HEAVY SEDIMENTARY SULFUR ISOTOPES AS INDICATORS OF SUPER-ANOXIC BOTTOM-WATER CONDITIONS

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 S-34. Most of these TRSs proba bly form in bottom waters, where reactive detrital iron phases encount er isotopically heavy dissolved sulfide. Enrichment in S-34 in Framvar en is a result of intense bacterial reduction of a sulfate reservoir w hose exchange with the open sea is limited by shallow sills and a low- salinity surface-water layer. S-34-enriChed TRS in the deposits of anc ient euxinic environments is commonly interpreted as evidence for earl y diagenetic sulfide formation where residual, less reactive iron phas es were in contact with isotopically heavy dissolved sulfide (sulfate- depleted pore waters) for prolonged periods during burial. However, th e Framvaren data indicate that isotopically heavy pyrite may form in r elatively shallow (80-180 m), super-anoxic bottom-water pools. In cont rast, the presence of isotopically light pyrite does not indicate an a bsence of restricted conditions, but only that most iron sulfide forma tion was completed before S-34-enriched sulfide could be encountered i n the water column.