Dear Max, Zach, Hans and others,

Sorry for the delay in jumping back into the fray, I was fully occupied yesterday.  I was a bit surprised at the flurry of response and at the same time reassured.  Isotope geochemists may nod as they age but they clearly never doze!

The question I was answering for Dr. Wang centers around what is SMOC and how it is defined.  If it is defined by going to the seaside and collecting some seawater and using that as your SMOC scale zero, we have a problem.  As Hans has pointed out, there are a couple of papers in the literature suggesting a maximum d37Cl of +0.94 per mill for ocean waters from the Central Indian ridge (Shirodkar, 2003) and a minimum value of -0.15 per mill (Kaufmann et al., 1988).  The conclusion from these published papers is that sea water can display up to 1 per mill variation in its d37Cl.  This was pointed out by Coplen et al. 2002 in their "Compilation of minimum and maximum isotope ratios of selected elements ..."

The d37Cl SMOC scale should not be defined by each geochemist taking their own random sample from off a pier or a small boat in one of the salt or brackish water areas of the world.  It needs to be defined by a universally available, isotopically homogeneous and well characterized material of suitable composition.  Consider the VCDT or VPDB scales for sulfur and carbon.  Neither are defined by people digging out troilite or belemnite tests from a rock now, even though that is how the scales originated.  VPDB is realized by analyzing NBS 19 and assigning it a value of +1.95 per mill while VCDT is realized by analyzing IAEA-S-1 and assigning it a value of -0.3 per mill.  

That brings me to the second part of my reply to Dr. Wang.  SRM 975a (or for that matter ISL 354) would be an excellent material to use to define SMOC (though this hasn't been done yet).  Note that the uncertainty in the absolute ratio of SRM 975a reflects the uncertainty in the calibration using  mixtures derived from pure end member enriched isotopes and represents a few parts in 10^4 total uncertainty.  This however is not the same as isotopic variability.  While the variability and therefore the uncertainty caused by isotopic heterogeneity is confounded with the calibration uncertainty, there is no reason we are aware of to assume that there is measurable isotopic variability in this material. It is suitable and it is available.  

Bob

Author(s): Shirodkar, PV; Xiao, YK; Hai, L
Source: CURRENT SCIENCE   Volume: 85   Issue: 3   Pages: 313-320    
Published: AUG 10 2003

Author(s): Kaufmann, RS; Long, A; Campbell, DJ
Source: AAPG BULLETIN   Volume: 72   Pages: 839-844    
Published: 1988

Author(s): Coplen, TB et al.
Source: WATER-RESOURCES INVESTIGATIONS REPORT 01-4222 Pages: 1-98    
Published: AUG 2002
   

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Hans Eggenkamp
Sent: Tuesday, October 06, 2009 4:19 AM
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] 37Cl/35Cl abundance ratio of SMOC?

Dear Cl isotopists,

I think that the variations in 37/35Cl ratios mentioned by Bob are  
based upon papers by Shirodkar et al.:

Influence of air-sea fluxes on chlorine isotopic composition of ocean  
water: Implications for constancy in delta Cl-37 - A statistical  
inference
Author(s): Shirodkar, PV; Xiao, YK; Sarkar, A, et al.
Source: ENVIRONMENT INTERNATIONAL   Volume: 32   Issue: 2   Pages:  
235-239   Published: FEB 2006

and

Boron and chlorine isotopic signatures of seawater in the Central Indian Ridge
Author(s): Shirodkar, PV; Xiao, YK; Hai, L
Source: CURRENT SCIENCE   Volume: 85   Issue: 3   Pages: 313-320    
Published: AUG 10 2003

I think isotope ratios for both studies have been measured by Y.K. Xiao

Best wishes

Hans

Quoting "Vocke Jr., Robert D." <[log in to unmask]>:

> Dear Dr. Wang,
>
> There is, unfortunately, no absolute 37/35Cl value for SMOC as it is  
>  in itself variable.  Standard Mean Ocean Chloride was thought   
> originally to be homogeneous but recent measurements have shown   
> variations of up to a per mill for ocean water.  Basing the Cl   
> isotopic scale on universally available but variable scale artifact   
> is not a good way to make precise measurements.
>
> I would recommend that you use the NIST SRM 975a. It has an absolute  
>  37/35Cl ratio of 0.31970 +/- 0.00048 and is as isotopically   
> homogeneous as any of the solid isotopic reference materials sold by  
>  NIST.  You can get a copy of the certificate and other information   
> on this SRM at the NIST SRM website:
> https://www-s.nist.gov/srmors/view_detail.cfm?srm=975A
>
> I hope this helps,
>
> Regards,
>
> Bob Vocke
>
> Robert D. Vocke, Jr., Ph.D.
> National Institute of Standards and Technology
> Analytical Chemistry Division
> Inorganic Chemical Metrology Group
> 100 Bureau Drive, MS 8391
> Gaithersburg, MD 20899-8391
> 301.975.4103 (work)
> 301.869.0413 (fax)
> [log in to unmask]
> http://www.cstl.nist.gov/nist839/index.html
>
>
>
> -----Original Message-----
> From: Stable Isotope Geochemistry [mailto:[log in to unmask]]   
> On Behalf Of Yi Wang
> Sent: Friday, October 02, 2009 12:30 PM
> To: [log in to unmask]
> Subject: [ISOGEOCHEM] 37Cl/35Cl abundance ratio of SMOC?
>
> Dear ISOGEOCHEMERS,
> Could anyone help provide the exact 37Cl/35Cl abundance ratio for the
> SMOC standard? I mean a more accurate ratio better than "0.324" itself?
> It is needed to develop a more convenient method for compound-specific
> chlorine isotope analysis on our GC-MSD.
> Thanks a lot,
>
> Yi Wang (Ph.D.)
> Director, Isotope Laboratory
> ZymaX Forensics
> 600 S. Andreasen Drive, suite B
> Escondido, CA 92029
> Email: [log in to unmask]
> Tel: 760-781-3338 ext 43
> Fax: 760-781-3339
> Cell: 609-721-2843
> http://www.zymaxusa.com
>



Dr. H.G.M. Eggenkamp
Centro de Petrologia e Geoquímica
Instituto Suporior Técnico
Universidade Técnica de Lisboa
Avenida Rovisco Pais
1049-001 Lisboa
Portugal
TEL: +351 21 841 9298
FAX: +351 21 840 0806
E-mail: [log in to unmask]