Guest post from Gwen Owen Jones – Biomarkers: what can they tell us about the past?

Throughout this course, you are learning lots about how the ocean behaves today but have you ever thought about how it behaved in the past? Both Matthew and myself are Palaeoceanographers and this is precisely what we study: how the oceans and climate have varied throughout time. Hopefully over the remainder of the course you will hear a bit more about this subject from me, Matthew and some other members of our research group. First up is Gwen who is lucky enough to share an office with us and is embarking on some really interesting work using biomarkers to understand the climate in the North Atlantic 34 to 23 millions years ago. I will leave her to tell you more…… 

Hello everyone! My name is Gwen and I’m a first-year PhD student at the National Oceanography Centre, University of Southampton. My research will focus on understanding the climate of the Oligocene epoch, a period of geological time from 34 to 23 million years ago. The Oligocene had similar CO₂ levels to the modern day and marked the beginning of the “Icehouse” climate state we see today, with the development of large-scale ice sheets over Antarctica. So although this was a very long time ago, we can apply what we learn about this period of time to understanding how climate change might affect us in the future.

One key tool I will be using in my project are biomarkers. These are special compounds produced by an organism in life in response to certain conditions, and they can tell us amazing things about past environments! Not only can biomarkers be used to identify the presence of certain organisms in the geological record, but we can also use them to reconstruct useful parameters such as temperature, pH, CO₂ and salinity. They’re even used to look for life on other planets – in the upcoming 2020 mission, the European Space Agency’s ExoMars Rover will drill down into the Martian soil to (hopefully!) collect organic material for biomarker analysis. Biomarkers are incredibly useful in the oceans, where they provide a measurable signature of biological activity even if the original organisms were made of soft parts (which are rarely preserved) or had carbonate shells that dissolved away.

One part of my PhD project will be using biomarkers to reconstruct North Atlantic oceanic temperatures way back in the Oligocene, using long-chain organic compounds known as alkenones. These compounds are produced by certain tiny micro-algae known as coccolithophores, which live in our oceans and form their shells out of calcium carbonate plates. These little shells build up on the seafloor and can eventually form huge carbonate deposits – exactly how the White Cliffs of Dover were made!

Emiliania huxleyi is one of the few species of coccolithophore that synthesises alkenones. It lives in the uppermost photic zone (the top layer of the ocean which is exposed to sunlight) and can be found in different environments all over the world, sometimes forming massive algal “blooms” that can be larger than the size of England! The cool thing about alkenones is that their structure changes depending on temperature – if Emiliania huxleyi grows in warmer temperatures the alkenones it produces will have fewer double bonds, and vice versa. Therefore, the relative abundance of certain types of alkenones can be used to figure out the sea surface temperature where these alkenone-producing algae lived. We can figure out an exact quantitative calibration of alkenone unsaturation (number of double bonds) to temperature in the laboratory by growing cultures of Emiliania huxleyi and analysing alkenone abundances at different temperatures. Alkenones produced by these organisms survive in the sediment for millions of years without changing their chemical structure, making them perfect for generation of climate records way back in geological time!

There are still limitations to this technique – for example, we have to rely on the assumption that ancient alkenone producers had the same relationship with temperature as modern-day Emiliania huxleyi. However, as with any uncertainties in paleoclimate research, it’s generally best to use what we’d call a “multi-proxy approach”. In this case, that means using multiple indicators of temperature at the same time and seeing how well they match – we often compare alkenone unsaturation with another biomarker-based indicator of temperature known as “TEX₈₆”. We can also analyse the chemical composition of certain organisms’ shells to get chemical indicators of past temperature, such as oxygen isotopes or the ratio of magnesium (Mg) to calcium (Ca) – but that’s a whole other blog post in itself!

I hope you enjoyed this short introduction into my PhD topic and that you all learned a bit about biomarkers. Feel free to ask any questions you might have!