Re: forams

For clarification, forams come in two kinds, benthic (bottom dwelling) and planktic (off the bottom) (NB: glossary says planktonic). The authors of the Chevron report state, "Foraminiferal biostratigraphy in the California Miocene has been based almost exclusively on the benthic fauna because associated diagnostic planktic foraminifers are rarely encountered."

And after Glenn Morton said that there are only about 150 foram species in toto, I rechecked those authors' list (see below) and found only 85 different species as candidates for their Miocene formation, not 150.

As for the really interesting questions, good luck! For such questions speculation is often as good as it gets.

Don

----- Original Message -----
  From: Randy Isaac<mailto:randyisaac@adelphia.net>
  To: asa@calvin.edu<mailto:asa@calvin.edu>
  Sent: Wednesday, October 12, 2005 6:15 PM
  Subject: Re: forams

  Thanks, Don. And I enjoyed reading Jim's links. It's fascinating but I still have lots of questions. I understand the observation of the sequence of species over time for these plankton. It is certainly amazing to see the progression and the fairly precise dating that can be done. But I couldn't find anything about the driving forces. What environmental factors drive natural selection for plankton? Have oceans change so much and so rapidly that over 66 million years it drove such changes? And why is the rate of change relatively constant? (if it is, of course. I couldn't tell for sure from the snippets I read) How are the differences in the sequential species affecting the ability of plankton to survive?

  Randy
    ----- Original Message -----
    From: Don Winterstein<mailto:dfwinterstein@msn.com>
    To: asa@calvin.edu<mailto:asa@calvin.edu> ; Randy Isaac<mailto:randyisaac@adelphia.net>
    Sent: Wednesday, October 12, 2005 10:05 AM
    Subject: Re: forams

    Randy,

    I'm not really the one to be tutoring you on this subject, because my relationship to paleontologists was never more than peripheral. However, not long after joining Chevron I became fascinated with the beauty and variety of the paleontologists' scanning electron micrographs of forams and diatoms, to the point that one of the few Chevron research reports I've kept to this day is one that has a large number (>100) of high-quality foram pictures. The authors list roughly 150 candidate species of forams found in their formation. The abstract states, "[This]...study was prompted by the need to relate lithostratigraphic boundaries and siliceous microfossil horizons to the benthic foraminiferal stages....Foraminiferal data is essential for correlating those intervals of the Mussel Rock section that are devoid of siliceous microfossils." Those "siliceous microfossils" are almost certainly diatoms, which are used in similar ways for dating rock. That report came out in 1985.

    The geologic ages determined from such microfossils are, of course, obtained by correlating their appearance with other sources of info on age. Among the reference ages would be absolute ages, obtained in suitable places from radiometric dating, and relative ages from rock layering. A formation of interest may be far from any rock that can be radiometrically dated and similarly far from any unbroken sedimentary sequence. The relative ages of index fossils are obtained where the sequences are unbroken. They are often then applied in dating isolated formations.

    You may also be interested in the definition of foraminifer from my Glossary of Geology:

    "Any protozoan belonging to the order Foraminiferida, characterized by a test composed of agglutinated particles or of secreted calcite (and rarely of silica or aragonite) and commonly found in marine to brackish environments from the Cambrian to the present. Precambrian forms and some freshwater forms are also known."

    Don

      ----- Original Message -----
      From: Randy Isaac<mailto:randyisaac@adelphia.net>
      To: asa@calvin.edu<mailto:asa@calvin.edu>
      Sent: Monday, October 10, 2005 4:26 PM
      Subject: forams

      I recently came across a Sept. 20 NYTimes article about fossils uncovered in Cuba. http://www.nytimes.com/2005/09/20/science/space/20mete.html?ex=1129089600&en=9bc3670e4e036958&ei=5070<http://www.nytimes.com/2005/09/20/science/space/20mete.html?ex=1129089600&en=9bc3670e4e036958&ei=5070>

       It included the following paragraph:

      "Microscopic study also revealed the presence of thousands of tiny fossil creatures, most especially foraminifera. Those one-celled animals have a bewildering array of minuscule shells. Forams, as they are known, evolve so fast that geologists, paleontologists and oil companies use their shifting appearance as reliable guides to geologic dating.

      "They told the age of the sediments," Dr. Alegret said. "So we've definitely confirmed the age of these deposits."

      I'm not familiar with forams or their use for dating. Can someone on this list please provide a brief tutorial about this technique or point me to a useful link?

      Randy
Received on Thu Oct 13 02:29:38 2005