Oxygen Isotopes and Temperature

If we want to know temperatures for the past hundred years, we can look them up in weather records. Determining the temperature thousands or even millions of years ago is not so straightforward. For this, we use stable isotopes in ice cores or marine sediments.

What is an isotope?

Isotopes are the same type of atom with different masses. The mass difference is caused by different numbers of neutrons in the nucleus. In the model shown, the round green marbles represent protons and the flat blue marbles represent neutrons. This is a model of oxygen, because the nucleus (glass bowl) has eight protons.

Most oxygen atoms have eight neutrons, so their mass is 16 atomic units. We call this Oxygen-16 and write it as ¹⁶O. Some oxygen atoms have ten neutrons, so their mass is 18 atomic units. We call this Oxygen-18 and write it as ¹⁸O.

Both ¹⁶O and ¹⁸O are stable isotopes. They do not release particles through radioactive decay. They behave the same in all chemical reactions, but ¹⁸O is heavier. This is important for finding out about ancient temperatures!

These models represent the two common isotopes of oxygen: ¹⁶O and ¹⁸O

The model of ¹⁶O has 8 neutrons; the model of ¹⁸O has 10 neutrons

It takes more energy to lift something heavy

We all know that it takes more energy to lift heavy items. The same is true for evaporation of water that contains the heavier ¹⁸O isotope. Scientists measure the energy of particles in a system by its temperature. When it is warmer, there is more energy. When it is cooler, there is less energy.

When the temperature is higher, there is more energy to lift the heavy ¹⁸O isotopes in water out of the ocean by evaporation. By examining the ratio of ¹⁶O to ¹⁸O, scientists can determine what the temperature was in the past.

Evaporation from the oceans falls on land as precipitation (rain or snow)
(NASA Precipitation Measurement Missions, 2017, Public domain)

Webpages:

• Palaeoclimatology: the Oxygen balance (NASA Earth Observatory) 
• Isotope analysis (Harvard) 

Isotope ratios in ice cores and marine sediments

Shell-building animals in the oceans preserve a record of oxygen isotope ratios at the time they died and became part of the marine sediment. This sediment turns into rock and can preserve a record of the ocean temperature dating back millions of years.

Ice cores on land are formed by layers of snow from the ocean water that evaporated. Ice cores preserve a record of oxygen isotope ratios dating back hundreds of thousands of years. Tiny bubbles trapped in the ice also provide samples of gases in the atmosphere so that scientists can measure the concentration of gases such as carbon dioxide and relate this to temperature.

Scientists examine sediment cores in the laboratory at the University of Bremen
(MARUM – Zentrum für Marine Umweltwissenschaften, 2018, Wikimedia Creative Commons)

A sliver of Antarctic ice shows tiny air bubbles in ice preserve the atmospheric gases from when the ice formed
(Atmospheric Research, CSIRO Science Image)

Explore isotope ratios at home

You can experiment with different amounts of energy and its effect on isotope ratios using a ping pong ball model. Watch the AusEarthEd video and have a go yourself.

You might also like to try the models from Earth Learning Idea.

Webpages:

• The oxygen isotope sweet simulation(Earth Learning Idea) 
• Interpret Earth temperatures from simulated deep-sea and ice cores(Earth Learning Idea)