BGS laboratory spotlight: isotopes as recorders of climate and environmental change

The National Environmental Isotope Facility (NEIF), a UKRI/NERC facility hosted at BGS Keyworth, recently took delivery of a new isotope-ratio mass spectrometer for measuring oxygen and carbon isotopes in very small (microgram-scale) samples of carbonate from fossil material. This instrument is helping scientists answer fundamental questions about climate and environmental change.

Evidence published in the latest assessment report from the Intergovernmental Panel on Climate Change confirms that human activity is warming our planet. The rate and scale of environmental change in response to warming is rapid, widespread and unprecedented over past centuries. Gaining a longer-term perspective on the types, rates and causes of previous climactic and environmental change, beyond the instrumental record, is crucial to enable us to make accurate predictions about the effects on our climate in the future.

A vital component of understanding past climates is the generation of robust data, which enables scientists to investigate the response of environments to climate change across thousands to millions of years of Earth’s history. Palaeoclimate archives — for example, ocean and lake sediments, cave deposits and corals — can be used to reconstruct how environments around the world have been shaped by climate change in the geological past. This provides essential information to better project future climate pathways.

How can isotopes help to unravel the history of past environmental and climate change?

Let’s take oxygen (O) as an example. Over 99.7 per cent of oxygen atoms contain eight protons and eight neutrons, giving oxygen a total atomic mass of 16. However, a very small amount (around 0.2 per cent) of oxygen atoms have two additional neutrons, resulting in a heavier mass of 18. These are the two main stable isotopes of oxygen, known as ¹⁶O and ¹⁸O.

Water molecules contain one oxygen and two hydrogen (H) atoms: H₂O. Through natural processes, for example when water evaporates from the ocean as part of the global water cycle, the difference in atomic mass between ¹⁶O and ¹⁸O causes a measurable change in the proportion of ¹⁸O compared to ¹⁶O. Water molecules that contain H₂¹⁸O are heavier and more difficult to evaporate. A preferential transfer of H₂¹⁶O from ocean to atmosphere occurs, leaving the ocean with more H₂¹⁸O.

When the evaporated water is transported to polar regions and falls as rain or snow, it becomes trapped in ice; the ¹⁶O is locked away and does not return to the ocean. Over geological time, the build-up of large ice sheets captures ever-increasing amounts of ¹⁶O, resulting in the global ocean having relatively more ¹⁸O. Whilst we can’t measure the isotope changes that take place over thousands to millions of years directly, we can use natural materials that contain ‘snapshots’ of the oxygen isotope ratio of the ocean from different points in geological time.

Some marine organisms grow shells while they are alive. When an individual dies, its shell can be preserved in sediments on the ocean floor, unlike the soft parts of its body that decay following death. The shell is made from calcium carbonate (CaCO₃) and the amount of ¹⁸O and ¹⁶O incorporated into the shell is linked directly to the balance of oxygen isotopes present in the ocean water during the organism’s lifetime. Over time, many of these shells accumulate on the sea floor, layer upon layer, capturing the relative proportion of ¹⁸O compared to ¹⁶O in the ocean. If the oxygen isotope composition of the ocean water changes (for example, due to a different global climate state), the shift is captured in the shells and reflected in the sediments.

We can extract, clean and measure the oxygen isotopes in fossilised shells found in cores of time-ordered sequences of marine sediment. These measurements help us reconstruct how the oxygen isotopes and, by extension, climate has evolved over time. This is known as a ‘proxy’ record, because the oxygen isotope composition of marine shells is used as a proxy for changes in global climate conditions.

Measuring materials at the micro-scale for oxygen isotopes

Isotope-ratio mass spectrometers allow us to measure very precisely the relative proportion of different isotopes of a particular element in a wide range of materials. In natural archives, this can be for the purpose of interpreting past climate and environmental change.

Very small samples weighing as little as 10 micrograms — for example, a singular microfossil just visible to the naked eye — are transferred into glass vial. These vials are tightly sealed and  then placed into a hot block. In the meantime, the mass spectrometer is checked and calibrated. To do this, we first introduce a reference gas against which the samples are compared. The reference gas has a known isotope composition and can therefore be used to ‘tune’ the mass spectrometer; we can calculate the isotope ratios in the sample by comparison against the reference gas.

Once we are confident the instrument is working accurately, we set it running. The run begins with a needle being inserted into the vial containing the sample. The needle has multiple holes so it can perform multiple functions without needing to be removed and re-inserted. The first hole in the needle draws out any air and creates a vacuum. It then dispenses very clean phosphoric acid (H₃PO₄) into the vial via a second hole. The acid dissolves the sample and liberates the carbon dioxide (CO₂) it contains. As the needle does not need to be removed, the reaction can take place without atmospheric gases mixing in with the sample gas. The spectrometer is fully automated and allows us to run as many as 40 samples overnight.

Once the reaction is complete, the liberated gas is taken across a ‘cold finger’, which traps water and any other gases that have also been liberated. This is a way of making sure that no impurities are included into the measurement. The mass spectrometer instrument measures how much CO₂ there is in the sample and its isotope composition, which can be converted to the isotope composition of the original calcium carbonate.

Ultimately, isotope compositions can tell us about climate change in the very recent past as well as deep geological time. This can help us make accurate predictions about the effects on our climate in the future and answer important questions about climate and environmental change.

Kotryna Savickaite

Geochemistry technician

BGS Keyworth

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Dr Jack Lacey

Stable isotope geoscientist

BGS Keyworth

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