Iron availability as a dominant control on the primary composition and diagenetic overprint of organic-matter-rich rocks

•We study mudstone geochemistry in marine ramp setting.•Enrichment in Fe, P, Mo, As and Sb, excluding U may be a proxy to iron shuttle.•Limitation in reactive iron increases sulfurization-induced protection of OM.•It may be also the case for marine environments with no strong productivity•Iron supply is a key factor for (in-)organic geochemistry of margin sediments.

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, δ34S, 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.