Exobiology and Evolutionary Biology

Laboratory Photochemistry Experiments to Identify the Source Reaction for the Archean Sulfur Mass-independent Isotope Effects

PI: Shuhei Ono

Signatures of mass-independent fractionation of sulfur isotopes (S-MIF) are found exclusively in sulfide and sulfate minerals older than 2.4 Giga years ago (Ga). The Archean S-MIF signature disappears from rock records at 2.4 Ga, reflecting a fundamental change in sulfur cycles due to the oxygenation of the early Earth’s atmosphere. The Archean S-MIF likely originated from atmospheric photochemistry driven by high-energy ultraviolet radiation that was unattenuated by oxygen and ozone absorption. Although S-MIF evidence currently places strong constraints on the Archean atmospheric oxygen level, the physical origin and the source reactions for these unique isotope effects are poorly understood. This represents a missing link in Archean geochemistry and limits the potential of this new proxy to further constrain the early Earth’s environment and its coupled biological evolution.
A series of laboratory photochemical experiments are proposed to identify which reaction(s) in the photochemistry of sulfur gases can produce multiple-sulfur isotope fractionation patterns consistent with what is found in the Archean rock record. Three sulfur gases, sulfur dioxide (SO2), carbon disulfide (CS2) and dimethyl sulfide (DMS) will be examined, as they could have played an important role in the Archean atmospheric sulfur cycle. S-MIF has been previously demonstrated in the photochemistry of SO2 and CS2. A series of photochemical experiments will be carried out as a function of light source, pressure, and the addition of an inert gas and/or chemical quencher. Carbonyl sulfide (OCS) is another candidate, but preliminary experiments show only mass-dependent fractionation during its photolysis. A broadband ultraviolet light source (an 150 W Xe arc lamp) will be used with a series of optical filters to isolate various regions of ultraviolet radiation. Three potential S-MIF reactions for SO2 will be evaluated by individually accessing three absorption bands: 1) photodissociation at <220 nm, 2) photoexcitation followed by intersystem crossing at 240 340 nm, and 3) the spin forbidden transition at 340-390 nm. Addition of an inert gas will quench reaction intermediates such as singlet SO2 and S, and will be used to manipulate the reaction channels. The triplet SO2 and S will also be extracted by reactions with hydrocarbons (e.g., acetylene, ethylene). Similar experiments will be carried out for two absorption bands of CS2, 180-210 nm and 290-340 nm, where excitation at shorter UV (~210 nm) leads to photodissociation. Photolysis experiments for DMS will test whether the photochemistry of this biologically important gas could have contributed to the Archean S-MIF. The photochemical reactions will be monitored by optical, gas chromatographic, and mass-spectrometric techniques, and multiple-sulfur isotope ratios will be analyzed by the SF6 isotope ratio mass spectrometer established in the PIs laboratory. Resulting data will be evaluated based upon known kinetic parameters for sulfur gas reactions. The isotope fractionation systematics (d33S/d34S and D36S/D33S) will be compared with the rock records to test 1) which reactions in SO2 photochemistry could have produced S-MIF, and 2) if sulfur gases other than SO2 may have played a significant role in the Archean sulfur cycles. This research effort will bring new insights into the Archean sulfur cycle and its photochemistry, which is strongly coupled to early biological evolution, and also may provide new constraints on the sulfur cycle on the early Mars. Therefore, the proposed research is designed to contribute to the research emphasis of the Exobiology Program: early Evolution of Life and the Biosphere, and is linked to Goal 4 of the Astrobiology Roadmap, understand how past life on Earth interacted with its changing planetary and solar system environment.