Initial Transgressive Phase of Leg 144 Guyots: Evidence of Extreme Sulfate Reduction

The initial transgressive phase at the Leg 144 Guyots is characterized by a typical association of sedimentary facies (from bottom to top): in situ weathered volcanic rocks; variegated clays, partly pyritic; gray clay, pyritic, homogeneous, or mottled; black clay, peaty, laminated, or bioturbated; and marine argillaceous limestone. Site 877 at Wodejebato Guyot represents the typical development of the initial transgressive phase. The black clay is rich in organic carbon (up to 40%) and sulfur (up to 25%). The organic matter is dominantly of terrestrial origin, but it has a significant marine, algal input. The variegated clays consist of a red, lower, sulfur-free part and a blue to gray-blue, sulfur-containing upper part. Organic carbon is not observed in this facies. The sulfur occurs as pyrite and organically bound sulfur. The isotope composition of pyrite varies from -50‰ to 0%e, which clearly points to bacterial sulfate reduction as the origin for the high sulfur content. Pyrite formation was limited by availability of reactive iron. Because of the origin of the clays as lateritic weathering products, the amount of reactive iron was high, and pyritization proceeded to high values. The following model is suggested for sulfur enrichment in the sediments at the initial transgressive phase: (1) marine flooding of an organic-rich back-reef mangroval swamp; (2) intense bacterial reduction of marine sulfate within the black clay; and (3) downward diffusion of H2S into the underlying clays, reduction of red iron oxides to blue-gray iron sulfides, and growth of pyrite.