Heavy sedimentary sulfur isotopes as indicators of super-anoxic bottom-water conditions

Abstract

Iron monosulfides and pyrite (total Cr-reducible S—TRS) in the bottom sediments of the exceptionally sulfide-rich euxinic fjord, Framvaren, south Norway, are unusually enriched in 34 S. Most of these TRSs probably form in bottom waters, where reactive detrital iron phases encounter isotopically heavy dissolved sulfide. Enrichment in 34 S in Framvaren is a result of intense bacterial reduction of a sulfate reservoir whose exchange with the open sea is limited by shallow sills and a low-salinity surface-water layer. 34 S-enriched TRS in the deposits of ancient euxinic environments is commonly interpreted as evidence for early diagenetic sulfide formation where residual, less reactive iron phases were in contact with isotopically heavy dissolved sulfide (sulfate-depleted pore waters) for prolonged periods during burial. However, the Framvaren data indicate that isotopically heavy pyrite may form in relatively shallow (80-180 m), super-anoxic bottom-water pools. In contrast, the presence of isotopically light pyrite does not indicate an absence of restricted conditions, but only that most iron sulfide formation was completed before 34 S-enriched sulfide could be encountered in the water column.