Holocene postglacial lacustrine clays in the northern Baltic Sea were studied for ichnofossils, petrography, and microscale elemental and 34S/32S composition to understand the impact of benthos-produced organic matrices on the early diagenesis of sediments. X-radiographs of these clays display intense burrow mottling with discrete Palaeophycus, ‘Mycellia' and rare Arenicolites. Small burrow-like and irregularly shaped concretions were separated from these clays. The burrow-like concretions are composed of framboidal pyrite and poorly crystalline FeS2 cement. They formed within worm-produced burrows by the decomposition of mucous coatings on the burrow walls by sulphate reduction. δ34SCDT composition of the framboids indicates saline inflows from the North Sea as the sulphate source. The irregular concretions are microcrystalline pyritic masses that formed in pore spaces with less reactive organic compounds. Their δ34SCDT values reflect precipitation from the ambient porewater sulphate by sulphate reduction. The burrow-like and irregular concretions are microcrystalline barite in the lower parts of the clays. ‘Mycellia' are mineral-replaced filaments produced by large sulphur-oxidizing bacteria or mycelial fungi. They concentrate in monosulphide bands in the lower substrate levels, but are associated with organic patches upward. This study demonstrates that substantial amounts of pyrite formed within organic matrices close to the sediment–water interface. This differs from previous studies that proposed a deeper-burial origin and allochthonous sulphur source.
Sulphur isotope data are available at http://www.geolsoc.org.uk/SUP18426.