Most of us think of ice as fleeting. It forms when it’s cold and melts once it warms up (or you pour salt on it). Yet glaciers are proof that ice can persist. The ice that forms glaciers takes centuries to form and can last for hundreds of thousands of years…and possibly much, much longer.
But that’s not all! Not only does the ice persist, but it is a excellent archivist. It can store information about all sorts of things in the Earth’s atmosphere such as particles (such as volcanic ash or windblown dust), aerosols (such as sulfuric acid) or gases (such as carbon dioxide trapped in the form of bubbles). All of this data can then be used to record changes in the Earth’s climate over the long term.
Glacial ice in the Gray Glacier of Southern Patagonia. Credit: Felipe Alarcon, Wikimedia Commons.
Why is that? To understand the accounting capabilities of ice, we need to look at how glacial ice forms. It’s not like the ice on your driveway, but rather something that forms over decades, if not centuries. All it takes is cold and time.
Glaciers and ice start with snow. In places that get a lot of snow each year, some of that snow may not melt during the hottest summer months. Then the next year’s snow falls on that old snow…and the process repeats itself year after year. Older snow warms and cools, possibly losing mass to melting (or sublimation – going straight to gas), but the snow will condense into ice pellets called firn, then ice and finally in glacial ice.
Glacial ice is as close to “rock” as water can get on Earth, forming an interlocking network of ice crystals that can look like granite under a microscope. This glacial ice could have started as many feet of snow but is reduced to less than an inch of ice. In this layer, there remain solid particles like ash, rock debris that has fallen on the ice, aerosol particles and whatever was there. Also, the air bubbles that sample the atmosphere when snow falls get trapped in the ice. In the end, you get layer after layer of ice that records the annual snowfall on this glacier.
Ice crystals in Antarctic sea ice under a polarizing microscope. Credit: Sepp Kipfstuhl, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
Glacial ice is weird. It’s solid, but it can flow sloping and under pressure. Glaciers and ice caps can be hundreds of feet to miles thick and all that pressure can warp the ice. Ice can flow when the ice at the bottom of a glacier descends. This is how a glacial advances, when snow accumulates at the top of the glacier and the accumulation causes the bottom to sink. Conversely, when ice melts down the glacier, it can retreat.
The Ice Carrot Record
So when scientists travel to places like Greenland or Antarctica, they can cut a drill core from the ice to read past conditions on Earth. The ice near the top is newer and the deeper you go the older it is. Some of the oldest continuous ice cores on Earth contain annual records that go back 800,000 years.
Now, there are a number of caveats when it comes to reading ice core climate records. If the ice has been heavily deformed, if the lower ice has been replaced by younger ice, or if the bottom of the ice has melted, then your record is much more difficult or impossible to interpret. You can also lose ice from above through melting or sublimation, leaving older ice exposed on the surface. The question then becomes how can you determine the age of the ice under your feet?
Find very old ice
One answer might be to look at the rare elements that form when quartz is hit by cosmic rays from space. A recent study in The Cryosphere by Marie Bergelin and colleagues attempts to use the sediments encased in ice cores to determine when the sediments were last on the surface, hence the age of the ice they are in. They may have ended up finding ice in the Ong Valley Antarctica which is over 4 million years old.
Ice core taken from the Ong Valley, Antarctica. There was a lot of variation in the ice cores the team recovered, some sections were clear, clean ice, while others were full of rocks and sediment. Credit: Jaakko Putkonen, United States Antarctic Program
Cosmogenic nuclides are isotopes of elements like beryllium, neon, and aluminum. They form when cosmic rays from space strike minerals, causing the elements to break down and form these rare isotopes. cosmic rays do not penetrate deep into rock or ice, so by looking at the amounts of these elements on rock surfaces and how these elements have broken down, you can model the exposure and burial ages of the material.
Antarctica’s Ong Valley is a deep glacial valley in the Transantarctic Mountains. It has active glaciers but also areas of stagnant old ice covered in rocks and debris. Bergelin and his colleagues cut ice cores from this old ice to examine the sediments in the rocks to determine when they were buried, giving a minimum ice age.
Interpret the ages
Now, that can get messy. The ice in these valleys may not have melted (the average temperature in the Ong Valley is -11F (-24C). Instead, the sublime ice cream in the sun, so that the young ice is lost over the years. In the Ong Valley, there could be more than 22 meters of ice per million years. That’s a lot of ice to spray.
In the roughly 30-foot (9.4-meter) ice core that was cut by Bergelin and his colleagues, they identified several lots of ice and surfaces. Using measurements of cosmogenic nuclides, they found the exposure and burial age of different surfaces. Some of the youngest ice was about 1.3 million years older. The deeper surfaces of the ice indicate that they were buried in the ice around 3 million years ago and even deeper around 4.3 to 5.1 million years ago.
To put that into perspective, if these ice ages are correct, then the oldest ice formed from snowfall when Australopithecines roamed East Africa. The middle layers formed near the time stone tools were first used by our deep ancestors and the surface represents the snow from the top of homo erectus.
The key to whether these new ages can be used to understand past climate is complicated. Measurements of cosmogenic nuclides can be fraught with difficulty depending on how the samples were taken and the type of assumptions used. For these ages to be accepted, the scientific community will look closely at what the authors have done to determine these dates.
If they’re accurate, then examining the material trapped in the Ong Valley ice could give us insight into Earth’s climate that dates back much further than what we’ve been able to do with ice. It could also give us new tools to find even older ice elsewhere.