Well, if the organic carbon coming out of rivers was a performer, I think it would be Houdini.
So, just as onlookers could watch Houdini make his way out from the wings and take position centre stage, we can measure the presence of organic carbon and track its movements, allowing us to ‘see’ it flow out of rivers as it makes its grand entrance into the coastal ocean.
And just like Houdini, one minute it’s right in front of us and the next, it’s vanished without a trace. Or has it?
(Before we go any further, if you haven’t read my last blog post, you can catch up here. Caught up? Then let’s go!)
For marine scientists, nothing ever really vanishes. Ever improving scientific techniques and equipment mean that we are continually getting better and better at being able to track the movement of things beyond what you might expect possible. You just need to know what to look for!
At the most basic level, everything is made up of atoms which make up molecules which make up compounds which make up substances which make up, well, everything. So, like every other living thing, the chemicals and compounds which make up microorganisms have to come from somewhere. We use isotopic tracking to find out where…
We think of carbon as being a singular thing, but it actually comes in 3 different forms. These different forms are called isotopes, and they work a bit like smarties (stick with me!)
Say there are two bowls of smarties, one at each end of a hallway – one is mostly blue, with some red and yellow, and the other is mostly yellow, with some blue and red. If you took a handful of smarties from one of the bowls and walked into another room, I could likely tell which bowl you had taken your smarties from (or which end of the hallway you were at when you picked them up), based on the mix of smarties you had in your hand.
If a plant grows on land, it will pick up a different ‘handful of smarties’ compared to one that grows in the ocean. This is that plants isotopic signature, and it’s like a chemical fingerprint specific to that species and location.
We can take a sample of water from anywhere – a lake, a river, the ocean – and filter everything else out until we are left with just the organic material dissolved in it. From there, we can use a technique called mass spectrometry to determine what that material is made up of. Using this technique, we can separate molecules based on their mass, including 3 different forms of carbon. It’s our way of looking to see what smarties the plant is holding – or the isotopic signature of the organic carbon in the sample, which gives us the fingerprint of where it came from.
And this is how we know that riverine organic carbon is going missing – when we look at samples from rivers, we find an isotopic fingerprint full of land-derived organic carbon, but when we look at samples from the ocean, we find much, much less.
Still with me? Great!
In my next posts, I’ll talk about some of the leading theories around why we don’t find as much riverine organic carbon in the ocean as we expect to, and how my research is trying to understand where it is going. In the meantime, if you have any questions please ask!