Isotope geochemistry of organic sulfur compounds with links to biogeochemical sulfur cycling and radical chemistry

TLDR: Oduro et al. as mentioned in this paper reported one of the first sulfur isotope constraints on the factors that control the expression on the S-isotope effects of VOSCs and their natural precursors.

Abstract: Title of Document: ISOTOPE GEOCHEMISTRY OF ORGANIC SULFUR COMPOUNDS WITH LINKS TO BIOGEOCHEMICAL SULFUR CYCLING AND RADICAL CHEMISTRY Harry D. Oduro, Ph.D. 2012 Directed By: Professor James Farquhar, Department of Geology and Earth System Science Interdisciplinary Center (ESSIC) Research on volatile organic sulfur compounds (VOSCs) such as dimethylsulfide (DMS), methanethiol (MT), carbonylsulfide (OCS), dimethyl disulfide (DMDS), and carbon disulfide (CS2) from aquatic environments has focused on the production and flux of DMS from the oceans into the atmosphere. In contrast, the biogeochemical connections between the atmosphere and the major reservoirs of VOSC species in freshwater, estuarine, wetlands and coastal marine environments are poorly understood. This thesis reports one of the first sulfur isotope constraints on the factors that control the expression on the S-isotope effects of VOSCs and their natural precursors. It describes ties to their formation, connections with inorganic and microbial processes, and chemical reactions that link the various productions of VOSCs in natural environments. Results from the four field sites studied in this research – Two Pacific Northwest Islands in the Washington State, the York River Estuary in Virginia, Fayetteville Green Lake in New York, and the Delaware Great Marsh – have demonstrated several strikingly different pathways for VOSCs production. In the Pacific Northwest Islands and York River Estuary, DMSP produced by marine algae and phytoplankton have δS values of +18.5 ‰ to +19.2 ‰, and ΔS and ΔS nearly similar to seawater sulfate. These values are slightly S-depleted relative to seawater sulfate. This observation is consistent with the origin of sulfur in DMSP being related to assimilatory pathways of sulfate. Analyses of VOSCs from Fayetteville Green Lake, a stratified freshwater system and the Delaware Great Marsh yield different δS, ΔS, and ΔS values of total VOSCs (consisting of MT, DMS, CS2, and DMDS) that are similar to but slightly Senriched relative to the compositions of coexisting sulfide produced via bacterial sulfate reduction (negative δS and ΔS, and positive ΔS) and reflect organic matter sulfurization pathways in addition to assimilatory sulfate pathways. Extension of chemical protocols to thermochemical sulfate reduction (TSR) process using a simple amino yielded sulfur radical adducts with uncompensated electron spins and S isotope enrichment of up to 13‰. These enrichments are hypothesized to originate from reactions involving sulfur radicals generated by thiyl-mediated thermolysis reaction via sulfur ion-radical pair mechanisms leading to the manifestation of magnetic isotope effect (MIE). ISOTOPE GEOCHEMISTRY OF ORGANIC SULFUR COMPOUNDS WITH LINKS TO BIOGEOCHEMICAL SULFUR CYCLING AND RADICAL CHEMISTRY