How do work the oxygen isotopes reconstructions of past temperature, ice volume and environment?

The atoms of every element on Earth are composed of 1 core, made of protons (positive electric charge), neutrons (no charge), and an electron cloud (negative charge). Each of these constituents have a mass, very low for electrons, more substantial for the protons and neutrons. Each atom element is defined by is proton and electron number, defining is size, mass and own chemical and physical properties. The mass of the elements is written as exponent before the element name.

For some of the elements, the atoms exist with a different number of neutrons, changing their mass, they are called isotopes. Some of these isotopes are stable (they remain in that state), some are unstable, they are radioactive and disintegrate at their own rate.

The oxygen exists with a mass of 16, 17 and 18, and all of these isotopes are stable and exists in different proportions in the universe. The most abundant one is the 16O (99.8 %), then the 18O (0.205%) and finally the 17O (0.038%).

The water molecule is constituted of 1 oxygen atom and 2 hydrogen atoms, and is written H2O. The water molecules are made of all the three different oxygen atoms. These molecules are moving from one reservoir to the other along the water cycle.

Because of their different masses, the different water molecules do not need the same amount of energy to be evaporated, more energy is needed to evaporate the heavier molecules than the light ones. Because in nature, for most of the phenomena, the processes consuming the less energy are favoured, more of the water molecules containing 16O are evaporated than the ones containing 17O and 18O, changing their proportions in the different reservoirs of the water cycle. It is called isotopes fractionation.

As a consequence, the water contained in the clouds, after evaporation, has a higher relative concentration of 16O, whereas the water remaining in the ocean is relatively depleted in 16O. The water contained in the clouds after the evaporation is transported to different location and different latitudes, and is then lost as precipitations (rain and snow).

The result of this is that when a lot of water issued from precipitations is stored as glaciers and ice caps during cold climates, the ocean is more and more depleted in 16O (and so relatively enriched in 18O) and during warmer climates, the ice sheets are melting, the sea level rises and the proportion of 16O increases.

Some organisms living in the ocean are building a calcareous shell. The calcite constituting their shells also contains oxygen, as the calcite is CaCO3. When they built their shell, these organisms take the oxygen in their environment (the ocean) and some of these organisms do not modify the 16O - 18O ratio at all when incorporating the oxygen. At their death, these organisms sink to the deep ocean and accumulates over time (see this article).

Thanks to these processes, by looking at the ratio between 16O and 18O constituting the shell of these organisms through time (using the fossil record) we can reconstruct the past temperatures and past ice volume (Billups et al., 1997; Lisiecki and Raymo, 2005; Petit et al., 1999; Zachos et al., 2008).

Recently, some laboratories developed instruments precise-enough to analyse the ratio of 17O as well. Thanks to this, it is now possible to evaluate the past climate atmospheric humidity (Alexandre et al., 2018). 

Bibliography

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Billups, K., Ravelo, A. C., and Zachos, J. C.: Early Pliocene deep-water circulation: stable isotope evidence for enhanced northern component deep water., in: Shackleton, N.J., Curry, W.B., Richter, C., and Bralower, T.J. (Eds.), Proc. ODP, Sci. Results, 154: College Station, TX (Ocean Drilling Program), vol. 154, 319–330, https://doi.org/10.2973/odp.proc.sr.154.115.1997, 1997.

Lisiecki, L. E. and Raymo, M. E.: A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records, Paleoceanography, 20, PA1003, https://doi.org/10.1029/2004PA001071, 2005.

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Zachos, J. C., Dickens, G. R., and Zeebe, R. E.: An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics, Nature, 451, 279–283, https://doi.org/10.1038/nature06588, 2008.

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