23 June, 2000
A Window to the Past: Core Samples
A couple of days ago we successfully launched the corer. We were able to bring on board a core that was just over thirty feet in length. The core itself was not as long as what we had expected or wanted. There could be many explanations of why the core sample was short. Two possible explanations include: the mechanism of the corer did not worked properly and the winch operation could have had an effect.
The core that was extruded will not be examined during this cruise. It will be refrigerated for the trip to the Healy's homeport of Seattle. On a normal science cruise, additional steps would be taken to prepare a sample for research. With the aid of Garry Brass of the United States Arctic Research Commission, I have outlined a broad summary of what might happen to these core samples after they have been collected.
First, the core liners are split lengthwise. A nylon fishing line then splits the core sample itself into two "D" shaped halves. Both halves are then wrapped in plastic kitchen wrap. One half of the core sample will be archived in a core repository, therefore it is placed in a D tube and capped. The other half of the core sample is called the working half. It will remain in the lab for additional work.
Most core samples are collected by a scientist, for a specific research need. But, since these samples are archived, other scientist can use the samples to complete their own research. One extraordinary discovery that was made because an archived core sample was available to other scientists was the evidence of the geologic time boundary between the Cretaceous and Tertiary periods. This boundary marked the extinction of the dinosaurs and the emergence of the mammals, which most scientists now suspect was caused by a huge meteorite impact off the Yucatan Peninsula.
The working half will first have a glass slide scraped along the flat side (cut surface) of the core sample. The purpose of this is to eliminate any smears or deformities to the cores "surface" that occurred when the core was split by the fishing line. After the scraping is completed, a description of the core is recorded.
The core describer records the internal structure of the sample; the depths of layers are measured and the colors of sediments are compared with a Munson color chart. The sediment type is also noted, whether it is composed of clay, silt, sand, or ooze.
To identify all of the visible grains in the sediment, a smear slide is taken. Then the scraping of the core is placed on a microscope. This process can be done in two ways, by simply adding a drop of distilled water to the scraping and smearing it out evenly over the slide, or by adding a plastic resin to the sample, which will spread it evenly over the slide. After the smear is set, the describer can then identify things such as mineral grains, plankton shells, plant matter, fish scales or teeth, and fecal pellets that may be contained within the core sample.
More detailed investigations can continue on board or at a research facility. These investigations may include testing of the physical properties of the core. The shear strength of the material can be determined using a probe. The probe determines the amount of force needed to move the material. How quickly sound travels through the sample and what magnetic properties is has can also be analyzed. Also, a pore analysis can lead to information on the chemical processes of the sediment.
Once all of the descriptions and direct measurements have been completed, the core then can be sampled. Samples of the core may be examined for the sediment and mineral composition. A type of fossil record can also be completed using the cores, by looking at what marine organisms have been preserved and where they are located in relation to one another. This information is useful in dating layers in the core samples. The dating record for single-celled fossils is very accurate and can date samples back as far as 60 million years ago. This information can then in turn be used to date events such as volcanic eruptions and the help determine the rates at which sediments are deposited.
Some very technical and complicated measurements can be completed to determine very specific information about the climate in which the samples were deposited. By comparing the ratio of oxygen isotopes 16O and 18O in calcium carbonate shells of foramimfera, the temperature in which organisms grew can be determined. This can help scientists map out climate changes over the past 150,000 years, for samples collected in Greenland or Antarctica. Samples of carbonates can help determine climatic conditions as far back as 100,000,000 years ago.
Core samples are important to scientist because they provide a window to the past. An almost continuous record of the past has accumulated on the ocean floors. Recorded in the sediment is the some of the history of the Earth's climate, geology and biology.
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