Volcanoes: our noble allies in the battle against export productivity

Finally, a blogospheric spat that actually matters. Craig McClain over at Deep Sea News has accused volcanoes of being the implacable enemies of marine life, based on new research linking them to some bouts of extreme ocean anoxia (where the deep oceans become severely depleted in oxygen, to the detriment of much of the life there). Maria jumped to rebut what she views as a vile libel (unsurprisingly, the Volcanism Blog has backed her up), and Craig has now hit back, rebutting her rebuttal.
It’s certainly true that volcanoes can be bad for life, both marine and non. A large-ish volcanic eruption such as the Mount Pinatubo eruption in 1991 has global effects on the atmosphere and climate, and that’s just a small pop compared to those times in the geological past where whole tracts of the Earth’s surface have been resurfaced by flood basalts (over a million square kilometres in the case of the Siberian Traps, which are widely – but not universally – implicated in the end-Permian extinction 250 million years ago).
But what Craig isn’t telling you is that the marine biosphere has it’s own dirty little secret. What do marine organisms do? They take carbon dioxide out of the atmosphere, either directly by photosynthesis, or more indirectly by taking bicarbonate from ocean water to make protective shells. Then they die, and sink to the ocean bottom, where all that organically fixed carbon is incorporated into carbonate- or organic rich sediments. Once buried, it’s pretty hard to remobilise it, and its especially hard to get it back into the atmosphere as carbon dioxide.*
Over geological time, this continual drawdown would leave us with an atmosphere devoid of all CO2, which would severely cool the planet by reducing the greenhouse effect (note: this fact has no bearing on the potentially deleterious effects of anthropogenic global warming. You can have too much of a good thing, and you can certainly have it too fast). In effect, marine export productivity (as it is called) has the potential to freeze all life on the planet to death.
Fortunately, plate tectonics comes to the rescue. When oceanic crust is subducted back into the mantle, the carbon-bearing sediments are heated, degassed, and the CO2 is incorporated into the ascending magma beneath…. volcanic arcs.
So perhaps we shouldn’t be renaming the Ring of Fire the ‘Arc of Evil’ after all. More seriously, life may sometimes be threatened by geology, but it is also intimately shaped by it, and I’m firmly convinced that life, and especially complex life, is dependent on vigorous geological activity to provide – and more importantly, maintain – the thermochemical gradients that drive interesting chemistry. Volcanoes may cause the odd extinction event or two, but they’re also a big part of why there are things to go extinct in the first place.
*This fact is the basis for all of the ideas about stimulating algal blooms in the ocean by seeding them with limiting micronutrients such as iron, to absorb anthropogenic CO2 (see the third item here)..