Sulfur is an important component of many biogeochemical processes, spanning atmospheric chemistry and aerosol formation, microbial ecology, ocean anoxia and mass extinctions, organic matter preservation, petroleum generation and souring, and the oxygenation of the ancient Earth's oceans and atmosphere. The stable isotopic compositions of sulfur species (i.e., their dS values) can provide considerable insight in these pursuits, because many redox transformations of sulfur induce large and diagnostic isotopic fractionations. These fractionations can be preserved in sulfur-bearing products, where they serve as fingerprints of past processes. In this regard, organic sulfur (OS) is among the richest "but least utilized" reservoirs of S-isotopic information. Until recently there has been no effective way to access the isotopic information recorded by individual OS compounds. The PIs have recently developed a novel GC/ICPMS analytical approach (Amrani et al., 2009) that now allows us to measure d34S in individual OS compounds containing just picomoles of S. They propose to study the progressive sulfurization of organic compounds during sedimentary diagenesis. Three specific questions will be addressed regarding the sulfurization of lipids: i) Do d34S values of individual compounds record assimilatory versus dissimilatory pathways of sulfur incorporation? ii) Are molecular d34S values offset from their inorganic S sources as a result of fractionations? iii) Do different compounds record sulfide and/or sulfate d34S values from different depths?
This proposal will help support the development and deployment of an important new analytical tool, namely compound-specific d34S analysis. The results of their research and the broader potential of the method are likely to impact our understanding of the global carbon cycle, and all of its far-reaching impacts on climate change, because the still poorly understood pathways of sulfurization exert a first-order control on the burial and preservation of organic matter in sediments. The proposal will also support the education of a PhD student at Caltech, who (upon completion) will presumably be the world's expert in compound-specific S isotope analyses and will carry this technique to other labs.
The goal of this project has been to employ a new measurement technique to analyze, for the first time ever, the sulfur isotope composition of individual organosulfur molecules from marine sediments. In doing so, we hoped to learn more about how and where these molecules form (are they biologic? formed in the sediments or water column?). For this initial study, we have focused our work on a set of marine sediments and particles (from sediment traps suspended in the water column) from the Cariaco Basin, a classic stratified and sulfidic basin. Cariaco has been the site of numerous previous investigations of inorganic sulfur cycling, and so serves as a good starting point for our study of organic sulfur. Our major finding has been the realization that -- based on 34S/32S abundance -- there are two major classes of sulfur-containing organic molecules in Cariaco. The first appear to be derived from the addition of H2S to lipids, and carry an isotope ratio similar to that of H2S. The second appears to be formed by the reaction of sugars with a different S source, possibly polysulfides in the water column, and having a different isotopic composition. Thus the inventory of sulfurized organic matter in Cariaco can be explained as a mixture of these two contributions. This has potentially important implications for the marine carbon cycle, because the preservation of organic matter is thought to be closely tied to crosslinking by sulfur. If in fact the reaction of sugar molecules with polysulfide in the water column is an important part of the process, this would help to explain the role of water-column anoxia in organic preservation. The next step will be to look at other locations, to determine whether the mixture observed in Cariaco is typical or extraordinary for marine systems. We hypothesize that systems without water-column sulfide will not contain sulfurized sugars as a major component of sedimentary organic matter.
