A NEW SULFUR ISOTOPE LASER MICROPROBE AND ITS APPLICATION TO BIOMARKERS AND PALEOATMOSPHERE PROXIES

Sulfur isotope geochemistry is important in the study of ore deposits, Half Zantop's chosen research specialty. New applications to biomarker and paleoatmospheric studies will be emphasized instead of ore deposits in this abstract honoring Half and Suzanne.

The use of sulfur isotopes as biomarkers has revealed a narrowing range of variation in d³⁴S with increasing age from Proterozoic to Archean. In contrast, laboratory experiments on microbial cultures show fractionations in d³⁴S of up to 20 - 30 ‰ by organisms that utilize sulfur redox reactions to gain chemical energy. A unique explanation of this disparate behavior has not been achieved. With the discovery of mass-independent S-isotope fractionations in Archean rocks, a new factor in the evolution of sulfur-metabolising organisms now can be investigated: the impact of atmospheric photochemistry.

Processes operating within the biosphere, hydrosphere, and lithosphere produce mass-dependent fractionated sulfur isotope distributions. Reactions between gas molecules in the atmosphere in the presence of ultraviolet light, however, result in mass-independent fractionation. Non-zero values of the quantity D³³S (defined as D³³S=d³³S - 0.515 * d³⁴S) measure the departure of a sample from the mass-dependent fractionation line (MDFL) followed by Phanerozoic samples.

The recent discovery by Farquhar et al. (2000) of mass-independent sulfur isotope fractionations in Archean rocks opened a new window on the history of atmospheric photochemistry. The discovery was called into question, however, because the accuracy of the measurements was challenged.

We have verified Farquhar's analyses by analyzing aliquots of his samples with a new laser fluorination sulfur isotope microprobe. Agreement is within 0.05 to 0.1 ‰ for D³³S. We also validated the existence of mass-independent fractionation by direct analysis of pyrite from Archean bedded chert, Pilbara, W. Australia, in samples provided by Y. Ueno and S. Maruyama.

High precision, high accuracy, analyses for four stable sulfur isotopes, ³²S, ³³S, ³⁴S, ³⁶S, have been made with a new microprobe using ultraviolet (UV) laser fluorination and computer controlled gas chromatography (GC) to purify the working gas, SF₆. Calibration of the microprobe was made by analyzing powdered IAEA and NIST isotope reference materials with a CO₂ laser. Results agree with other laboratories within 0.1 to 1.0 ‰ in d³⁴S over a range of 50 ‰. The slope of the MDFL in a plot of d³³S vs. d³⁴S is 0.515 (R²=0.9999) with D³³S=0.03 (+/- 0.05 ‰) for 110 analyses of 12 minerals.