33S

33-Sulphur, the colloquial name for the atom with 16 protons and 17 neutrons in its nucleus, is one of science’s more interesting characters. Many elements have multiple isotopes. Sulphur, for example, has four (mass 32,33,34,36). And while isotopes are chemically similar to each other, they can be separated to some degree in biological of physical systems. These effects produce what is called mass-dependent fractionation, where the degree of enrichment (or depletion) of the various isotopes is a function of their mass.

Mass dependent fractionation of various isotopes is used for everything from making nuclear bombs to predicting the mass of ice-age glaciers. In the case of Sulphur, it is generally measured by comparing the ratio of ³²S (the most abundant isotope) to ³⁴S (the second most abundant). Calculations show that the fractionation of ³³S from ³²S should be about half of the fractionation of ³⁴S from ³²S. Since ³³S is about 8 times less abundant, and the signal is expected to be half the size, measuring it seems like a silly think to do.

That all changed in 2000 when Farquhar et al. showed that Archean sulphides showed a ³³S anomaly that was not consistent with mass-dependent fractionation. This anomaly is expressed as Δ³³S, the difference between the expected value (based on extrapolation from the measured ³⁴S/³²S ratio, and the measured ³³S/³²S ratio, and is generally only present in archean sedimentary rocks.

One of the few processes that produces mass-independent fractionation (MIF) is photolysis of sulphurous gasses by UV radiation (the exact mechanism is still being researched). So the presence of these anomalous ratios in early sediments is generally thought to be an indicator that there was no ozone or other UV-blocking layer in the atmosphere at that time, and there were enough atmospheric suphur-bearing compounds to be dissociated by that radiation.

In 2003 Mojzsis et al. did some classy ion probe work (using a non-SHRIMP instrument) on in-situ sulphide grains so show where in the rock the anomalous D³³S was located. Our approach was somewhat different.

My hypothesis was that the Monica Lewinsky scandal of the late 1990’s caused enough hyperventilation to such all the oxygen out of the atmosphere and return the surface conditions of Earth to those of the late Archean. So I chose to test this hypothesis my analyzing sulphide minerals grown in the 1998-2000 time period, to see if a non-zero Δ³³S was present. We are breathlessly awaiting the results...