Reed and everyone else:
To add to what Max, Scott, and Steve have written, I performed sample
treatment experiments with foraminifera (Grossman, 1982) and with
aragonitic pteropods and atlantids (Grossman et al., 1986). The results
are summarized below. The data can be accessed as a jpeg file in my web
page:
//geoweb.tamu.edu/faculty/grossman/EGpubs.html
Grossman (1982): Generally, roasting at 450 C had a small effect on the
d13C and d18O of live calcitic foraminifera (about 0.15 o/oo lower)
compared with no treatment. Bleaching and ashing in a low temperature
plasma (Fairbank's lab) did not yield significantly different results
Grossman et al. (1986, Table 2 [on web-site]): Bleaching yielded data
similar to untreated samples. Samples roasted at 470 C (overkill I'm
afraid; 350 C would have sufficed) yielded similar d13C values as bleached
samples, but d18O values were 0.4 +/- 0.3 o/oo lower.
In my dissertation (Grossman, 1982, Table 3 [on web-site]) I also compiled
data from about 10 sources on the effect of sample treatment on isotopic
composition (also reproduced in Wefer, 1985, p. 24-25 [in German]). The
results are varied but generally show that roasting (often done at 450=B0C)
lowers calcite d13C and d18O values 0.1 to 0.2 o/oo. Aragonite d18O values
are lowered on average by about 0.5 o/oo. By the way, that is why Epstein
et al. (1953) found no significant difference in d18O between aragonite and
calcite precipitated under the same conditions.
Bleaching is a good technique when dealing with relatively clean shells
because it does not alter isotopic composition. Thus, fractionation
relations based on abiotic precipitation experiments like those in O'Neil
et al. (1969; revised in Friedman and O'Neil, 1977) and Romanek et al.
(1992) can be used with confidence. Bleaching is not practical with whole
rock samples because there may be a variety of phases with which the bleach
can react (e.g., sulfides).
If you or anyone else is interested, I can send the section of the thesis
or a reprint of the paper. Don't forget to send the request directly to me
([log in to unmask]).
Regards,
Ethan
***************
Selected References
Grossman, E. L., 1982. Stable Isotopes in Live Benthic Foraminifera from
the Southern California Borderland. Ph.D. dissertation, University of
Southern California, Los Angeles, 164 p.
Grossman, E.L., Betzer, P.R., Dudley, W.C., and Dunbar, R.B., 1986. Stable
isotopic variation in pteropods and atlantids from North Pacific sediment
traps. Mar. Micropaleontol., v. 10, p. 9-22.
Wefer, G., 1985. Die verteilung stabiler isotope in kalkschalen mariner
organismen, Geologisches Jahrbuch, Reihe A, Heft 82, p. 1-114.
Department of Geology & Geophysics (409) 845-0637
Texas A&M University fax (409) 845-6162
College Station, TX 77843-3115
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