|MadSci Network: Earth Sciences|
Kelly: This is an interesting question and it has many answers. The simplest way to identify a fossil as freshwater is to recognize it as identical to a living species that lives only in freshwater. Failing this, it may belong to a group that has living representatives, all of which live in freshwater. Or, the fossil may always be found in association with some other fossils that are already known to be found only in freshwater. Sometimes we know enough about the rocks that contain the fossils to identify the deposit as a freshwater deposit. For instance, if the fossils are found in the bed of an ancient river, at a point far from the river mouth, they are likely to be freshwater fossils. Fossils found in lakes are commonly freshwater fossils, though some lakes are salty. This method is not 100% reliable so one must use good judgement. For instance, coal is a freshwater deposit, but many coals form near the sea. The gradual back and forth movement of the shoreline can result in interlayering of coals and marine shales. The best way to answer this kind of question is to use multiple lines of evidence. That way, if one piece of evidence is misleading, the other pieces of evidence will reveal that fact. This method works as long as more than half of your lines of evidence give the expected results. One of the most reliable methods of identifying fossils as freshwater only works if the specimens have not been changed too much (for example, turned to silica or pyrite). Usually, it is obvious that a fossil is pristine (good as new) because the microscopic structure is the first thing to be lost during changes that affect the fossil underground. If the microstructure looks normal, then the isotopic ratios (for example, amount of light and heavy carbon atoms) and trace element geochemistry (for example, amount of manganese atoms in the fossil) of the fossil are like they were when the organism was alive. For example, seashells can contain different amounts of light and heavy oxygen atoms depending on whether the shells formed in salt water or freshwater. Trace elements like Boron and Sodium also are present in different amounts in shells formed in salt- and freshwater. Sodium is part of salt (sodium chloride) and it is very common in salt water. This kind of analysis can be expensive and difficult, but it has the potential to show a lot about how the fossil formed (and what happened to it after burial, if indeed it has been altered underground). Paragraph one is pretty much common sense. Geochemistry, the subject of paragraph two, is very complicated, but you can learn a little about it by reading an undergraduate geology textbook (a modern one; old ones don't have much about geochemistry because it is a new field). I know that you are still pretty young, and you might have difficulty with a college-level textbook. Another possibility is to talk to a local geochemist. If you can find one who is good at explaining his or her work, you may learn a lot! Good luck, David Kopaska-Merkel Geological Survey of Alabama PO Box 869999 Tuscaloosa AL 35486-6999 (205) 349-2852 fax (205) 349-2861 www.gsa.state.al.us
Try the links in the MadSci Library for more information on Earth Sciences.