60th Annual Report on Research 2015

Organic sulfur (OS) is the second largest pool of reduced sulfur in sediments. The record of varying organic carbon burial, with its implications for carbon and nutrient sequestration, surface redox balance, and oxidation of the atmosphere potentially depends in large part on details of marine sulfur chemistry. OS is of particular interest due to its impact on petroleum formation and refining and organic carbon accumulation. Understanding how and why OS forms in oils and source rocks is a key question, and the application of compound-specific S isotope analysis – an emerging technique that has just recently been developed - to such questions is likely to be informative.

Sulfurization has been shown to preserve labile organic matter that would ordinarily be rapidly remineralized. For example, reaction with sulfidic ocean waters can potentially sequester carbohydrates in sediments, and this process mostly likely occurred during the deposition of the Jurassic Kimmeridge Clay Formation (Sinninghe Damste et al., 1998; van Dongen et al., 2006). These findings are critically important, because carbohydrates had previously been thought to make only minor contributions to sedimentary organic matter. However, the source of the sulfur remains enigmatic.

Preliminary data applying compound specific S isotope analysis indicate that the sulfur isotope composition of specific organic sulfur compounds (OSC) vary substantially and cover a wide range of values in different environments (Raven et al., 2015). Thus, sedimentary OS must derive from multiple pools of inorganic sulfur with sulfur isotope compositions that vary in time and space. Furthermore, different reaction pathways involving different substrates will impart additional fractionations, leading to a substantial range of sulfur isotope compositions in individual compounds.

This study is designed to (1) determine the sulfur isotope fractionation associated with the sulfurization of dissolved organic matter and (2) investigate an organic rich sedimentary systems to determine whether the sulfur isotope signals expected are preserved in geological samples or overprinted during diagenesis. To accomplish this, we are sulfurizing a suite of compounds, including carbohydrates and natural DOM samples. Sulfurized reaction products will be identified and analyzed for their sulfur isotope composition, as will the inorganic sulfur reactants, in order to determine the sulfur isotope fractionation associated with organic matter sulfurization. We are also extracting and analyzing OSC in the Kimmeridge Clay Formation, and comparing the compound specific sulfur isotope signature of this system to the laboratory-sulfurized samples to confirm whether the OC enrichment observed is indeed caused by sulfurization of carbohydrates in the marine water column.

Progress to date

The sulfurization reactions we are studying require anaerobic conditions, so before we could actually proceed with the sulfurization experiments, we had to assemble an anaerobic chamber, and test it to ensure proper operation. Once that was accomplished, we could begin creating our inorganic sulfur mixtures in order to sulfurize carbohydrates and other compounds using polysulfides. These experiments are now underway.

Second, and much more challenging, was the development of analytical protocols for the S isotopic analysis of individual polysulfides. Inorganic polysulfides are important reactive intermediates in the sulfur cycle, resulting from various processes including the oxidation of hydrogen sulfide, the reaction of hydrogen sulfide and sulfur, and the disproportionation of thiosulfate. Despite their low concentrations in most natural environments, polysulfides likely play a significant role in the sulfurization of organic matter because of their redox reactivity and nucleophilic tendencies. These same attributes have also made determining polysulfide speciation in the natural environment challenging, and as such published measurements of polysulfide species derived from natural samples are limited. Because our ultimate goal is to go from analysis of polysulfides and sulfurized organic sulfur compounds in the laboratory (at high concentration) to the field (at low concentration), we needed to develop the capability to “quench” polysulfide reactions, and derivatize the polysulfides in a manner that makes them amenable to analysis by gas chromatography (GC). GC analysis not only allows for the complete separation of each species (S₂^2- to S₈^2-), but also the concentrations of each at low levels present in environmental samples. Furthermore, it is required due to the method of isotopic analysis we will employ, that of a GC coupled to a multi-collector ICP-MS via a heated transfer line (Amrani et al., 2009). Our approach is based on the derivatization method of Kamyshny et al. (2006) using methyl trifluoromethanesulfonate (methyl triflate) for rapid methylation of polysulfides and the subsequent determination of dimethylpolysulfides using optimized GC techniques originally considered by Heitz et al. (2000). We have improved the methods of polysulfide analysis; however, derivatized polysulfides are only stable for ~2 weeks, so must still be analyzed soon after derivatization if the distribution of individual polysulfides is required.

Finally, we have completed extraction and chromatographic separation of samples from the Kimmeridge Clay Formation. They will be analyzed via off-line pyrolysis in the laboratory of collaborator Dr. B. van Dongen (Univ. Manchester) for organic sulfur compounds present in the polar fractions and kerogen, and then by GC-MC-ICPMS in the laboratory of Dr. A. Sessions at Caltech for the sulfur isotope composition of individual compounds.

Impact of research

The research supported by this ACS-PRF grant represents a significant step forward in our understanding of the sulfurization of organic matter in natural systems. The development of improved methods will facilitate the first ever sulfur isotope analyses of individual polysulfides, which is a key factor in understanding this process. The support provided to two graduate students has allowed them to focus full time on their research into organic and stble isotope biogeochemistry, particularly related to sulfur. References

Amrani, A., A. Sessions, J. Adkins (2009) Analytical Chemistry. 81:9027-9034.

Heitz, A., Kagi, R., and Alexander, R. (2000) Water Science and Technology. 41(4), 271-278.

Kamyshny, A., Jr., Ekeltchik, I., Gun, J., and Lev, O. (2006) Analytical Chemistry. 78:2631-2639

Raven, M., A. Sessions, J. Adkins, T. Lyons, J. Werne (2015) Org. Geochem.  80:53-59.

Sinninghe Damste, J., Kok, M., Koster, J., Schouten, S., 1998. Earth Planet. Sci. Lett. 164:7–13.

Van Dongen, B., S. Schouten, J.S. Sinninghe Damste (2006) Org. Geochem. 37:1052-1073.