Dear Ethan,
Thanks for your comments! I agree that the oxygen isotope fractionation
effect for aragonite is probably very close to the value found by Tarutani
et al. (1969).
In our studies of coralline sponges ("sclerosponges") we also found some
confirmation for a similar effect on high Mg calcite (HMC). We compared
oxygen isotopes of a "HMC sponge" and an "argonite sponge" from two
neighbouring reefs in the Coral Sea (Great Barrier Reef). The HMC has about
15 mol% of MgCO3. Both sponges show similar d18O values: -0.8 for HMC and
-0.9 for aragonite (permil VPDB). Equilibrium low Mg calcite would have a
d18O of about -1.7 permil (T=25, d18Owater=+0.35).
Our HMC offset agrees quite well with the value given by Tarutani et al.
(1969). For 15% Mg it should be +0.9 permil in d18O.
Unfortunately our data are not that conclusive for carbon isotopes. The
difference between HMC and aragonite is 1.3 permil, while the offset
between aragonite and LMC found by Romanek et al. (1992) is 1.7 permil. Our
value would agree with the offset between LMC and HMC found by Gonzalez &
Lohmann (1985) (difference of 0.4 permil). However, there also may be vital
effects or local differences in the d13C of the dissolved carbon
contributing to our measured values. Therefore, I still wonder, if the HMC
offset in d13C is real and also present in inorganic equilibrium systems.
Refs.:
Gonzalez, L.A., Lohmann, K.C. (1985): Carbon and oxygen isotopic
composition of Holocene reefal carbonates.- Geology, 13, 811-814
Romanek, C. S., Grossman, E. L., Morse, J. W. (1992): Carbon isotopic
fractionation in synthetic aragonite and calcite: Effects of temperature
and precipitation rate.- Geochim. Cosmochim. Acta, 56, 419-430
Tarutani, T., Clayton, R. N., Mayeda, T. K. (1969): The effect of
polymorphism and magnesium substitution on oxygen isotope fractionation
between calcium carbonate and water.- Geochim. Cosmochim. Acta, 33, 987-996
At 15:04 14.8.97 -0500, Ethan L. Grossman wrote:
>We have done some work examining isotopic fractionation in biotic and
>abiotic calcite and aragonite. The Tarutani et al. (1969) for oxygen
>isotopes in aragonite agrees well with data from aragonitic foraminifera
>and mollusks (Grossman and Ku, 1986). You are right about the effect of Mg
>content on d18O. I have not seen convincing confirmation of 18O enrichment
>in high Mg calcite (HMC). 18O data for Miliolina (HMC forams, >10% mol%)
>show a tendency for lower-than-equilibrium values except for deep-water
>(<1000 m) specimens (see Grossman, 1987 for review). Interestingly, these
>higher-than-equilibrium cold-water specimens tend to be low in Mg content
>(<10 mol%)(Blackmon and Todd, 1959).
>
>With regard to carbon isotopes, the Emrich et al. (1970) study was hampered
>by the coprecipitation of calcite and aragonite. Naturally, their 25 C
>enrichment factor (1.85o/oo) falls between those for calcite (0.9 -
>1.0o/oo) and aragonite (2.7o/oo)(Rubinson and Clayton, 1969; Romanek et
>al., 1992). As for 13C in HMC, Chris Romanek performed some precipiation
>experiments. You might want to talk to him about it.
>
>
>Department of Geology & Geophysics (409) 845-0637
>Texas A&M University fax (409) 845-6162
>College Station, TX 77843-3115
>Web page: geoweb.tamu.edu/faculty/grossman
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GEOMAR
Forschungszentrum für Marine Geowissenschaften
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email: [log in to unmask]
http://www.geomar.de/personal/fboehm/
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