『Abstract
　Ferromanganese nodule fields and hardgrounds have recently been discovered in the Cadiz Contourite Channel in the Gulf of Cadiz (850-1000 m). This channel is part of a large contourite depositional system generated by the Mediterranean Outflow Water. Ferromanganese deposits linked to contourites are interesting tools for palaeoenvironmental studies and show an increasing economic interest as potential mineral resources for base and strategic metals. We present a complete characterisation of these deposits based on submarine photographs and geophysical, petrographic, mineralogical and geochemical data. The genesis and growth of ferromanganese deposits, strongly enriched in Fe vs. Mn (av. 39 % vs. 6 %) in this contourite depositional system result from the combination of hydrogenetic and diagenetic processes. The interaction of the Mediterranean Outflow Water with the continental margin has led to the formation of Late Pleistocene-Holocene ferromanganese mineral deposits, in parallel to the evolution of the contourite depositional system triggered by climatic and tectonic events. The diagenetic growth was fuelled by the anaerobic oxidation of thermogenic hydrocarbons (δ¹³CPDB = -20 to -37‰) and organic matter within the channel floor sediments, promoting the formation of Fe-Mn carbonate nodules. High ⁸⁷Sr/⁸⁶Sr isotopic values (up to 0.77993±0.00025) observed in the inner parts of nodules are related to the influence of radiogenic fluids fuelled by deep-seated fluid venting across the fault systems in the diapirs below the Cadiz Contourite Channel. Erosive action of the Mediterranean Outflow Water undercurrent could have exhumed the Fe-Mn carbonate nodules, especially in the glacial periods, when the lower core of the undercurrent was more active in the study area. The growth rate determined by ²³⁰Thexcess/²³²Th was 113±11 mm/Ma, supporting the hypothesis that the growth of the nodules records palaeoenvironmental changes during the last 70 ka. Ca-rich layers in the nodules could point to the interaction between the Mediterranean Outflow Water and the North Atlantic Deep Water during the Heinrich events. Siderite-rhodochrosite nodules exposed to the oxidising seabottom waters were replaced by Fe-Mn oxyhydroxides. Slow hydrogenetic growth of goethite from the seawaters is observed in the outermost parts of the exhumed nodules and hardgrounds, which show imprints of the Mediterranean Outflow Water with low ⁸⁷Sr/⁸⁶Sr isotopic values (down to 0.70693±9.99981). We propose a new genetic and evolutionary model for ferromanganese oxide nodules derived from ferromanganese carbonate nodules formed on continental margins above the carbonate compensation depth and dominated by hydrocarbon seepage structures and strong erosive action of bottom currents. We also compare and discuss the generation of ferromanganese deposits in the Cadiz Contourite Channel with that in other locations and suggest that our model can be applied to ferromanganiferous deposits in other contouritic systems affected by fluid venting.

Keywords: Gulf of Cadiz; Mediterranean Outflow Water; Ferromanganese nodule; Palaeoenvironmental indicators; Fluid venting; Contourite』

1. Introduction
2. Oceanographic and geological setting
3. Materials and methods
　3.1. Oceanographic cruise data
　3.2. Field site and sample suite
　3.3. Laboratory analyses
4. Results
　4.1. Nodule fields: bathymetric, photographic and seismic characterisations
　4.2. Structural and textural features
　4.3. Petrography and mineralogy
　4.4. Geochemistry
　4.5. Growth rates and age
5. Discussion
　5.1. Origin and evolution of ferromanganese deposits in the Cadiz Contourite Channel
　　5.1.1. Stage one: the diagenetic process
　　5.1.2. Stage two: the influence of the Mediterranean Outflow Water and the North Atlantic Deep Water
　5.2. Analogous ferromanganiferous deposits on continental margins
6. Conclusions
Acknowledgements
Appendix A. Supplementary data
References