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Abstract

The Black Sea offers the opportunity to study the fundamental processes which have to be considered in the interpretation of the modern and fossil sedimentary isotope record. We analyzed Holocene/Pleistocene sediments of the western Black Sea (cruise M51/4 of RV METEOR in 12/2001) which were deposited both below an oxic and anoxic water column for the abundance and stable S isotopic composition of different dissolved and solid species. Dissolved sulfate and sulfide reflect in-situ microbial isotope discrimination and in deeper limnic sediments transport processes. Activity of sulfate reducing bacteria in surface sediments and the zone of anaerobic methane oxidation was analyzed using radiolabeled sulfate. In-situ isotope fractionation between sulfate and sulfide in surface sediments (upper 15 cmbsf) on a transect ranging from 80 to 2200 m water depth showed isotope discrimination up to 60‰ similar to water column results. Decreased values (about 44 ± 4‰) are associated with turbidites. S and O isotope fractionation in anoxic brackish sediments is dominated by microbial sulfate reduction, which is influenced by several factors, such as the microbial SRR which is controlled by the availability of reactive organic matter, sedimentation rate, bottom water conditions, and probably the microbial community structure. Additional factors at the chemocline and in surface sediments under oxic bottom waters may include the metabolism of intermediate sulfur species via disproportionation processes. In limnic sediments counter diffusion processes (sulfate – methane and sulfide - iron(II)) and in-situ microbial activity due to anaerobic methane oxidation lead to an overprint of the original S isotope signals by extremely heavy sulfide, leading to the precipitation of iron sulfides with characteristic d34S values up to +40‰.