AbstractsEarth & Environmental Science

Sulphur isotope fractionation in modern and ancient sediments: field and laboratory experiments

by Thi Hao Bui




Institution: McGill University
Department: Department of Earth and Planetary Sciences
Degree: PhD
Year: 2015
Keywords: Earth Sciences - Biogeochemistry
Record ID: 2062003
Full text PDF: http://digitool.library.mcgill.ca/thesisfile130562.pdf


Abstract

Modern and ancient sediments are complex biogeochemical systems where multiple sulphur isotopes may be useful in tracing previously unseen processes. Multiple sulphur isotope signatures of pore water sulphate from methane-rich marine sediments indicate different microbial pathways involved in the overall sulphur cycling. Particularly in chapter 2, we found that sulphate reduction is the only microbial process controlling the sulphur cycling at station 12 but at station 5 up to 60% of sulphide produced from sulphate reduction re-oxidizes back to sulphate. We are only able to tell this difference with multiple sulphur isotopes. (Chapter 2- Hidden sulphur cycle stimulates the microbial methane biofilter in deep marine sediments)Beside biological processes, transport of material may cause isotope fractionation. We analyze multiple sulphur isotope composition of dissolved sulphate and sulphide when these ions diffused through an acrylamide gel column. The experimental results showed that the diffusion-associated isotope fractionation of ion sulphate is insignificant within the study size (~20 cm length) but the diffusion-associated isotope fractionation of ion sulphide is clearly observable (34α = 0.9990±0.0005). With that fractionation factor, when sulphide diffuses 1 meter away from the source, it is at least 10‰ lighter than the original isotope composition. (Chapter 3 - Sulphur isotope effects of 〖"SO" 〗_4^(2-) and 〖"HS" 〗_^- diffusion in water)In chapter 4, we apply multiple sulphur isotope techniques to study the sulphur cycling during the end-Permian mass extinction. Minor sulphur isotope signature suggests the mixing of at least two distinct sulphur sources during that time period. We have proposed possible sulphur sources even though further studies are required in order to identify exactly which sources contributed to the sulphur enrichment at Permian-Triassic period (Chapter 4 - Sulphur and carbon isotope records across the terrestrial Permian-Triassic (P-T) boundary). Les signatures isotopiques du soufre, dans les sédiments marins riches en méthane indiquent que différents processus microbiens jouent un rôle important dans le cycle global du soufre. En particulier dans le chapitre 2, nous avons constaté que la réduction de sulfate est le seul processus microbien qui contrôle le cycle de soufre à la station 12, mais à la station 5 à 60% du soufre réduit est ré-oxydé en sulphate in-situ. Cette conclusion est seulement possible grâce à la mesure de plusieurs isotopes du soufre (Chapitre 2 - Hidden sulphur cycle stimulates the microbial methane biofilter in deep marine sediments).À part les processus biologiques, le simple transport par diffusion peut aussi causer un fractionnement isotopique. La composition isotopique du soufre de sulfate et de sulfure dissous varie lorsque ces ions diffusent à travers une colonne de gel d'acrylamide. Les résultats expérimentaux ont montré que le fractionnement isotopique associé à la diffusion d'ion sulfate est négligeable à l'intérieur de la plage d'étude (~20 cm de…