• Kenneth M Towe added an answer:
    What are the best methods for determining the extent of alteration in a rock sample prior to isotopic analyses?

    I am undertaking some chemostratigraphic work with carbon and oxygen isotopes in carbonates. I have used basic petrographic techniques and (cold) cathodoluminescence to gauge diagenesis and alteration in my samples, though I am interested to hear about experiences other researchers may have had and what techniques they have used. Cheers!

  • Daniel Garcia added an answer:
    Why is Th an indication of diagenesis?
    After reading around the subject, I found a lot of literature suggesting that Th is an indication of diagenesis and not a factor of province. Why is this?
    Daniel Garcia

    The basis for using Th/U as en environmental fingerprint, is indeed that U is redox-sensitive while Th is almost insoluble.

    Actually, U is easily displaced in early diagenesis, particularly in oxic conditions. But one shoud take care that U is not necessarily removed from the formation; it may be just displaced from an oxidised bed (say a sand flushed by meteroric water) towards the nearby (reduced) shale beds where organic matter is preserved.

    The second difficulty when using Th/U is that Th is not constant at all in sediments. Th content is generally moderate in distal claystone (say 20 ppm), and often very low in coarse sands, but fine grained sediments (say silts) often have a much higher Th content if they contain detrital monazite (from granitic or high grade metamorphic sources). The main control on Th content is the granulometry of the sediment (if siliciclastic, of course), and it depends also very strongly on the source mineralogy.

  • Patricia Zalba added an answer:
    Can anyone give basic informations about Opal-A and Opal CT?
    Those subjects are related with diagenetic BSRs (bottom simulating reflector).
    Patricia Zalba

    Perhaps you can find some useful information for your question in our paper (in this site): Preservation of biognerated mixed facies...Zalba et al, 2010, Table 2. Paragenetic sequence. We propose a diagenetic origin for opal CT drived from opal A in Neoproterozoic sequences of Argentina

  • Abhijit Chakraborty added an answer:
    Are there any methods that offer specific signatures for specific bacteria in the Archean?

    Since the C-Isotopes are not really that helpful, I was wondering if anyone has some suggestions on this. Is it possible to distinguish, let's say, cyanobacteria from anoxygenic photoautotrophs?

    Thanks for the Replies,


    Abhijit Chakraborty

    Thank You very much Dr. Chi Fru. I would like to mail you on more detailed case specific issue of my study area.

  • Andreas Paul added an answer:
    Can anyone give me any pointers on using cathodoluminescence to determine extent of alteration/diagenesis in carbonates?
    I’m keen to use CL as a quick method to identify appropriate samples for isotope analyses. From what I have read, CL is mostly used to analyse very small parts of a sample. However, I would like to see a large-scale effect on whole thin sections (75x50mm) so I can be confident about drilling sample that is as unaltered as possible. Advice?
  • Agnes Tello added an answer:
    Has anyone used 0.1% SDS sonication in diagenode?
    0.1% SDS sonication in diagenode:
    I know 1% SDS works great for diagenode bioruptor. Now I need to use a lower SDS concentration like 0.1% SDS. I haven't tried that yet, but I prefer to know if somebody here use that before.
    Another problem may be a longer time sonication process in 0.1%SDS may cause protein degradation?
    The rationale for 0.1% lysis buffer is to avoid using dilution buffer, which gives a large volume for IP. If anybody has tried any ultracentrifuge to reduce the volume, please tell me.
    Thank you all.
    Agnes Tello
    I have used 0.1% sds in the past and it greatly reduced the sonication efficiency (I had to sonicate for 50 min), however I did not find any problem with doing the ip afterwards (some antibodies won't work if you sonicate in 1%, even though you dilute it out, not sure why this is the case, but I found it to be). So I would try 0.1% if I were you and retitrate the sonication time.
  • Jeff C. Clements added an answer:
    Is there any quantitative evidence regarding the degree of precision in using DIC over pH in predicting carbonate mineral saturation states?
    I'm assessing sedimentary carbonate saturation states with respect to aragonite and was wondering if it may be worthwhile to spend some money on DIC analysis rather than measuring pH for predicting aragonite saturation states? Given that DIC is quite expensive, I was wondering if there is any available literature outlining degrees of precision of the two predictors on which to base a decision?
    Jeff C. Clements
    Much thanks Greg! I've been searching for a while now but could not find anything. Thanks for your input!
  • Bjørn Eske Sørensen added an answer:
    Looking for a software program in free domain that I can use to obtain digital spectra intensities for cathodoluminescence colours I have for my images?
    I have a set of images taken under CL and will like to obtain their digital spectra intensities to see how they vary with the chemical composition of the mineral phases.
    Bjørn Eske Sørensen
    are they hyperspectral images or RGB?

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