Mapping and Sampling

As I hope you gleaned from earlier posts, Roberts Massif is a truly massive place. For scale, our farthest sampling site was ~5 miles away as the crow flies. This made for some really long days, strong legs, and hungry bellies. We averaged hiking 10 miles a day over rocky, uneven terrain. Despite this handicap, we collected over 160 samples in only 9 days. The glacial geology was too exciting not to!

The first few days were spent getting the lay of the land. We needed to find the moraines, pick the ones we wanted to sample, and draw schematic maps. This helped us figure out what all these moraines meant and how they related to each other. For those who need a refresher, a moraine is a pile of sediment that a glacier deposits at its edge. In the Antarctic, moraines are usually lines of boulders that have dropped out of the glacier margin and are only ~1-2 m high. Here are some pictures of moraines forming today:

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We found moraines that had been deposited directly by the East Antarctic Ice Sheet at some point in time when it was larger than it is today (i.e. covering some of the exposed rocks that we were able to walk on). We also found several alpine glaciers and associated moraines that we weren’t expecting to see. There were actually too many moraines to sample—a glacial geologists dream!

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Once the mapping was complete, we began to take rock samples from each moraine for dating. Dating happens in the lab back at UMaine, and allows us to determine when exactly the ice sheet margin sat at the location where the moraine was deposited. Cosmic rays, which originate in outer space, bombard the earth all the time. They go right through soft material, like humans. In rocks, however, they induce nuclear reactions within certain elements, creating isotopes that we can measure in the lab. When a boulder is covered by or being transported by glacier ice, it is not exposed to these reaction-inducing cosmic rays. However, when the glacier is no longer covering the boulder, the cosmic rays will bombard the rock, make those nuclear reactions happen, and create the isotopes at a specific rate per year. Therefore, when we measure the concentration of isotopes in a rock sample in the lab, we can calculate how long the rock has been exposed to the atmosphere. In other words, we can determine how long the boulder, and the spot on the landscape where it rests, has been ice-free. This information allows us to learn something about past changes in climate, which drove the advance and recession of glaciers.

After trekking over to a moraine, our first step was to pick boulders to sample. We aimed to collect 7 samples on each moraine from boulders that looked like they had been stable in their position since they were deposited by the glacier. These were usually boulders that were large and well embedded in sand and gravel (i.e. stable), or boulders that were perched atop other boulders. As you can see in the photo below, perched boulders look as if they were delicately placed in their precarious positions. And while this isn’t entirely true—the glacier is just dumping them wherever it pleases—you can imagine that there’s no other way to get a boulder in that position.

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Boulder perched on other boulders. Perfect for sampling!

Once our boulders were chosen we would draw a picture of each one and describe it, answering the following questions: What is the rock type? How large is the boulder? Is there weathering? What is the color? How angular or rounded is the boulder? How is it placed on the moraine (embedded or perched)? How relatively “old” does it look compared to other boulders? Does it look like it had been stable since deposition? Is there anything unusual about this boulder? Here’s a page from my field book (don’t judge my handwriting, I was wearing 2 pairs of gloves!):

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Other tasks at each boulder included taking a GPS point, measuring the boulder’s elevation above sea level, and measuring the angle of the horizon from the boulder’s point of view. The horizon measurements are used to see if any cosmic rays are blocked from reaching the boulder by surrounding mountains, which would change the number of isotopes atoms produced within the rock over time. Lastly, we would sample the boulder using a hammer and chisel! Sometimes we were able to use a drill, but because it was so cold the battery didn’t last long.

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Once all of our samples were collected, we’d throw them, and all of our equipment, into our backpacks to haul back to camp. Repeat this for a few days, and you’ve definitely got sore shoulders. Here’s a photo of Gordon adding a sample bag to our ever-growing pile of samples:

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Well, that’s about it for sampling! I know parts of this post may have been technical, so comment below if there are any questions. I hope that gives you all a better idea of what we actually did in the field. We’ll continue posting periodically to show you what we do in the lab, or share any more stories we may have from our season in Antarctica. Thanks for reading!

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