ANOMALOUS ACCUMULATION OF ACID-VOLATILE SULFIDES (AVS) IN A COASTAL MARINE SEDIMENT, SAGUENAY FJORD, CANADA

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:FeS2-S 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, S-0, reactive iron (Fe-HCl), C-ORG, 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 (Sigma H2S) in the porewaters as a result of AVS-S precipitation. In the absence of a strong Sigma H2S gradient, slow diffusion to the redox boundary and limited formation of soluble intermediate reduced sulphur species may hinder the conversion of AVS to pyrite.