Sulfur distribution and isotopic composition in peats from the Okefenokee Swamp, Georgia and the Everglades, Florida

Abstract

Peat cores from three aquatic environments (freshwater, brackish and marine have been analyzed for organic, pyritic and sulfatic sulfur contents and isotope ratios. Peat collected from each environment has a unique combination of sulfur content, distribution and isotope ratio. Freshwater peat from the Okefenokee Swamp contains the least sulfur (0.26 to 0.48%, primarily organic sulfur) and is vertically uniform in sulfur content and isotope ratio. The organic sulfur isotope ratio of the Okefenokee pear (+ 12.3 to + 12.8 per mil) is similar to that of sulfate in associated surface waters. Cores from presently brackish and marine peats from the Florida Everglades have high organic, pyritic and sulfatic sulfur contents with vertical variability in sulfur content and isotope ration. The isotope ratio of organic sulfur in the brackish and marine peat is lighter than that of sulfate associated with their respective peat-forming environments. Organic sulfur in the marine peat is isotopically lighter than in the brackish peat. Freshwater peats subsequently overlain by sediment associated with brackish to marine waters may have a sulfur content, distribution and isotopic ratio indistinguishable from peats derived under brackish or marine conditions. The primary source of sulfur in peats is dissolved sulfate associated with the peat-forming system. The sulfate concentration of the waters in which a peat forms is of major importance in determining the isotopic composition of the sulfur form found in peat. The content, distribution and isotopic ratio of low and high sulfur peats are similar to those reported for coals, indicating that the fundamental sulfur geochemistry of coals may have been established during the peat-forming stage of coalification. The organic sulfur content and isotope ratio of a coal may be useful in estimating the relative sulfate ion concentration of the waters present during the peat-forming stage of coalification where transgressive sequences are not involved. The sulfate ion concentration of paleowaters could be used to interpret the paleoenvironment during the peat-forming stage of coalification and thus, information about the organic sulfur content and isotope ratio of coals may prove useful in helping to assess coal quality in advance of mining.