Below in the picture, you’ll see the first “run” of the drill. Every time the drill goes down into the ice sheet, we bring back a piece of firn (unconsolidated snow) or ice about 1.5 to 2 meters long. At the South Pole, we won’t technically hit “ice” until around 130 meters deep. We call the top portion of dense snow before actual ice, firn. In the firn layer, air can still mix and diffusive with the modern atmosphere. Below the firn-ice transition, ancient air is trapped within the ice and this feature is how ice core scientists, like myself, can study the ancient atmospheric composition! The depth of the firn layer is different for every site on an ice sheet and depends on the local meteorological features such as yearly snow accumulation. If you think about it, a location where it snows a lot (say 20 cm/year) is going to have a lot of snow piled up much faster than a location where it doesn’t snow as much (say less than 8 cm/year, similar to South Pole). With more snow each year, the snow below becomes more compacted until eventually it turns to ice and the air cannot move freely and gets trapped as air bubbles! Therefore, locations with more snowfall each year, will have a shallower firn layer (~60 – 70 meter depth) compared to a location with very small amounts of snowfall like the South Pole.
During our last run of the day, a piece of ice from the ice core showed that we are VERY close to the firn-ice transition. We could see some trapped air bubbles mixed in with open spaces. I tested the quality of the ice by eating it (see picture!). For being around 900 or so years old, it was delicious!
Hope you all are having a festive holiday season!
Until next time,