SULFUR ISOTOPE RECORDS OF WATER-COLUMN ANOXIA: A CRITICAL EVALUATION OF PALEOREDOX POTENTIAL

Data from modern anoxic-sulfidic (euxinic) marine basins, such as the Black Sea and Cariaco Basin, reveal that extensive (syngenetic) pyrite formation in the water column can result in uniform sulfur isotope values for pyrite accumulating in the underlying microlaminated sediment. Furthermore, the large size of the water-column sulfate and sulfide reservoirs results in sulfur isotope ratios that are commonly 50 to 60 permil lower than those of coeval seawater sulfate and are generally coincident with the isotopic composition of water-column dissolved sulfide. Similar observations from the ancient black shale record have been invoked to argue for euxinic paleoredox conditions with largely syngenetic rather than diagenetic pyrite formation. The universal relevance of these observations, however, lies with factors beyond the simple presence or absence of a euxinic water column. More specifically, sites of rapid euxinic deposition bear a larger diagenetic S signature, in part as a consequence of suppressed iron monosulfide to disulfide transformation under the conditions of rapid sediment accumulation. This comparatively large diagenetic contribution with the associated reservoir effects of the pore-water environment can result in bulk pyrite that is isotopically more variable and is enriched by 10 permil or more relative to the coeval, dominantly syngenetic pyrite accumulating in the siliciclastic-starved basin center.

The isotopic character of pyrite forming within euxinic basins can also be affected by the nature of the water-column reservoir. Consequently, settings less open to exchange or globally more limited than the modern Black Sea and Cariaco Basin can show isotopic reservoir effects akin to those of diagenetic pyrite formation. The goal of this presentation is a critical synthesis of S-isotope records from a wide range of modern and ancient euxinic settings. This summary has particular relevance in light of recent models invoking a sulfide-rich but ultimately sulfate-limited Proterozoic ocean.