Limitations of 
 Isotopic Methods

     Any isotopic measurement is only as good as the sample from which it was taken.  If a sample has been altered during diagenesis, its isotopic integrity may be jeopardised.  Holmden et al., (1996) analysed a hand sample of Ordovician fossiliferous limestone, and its fossil content for its strontium ratio.  One would expect to find a uniform ratio shared by the fossils and the carbonate cement which would be indicative of the seawater at the time.  What they found was quite different; their fossils showed different ratios than each other and than the whole rock value

     Strontium is very soluble and diagenesis can overprint original strontium isotopic ratios with the ratio of the diagenetic fluid Holmden et al., (1996).  What this means for isotope studies is that not all values from a fossil can be trusted.  An altered signal will not yield information about the original conditions, and this can lead to erroneous conclusions being made.

    It would be wrong to take isotope values out of their context.  Paleontological and sedimentological evidence cannot be ignored when interpreting isotopic values.  Strontium seawater dating techniques are fascinating, but it would be easy to make errors in dating if one analysed a sample that had been altered very slightly.  Biostratigraphy is still a reliable dating method for much of the Phanerozoic, and a good index fossil can yield an age within a million years.

     For isotopic methods of analysis to be useful one must first have a trustworthy original isotopic signature, and secondly one must know all of the factors which could affect an original isotopic signature.  All things being said isotopic methods are a very welcome and useful tool for paleoscientists, but one still needs to be critical and possibly a little skeptical now and then.

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