Citation
Torres Pérez-Hidalgo, Trinidad José
(2010).
Internal features, mineralogy and geochemistry of ferromanganese nodules from the Gulf of Cadiz: the role of the Mediterranean Outflow Water undercurrent.
"Journal of marine systems", v. 80
(n. 3-4);
pp. 203-218.
ISSN 0924-7963.
Abstract
A large suite of Fe–Mn nodules (561 samples) were recovered during the Anastasya 2001 cruise (TASYO
project) along the continental margin of the Gulf of Cadiz (Eastern Central Atlantic), at the confluence of the
Mediterranean Sea with the Atlantic Ocean, where extensive nodule fields were discovered. Based on wide
previous studies that included swath bathymetry, multi-channel and very high-resolution seismic reflection,
gravimetry, magnetism, heat flow probes and underwater photography surveys, nodules were collected at
water depths ranging from 850 to 1000 m, associated with hydrocarbon-derived Mg-calcite, ankerite and
dolomite chimneys and crusts. Forty-six selected samples among the various morphological types were used
for the laboratory analysis of physical properties (morphology, color, surface texture, sphericity, weight and
size), mineralogy (XRD, optical and electronic microscopy), geochemistry (XRF, AAS, ICP-MS, ICP-AES, EPMA,
and GC-MS) and stable isotopes. The nodules show a wide range of sizes, densities, weights and
morphologies. They are formed by multiple millimeter-thick layers of Fe and Mn oxyhydroxides surrounding
the nucleus composed of Early–Middle Miocene plastic marl and sediment, which were derived from
underlying units by fluid venting. Massive, laminated, detrital and mottled to dendritic textural features
were developed by the Fe and Mn oxyhydroxide layers. The main components are Goethite, lepidocrocite,
Mn oxides (7 Å manganates and 10 Å manganates), quartz, and phyllosilicates. Accessory minerals are
calcite, dolomite, siderite, rhodochrosite, kutnahorite, pyrite, chalcopyrite, potassium feldspar, zircon, rutile,
ilmenite and chlorite. Fe–Mn carbonates from the siderite–rhodochrosite continuous series are the principal
constituent of the nuclei. Framboidal, filamentous and globular textures are observed in Fe–Mn oxides and
pyrite, suggesting biogenic origin. The nodules show a high mean abundance of Fe (38.6%), moderate Mn
(6.0%) and low contents of trace metals and REEs compared to the average content of deep-seabed
polymetallic nodules from other oceanic areas. The Mn/Fe ratio ranges from 0.07 to 0.25. The studied nodules
hold hydrocarbons (n-alkanes) derived from marine bacterial activity in their oxide layers, with the presence
of aromatic hydrocarbons as phenanthrene as well, characteristic of mature petroleum. The structure,
mineralogy and chemical composition in the studied nodules are more similar to those of diagenetic–
hydrogenetic continental margin nodules rather than deep-sea nodules. We suggest that the formation of
this type of nodule could respond to a combined diagenetic–hydrogenetic growth process, where the fluid
venting from deep-seated hydrocarbon reservoirs, the bio-mineralization processes and the bottom currents
erosion and chemistry could have played an important role.