Anomalous accumulation of acid-volatile sulphides (AVS) in a coastal marine sediment, Saguenay Fjord, Canada

Abstract

Sulphate reduction usually leads to the formation and accumulation of authigenic pyrite (FeS2) in organic-rich coastal marine sediments. Iron monosulphides (FeSx) are most often found as a minor constituent of the reduced sulphur pool in these sediments and are believed to be precursor phases which convert to pyrite during early diagenesis. Abnormally high ratios of acid-volatile sulphides (AVS) to pyrite, however, are found in the sediments of the Saguenay Fjord, Canada. The AVS-S:FeS2S ratio observed throughout the anoxic zone in three sediment cores collected along the main axis of the fjord generally exceeds 1 and reaches values as high as 7 at the head of the fjord. To document and explain this anomaly, solid phase components (AVS, FeS2, S0, reactive iron (FeHCl), CORG, and total N) as well as porewater constituents (SO4, Fe, and Mn) were analyzed. Degree of pyritization (DOP) values were low (0.1–11%) and generally decreased toward the mouth of the fjord. In contrast, degree of sulphidization (DOS) estimates were high and reached values of up to 43% at the head of the fjord. These latter values indicate that pyrite formation is not limited by iron availability but a large excess of reactive iron may in fact inhibit the build-up of reduced sulphur (ΣH2S) in the porewaters as a result of AVS-S precipitation. In the absence of a strong ΣH2S gradient, slow diffusion to the redox boundary and limited formation of soluble intermediate reduced sulphur species may hinder the conversion of AVS to pyrite.