Snow Pits!

WAIS Divide, Antarctica, Jan. 12, 2010 — After work today, I helped to dig a snow pit. This is the third snow pit we’ve dug out so far this season. A snow pit consists of one main large rectangular hole in the snow, about 6 to 7 feet deep and wide enough for several people to stand in it, and another thinner rectangular hole that borders one side of the main hole (usually the back wall). The thinner hole allows sunlight to illuminate the fine layers in the snow. Not only does this create a beautiful glowing wall of snow layers, it is also useful scientifically to study snow composition and accumulation over the most recent years.

Digging out a snow pit. Shovels, saws, a chain saw and multiple people were used
to create one large pit and an adjacent, narrower, and smaller pit (current
covered with a board behind the pit where people are digging). The smaller
pit allows sunlight to shine through the layers in the wall of the larger pit.
The smaller pit is covered with a board to keep snow from blowing in and filling it.
Other helpers are loading snow onto a sleigh to be transported to a dumping
ground in the background of the photo.

Constructing a roof for the snow pit. The main snow pit needs a solid roof to
keep out blowing snow and sunlight. It must be dark in the pit in order to see the
layers in the snow illuminated by sunlight shining in through a smaller pit
behind one of the walls of the larger pit. The flags mark the location of the pit and
warn people to be careful in this area. In the forefront of the picture is the
entrance to the pit which, although you cannot see them here, has snow stairs!

Outside view of a finished snow pit. The wooden roof covering the smaller pit
(behind the main large pit) is removed to allow sunlight to illuminate
the snow layers composing the back wall of the main pit.

Inside a finished snow pit. I am pointing to a bright layer potentially deposited
during an Antarctic summer season just a few years ago. The layer is brighter
because the snow is less dense due to the larger snow crystals that form in
the warmer summer temperatures.

One of my fellow core handlers and a graduate student working with the Ice and Climate Group in the Department of Geosciences at The Pennsylvania State University, John Fegyveresi, has been doing some work with snow pits here at WAIS Divide:

“The snowpit project is simply a side project that my advisor wanted me to do to get familiar with the process of picking out seasonal and annual layers near the surface.  There are several things he wanted to me to look for and be able to identify, such as wind crusts, depth hoar and hoarfrost layers, individual storm events, annual layer groupings, surface sastrugi, and any other notable features.  As an additional step, I decided to take density and water isotope samples from the pit.  Using the density samples, I should be able to get a better idea of the annual layers based on the principle that the winter snow consists of smaller flakes (due to colder temps), and therefore packs more densely.  Conversely, the summer snow forms larger flakes (due to warmer temps), and thus packs less densely.  A density plot for a snow pit should take the shape of a rough sine curve, with the peaks representing winters, and the valleys summers (with an overall increasing density trend with depth).  Generally, with the known average accumulation rate at WAIS Divide, one can see about three years of history in a 2 meter snow pit.   The water isotope samples will be analyzed to get a ratio of ¹⁸O to ¹⁶O isotopes, which can be used as  a proxy for temperature.  So this plot should also coincide with the density plot with the more negative ¹⁸O/¹⁶O ratio equating to colder temperatures in the winter.  This work will probably not be incorporated into any type of publication, but is a great learning tool for physical properties studies and helps me to understand what we may see in the ice core.”

John Fegyveresi, a core handler at WAIS Divide, is smoothing out the wall of a snow pit.

Other interesting research that John is working on while at WAIS Divide involves studying the physical properties of the ice.

Studying the Physical Properties of an Ice Core

This section was contributed by John Fegyveresi, a core handler currently at WAIS Divide and also a graduate student working with the Ice and Climate Group in the Department of Geosciences at The Pennsylvania State University:

The physical properties work that I’m doing with the WDC06A core (the core being drilled at WAIS Divide) is based on a separate grant and project number (I-168) through my advisor, Richard Alley, and Joan Fitzpatrick (USGS).  The overall goal of the project is to look at various physical aspects of the core to reconstruct climates, determine information about the direction and properties of ice flow, look for stratigraphic deformation of the ice near the bed, analyze the c-axis fabrics of the grains, and hopefully try to learn something about clathrates (a solid compound in which molecules of one substance are physically trapped in the crystal lattice of another) and their relationships to the bubbles they formed from.  The easy way of thinking about physical properties is to simply think that we are looking at the geology of the ice.  Just like the chemistry folks look at isotopes or gas concentrations, and the microbiology folks look at bacteria, we look at how the ice has moved/flowed, deformed, and what’s going on inside it (physically speaking, i.e. bubbles, clathrates, crystals, etc).  More specifically, my task here on-site is to obtain both a horizontally and vertically oriented piece of ice from the core from a depth of every 20 meters.  This sample I shave down smooth with a medical microtome and mount between two glass plates.  One side is mounted with a small bead of water around the edge, and the other with a special cold temperature "super glue".  The samples are then sent back to the National Ice Core Lab, where they are cut in half, and each half is used to make a separate thin section.  The glued side is shaved down to about a half a millimeter, and put into a special machine that is used to determine the c-axes of the ice crystals.  The water-tacked side is shaved down to about 1.5 millimeters and is digitally photographed so that the bubbles and/or clathrates can be studied/analyzed.  My masters thesis project involved looking at the bubble number-density (number of bubbles per given volume), and then using both grain-growth and firn-densification models, combined with a new paleoclimate technique developed by another one of my advisor’s former students, Matt Spencer, to reconstruct climates over the 2 millennia prior to 1700 C.E.