Iron is known to stimulate surface ocean productivity, as well as intervene with bacterially-mediated processes of organic matter remineralization, during early diagenesis. In this paper, we examine the influence of iron supply on the geochemistry (trace metals, δ 34 S, organic matter) of sedimentary rocks deposited in a clastic-dominated marine ramp environment. To this end, we studied two Late Jurassic formations of the Boulonnais area (North-France). Both formations were deposited under quite similar conditions, but they differ in the reactive-iron supply they received. Only one of the two formations was affected by the particulate iron shuttle process. Our results indicate that 1) the iron shuttle may be recorded through concomitant enrichments in P, Mo, As and Sb; 2) a limited reactive-iron supply will allow the sulfurization of organic matter, even in a context of moderate productivity. Thus sulfurization can be a factor favoring a noticeable accumulation of organic matter: iron may thus be an important agent in the C cycle.
IntroductionThe geological rocks of the Late Jurassic times (Kimmeridgian-Tithonian), cropping out along the Boulonnais shore (Strait of Dover, Northern France; Fig. 1), represent a proximal, lateral equivalent of the Kimmeridge Clay Formation (when used with a proper name, the word Formation will be abbreviated as Fm.), famous as a major petroleum source rock. These formations accumulated in a clastic dominated ramp environment where the sedimentary succession is made up of an alternation of marlstone-dominated formations (called "Argiles") and sandstone-dominated formations (termed "Grès") reflecting a range of water depths from lower offshore to shoreface settings, respectively (e.g., Wignall, 1991;