Ž . Journal of Marine Systems 25 2000 299–312 www.elsevier.nlrlocaterjmarsys Biogeochemistry of sulfur in a sediment core from the west-central Baltic Sea: Evidence from stable isotopes and pyrite textures Michael E. Bottcher a, ) , Aivo Lepland b ¨ a Department of Biogeochemistry, Max-Planck-Institute for Marine Microbiology, Celsiusstr. 1, D-28359 Bremen, Germany b Scripps Institution of Oceanography, UniÕersity of California, La Jolla, CA 92093-0220, USA Received 15 January 1999; accepted 18 August 1999 Abstract Ž . The biogeochemistry of the sulfur cycle in a ca. 5-m-long sediment core from the eastern slope 221 m water depth of the Landsort Deep in the west-central Baltic Sea was investigated by analyzing the solid phase records of sulfur isotopes and pyrite textures, besides selected main and minor elements. The sediments were deposited during post-glacial history of the Ž . Ž Baltic Sea when the basin experienced alteration of brackish Yoldia Sea, Littorina Sea and freshwater Baltic Ice Lake, . Ancylus Lake conditions. The stable isotopic composition of total sulfur was analyzed as a function of depth. In selected Ž . Ž . Ž . samples pyrite FeS , greigite Fe S , and barite BaSO fractions were separated for isotope analyses. Pyrite textures 2 3 4 4 were analyzed by SEM and optical microscopy. Microbial reactions associated with the oxidation of organic matter resulted in assemblages of authigenic sulfide minerals which for the post-Ancylus Lake brackish water environment are dominated by pyrite and for freshwater environments by Ž 34 Fe-monosulfides. The sulfur isotopic composition of the brackish water Littorina Sea sediments d S between y40 and . y27‰ vs. V-CDT is believed to be determined by cellular sulfate reduction rates and reactions involving intermediate sulfur species. The availability of reactive iron and decomposable organic matter as well as sedimentation rate and the chemocline position are important variables upon the d 34 S values of sulfides in brackish water environment. The syn-depositional abundance of sulfur and organic matter, and transport of dissolved sulfur species vs. rates of microbial reactions determine d 34 S in the freshwater sediments. The upper part of the Ancylus Lake sediments is sulfidized by downward diffusing H S andror sulfate from overlying brackish water sediments. Minor concretionary barite formation in 2 the freshwater sediments is most likely due to the reaction of pore water sulfate diffusing downward from brackish water sediments with barium desorbed from freshwater sediments. The size distribution of pyrite framboids in the brackish sediments indicates that the formation mainly occurred from anoxic pore waters, although some pyrite formation in an anoxic water column cannot be excluded. q 2000 Elsevier Science B.V. All rights reserved. Keywords: sulfur cycle; biogeochemistry; sulfur isotopes; iron sulfides; barium sulfate; Baltic Sea ) Corresponding author. Tel.: q 49-421-2028-632; fax: q 49-421-2028-580. Ž . Ž . E-mail addresses: mboettch@mpi-bremen.de M.E. Bottcher , alepland@ucsd.edu A. Lepland . ¨ 0924-7963r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0924-7963 00 00023-3