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Isotopes:the difference between fractionation and selection processes

  1. Jul 21, 2011 #1
    Isotopes:the difference between "fractionation" and "selection processes"

    I'm reading a book about Isotope Hydrology and it differentiates between "isotope fractionation" which denotes any situation where changes in isotopic abundances result, and "selection processes" which are situations such as when the average isotopic composition of a compartment changes due to the compartment not being well mixed and then partially utilised.

    However, I am unclear of the difference between the two, what is fractionation exactly? Under which bracket does evaporation occur? eg. after precipitation, you could argue that some is "partially utilised" in evaporation, making it a selection process, or you could say that the mass effect reduces the vapour pressure of the heavier isotope, therefore making it a fractionation process. If anyone could clarify the definitions it would be great.

    (The book is Isotope Hydrology, A study of the water cycle, Joel R. Gat.)
     
  2. jcsd
  3. Jul 21, 2011 #2
    Re: Isotopes:the difference between "fractionation" and "selection processes"

    Fractination, if you have a certain ratio of fat and skinny boys in class and you call them out for getting candy, what would that do the the fat /skinny ratio of the first arriving boys at the point of distribution?

    Back later, just an intro :smile:
     
  4. Jul 21, 2011 #3
    Re: Isotopes:the difference between "fractionation" and "selection processes"

    Sorry, I thought it was going to be easy to answer this but it seems not so.

    Anyway isotope fractination happens during chemical reactions and/or physical processes (diffusion, evaporation, condensation, sublimation etc). The difference in masses (fat and skinny boys) make that there are different affinities for the different states

    I could not find however a clear definition of selection processes. However what comes to mind for instance is the vertical transport of 13C in the oceans. Biota are usually depleted with 13C due to the higher affinity for 12C in the photosynthesis. Dead organisms sink to the ocean bottom and decay causing a selection of less 13C in the deep oceans. Hence limestone and foraminifera shells from the deep are depleted in 13C due to that selection process by gravity.

    On the other hand 'selection' could also refer to kinetic fractination rather than equilibrium fractination

    But I understand the confusion.
    my two cents. it may still be something else.
     
    Last edited: Jul 21, 2011
  5. Jul 22, 2011 #4
    Re: Isotopes:the difference between "fractionation" and "selection processes"

    Thanks for your input, glad I'm not the only one confused :)

    I see what you mean about the biota, fractionation being the process by which the biota are depleted in 13C however, the selection process being that they decay at the bottom of the ocean.

    I don't think that 'selection' could refer to kinetic fractination rather than equilibrium fractination, because the book I'm reading describes the two types of fractionation.

    Thanks for your help though!
     
  6. Jul 22, 2011 #5
    Re: Isotopes:the difference between "fractionation" and "selection processes"

    Actually the decay would lead to some fractination, but I was explicitely thinking of the transport of 13C depleted remains by sinking, that is no fractination but still a process that alters the general d13C of the local environt, not fractination but transport, could that be 'selection'?

    One can also think of the Rayleigh effect or Rayleigh fractination maybe where the fractinated bit is removed from the environment after the process, altering the ratio of the remaining species, evidently as a cloud rains out, the heavy isotopes go out first, leaving the cloud lighter and lighter in isotopes, not by fractination but by ..selection?

    More two cents.
     
  7. Jul 25, 2011 #6
    Re: Isotopes:the difference between "fractionation" and "selection processes"

    Maybe, but as the formation of a cloud involved a phase change, I would have thought it would be considered as fractionation....

    Difficult, it seems the more I read, the more people seem to have a different take on what fractionation is
     
  8. Jul 27, 2011 #7
    Re: Isotopes:the difference between "fractionation" and "selection processes"

    Yes obviously, but during the life time of the cloud, when the heavier isotopes rain out first, the remaining watervapor gets lighter and lighter (Rayleigh effect), without going through a phase change.
     
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