My Research: Heather Goring-Harford
I am a chemistry graduate who has moved into the ocean and earth sciences for my PhD. As a chemist, I have some specialist skills and knowledge which are particularly useful for investigating how chemical elements behave in the oceans. For my PhD project I am studying how chromium stored in ancient rocks can be used as a tool to measure oxygen levels in past oceans.
Chromium is weathered from rocks, transported by rivers into the sea and deposited in ocean sediments. Along this journey, it undergoes different reactions which depend on the presence or absence of oxygen. These reactions cause some isotopes of chromium to be stored in sediments and rocks preferentially to others, and this creates an isotopic ‘signature’. By measuring these signatures in ancient rocks, we may be able to find out what levels of oxygen the chromium was exposed to, providing an indirect measurement of atmosphere or ocean oxygenation at the time the rock formed. We want to know more about how the oxygenation of the oceans affects climate and vice versa to help predict future changes, so this type of measurement should be very useful. The problem is, we don’t yet know exactly how much the oxygenation of seawater affects the isotopic signature of chromium, even in modern seawater. This means we can’t yet use isotopic measurements to their full advantage, but that’s where I come in! To fill this gap in our knowledge, I am measuring the isotopic signatures of chromium in modern seawater to see how they change in waters with different oxygen levels.
Because chromium is present at very low concentrations in seawater, sophisticated techniques and very sensitive equipment are needed to measure its isotopic signature. Few scientists have tried it up to now, so the first year of my PhD has involved lots of work in the lab trying to find the best way of measuring the signatures. Geochemists like me often work in specialist clean labs which are designed to keep out any particles and other contamination that could affect the measurements we make. This is the one I work in:
To extract the dissolved chromium from seawater, I do a redox reaction which involves adding iron to my samples. The reaction causes chromium and iron to come out of solution and form a solid precipitate. Here it is in action:
The colour change you can see is due to oxidation of iron. The reaction only takes a couple of minutes! After filtering the orange precipitate, I need to purify the chromium in it. I use column chromatography to separate chromium from other elements in the sample:
Most elements stick to the orange resin in the columns, but chromium passes straight through for collection. This process removes iron, so the samples go into the columns yellow (like the ones at the bottom of this picture) and come out completely colourless.
The next step is to analyse the samples using a mass spectrometer. The data from this machine needs to be processed using lots of calculations to get a final isotopic signature. This is done using some rather complex MS Excel spreadsheets, making my life a bit easier!
As a PhD student, I don’t spend all day in the lab and reading papers – there are lots of other activities to get involved in. Here at NOC, there are always seminars, conferences and other talks going on. They span a wide range of topics from submarine landslides to pterosaur flight, and everything in between! They can really help to broaden your scientific knowledge. It’s also good to share your own knowledge and skills with others by giving talks or helping out with undergraduate teaching. For some projects, there are opportunities to go out on research cruises, something which I am preparing for next year on the new ship RRS Discovery!