we saw not ten, not a hundred, but truly massive numbers of lines all disappearing in a very short span. These events are referred to as mass extinctions. The most celebrated is the one that saw the end of the dinosaurs, but it wasn't the most severe.
The most ancient and the second-worst (as far as marine life is concerned) was the Ordovician-Silurian extinction event, which occurred 450-440 million years ago. Paradoxically, this event put an end to a great expansion in animal life forms, as the different phylla that had appeared during the Cambrian saw a spectacular diversification. More than 60% of marine life form were to disappear during this extinction event.
Perhaps less brutal because it was more of a slow burn than a bang (or consisted of several events happening over a relatively short time, geologically speaking), the late Devonian extinction, which likely took place over a few million years around 360 million years ago, killed half the genera living at the time. Among the most severely affected the reef-building corals; there were back then massive reefs in the oceans. Trilobites were also pretty hard hit, but a few lines manages to hang on.
Then came the Great Dying, the Permian extinction, which ushered in the Triassic period and the start of the Mesozoic era. Roughly 252 million years ago, this dreadful moment saw up to96% of all marine life and 70% of land vertebrates vanish from the face of the Earth. (Well, forgive me the lyricism of the image; they didn't actually vanish; I assume they more likely dropped dead). Marine invertebrates took the worst of it and that was pretty much it for the remaining trilobites, who had been around for 270 million years. Compared to that kind of death toll, concepts like decimation (by which the Roman army inspired discipline in its men by killing one legionary out of ten) seem tame indeed.
Then came the Mesozoic era, starting with the Triassic period. This would be the era of the large reptiles. At the end of the first period of the era, the Triassic-Jurassic extinction event, 201 million years ago, would once again clear the table. The fifth of all marine families disappeared, and on land almost all the non-dinosaurian archosaurs died off, with the exception of the crocodiles and a few therapsids. The dinosaurs would then rule the land for the two following periods, the Jurassic and the Cretaceous. (That they did so for more than 180 million years tells us that they were remarkably good survivors; hence, the derogatory comparison of any obsolete thing or person with a dinosaur strikes me as particularly ill-informed. When our modern hipster cell phone-carrying scoffers will have endured 180 million years, they'll have the right to sneer. Not before).
But tough as they were, dinosaurs were to know their Götterdämmerung, leaving only the lovely bit mostly unthreatening birds as their descendants. This extinction is known as the Cretaceous-Paleogene event, but because the Paleogene (with the Neogene) used to form what was known as the Tertiary period, we often informally call the event the Cretaceous-Tertiary, or K-T, crisis.
We're fairly sure that this event was caused, or at least greatly facilitated, by the impact of a very large asteroid (more than 10 km in diameter) that left us the Chixculub crater in Mexico. Another smoking gun is the presence of a thin layer of clay highly enriched in iridium at the boundary between the Cretaceous and the Paleogene. Iridium is rare at the surface of our planet, but more abundant in certain asteroids and other rocks hitting us from space; our hypothesis is that as it was pulverized upon impact, the Chixculub asteroid spread its iridium all over the globe.
Mammals appeared roughly at the same time as the early dinosaurs, but it is only with their passing that we finally got our chance to shine.
Researchers estimate that each of these massive extinctions accompanied important changes in the levels of atmospheric CO2. This gas has different effects: we are all aware that it is a greenhouse gas that can cause the warming of our atmosphere, but it is also the cause of ocean acidification according to the reaction CO2 +H2O -> H2CO3. The carbonic acid thus formed attacks and demineralizes the calcium-rich shells of marine invertebrates... bad news for all involved.
As great CO2 producers, we'd probably be well-advised to pay attention to what we do with our world.