Fe-Mn nodules associated with hydrocarbon seeps: A new discovery in the Gulf of Cadiz (eastern central Atlantic)

Abstract

The Gulf of Cadiz is situated geologically at the Gibraltar Arc, the westernmost arc of the Alpine- Himalayan orogenic belt. Based on extensive previous studies that include swath bathymetry, multi-channel and very high-resolution seismic reflection, gravimetry, magnetism, heat flow probes, and underwater photography surveys, more than 500 polymetallic nodules were collected at water depths ranging from 850 to 1000 m, associated with hydrocarbon-derived carbonate chimneys, slabs, and crusts. Nodules show a wide range of sizes, densities, weights and morphologies. Nodules are composed of multiple millimetre- thick layers of Fe and Mn oxyhydroxides surrounding a nucleus composed of Early-Middle Miocene plastic marls, which were expulsed from underlying units by fluid venting. Nodules show a high mean abundance of Fe (39.03%), moderate Mn (5.84%), and low contents of trace metals and REEs compared to the average content of deep-sea polymetallic nodules. They display fast growth rates (av. 2,500 mm Myr-1) which are probably the main cause for the low contents of transition metals. The oxide layers contain both bacterial-derived hydrocarbons and aromatic hydrocarbons such as phenanthrene, characteristic of mature hydrocarbons. We propose both diagenetic and hydrogenous processes for nodule, beneath and on the seabed, as consequence of alternating episodes of burial and exhumation. Diagenetic processes beneath the seabed are fuelled by deep-seated hydrocarbon seeps probably through microbial-mediated anaerobic oxidation of hydrocarbons. On the other hand, hydrogenous nodule growth on the seafloor is controlled by mineral precipitation from the Mediterranean Outflow Waters.