Sedimentary sulfur and iron chemistry in relation to the formation of Eastern Mediterranean sapropels

Abstract

Detailed analyses of total sulfur, pyritic sulfur, humic sulfur, NaCl-extractable sulfur, elemental sulfur, acid-volatile sulfi de, and organic polysulfide in and around the most recent Sapropel S1 (maximum C org = 2.3%) and two other sapropels recovered during Ocean Drilling Program Leg 160 (maximum Corg = 7.4% and 23.5%), show that the main sulfur species in and immediately below each sapropel is pyrite. Directly above each sapropel sulfur is rarely present in the solid phase, but occurs as po rewater sulfate (SO 4 2‐ ). Microbial SO4 2‐ reduction took place in the sapropels during sapropel formation. Addition of reactive iron to sapropel layers occurred via upward diffusion of Fe 2+ from underlying sediments and/or through water-column iron sulfide precipitation. All reactive iron that was available in the sediments was used for pyrite formation. As a result, diffusion of sulfide out of the sapropels and sulfidization of the sediments underlying each sapropel have occurred. Only in the most C orgrich sapropel did large-scale uptake of reduced sulfur by organic molecules occur, and SO 4 2‐ reduction probably still continues.