Extinction

A theme that reoccurs in these pages is to do with the notion of extreme events in planetary environments or in organism populations. I've talked before about whether we're more likely to spot terrestrial type planets and biospheres in states of flux rather than cozy stability, or indeed whether cozy stability is just a fleeting illusion based on our biased worldview of modern earth.

Among the various characteristics of life on this planet are episodes or periods where life has undergone dramatic down-sizing. Minor and mass extinction events pepper the fossil record, and indeed define the labels we assign to the great periods of Earth history. One of the grand-daddies of all such events occurred in the late Permian about 250 million years ago. An astonishing 95% of global marine life (at least of the larger variety) was extinguished, along with something like 70% of surface life. Imagine visiting a zoo of the late Permian, a couple of trilobite relatives and some insects, maybe a fern. Exactly what caused this massive die-off has been, and still is, a matter of intense debate. Many have long pointed the finger at places like the 'traps' (step-like terrain from ancient volcanic basalt flows) in what is now Siberia as a site of enormous geophysical activity that could have profoundly altered the planetary surface and marine environment. Covering a couple of million square kilometers this active region could easily have a global impact.

A key clue as to why the Siberian Traps could have profoundly affected the planet has been the suggestion that this volcanic region would have ignited massive coal deposits. As these burned they would have dumped colossal amounts of ash, carbon dioxide and other combustion byproducts like sulfuric acid, and even methane into the atmosphere. Now a new work in Nature Geoscience by Grasby et al. describes the discovery of precisely the kind of ash deposits in the rock record of far northern Canada that would have been produced by the Siberian volcanoes. Not only that, but the nature of this ash is very similar to that produced by modern industrial coal use, suggesting that toxic slurry would have been pouring into the late Permian marine environment. 

What is particularly interesting to my mind, which harks back to earlier posts, is that obviously the coal deposits that the Siberian lava ignited were themselves the product of a much earlier era of rich plant life - perhaps during the Carboniferous period some 300-360 million years ago. At the risk of greatly oversimplifying things it nonetheless seems that the exuberant growth of plant life, together with circumstances that led to burial and fossilization as coal, some 50 million years before the late Permian helped set a time-bomb for future organisms. I think we (astronomers, exoplanetary scientists) tend to ignore this kind of factor when we discuss planetary habitability. Extinction events are sometimes seen as random or disconnected from the deeper planetary history. Did it really matter what happened a hundred million years earlier if an asteroid comes plunging in or a super-volcano erupts? For the late Permian it looks like it did matter.

For discussions of the long-term habitability of planets this brings in a whole new piece to contend with. If life itself can actually lay the foundations for disaster tens to hundreds of millions of years down the line then we're dealing with a much more interconnected and potentially chaotic type of system than perhaps we thought. As always, stepping back from our incredibly narrow worldview of the modern Earth, is critical.