Hi Gerry,
 
This is indeed a difficult matter and Iím not sure that isotopes could differentiate between abiotic or biotic processes in this case. I would ask the following questions before proceeding with any isotopic measurements:
1.                  Have you cut a thin section of the material to see what the relationships are among the grains?
 
2.                  Have you done CL work to identify small scale zoning processes?
 
3.                  Has the sample been examined for fluid inclusions?
 
4.                  What exactly is the chemical composition and how is it determined?
 
5.                  Have you examined the mineralogy using electro-microprobe techniques?
 
Until these have been done, I would be reluctant to do any isotopic work because you would never know what it is you are analyzing.  If you have more information that includes some of the above, please let me know and we will see what we can do.  Thanks for consulting us and keep us in mind.
Cheers,
Kurt



At 06:26 PM 7/23/2004, you wrote:

Hi,

We were referred to this list server by Prof. Robert Clayton (Univ. of Chicago), since this is not our field.

We wonder if we can trouble you with a question on a rock specimen we have analyzed recently.

A few of us in the analytical group at BP research in Naperville IL have characterized a rock specimen from the Kobuk desert taken by Mr Ray Pawley formerly of the Brookfield Zoo. The rock specimens were taken from part of the vast Kobuk desert in northern Alaska. This sample appears to be an aggregate of silica (sand) and primarily calcium carbonate which is intergranular and acting as a cementum.

The deposits were observed to be stalactite-like formations penetrating the sand surface of the desert. The origin of these formations were of interest and x-ray diffraction and x-ray photoelectron spectroscopy analyses revealed the presence of the aggregate nature of the silica-calcium carbonate cementum. However, XPS observed several thousand ppm of a nitrogenous compound which was consistent with an organic species. The surrounding silica sand had very little detectable nitrogen species.

Mr. Pawley hypothesized that the origins could be related to biological activity such as migrating caribou or the like as a result of urination or other biological activity.  C-14 dating (LLNL) performed on CO2 formed by acid digestion of this specimen has shown that the carbon-14 of the calcium carbonate fraction of the aggregate dates from ~ 9.5 kyrs ago.  It is known that urine samples of mammals contain (C, N, O, Ca, Cl, Na, K, Si and S, among other cations).

The reason that we are e-mailing you is to ask if isotopic abundance measurements which is your specialty might be of some value in approaching this problem. Previous studies on chlorine-36 in fossil rat urine(1) has shown that 36-Cl/Cl ratio could be used to track temporal patterns dating back ~ 38,000 years before the present (as measured by 14-C).  According to ref. 1, the 36-Cl/Cl ratio shows a discontinuity around 12,000 years before the present with a ratio between 125 %- 200% of modern pre-bomb values whereas during the past 10ky the ratio ranged from 50-100% of modern values. 

Since we appreciate this is a difficult matter and the cementum could have abiogenic origins, we wonder if isotopic abundance can be employed to ascertain if this rock specimen cementum is abiogenic or biogenic in origin in general.  We have ample specimens of the rock for analysis, if there is any interest.

All helpful comments and/or offers of assistance would be greatly appreciated by us.  Please reply off list.

Sincerely

Gerry Zajac
BP research
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