Sulfur isotopic analysis of individual organosulfur compounds from Cariaco Basin

Organic matter sulfurization reactions in marine sediments influence global cycles of carbon, oxygen, and sulfur. We investigated sulfurization processes by exploiting the sulfur isotopic gradient generated by microbial sulfate reduction. A novel analytical method, coupled GC-ICPMS, provides a previously inaccessible level of information about the controls, mechanisms, and sulfur sources for organic sulfur compound (OSC) formation. Lipids were extracted from eight depths in a six-meter-long sediment core from the Cariaco Basin. The polar fractions of these complex extracts contained individual OSC with δ34S values ranging from -37.0 to -20.9‰. The average standard deviation for triplicate analyses was approximately 1.5‰. Organosulfur compounds are more isotopically depleted than coexisting sulfide and represent the most depleted component of the Cariaco sulfur system yet observed. Extreme 34S depletions may record significant syngenetic OSC formation in a zone of intensive sulfur recycling near either the sediment-water interface or in the water column. OSC δ34S trends with depth indicate the onset of sulfurization between 140 and 265 cm; apparent diagenetic OSC formation continues to at least 535 cm, beyond the previously described sulfurization window for some non-polar compounds. Diverse δ34S values for OSC also provide important constraints on the diagenetic histories of different compounds and may record reaction with distinct inorganic sulfur pools. These results will be presented in the context of available inorganic and organic geochemical data to describe the processes of organic sulfurization in marine environments.