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Conodont Fossil Colors

Conodont fossils can be found in vast quantities in marine sedimentary rocks from almost 300 million years of Earth history. While naturally a light amber color, the organic elements of conodont fossils change color due to heat and pressure in a consistent way that is expressed in the conodont color alteration index (CAI). Geologists are able to use this index to determine how much heat a sedimentary rock was exposed to throughout its development, which can aid in the discovery of fossil fuels like oil and natural gas.
  1. Conodonts and Their Extraction

    • Conodonts are extinct eellike organisms that lived in marine environments from the late Cambrian to the late Triassic period. Most knowledge about them comes from microscopic fossils of jaw and teethlike samples found in sedimentary rocks. Conodonts are easily extracted from their surrounding rock by dissolving the rocks in a weak acid like vinegar or acetic acid because the acid can dissolve the rock without damaging the apatite or phosphatic chemical structures of the fossils.

    Conodont Alteration Index

    • Observation under a microscope shows that conodonts appear to be light amber in color, so stage one of the scale correlates a light amber, light brown or yellow appearance to fossils which experienced a maximum heat of less than 176 degrees Fahrenheit (80 degrees C). Conodonts which appear to be dark brown experienced a maximum heat of 176 to 284 F. Conodonts which appear to be dark grayish-brown experienced a maximum heat of 284 to 392. Conodonts which appear to be dark gray experienced a maximum heat of 392 to 572 F. Conodonts which appear to be black experienced a maximum heat of 572 to 896 F. Conodonts which appear to have turned pale gray or white experienced a maximum heat of at least 752 F.

    Uses for the Conodont Alteration Index

    • Because the colors of conodont fossils correlate accurately to the exposure of surrounding sedimentary rocks to heat, the CAI can be used to help scientists better understand the geological history of the rocks. This allows geologists to determine if these rocks were beginning the process of change to metamorphic rocks, or are located near metamorphic rocks, which could mean proximity to fossil fuel resources or other valuable mineral deposits formed under extreme heat and pressure. In particular, examining the color of conodont fossils is essential in the evaluation of oil shales and tar sands.

    Advantages and Disadvantages of the Conodont Alteration Index

    • One advantage to using conodont fossil colors as a way to determine the exposure of sedimentary rocks to heat is that the process of extracting and analyzing conodont fossils is simple and requires cheap and easily accessible materials. Helsen, David and Freemont also note that the process of conodont fossil color analysis is made more exact through the use of computerized Color Image Analysis. However, conodonts as a class of organisms go extinct and disappear from the fossil record at the end of the Triassic, and so the CAI cannot be applied to rocks less than 200 million years old.


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