Old Whiskey, Very Old Air and Even Older Ice
Ancient bubbles in the ice
The world has a lot of ice. There is the Antarctic Ice Cap, the Greenland Ice Cap, assorted smaller ice caps, thousands of mountain glaciers, and the sea ice in the Arctic and Antarctic Oceans. Indeed, most of the world’s fresh water is locked up in ice. Except for sea ice, this ice forms gradually through the accumulation of snowflakes, tiny bits of crystallized water. As the snow falls and accumulates, air fills the space between the snowflakes, and the snow is initially fluffy, but as the snow compacts it changes from fluffy to firm. What happens to this air as the snow compacts more and more to form ice? Is it squeezed out back into the atmosphere, or is it captured in the ice in some form? This question loomed large on a recent cruise Vicki and I took to the high Arctic of Greenland and Svalbard where glaciers and ice caps are a dime a dozen.
Whenever we were off the ship, transportation was by means of zodiacs, giant inner tubes with an outboard motor. On one zodiac cruise close to the front of a calving glacier, I asked our guide if she would collect a hunk of glacial ice for my whiskey later that day. Inspecting several pieces of floating ice from house-sized to little chunks, she stretched on the side of the zodiac and, almost tipping into the water, hauled in a nice clear piece weighing about ten pounds: my Found Object for this essay. A few weeks before our zodiac tour, this ice hunk had been part of the calving glacier in front of us. It was mostly clear but also had some air bubbles, some of which might have been along fracture lines.
Once back onboard the ship, the bartender put a chunk of my Found Object into a glass and poured in some fine, Talisker scotch whiskey--- I was now ready to enjoy what I had set out to do, listen to ancient air exploding from the ice as it melted. Putting the glass up to my ear, I heard sharp pinging and crackling sounds as tiny bubbles of air that had been compressed by the weight of a several-millennium accumulation of snow and ice broke free explosively from their tiny prisons in the ice.
The next day while floating quietly in a zodiac next to a piece of glacial ice, we saw and heard the same phenomenon again on a scale much larger than my whiskey glass. The floating ice emitted a continuous crackling and snapping sound as the compressed air in the ice bubbles exploded into the water. There was no need to put my ear close to the ice (that would have been tricky) --- my pinging scotch on the rocks seemed pitiful by comparison.
What I was enjoying were tiny examples of a phenomenon that has given science a window on past climates extending back one million years or more, because the bubbles preserved bits of the atmosphere under which they were formed, and when they burst, released a tiny sniff of ancient air, air from atmospheres that were possibly quite different than the one we were puttering around in.
The creation of this Air-Library-in-the-Ice is pretty straightforward. When snow settles on a glacier or ice sheet, its density is low because there is a lot of air separating the snowflakes, and the ice is about 65% pore space. As more snow accumulates and settles, the weight of the added snow compresses the layers below, so that by the time it is 10 to 30 m deep, the density increases by about 60% greater. This forms a granular ice called firn in which the air in the spaces between grains is still continuous with the atmosphere. But once 30 to 50 meters of ice press downward on the firn, it gradually transforms into solid ice, and the air within it is trapped as bubbles separated forever from their natal atmosphere and preserved for the life of the ice. The closed bubbles are a multi-year sample over the period when the firn was still porous. When the rate of snow accumulation is low (as in the Antarctic) the gas can be as much as 7,000 years younger than the ice in which it is enclosed. With higher accumulation rates (as in Greenland), it can be as little as 30 years younger.
Below this bubble-closure zone, the weight of the accumulating overburden of ice continues to compress the bubbles until, under about a kilometer of ice, bubbles are no longer visible, for the air is now dissolved in the ice as air hydrates called clathrates. The deepest Antarctic ice cores are a 1.5-million-year record of past atmospheres that can help us understand past and present climates.
But I was really interested in knowing the age of the air popping in my whiskey, and Antarctic ice cores were of no help whatsoever. I found the answer in the published glacial history of Svalbard as revealed by several hundred dated samples of sediments, ice, plant material and animal material. The last glacial period ended about 12,000 years before present, a time referred to as the Holocene warming. At that time, all the islands of Svalbard as well as a good deal of the sea to the east was under ice, as in the image below. Warming continued irregularly with the result that between about 8,000 and 4,000 years ago, Svalbard was almost ice free. All that remained were three modest ice caps. Then, beginning about 4,000 years before present, all the glaciers grew again so that today about 60% of the land area of Svalbard is under ice, with three major ice caps from which ice rivers (glaciers) make their way to the sea.
One of these glaciers was the mother of the ice chunk that cooled my Talisker. Sadly, this means that it could not have been older than 4,000 years, and the air popping in my glass was even younger, let’s say 3,000 years. Not a cool million years like ice from the bottom of the Antarctic Ice Sheet but pretty respectable anyway, and it smelled old to me, certainly older than the ten-year-old Talisker.
I tried my hand at making ice cores of sea ice, not to reveal ancient climates or to get ice for my whisky, for the ice was not more than two or three years old, but to determine if the sea ice in which our ship was parked was thick enough to walk on. At a thickness of 1.4 meters, my opinion as an expert in ant biology was that it was indeed safe to walk around on it. By licking this core, I also confirmed that despite it being sea ice, the ice was not salty. Of course, in high school, I had learned about how water freezes, so I can’t claim to have been surprised. Indeed, any apple farmer who has frozen fermented apple cider to make apple jack knows that only the water freezes, thereby raising the alcohol content of the jack. Some things, including sea ice and apple jack, are just simply universal truths.
I hope all this ancient ice survives in a time of global warming. Whether it is your glacier ice with Scotch, the sounds of melting ancient ice bubbles or your core sample, these instances contribute to my knowledge and curiosity about what is buried in and under the ice.
marvelous essay!!!! true, the text under the samples reversed right and left, but these things can happen! self-editing is the most dangerous thing-- i know it too well!