When organic matter is deposited in sediments, it undergoes a series of reactions that condense and transform the original organic molecules into a denser, more resistant set of compounds that form a material called kerogen: the material from which petroleum is formed. Our present understanding of how the original organic molecules change and crosslink (i.e., polymerize) to form kerogen during burial in sediments is poor. It has been hypothesized that the element sulfur plays a large role in the crosslinking; however, so far there is little direct evidence due to the absence of analytical techniques that can appropriately study the role of sulfur in this process. This research uses a newly developed, novel, analytical technique to measure the sulfur isotopic composition of individual molecular compounds in organic matter. This will allow the identification of the origin of the organic-bound sulfur and has the potential to revolutionize our understanding of the polymerization of natural organic matter, and hence formation of kerogen. Broader impacts of the project include determining fundamental physical and chemical relationships that allow better understanding of the carbon cycle, which has implications for the formation of kerogen and petroleum; building infrastructure for science via the development and refinement of a new and potentially powerful analytical technique and methodology; and graduate student training. It will engage under-represented minority undergraduate students via the California Institute of Technology Minority Undergraduate Research Fellowship program in order to help broaden participation of minority students in state-of-the-art research.
The condensation and polymerization of biologically generated organic matter into kerogen is one of the key steps in the global carbon cycle because it determines whether organic matter will be remineralized (dissolved) or preserved and buried (turned into kerogen). Although sulfur is thought to play an important role in this process by crosslinking organic monomers, the precise mechanistic details of how this happens is unknown. This research uses a newly developed technique that couples gas chromatography with multi-collector inductively-coupled-plasma-mass-spectrometry (ICPMS). This allows analysis of the isotopes of sulfur of individual organic compounds. Using this technique it is possible to document compounds resulting from: (1) the reaction of lipids with hydrogen sulfide (HS-) in sediment porewaters over long timescales, and (2) the reaction of carbohydrates with polysulfide in the euxinic water column on very short timescales. Because these two processes have sulfur isotope (S-34) signatures that differ by almost 30 permil, the two different processes can be easily distinguished. Organic-rich sediments deposited in the Santa Barbara Basin will be analyzed and the results compared to those from sediments in the Cariaco Basin to see if the pilot study in the Cariaco, which is a sulfide-rich water column environment, hold true in more normal organic depocenters. Goals of the research include: (1) understanding how sulfurization, a main process in the cross-linking of organic molecules to form kerogen, works and how it relates to organic carbon preservation (2) seeing if the results from the Cariaco Basin are generalizable to other organic carbon depocenters, (3) examining whether the abundance of disulfide-bound compounds relate to the presence or absence of water column exunia, (4) determining whether disulfide-bound organic sulfur is always enriched in S-34 compared to unbound sulfurized lipids, and (5) seeing if the sulfur isotope values of individual organic sulfur molecules are representative of bulk organic sulfur.
