Ice and Medicine at the end of the earth

Shoveling Snow For Science

Digging snow pits provides an interesting look at the ice sheet.  These photos were taken in a backlit snow pit.  Two snow pits were dug with a thin wall between them. They were a little over 2 m deep.  The deep blue light comes from backlighting the snow pit wall. 

You cover the first snow pit, the one you walk down into, with plywood. The second snow pit is left uncovered. This allows the light to travel through the snow pit wall into the dark snow pit that you’re standing in. 

What pops out is all the different layers in the snow.  You can see the history of snow being deposited on the ice sheet.

In the photo above, I’m pointing to what is most likely last year’s summer surface. There are bigger snow crystals which get formed by the strong temperature gradients at the surface.  The thin dark layers are likely wind crusts deposited during the storms of winter. 

All of this snow gets compressed into ice.  It snows about 70 cm a year here, which compresses down to about 20 cm of ice per year. These “annual layers” are one of the primary ways we date the ice core. From the surface to 1950 meters depth, I’ve counted 11622 years (with an uncertainty of about 1%).

2 responses

  1. Marcia

    Great photos! How do you count the layers?

    January 20, 2011 at 6:35 PM

    • From T.J.:
      There are a lot of ways to count annual layers. What you need is something that gets recorded in the ice core that has a seasonal signal.

      In my case, I measure the acidity of the ice. More acid reaches the ice in the summer than the winter, so there is a nice annual cycle (more acid reaches the ice in summer because the sea ice does not extend as far).

      There are other things with a seasonal cycle too: dust, visual stratigraphy (ice crystal size), sulfate, oxygen isotopes, etc. The best annual layer counts use multiple different things.

      January 23, 2011 at 11:55 AM

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