Snow Sampling

2007-12-14

Well, the ceaseless winds and endless nights of white did, in the end, relent. After a week of having to find work to do indoors but having lots of work to do outdoors I get to feeling very cooped up indeed. Cooped up to the point that I volunteer to dig the melt tank, or go for a ski in a 30 knot blizzard with just enough visibility to see two or three drums of the perimeter ahead. In winds like that you can't really ski, as any glide instantly gets knocked back by a gust, but it's nicer to walk with sticks on my feet over the soft heaps of fresh snow than wade through them in my mukluk boots.

The good weather was essential for one job we'd had lined up for a while, digging and sampling the snow that had fallen at Halley over the last year or so. Every couple of months one of these holes has been dug by the met team, and I helped them out for the final hole of the year. It's important for ice core scientists to understand how snow that falls is transformed into snow that gets squeezed into ice and so into ice cores. As it settles on the ground various gases, salts and dust particles are trapped in the gaps between the snow flakes. After a while chemical reactions might cause changes in these gases. Eventually, after enough snow has fallen, the trapped gases form bubbles in ice cores or dissolve into the ice. As we know the current conditions at Halley through measurements of atmospheric gases by the CASLab, we have a chance to follow the processes that occur in the snow as it compacts.

Halley gets a lot more snow than the places where most ice cores are taken from. We have something like a meter a year, while many ice core locations only get a couple of centimeters, but the general principles will be the same. Understanding the snow at Halley will help us calibrate ice cores elsewhere in Antarctica, and so make deductions about previous climates with confidence.

To do this, of course, you need some snow. More than that, you need some snow that hasn't been contaminated by anything that wasn't there when it fell. We get this by maintaining a clean air sector which is isolated from the rest of the base and so less likely to be polluted by vehicle emmisions. When we dig the hole we take a huge amount of care to keep our equipment and ourselves from getting anything into the samples. First we wear barrier suits, face masks, and four layers of plastic gloves. Second we clean everything: shovels, saws and boots all get wiped down with isopropanol in a clean cupboard. Finally, we try not to sneeze while we're digging.

Suiting up for science

Having dug a hole a couple of meters down, we carefully prepare one wall, then push small ultra clean sample vials into the snow every two centimeters. These trap the snow at that layer, then get sealed with caps, put in a sealed bag, then put into another sealed bag. All along we take great care not to get any flakes of snow into the sample that shouldn't be in it. Eventually these samples will get shipped (still frozen) back to the UK where they will be analysed by BAS chemists.

Tamsin takes the 10cm sample

I end up doing the ones at the bottom of the hole

Taking the samples is a bit of a faff (opening plastic ziplock bags with plastic gloves is painfully frustrating), and it takes the best part of a day, but as my job mostly features computers and complicated electronics that blast invisible waves at elemental particles hovering somewhere at the edge of space it's satisfying to do something a little more tangible from time to time.