Often layers in the ice, such as the dark layer discovered on December 18 (see December 18 blog) are the result of aerosols falling out of the air and landing on the snow. The aerosols are eventually covered with additional layers of snow and are preserved in time. Aerosols that create layers such as this can be dust, sea salts or ash from a large forest fire or volcanic eruption. Since this part of Antarctica is so remote, any aerosol has to travel a long way to deposited in this location. Consequently, the concentrations of aerosols that we see in the chemistry of the ice are typically very low and require high tech instrumentation to be detected. Hence, the fact that one can actually see this dark layer with the naked eye is a very rare and exciting event. We won’t know for certain what created the layer in the ice core from December 18 until we examine its chemistry. However, similar layers discovered in other Antarctic ice cores at similar depths indicate that this layer is most likely composed of ash from the eruption of Mt. Takahe (a volcano on the West Antarctic Ice Sheet) roughly 8,200 years ago (determined from Argon dating).
Why is the Chemistry of Ice Cores Important for Climate Research?
Ash layers and ice chemistry can tell us interesting things about past climate. Imagine if you have a storm cloud that travels over the ocean. This cloud will pick up sea salt along the way. If that cloud goes over the Sahara desert, it will also pick up some dust. If it travels over a city, it will pick up pollution from cars, factories, etc. When that cloud reaches Antarctica, it will rise in elevation causing the cloud to cool in temperature and release precipitation, in this case in the form of snow. Along with the snow, some of that sea salt, dust, pollution or whatever else was in that cloud, is also released. Because Antarctica never gets above freezing, the snow never melts. This means that the snow eventually becomes buried by the next storm and any subsequent storms. So, if you want to actually see the snow that fell in Antarctica about 10 years ago, dig down 5 meters or so and you can see the actual snow (of course this depth will vary depending on how much it snows at your site). In the WAIS Divide project, we will eventually dig down 2 miles. At this site, this translates to snow (which has now been transformed into ice) from 100,000 years ago. We will examine the chemistry along the way, along with the gases that are trapped in the bubbles, to determine information about past climates.
Aerosols in ice are important for understanding past climates and wind patterns because they are tracers of different sources. For example, sodium is a tracer of sea salt (think of your table salt which is sodium chloride). Another example, aluminum, is a good tracer of dust. By looking at multiple aspects of chemistry (for example isotopes) we can begin to understand where the aerosols came from and how they got here.
Ryan Banta is my ice core processing partner at our field site. When he is not in Antarctica, he is a scientist at the Desert Research Institute in Reno, Nevada. He studies the chemistry of ice cores and finds ash layers in ice cores particularly interesting. In his lab, he measures concentrations of aerosols that are extremely low. Some of the concentrations are in the range of 0.000000000000001 grams per gram! However, even at these low concentrations, scientists like Dr. Banta can clearly see natural cycles that the earth undergoes on both seasonal and multi-year cycles. You can imagine how excited Dr. Banta was to find a layer of aerosols so high in concentrations that he could see it!
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