(890 words, about 4 minute read)
One of these pieces of ice was made in a lab and the other was made in a glacier. Can you guess which is which?
If you guessed that the ice sample on the right came from a glacier, you’re correct! (Bonus question: can you guess how old those bubbles are?) So, why do these two samples look so different, and what do we use them for?
Glacial ice forms on top of a glacier, starting as fluffy snow and becoming compacted as year upon year of snowfall accumulates over time. When the snow first falls, there is a lot of space for air between the individual snow grains. Over time, additional accumulating snow compresses the layers below. Under all that weight, individual snow grains are pushed closer together. These top layers of a glacier are known as firn, the name glaciologists give to multi-year snow. Within the firn, air from the atmosphere can travel freely between snow grains. Firn officially transitions into ice when the spaces between snow grains are no longer interconnected, closing-off air bubbles in the ice and isolating them from the atmosphere. These bubbles are like time capsules, filled with a sample of the atmosphere as it was when they formed. Thousands of years later (up to 800,000), we can dig up these tiny time capsules by drilling ice cores on the Greenland or Antarctic ice sheets. The chemical composition of gases in the bubbles helps us learn about the earth’s atmosphere and how it has changed over past millennia. This is one way that we are able to reconstruct the earth’s climate history (find more info on our lab website).
So, then what’s up with the clear ice sample? Well, that’s a piece of gas-free ice, which we make ourselves in the lab. You’ll probably notice that it’s completely clear – unlike the glacial ice there are no bubbles and unlike the ice cubes in your freezer it’s not cloudy.
When you make ice cubes in your freezer, the water you use has dissolved gases and minerals in it. Ice can be really picky—as it forms it likes to exclude impurities in the water from becoming part of the ice structure. In an ice cube tray, however, the water starts freezing from the outside and moves inward. This happens on all sides, so the impurities are pushed into the middle of the ice cube and get trapped there—this is why you might have noticed the ice cubes in your freezer look cloudy in the middle.
When making gas free ice, we boil purified water in a vacuum-sealed vessel to drive the dissolved gases out of the water without letting any more ambient gases dissolve into the water. Then, we place our container of degassed water in a cold bath, forcing the water to freeze slowly from the bottom up. You can see this setup in the picture below.
As the ice front advances upward, it pushes any remaining impurities above the ice so that they end up in the remaining water or in the headspace of the vessel. After a few days, we have a sample of ice that’s just like our glacial ice samples but without any gases in it! But why go through all of this trouble just for some super clear ice?
The gases that we try to measure in glacial ice samples are present in trace amounts: parts per million for CO2 (carbon dioxide) and parts per billion for CH4 (methane) and CO (carbon monoxide). This means we have to pay attention to small additions from other sources, such as our lab equipment or modern ambient air. We treat our gas-free ice the same way we would a real sample, and can then compare the results to quantify those extra additions (we call this a “blank”). For example 14C, a radioactive isotope in the gases that we measure, has many useful applications. 14CO2 in particular is produced in the atmosphere by cosmic rays and gets incorporated into the air bubbles trapped in glacial ice. Cosmic rays also produce 14CO2 directly in the ice itself. To be able to use 14CO2 meaningfully to study the atmosphere, we need to quantify the procedural inclusion of 14CO2 at various stages of the ice core collection and analysis process. We can do this by comparing gas free ice made in the field, gas free ice made in the lab, and glacial ice samples. As we extract and measure air from our ice samples in the lab, the gas free ice acts as an essential diagnostic tool to quantify how well our gas extraction systems are working and to identify the background amount of 14CO2 from the system, which we can subtract from our sample measurement.
So, to summarize! Aside from looking really cool, both ice samples are central to the research we do at the Ice Core Lab. We study the atmospheric trace gases trapped in bubbly glacial ice in order to learn about how Earth’s climate works. To accurately interpret this information we manufacture pure gas-free ice, allowing us to double-check that our equipment and instruments don’t dirty up these valuable, untouched time capsules!