Volcanoes and the carbon cycle

When volcanoes erupt, huge amounts of carbon are released into the atmosphere. Historically, we have believed this carbon came from deep inside the Earth, however new research from Cambridge University has upended this notion. Georgia Mills spoke to Sasha Turchyn...

Sasha - We’re really interested in understanding how carbon moves around the planet. When we think about the carbon cycle you often think about the movement of carbon between the atmosphere, or into the biosphere, or into the oceans. On geological timescales, so tens of millions of years or hundreds of millions of years, we think about the carbon cycle as carbon that comes from volcanoes into the surface of the planet and, eventually, after a certain amount of time will become a rock or a mineral and return back down into mantle, or back down into a rock form where it’s locked up for a while. That geological carbon cycle that’s been an open question for a little while to understand exactly how carbon is coming out of the surface of the planet and how it actually gets removed from the surface of the planet.
Georgia - So you’re interested in where this carbon was actually coming from - the volcano shoots is out into the atmosphere but where is it from originally? So how did you go about looking into this?

Sasha - We used something called isotope ratios. When we think of carbon, we think of carbon having 6 protons and 6 neutrons, but actually 1% of all carbon has an extra neutron in the nucleus. So, because of that we can use that ratio of the heavy carbon to the light carbon to understand the source of that carbon.

We had this question a long time ago, maybe 7 or 8 years ago and there weren’t many measurements made just yet, and over the last 7 or 8 years there’s been many more measurements. There’s been an explosion in the number of measurements that are reported in literature. So, when we went and revisited this question with a Master’s student, Emily Mason, who’s the lead author of the study, we went back this past fall and we could see that there was enough data that had been developed of this isotope ratio, which is effectively a chemical fingerprint of where the carbon comes from to be able to say something that was significant.

Georgia - I’ll ask you where you found the carbon was from in a second, but I’m curious. I’m imagining someone with a test tube standing over a volcano - how do you get these samples?

Sasha - Well, there are some test tubes over volcanoes, but no-one falls in any more. One of the advances that’s happened in the last 7 or 8 years is that the analytical capabilities have meant that you need smaller and smaller samples sizes. And that has allowed people to take very small vials, either from fumeral gases or through bags attached to pumps on the ends of camping poles. I’ve sampled things like methane in saltmarshes, and there we can take a tent pole with a bag and you can swirl it using a small vacuum pump and you can sample it in about a minute or two. In a similar way you can do this, you don’t do this in super hot parts of the volcano, but in some of the more dispersed parts of the volcano you can sample the gases pretty easy.

One other advance that’s been great in the last few years is you can make some of these measurements on site, which has allowed for more measurements to be made.

Georgia - So what did you find? Where is this carbon that the volcanoes are shooting out coming from?

Sasha - A lot of it is coming from the surface. A lot of it is from other rocks that are very close to the surface and not from as deep in the mantle. It doesn’t have that characteristic sample of deep mantle carbon that we were expecting to find. But what was most interesting is the volcanoes that are putting out the most CO2 - things like the Italian volcanoes - they’re very, very big CO2 emitters, so is Papua New Guinea for example. Those ones had the most carbon that was coming from the surface. Whereas other volcanoes like the Alaskan Ark, they were putting out mantle carbon, they’re not putting out quite as much CO2. So once you did a weighted average of both the amount of CO2 and its chemical fingerprint, it was very heavily weighted towards the crustal recycling.

Georgia - Why is it important to know this?

Sasha - That isotope fingerprint is the primary tool that we use to reconstruct the carbon cycle over geological time. So if I’m interested in 500 million years years ago, or a billion years ago understanding how much carbon was coming out, how much carbon was going down, how was it going down, was it related to animals or not? I’ll use that carbon ratio. So if the ratio in volcanoes can change systematically over time then that implies that we can’t trust those measurements from deep in geologic time for understanding the carbon cycle.

Georgia - I see. So it’s kind of changing what we understand the history of the carbon cycle will have been?

Sasha - We’ll have to rethink some of our interpretations. We have interpreted changes that have been measured to be attributed to say changes in some process at the surface, but it may be a change in the type of volcanoes that are around.