Friday, November 30, 2007

Following the tracks

In the roads around town, there are many interesting tracks made by the wide variety of heavy equipment and all-terrain vehicles used here. The picture above shows the tracks and some of the vehicles they were made by.

It isn't always easy to match them up. I still haven't seen the vehicle that made these:
Based on the marks it has left in the snow and dirt, I can guess what the tire or tread surface that made it might have looked like and I can get an idea about the path it followed as well as how large/wide a vehicle it might be.

Similarly, we can use the tracks and paths left behind in the marine sediments of the core by small animals to figure out what type of animal (often worms) might have left them, their size, how they were moving or behaving, and perhaps how many were active in an area (population). These sections of sediment that were disturbed by biological organisms post-deposition but pre-consolidation (after the sediment settled but before it turned into rock) are described as being bioturbated - mixed by living organisms. The most important information that the ANDRILL project gains from these trace fossils are from those that can be used as index fossils - giving a date or date range for the rock in which they appear. Many of these characteristic trace fossil types have specific names even though we don't know the exact species or genus that created it. One type, called escape traces or Fugichnia, are created when a shallow burrowing, near surface, or surface marine animal gets burried suddenly under a quantity of sediment and has to dig furiously for the surface to survive. (Look up "trace fossils" or "trace fossil classification" for more information about these fascinating fossils!)

In the photo of bioturbated sediment in the core above and to the left, see if you can find burrows and other evidence of the movement of organisms through the sediment. What might you be able to tell about the organism involved?

Notice what the movement of organisms has done to the layering of the fine sediments. Compare it to the picture on the right of soft sediment deformation caused by physical or mechanical stresses experienced by the sediment over time. What similarities and differences can you find?

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