THE INFLUENCE OF BOTTOM WATER OXYGENATION AND REACTIVE IRON CONTENT ON SULFUR INCORPORATION INTO BITUMENS FROM JURASSIC MARINE SHALES

Sulfur isotope studies of bitumens (dichloromethane-extractable) from Jurassic marine shales indicate that most sulfur is derived, at least initially, from diagenetic sulfate reduction. Under euxinic or semi-eu xinic conditions sulfur incorporation in the jet Rock and Posidoniensc hiefer bitumens appears to post-date formation of most syngenetic pyri te but occurs contemporaneously with the formation of small concentrat ions of later pyrite, relatively depleted in S-32. The most reactive i ron minerals were mainly pyritized before any significant incorporatio n of sulfur occurred, and subsequent sulfidation reactions only occurr ed in response to prolonged contact between dissolved sulfide and resi dual, poorly reactive iron minerals (forming the later pyrite) and org anic matter (incorporating sulfur into bitumen). In these circumstance s, which are typical of most euxinic or semi-euxinic sediments, the ab undance of reactive iron is not the main control on the incorporation of sulfur (0.8 wt percent in the jet Rock, 1.5 wt percent in the Posid onienschiefer) into bitumen. Consistent with this, variable concentrat ions of bitumen S occur in the Alum Shales (2.48 and 0.86 wt percent) which have uniform reactive iron contents. In the Alum Shales, the wea kly bioturbated and more oxygenated depositional environments appear t o favor sulfur incorporation into bitumens, which may occur here via p artially oxidized sulfur species. Bitumen compositions suggest that ox ygen is eliminated as sulfur is incorporated, possibly due to the dehy dration of polar compounds followed by nucleophilic addition of sulfur .