Stable isotopes

We’ve discussed elsewhere how most of the bogs we work with are ombrotrophic, or rain-fed, obtaining all their water from precipitation. We’ve also described how many of these bogs are covered in Sphagnum mosses. Now, these mosses are very simple plants – they have no stomata and so are unable to regulate water uptake and loss. Rain that falls on the surface of the bog is taken up by these plants and incorporated into their tissues very quickly. This is great news for us, as it means that we can analyse the chemical composition of moss tissues preserved down through our peat cores, looking for changes over time that we might be able to link back to climate.

To be more precise, we’re looking for chemical evidence that changes have taken place in the stable isotopic composition of the rainwater. Isotopes are different versions of the same element, made heavier or lighter by the addition or subtraction of neutrons. They occur in in the main chemical components of rain and air that are taken up by plants (the important ones to us are oxygen, hydrogen and carbon). Oxygen, for example, has three different stable isotopes known as oxygen-16, oxygen-17 and oxygen-18. More than 99% of all oxygen is oxygen-16, which contains eight protons and eight neutrons. Only about 0.2% is oxygen-18, which has two extra neutrons, making it heavier than oxygen-16.

Here, we’re discussing ‘stable’ isotopes. This means that their chemical structure remains the same over time, but it is important to remember that some isotopes are radioactive, meaning that they decay over time – a process central to radiocarbon dating.

One key factor to us is that the isotopic composition of rain varies slightly according to where the moisture originated and how far it has travelled. For example, because oxygen-16 is lighter, it will evaporate first in warmer conditions.  In the same vein, the heavier oxygen-18 atoms will condense quicker in clouds and fall out in rainwater before oxygen-16 atoms. If we can identify changes in the ratios of the heavier and lighter isotopes from our moss tissue records then, in theory, we can reconstruct how atmospheric circulation and, therefore, climate has changed over time – and we’ve talked elsewhere about how important these circulatory systems are.

We also exploit the lighter and heavier carbon-12 and carbon-13 isotopes that occur in atmospheric carbon dioxide (CO2). Changes in these atoms can give us an indication of the environmental conditions at the time that the CO2 was incorporated by the Sphagnum moss through the process of photosynthesis. For example, when wetter conditions prevail there tends to be a higher proportion of ‘heavy’ CO2 (i.e. carbon-13 atoms) assimilated and preserved in the plant’s tissues.

Of course, it’s not quite this straight forward – there are plenty of other factors that we have to consider, but that’s another blog post for another day!

“But wait”, I hear you cry, “you’ve already told us that some bogs aren’t dominated by Sphagnum moss, but other plants entirely! How do stable isotopes work here?”. Well, good question! The short answer is that we’re not quite sure. Clearly though, with our insatiable quest for Bogological knowledge, we’re not happy with this! As a result, Matt is currently working on a project based in the peatlands of New Zealand, where rushes rather than mosses dominate, to see if we can expand the technique into these systems too.