How do we know about the Earth's past climate? Part II: Ice cores

This post follows on from the previous post about the Earth’s past climate. So far, we have seen how the rock and fossil record can provide information about changes in the Earth’s climate, and now we will move onto another huge information source: ice. Falling snow slightly melts, and is compacted over time to form the ice we see in ice caps and glaciers. Geologists can examine ice cores taken from around the world to gain an insight into past climates; they can look at several features of the core, including trapped gases and the different isotopes of oxygen present.

Trapped gases in the ice

As the snow falls, small pockets of gas will be present that represent the atmosphere at that time. As the snow compacts and forms ice, these pockets will be preserved as bubbles within the ice; these contain greenhouse gases such as carbon dioxide and methane. As we’ve all seen in the news, increasing temperatures are linked to an increase in these gases, with carbon dioxide the most commonly cited. These gases are therefore used as a proxy for temperature, so if we can measure the proportions of gas within these bubbles, we can get an idea of what the temperature was like in the location of the ice core over time. These measurements are taken all the way down the ice core to get an idea of how temperatures have changed over time.

But how do we know the exact ages that these temperatures correspond to?

Annual snow fall in an ice core can be counted, similar to how you count the rings of a tree to determine its age. Where it is not possible to count annual snow fall, mathematical models of ice flow can be used. This is as well as looking for certain seasonal chemical signatures, or correlating with other records for larger, global events that took place.

Oxygen isotopes

The isotopes of oxygen that we are concerned with here are O-16 (8 protons, 8 neutrons), and the rarer O-18 (8 protons, 10 neutrons). The latter are marginally heavier due to the 2 extra neutrons, so this isotope is referred to as ‘heavy’, and the O-16 are referred to as ‘light’.

First, a quick reminder of the water cycle; water (mostly) evaporates from oceans, condenses in clouds, and later falls as precipitation on land. In the initial evaporation, it is easier for the lighter O-16 isotopes to overcome the barriers for evaporation, leaving the oxygen in the seawater enriched in the heavier O-18. The heavier oxygen isotopes will condense at lower latitudes, leaving the remaining water vapour moving towards the poles enriched further in O-16. Let’s consider a pack of skittles; maybe your favourite colour is the red, so you’ll preferentially take these out to eat first. Your pile ‘to eat’ will be enriched in the red skittles, whereas your pile that is left (for the moment) will be depleted in red, and enriched in other colours.

Relating this to ice cores, the precipitation/snow falling to form the ice caps and glaciers will therefore be enriched in the lighter O-16 compared to the sweater. During colder, glacial periods in the Earth’s history, glaciers will be more enriched in the lighter O-16. This is because ice sheets will extend further towards the equator, and the O-18 will precipitate even closer to the equator, leaving the remaining water vapour increasingly enriched in the lighter O-16. However during the warmer interglacial periods, melting ice will return O-16 in the oceans, resulting in ocean water that is less enriched in O-18 compared to a standard value (see below).

How the relative concentration of oxygen isotopes changes with latitude [1]

This balance of oxygen isotopes in the ice can be related to a standard value, and changes from this can indicate changing temperatures. This is coupled with the age of the ice (as detailed above) to see how these temperatures changed over time. This isotope record is also preserved in shells and sediments formed, so we can analyse these alongside the ice cores to produce a more complete record of the temperature in different areas. However measurements in shells can be quite difficult - scientists need to take into account the biological and chemical processes taking place that may affect the oxygen isotope values.

References

[1] NASA (2005). [image] Available at: https://earthobservatory.nasa.gov/features/Paleoclimatology_OxygenBalance [Accessed 31 Oct. 2019].