ze Science of ze Pits

Written by Zoe Courville

While the guys are busy setting up the second automatic weather station (AWS), I have a chance to wax poetic about the science one can obtain from snow pits.  It is easy for me, sitting in my warm office, to daydream about digging and sampling a cold snow pit, but if truth be told, it is not that bad of a place to spend an afternoon.

On many of my previous field trips, I have been the field member tasked with completing the snow pit studies, and gained, through this task, the reputation of being “tough.” But once again, to be completely truthful, a snow pit, usually dug to around 2 meters depth, is generally out of the maddening wind that so often plagues field teams.  And while the pits start out fairly cold, the small space is generally warmed up just by the person working in it to the point that it becomes fairly comfortable.  I’ve spent countless pleasant hours in snow pits, and have even been known to take a quick nap or two.

So what would make such a past time worthwhile?  After all, comfort aside, is the effort required to dig and sample a pit, easily several hours worth of hard labor, really worth it?

The top 2 meters of the polar ice sheets is usually fragile enough that efforts to sample it through coring result in a crumble of snow grains, so even for field efforts that involve drilling deep ice cores, a snow pit or series of snow pits is needed for the top two meters.  For a field project like we are conducting, where speed and weight are of the upmost importance, collecting ice cores and bringing them back to the lab, which requires dragging along the extra weight of both equipment and the collected cores, is too prohibitive.  So pits it is.  Many of the giants of polar research, Dr. Tony Gow, Dr. Richard Alley, Dr. Carl Benson, have spent many, many hours working in snow pits, and we owe much to their pioneering work.  Chris is lucky in that Carl himself gave him a few pointers on snow-pit work before he came to Greenland (i.e. “nobody digs a proper four-wall pit anymore”)

And how do we interpret the climate signal left in the snow?

I’ve been working with a freelance artist, Sam Carbaugh, a graduate of the nearby Center for Cartoon Studies in White River Junction, VT, who put together this illustration of how ice sheets record past and current climate conditions (http://samcarbaugh.tumblr.com/).

Snowpack

ice sheet cartoon by Sam Carbaugh

In the interior of polar ice sheets  (Antarctica and Greenland), snow falls on the surface and rarely melts.  Over time, because the snow accumulates, layer after layer as each storm and snowfall comes, the ice sheets build up.  The top part of the ice sheet is made up of what is called “firn,” or snow that is older than one year, and is permeable (or open to air flow).  At a certain depth, generally just under 100 meters, the firn becomes compressed enough from the weight of the snow above that it becomes ice, where instead of being permeable, the air in the pore space becomes isolated into bubbles.

The snow and firn can be sampled, as Chris and Mike and Nate are doing at the moment, in a pit by extracting small (around 100 cubic centimeters, about the volume of a pack of cards) samples at a few centimeters depth per sample, and collecting the samples in carefully cleaned sample bottles.  Generally, we take one sample every 1 to 3 cm down the height of the snow pit wall (and yes, that adds up to a lot of samples).  Because the snow that makes its way to the interior of the ice sheets in Greenland and Antarctica is so clean, it is easy for the person collecting the samples to contaminate it.  We generally have to wear a clean suit, plastic gloves, and use carefully cleaned sample cutters.  The small snow samples are then run through a series of instruments back at the lab (most generally some sort of mass spectrometer) that can determine what concentration levels are present of a whole suite of chemical species.  You name it, a geochemist is probably looking for it.  Dust (which is made up of different chemical species depending on where it originated) is one example of an aerosol, or particle, frozen into snow that can be studied, for instance.  Black carbon, or soot, is another.  Concentrations of sea salt which serve as indications of storminess and strong winds are another source of information locked in the snow.

In Greenland, it snows enough each year that many chemical species have seasonal differences between summer and winter that can be distinguished from one another and used to discern annual layers.  Large volcanic eruptions leave a layer of tephra in the snow, which can be used to help check the counting of annual layers done through seasonal chemical analysis, if the date of the eruption is known.  For instance, the 1815 eruption of Tambora in Indonesia is one eruption that can be used to help date ice cores.  Another common technique used to help date ice cores is by measuring radiation found in cores…when a peak in beta radioactivity is reached in the measurements of radiation with depth, that layer can be attributed to the run on nuclear weapons testing that occurred just before the 1963 global ban on above-ground testing.

The local temperature when the snow fell can be determined indirectly through the isotopes of water at a given site, based on the fact that there are generally smaller amounts of the heavier isotopes of water as the temperature gets colder.

Another property of the snow comprising the polar ice sheets that can be used to interpret past local climate and weather conditions is the actual physical structure of the snow itself.  In Greenland, the summer and winter snow layers not only have different chemical signatures, but the snow grain size and shape and the density of the snow is different from one season to the next.  Winter snow tends to be denser, with finer grains, than summer snow, which is less dense and has coarser grains.  Here I am in the bottom of a 3 meter snow pit near Summit, Greenland, where Chris, Mike and Nate are starting and ending their journey.

Zoe in a 3 meter backlit snow pit near Summit.

Zoe in a 3 meter backlit snow pit near Summit.

This pit is actually a backlit pit, where a group of us dug first one pit, and then left one wall of about 20 cm, and then dug a second pit.  Yes, lots of work.  We covered the first pit, and what you see is the sunlight from the second, open pit, filtering into the pit I am standing in.  The blue light is caused by the snow preferentially absorbing other wavelengths of light, most notably red.  The lighter bands of snow are summer layers, while the darker bands of snow in the picture are winter layers.  This was actually an area that I had revisited over the course of 3 years.  The first year (in the summer), I had left a string marking the snow surface that you can see in the picture if you look near my shoulder.  The second year, I left another string, which you can see near the top of the picture.  It snows an average of 65 cm of snow per year here.  If you look even more closely at the layers of snow that make up each seasonal layer, you can see individual weather events—storms, layers of frost that form, wind scour layers.  Over time, these 65 cm thick layers are compressed by the weight of the snow that falls on top of them, undergo metamorphic changes due to the transient and ever-changing nature of snow, and end up being squished down to less than 1 cm towards the bottom of the ice sheet which can be 3-5 kilometers deep (and then undergo melting, folding and deformation due to the heat of the earth insulated by the ice and movement of the ice sheet as it flows over the bedrock…but that is a whole other topic!)  But at the surface, they start out as these wonderfully clear markers that we can marvel at before they begin their journey to the base of the ice sheet.

Finally, I share with you the following link to a video a group of us (Maria Hoerhold from the University of Bremen and two students, Elyse Williamson from Hamiliton College and Kristina Sorg from Bowdoin College) shot extolling the virtues of snow pit digging.  We called it “SnowFreaks.” Enjoy.

http://www.exploratorium.edu/tv/index.php?project=9&program=1075

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