Popular stable carbon isotopes

I teach a module in which the focus is on us inviting guest speakers from all sorts of directions in academia and the corporate world, who do something with marine geology. They just provide a lecture about their field of expertise, and the students can then take one of these topics and read up on it a bit more. They write a popular scientific article about it, and give a presentation. The idea is that they push the topic further than the guest lecturer did. We the resident staff provide a series of lectures that we use for trying to make sure that the students have all the background knowledge they need an order to understand these guest lectures.

I tend to talk about all the most common methods we are using to try to get useful information out of sediments or rocks. And in my general line of science, you can't get around stable isotopes. They are so incredibly useful! All elements have more than one isotope, and the isotopes are chemically identical but physically slightly different. And typically, when these elements go through a phase change, not only do some of the isotopes preferentially make that phase change, but also, the strength of this preference depends on temperature. So if you are reading this, and you have the faintest idea of what the temperature of any aspect of the system Earth was at some point in the past that is beyond historic measurements, you probably know this because of stable isotopes. So I had given lectures about that. And I know it comes back in at least one of the guest lectures.

What I said above about phase changes and isotopes also holds for carbon, but carbon is special; any creature that engages in photosynthesis prefers light carbon isotopes. I'm not sure why! But they do. As a result, organic carbon is isotopically a lot lighter than inorganic carbon. And it also means that you can basically find out whether there have been big fluctuations in photosynthesis in the geological past. Especially in the ocean. Many people might think of photosynthesis as something that mainly happens on land, but don't forget plankton! And plankton blooms!

A plankton bloom near Cornwall. Pic by NASA

You can probably imagine that if you have an awful lot of planktonic algae photosynthesising away at the sea surface, they can take up a lot of carbon. And if they then subsequently die, and then end up locked away in the sediment rather than being recycled somewhere along the way, you have effectively drawn carbon from the atmosphere. Anything that does that is currently of high societal relevance, of course!

So how would we know that that that happened? Well, if you have a lot of photosynthesis going on in the ocean, all the algae will pull all the isotopically light carbon out of the ocean (there is a lot of CO2 dissolved in it, and they can take it out) and then the ocean water ends up relatively isotopically heavy. If any creature is then precipitating calcite in that seawater, the carbon in that calcite will automatically also be quite isotopically heavy. And that calcite is quite likely to survive. Possibly for tens of millions of years! So in that sense, you can track the productivity of the ocean quite far back in time.

There is a flipside to this, of course. If you store all that isotopically light carbon somewhere, it might escape its  storage and come back to haunt you! Marine sediments contain a lot of methane. And that methane can also be released. It tends to be stable in low temperatures. If you raise the temperature of the sediments too much, your methane will basically just burp out. This has happened before! And you can imagine that that would turn both the atmosphere and the ocean suddenly quite isotopically light when it comes to carbon. And that, as well, is something you would quite easily see in marine calcite.

How all this carbon moves between the atmosphere (where we don't want it) and somewhere safely out of reach, like in marine sediments (where we do want it) is something we really want to know about. Some 55 million years ago, something went catastrophically wrong in the Earth produced the biggest burp known to man. It wreaked climate havoc! And the thing is; we are now on our way to create a much worse event. It's just not yet quite apparent; that event took about 10.000 years to unfold, which in geological terms is just a blink of an eye, but for a humans is an unimaginable amount of time. And we are releasing carbon into the atmosphere at about 10 times the speed that was seen back then. But we have only been at it for some hundred years! So the effects are only beginning to be felt.

Altogether, I hope you agree with me that stable carbon isotopes are important. And there used to be a guest lecture in which they were very important. It was the one by my old PhD supervisor Dick Kroon. He presented an amazing lecture where he went into detail about how often this kind of world burp takes place, and under what circumstances. Fascinating but also cause for concern! But when the pandemic started, he decided to stop giving this lecture because he was a big cheese in an ocean drilling program, and he needed all his energy to somehow mitigate the damage the pandemic was doing to that. And he has since died, so he never will give the lecture again.

So where am I going with all of this? I normally teach the students about stable carbon isotopes, but this year I figured none of our guest lecturers would actually discuss it. So then what was the point? I had my students for a double lecture slot, and I decided to use the first slot for things that I knew they would be needing for the guest lectures. And then we would have a tea break, and after that break I would tell them about stable carbon isotopes. I also told them that there would be literally no assessment at all within this module in which that would come back. So if they thought it was a waste of their time, they were totally welcome to go away and do something else! But if anyone would stay on, I would deliver that lecture.

I seriously considered the possibility that none of them would come back. But after I came back from my convenience break, I saw that almost all the students still there! I thought that was an uplifting view. And I quite happily delivered my lecture!

I think it is lovely that the students just want to know about these things just for the sake of knowing it! And not because it has any particular positive effect on their grades. Who knows what they will be doing with this knowledge in the future. They are part of a generation that will have a much tougher time dealing with carbon in the climate system than my generation does! And knowing what sort of havoc it has created in the past is, of course, by no means a guarantee for mitigating the damage in the present day in the future, but it is a start. There is hope!