Connecting volcanoes to Earth’s ancient climate | Emily Mason
When it comes to both the life and climate of our planet, you’d be hard pressed to think of a more important element than carbon. It forms the basis of all life and in carbon dioxide (CO2) it plays a fundamental role in the variation of Earth’s temperature (think the Greenhouse effect). So we can agree that carbon plays a crucial role in processes at the surface of our planet. However, the vast majority of Earth’s carbon actually resides in the deep Earth (mantle and core). This means that even a small imbalance in the cycling of carbon between the surface and the deep could have a very large impact on Earth’s climate and the organisms that call it home. Carbon is released from the deep Earth from through volcanoes as CO2 and is returned at particular tectonic settings called subduction zones. Volcanoes in these settings form volcanic arcs – chains of volcanoes that often form an arc shape over the Earth’s surface. Where the overriding crust in a subduction zone is continental, this is a continental arc, and when it is oceanic crust, it is an oceanic arc. When the processes of subduction and volcanic outgassing are balanced, CO2 contents in Earth’s atmosphere, and therefore atmospheric temperature, are stable over long timescales.
Schematic displaying the differences between Oceanic Arc and Continental Arc Volcanic Settings. Created by Emily Mason for use in her masters project. It is worth pointing out that release of carbon from the deep Earth has the potential to modulate Earth’s climate over long, or as we like to say ‘geological’, timescales (reset your stopwatches, we’re talking millions to tens of millions of years here). So, just to clarify, there is no way we can lay the blame for recent warming on volcanoes! While volcanoes are estimated to release around 540 mega tonnes (that’s 540000000000000 kg) of CO2 per year1, anthropogenic CO2 emissions exceed 35,000 mega tonnes per year2. Different volcanoes emit different amounts of CO2 and therefore have a more or less significant influence on Earth’s long-term climate. One of the biggest carbon emitters is Mount Etna, on the island of Sicily, Italy. Etna releases on average about 20, 000 tonnes of CO2 per day1. While this pales into insignificance when considered next to anthropogenic emissions, on the pre-industrial Earth, volcanoes like Mount Etna would have represented the largest point sources of CO2 on the planet, with a correspondingly large influence on Earth’s climate. Let’s forget about anthropogenic emissions for a while (it’s nice to think about an Earth that is not affected by global warming for a few minutes!) – what makes one volcano a bigger carbon emitter than the rest? There are a lot of factors that could be contributing to the carbon emissions of any one volcano (e.g. higher magma supply rate, differences in magma source composition) but for volcanoes like Etna, the presence of carbon in the form of carbonate platforms (i.e. limestone) in the crust overlying beneath the volcano (or in the crust overlying the magma source region) might have a significant effect. One way to determine whether interactions between magma and limestone do control the amount of CO2 emitted from a volcano is to use isotope proxies. An isotope is an atom of an element e.g. Carbon, that has the same number of protons but a different number of neutrons – giving the atom the same relative atomic mass but different chemical properties. Different sources of carbon will have a different isotopic signature and so we can use analysis of these isotopes to understand what is feeding the carbon in our volcano. Using a combination of carbon and helium isotope proxies (for more detail see Mason, Edmonds and Turchyn, 2017, Science)3 we showed that volcanoes that display evidence of interaction with carbonate in the overlying crust do also appear to emit the largest volumes of CO2 (by some margin!). So what does this mean for the history and modulation of Earth’s ancient climate? Well what if we could somehow have a version of Earth where we had many more volcanoes like Etna. For this we would require many more subduction zone (arc) volcanoes that lie above continental crust containing thick carbonate platforms. Lee et al. (2013)4 suggest that the cycle of supercontinent growth and dispersal provides a means of increasing the proportion of continental arc volcanoes (because during continental dispersal, continental crust is in the right position for volcanoes to erupt through it). To put this into context, this means that during the break-up of supercontinents like Pangea and Gondwana, continental arcs would be more abundant than oceanic arcs. Lee et al. (2013)4 also show that today we actually live in an ocean arc dominated world. This means that volcanic CO2 emissions have the potential to be much higher in the past when continental arcs were more dominant. For example, in the Cretaceous (~145 to 66 million years ago), where we have inferred higher CO2 levels and a warmer world, a very long, carbonate-rich arc existed that is no longer present today. This subduction zone was related to the closure of an ancient ocean called the Tethys and ran from the present day Mediterranean, across the yet to be built Himalayas and down through land masses that now make up Thailand. Could the CO2 release from volcanoes on this continental arc explain why we see a warmer world in the Cretaceous? And could this be extended deeper into Earth’s history? Might we see a repeated long-term CO2 modulation related to supercontinent building and break-up? Arc volcanic settings are not the only places that might change CO2 outgassing behaviour in these supercontinent cycles and exciting new research has shown that continental rifting (e.g. in places like the East African rift today) might also have a significant role to play5. We are really starting to get to grips with how these processes work and exciting new developments continue to arrive quickly – but there’s still lots more work to do to figure out how volcanoes have acted as Earth’s ‘exhaust pipe’ over geological time!
To find out more about Emily’s research click here. References:
1. Burton, M.R., Sawyer, G.M. and Granieri, D., 2013. Deep carbon emissions from volcanoes. Reviews in Mineralogy and Geochemistry, 75(1), pp.323-354. 2. Friedlingstein, P., Houghton, R.A., Marland, G., Hackler, J., Boden, T.A., Conway, T.J., Canadell, J.G., Raupach, M.R., Ciais, P. and Le Quere, C., 2010. Update on CO2 emissions. Nature Geoscience, 3(12), pp.811-812. 3. Mason, E., Edmonds, M. and Turchyn, A.V., 2017. Remobilization of crustal carbon may dominate volcanic arc emissions. Science, 357(6348), pp.290-294. 4. Lee, C.T.A., Shen, B., Slotnick, B.S., Liao, K., Dickens, G.R., Yokoyama, Y., Lenardic, A., Dasgupta, R., Jellinek, M., Lackey, J.S. and Schneider, T., 2013. Continental arc–island arc fluctuations, growth of crustal carbonates, and long-term climate change. Geosphere, 9(1), pp.21-36. 5. Brune, S., Williams, S.E. and Müller, R.D., 2017. Potential links between continental rifting, CO 2 degassing and climate change through time. Nature Geoscience, 10(12), p.941.