Q&A - Stable isotopes: signatures of life and times past

Almost every branch of NIWA's science employs stable isotope analysis, but what are they, why are they so valuable, and how are they measured?

Almost every branch of NIWA's science employs stable isotope analysis, but what are they, why are they so valuable, and how are they measured?

What are stable isotopes?

Isotopes are naturally-occurring atoms of the same element that have equal numbers of protons and electrons, but different numbers of neutrons.

The number of neutrons partly determines the isotope's atomic mass. More neutrons mean a 'heavier' isotope: for instance, carbon-12 has six neutrons, but the heavier carbon-13 has seven.

It also helps determine the energy in the isotope's nucleus. If excess energy is present, the isotope 'decays' radioactively over time, and is said to be unstable. If the energy level is low, the isotope doesn't change, and is known as a stable, or non-radioactive, isotope.

Why are they so valuable to scientists?

In their natural state, stable isotopes exist in constant proportions. Atmospheric carbon, for example, is 98.89 per cent carbon-12 and 1.11 per cent carbon-13. A good stable isotope has a large relative mass difference between heavy (rare) and light (abundant) isotopes.

During certain chemical and physical processes, however, those ratios change. This is called isotopic fractionation, and occurs because chemical bonds formed by lighter isotopes are weaker than those formed by heavier ones. As a result, some stable isotopes are taken up more readily than others.

Fractionation acts on all isotopes, but crucially for scientists, the large mass ratio of some (for instance, hydrogen, carbon, nitrogen, oxygen and sodium) means the ratio of light to heavy isotopes can be measured.

Processes that leave recognisable 'signatures' of isotopic fractionation include photosynthesis, temperature changes in seawater, salinity changes (or migration between marine and freshwater systems), decomposition of organic matter, changing diet and/or metabolism and the formation temperature of rock and mineral systems.

When matter has undergone one or more of these processes, scientists can learn an enormous amount about it by studying a range of stable isotope ratios within it. They can work out how – and how quickly – the environment changed: over time scales of decades, tens of thousands of years, or just a few seasons.

Stable isotope signatures pass into the food web, and animals take in a chemically-imprinted life history of the foods they eat.

This can tell us much about a creature's life: how far it travelled, what it ate at different times of the year, where it got that food from, and how quickly it was metabolised.

Combining such information with other research helps scientists make deductions about all manner of biological and geological processes. Beyond atmospheric and aquatic applications, for example, stable isotopes can help identify the origin of illicit drugs, or detect human stomach ulcers.

Scientists can also introduce synthesised isotopes into natural environmental systems, to trace pathways of adsorption, diffusion or assimilation. For example, a synthetic nitrogen-15 substrate is used to measure nutrient uptake and identify transport pathways in ecosystems.

What do NIWA scientists use them for?

Almost every branch of NIWA's science employs stable isotope analysis.

In the sea, stable isotopes form a vital part of NIWA's contribution to palaeoceanography – the study of the evolution of ocean systems.

When seawater cools as a result of seasonal transition, long-term climate change or ocean circulation, it becomes isotopically heavy, with a higher ratio of oxygen-18 to oxygen-16. The reverse happens when seawater warms.

What's more, when aquatic creatures absorb sea water over their life, they take up the isotopic signature of that water, and the various temperature changes it's undergone. 

As a result, stable oxygen isotope ratios unlock a wealth of natural resources information about the migration patterns and life histories of certain species of fish, about seasonal and long-term climate change (including glacial and interglacial periods), and about broad-scale ocean circulation.

NIWA scientists can even assess the age of paua, based on the number of seasonal cooling and warming sequences 'imprinted' in the stable isotopes of carbon and oxygen in their shells.

In the atmosphere, NIWA is working with stable isotopes to understand seasonal and long-term changes in concentrations of atmospheric methane, measured at Baring Head in Wellington and Arrival Heights in Antarctica. NIWA's measurements of stable carbon isotopes in atmospheric methane are recognised as among the most precise in the world.

The ratios of stable carbon isotopes in these samples help scientists explain what may have caused short-term peaks in methane concentration (e.g., wetland activity, forest fires, and destruction of 'sinks') and from where that methane may have been transported.

They also hold clues to a marked long-term increase in carbon dioxide (CO2) concentrations observed over the last 50 years. Stable isotope ratios, combined with other analyses, indicate that fossil fuels are squarely to blame.

How are stable isotope ratios measured?

Measuring stable isotope ratios is a complex process using an Isotope Ratio Mass Spectrometer.

Because samples at NIWA are measured as CO2 gas, original material such as carbonate and methane is usually prepared to first form that gas.

A small amount of CO2 gas is then drawn into a high-vacuum chamber and directed toward a filament, where it's ionised. A series of metal plates at high voltage (10,000V) then accelerate and focus the ions into a narrow beam. This beam of ions enters a flight tube, where it encounters a very strong magnetic field. As the charged particles move through the field, they're bent in arcs, rather than following straight lines. Ions from the lighter isotopes are bent in tighter arcs than the heavier ones. Precisely-positioned collector cups then capture and count the ions as they arrive. Software then calculates the isotope ratio of the sample, based on the data obtained in each cup against a known reference material and the sample itself.