21 July, 2000

July 21, 2000

Matanuska Glacier, Alaska

Today’s time has been occupied primarily with analyzing samples that were collected during yesterday’s dye experiment. After the samples were drawn from the ISCO bottles they were filtered to remove whatever parcticles remained suspended in the water. They were then transferred to special holders called cuvettes and placed into the fluorometer. The concentration of dye was then recorded. So far there has been no measurable trace of dye in the samples that have been tested. There are still a lot more to be measured so it’s wait and see time.

The water emerging from the vents carries with it many types of solid parcticles. It appears to be similar in appearance to the muddy floodwaters in Indiana except that it has a very gray color. The size of these parcticles varies but it is very obvious that there is lots of suspended sediment called “glacial flour” that emerges. Suspended sediment consists of parcticles of any material that are small enough that they will stay mixed with the water for some time, but eventually settle to the bottom when allowed to sit still for a period of time. The Matanuska River which is formed from these waters keeps the same gray color of the water emerging from the vents all the way to Cook Inlet where its flow finally stops. At this point the waters are stilled and the sediment settles to the bottom, slowly but surely filling in this inlet.

The water rushing into the moulins is extremely pure and yet somewhere along the way it has to pick up this sediment. The moulins are simply holes at the surface of the glacier which funnel the water into a complex system of conduits. The drainage system eventually makes its way down to the interface between the basal ice at the bottom of the glacier and the earth where it joins the subglacial drainage system. The subglacial drainage system is loaded with sediment. Where does this sediment come from? As a glacier moves along it dislodges rocks and boulders along the way and carries them with it. As these rocks move they scrape and scratch the bedrock beneath them as well as each other. If you were to rub two rocks together they would produce a fine powder. Under the glacier these rocks are worked so much that they lose their jagged edges and become rounded instead much like rocks that get rounded by rolling surf at the ocean or a large lake. The powder that gets produced by this scraping gets washed away by the subglacial waters that flows beneath the glacier.

We need to remove these suspended parcticles because they cause scattering and to some extent blocking of the UV light that the fluorometer produces. In my July 13 journal I mention that this UV light will cause certain materials such as our dye to give off light or fluoresce. When the UV light is scattered the scattered light interferes with the sensor and prevents it from giving an accurate readout. To some extent the parcticles also absorb or block the UV from hitting all the dye molecules. The more molecules that are exposed, the more fluorescence will be produced. A somewhat impure sample would produce less fluorescence than the same sample after filtering due to this interference. Obviously we want to have all samples free from these problems that would give inaccurate results. Hopefully tomorrow we will be able to finish the analysis and get some good results.

Marvin Giesting

This is just part of a huge boulder I saw yesterday on my hike back from reprograming the Little River ISCO. It was maybe eight feet wide and twelve long and six feet thick. You can see scratches all over its surface from being scraped by other rocks as it was moved along by the glacier. All these scratches produced lots of small parcticles and sediment that easily wash out in the subglacial drainage system to give the stream here a gray color. Other colors of rock will of course produce a different color of sediment but this is the predominant color of the rocks in the Chugach Mountains.

This is water at North Vent and shows the color of the glacial flour in the streams flowing out of the Matanuska Glacier. I have seen streams with other colors coming down from the Talkeetna Mountains to the north on our way back and forth to the grocery store. The color depends on the predominant rock type in the source mountains that the glacier scours. It's interesting to see where streams with different colors meet. The resulting stream is half and half for awhile until the mixing is complete.