A magnetite-driven cryptic iron cycle

Iron is essential for the proper functioning of the entire production chain of marine biomass and of the ocean’s food web. However, its biogeochemical behavior often makes it a limiting factor in ocean functioning. On a geological timescale, the initially reactive-iron reservoir is most often stored as iron sulfides in the sedimentary record. This study focuses on episodes occurring during the earliest stages of diagenesis. It shows that there is a cycle of reversible transformations of iron states before the situation becomes fixed by the formation of iron sulfides, the most emblematic of which is pyrite. The material studied here is an alternation of diagenetic limestone beds and marly interbeds of Tithonian age, observed along the cliffs of the Boulonnais region (Strait of Dover, France), and known as the Assise de Croï Formation. The early, authigenic carbonates of the limestone beds trapped iron-bearing, authigenic minerals, notably magnetite. This made visible the iron cycle, which can be described as cryptic because it goes unnoticed if nothing reveals it. This “fossilization” of the early stages of iron distribution, through the precipitation of diagenetic limestone, allows for a more refined understanding of the carbon cycle, particularly in its shallow marine compartment: indeed, the cryptic iron cycle actively participates in the remineralization of fragile (labile) organic matter.