Answers · Sharp, Principles of Stable Isotope Geochemistry, 1st ed. (2007) · ← Problems
Not printed in the 1st ed.
The abundance of 36S is only 1/40 that of 34S, so that the low counting statistics dramatically decrease the precision and outweigh the benefit of the larger fractionations. (Table 10.1.)
[Worked grid (constructed); the mass-difference factor is 2.00, as the problem states.]
The two major isotopes of sulfur are 32 and 34. Fluorine has a mass of 19. The analyzed masses would be 19 × 6 + 32 = 146 and 19 × 6 + 34 = 148.
Assimilatory sulfate reduction has a small fractionation; dissimilatory sulfate reduction has a large fractionation. (See Section 10.4.)
As light sulfur is removed from the system as sulfide minerals, the remaining sulfur in solution gets heavier with time. Thus the rims of sulfide grains should be very heavy.
Unlike sulfates, which precipitate with nearly the same δ34S value as the dissolved sulfate, sulfides precipitate with a kinetic effect associated with biogenic sulfur reduction. As with carbon, the kinetic effect is not constant.
In the first case, there would be very little effect. The fractionation between evaporites and seawater is small, so that the removal of sulfur in the form of evaporites would have little effect on the remaining dissolved sulfate in the ocean. Shale formation includes the precipitation of sulfides, which strongly incorporate the light isotopes of sulfur. Therefore, sulfate formed during periods in which large shale deposits formed should be characterized by high δ34S values.
Transcribed from Z. Sharp, Principles of Stable Isotope Geochemistry, 1st ed. (Pearson Prentice Hall, 2007), end-of-chapter problem set (published source). Reference-scale names are written VSMOW, VPDB and VCDT where the original used SMOW, PDB and CDT, with the original numerical constants unchanged; historical usages (e.g. Craig's definition of SMOW) are kept as printed.
The source PDF's text layer scrambles mathematics, so equations and some values were reconstructed during transcription. Bracketed notes and anything tagged reconstructed or check vs print are editorial (constructed), not part of the original, and the warn-tagged items should be verified against the print copy.