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> August 2, 2006 - Polar Bears and Lake Cores
post Aug 4 2006, 11:52 AM
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Today was a river and glacier day, so the two groups that head up-valley headed off to study their field sites. This was also the first day that Caroline and Mike were able to collect a core from the Kongress Lake bottom. This was a pretty big deal, as the conditions have to be just right. It also requires some specialized equipment and patience.

Me Working at Kongress
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In order to collect these cores, a fish-finding device was used to locate the deepest part of the lake. Caroline and Mike located their first core spot where the lake depth was 52m, at nearly it’s deepest point. Their second core spot was much shallower, at a depth of about 15 meters deep.

They chose these two depths because of a line that seems to separate the lake into two distinct levels. This line is at 45m depth and is called a chemocline, or a line where the chemistry is very different above it than it is below it. They wanted to make sure to take cores both above and below the chemocline.

Aquatic Research Universal Corer

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They lowered down a long rope connected to the Universal Corer, attached to a cylindrical plastic coring tube. Attached to this tube was a weight, so that once Mike felt the core touch the bottom, and he released the line, the core could sink into the bottom mud. This sounds really easy but it is actually really difficult! Mike has done this thousands of times on lakes all over the arctic and in New England, and has the feel for the rope and when to drop the core down, and more importantly, when to pull it back up! Every lake is different, and he is really skilled at collecting good cores.

What Cores Look Like

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One of the difficulties is that the boat does not stay in spot, and so the core may not go into the mud vertically. If it goes in crooked, it will be a bad core. It is really hard to keep a boat from drifting in the wind, so they worked as best as they could. Out of five tries, two attempts brought up enough mud to be considered good quality. As soon as it got close to the surface, Mike reached in deep under the water and slid a cap on the bottom of the tube. It is really important to keep the sediment-water interface undisturbed so that the layers of mud are not changed at all. If the sample is suspended in the tube, with water both above and below it, the core can begin to crumble away and become unusable.

Caroline with her Cores
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Caroline hopes to be able to work with these cores once she gets back to her lab at college. They have very thin layers and can be very difficult to read. Here is a picture of a core collected by Emily Pratt in 2004.

One of Emily's Cores
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Caroline will prepare thin sections of the core by first filling it with epoxy to make it hard so it won’t crumble away. Then she can slice it into thin layers to make microscope slides. Here is a picture of Emily’s thin section… you can see how tiny the layers are – the scale on the left is millimeters.

One of Emily’s Thin Sections
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Finally, she can look the whole length of the core under a microscope, slide by slide, to see the tiny annual layers and gradation of grain size. Other tests will tell her the mineralogy of different parts of the core. By matching mineralogical and trace element “fingerprints” in the core to the same “fingerprints” in rocks on-land, she may be able to discover something about where sediment has come from during different times.

Different types of rocks have entered the lake at different times over the past several thousand years. Much of the black shale that contributed black sediment to the lake was moved down to the lake level by an ancient glacier between 100 and 500 years ago. This glacier has since melted away. Lighter colored limestone rock has been coming down toward the lake since the glacial period. The black sediment and the white sediment have different “fingerprints”, so when we see that the “fingerprint” of the black sediment is present in the cores, we will know that it must have been moved there by the glacier!

We can also find some absolute dates in the core by searching for a spike in the cesium-137 concentration. This was a radioactive element that circulated in the atmosphere and settled around the world during atomic bomb tests in 1963-4. Although the bomb testing was an unfortunate time in our world history, this radioactive layer is really helpful to geologists in pinpointing a timeline along sediment layers. It is kind of like having an item in your laundry basket with a date on it, like a movie ticket, that you can be sure separates items that came in before you saw the movie, and after you saw it. This is considered an “absolute” date, while the sequence of layers in other parts of the column represent “relative” dates.

And now, for the bear story. On the way back from the field, the group was walking along the beach a few hundred meters from Isfjord Radio. In the sand, Caroline and Mike spotted a print of a polar bear. It was huge. Caroline and Christina took some photos. I have included the best 2 here, with a hand or a boot for scale. Pretty cool, eh? No, we didn’t see the bear, but it must have been passing through in the night. Leif said he heard the dogs barking like crazy sometime that night, so it was probably when the bear was around. We all really want to see a polar bear, but that would be dangerous as well as thrilling, so the print was pretty cool.

Polar Bear print with Hand for Scale
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Polar Bear print with Boot for Scale
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View of Kongress - a very beautiful place
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