Ferromanganese nodules and micro-hardgrounds associated with the Cadiz Contourite Channel (NE Atlantic): Palaeoenvironmental records of fluid venting and bottom currents

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 palaeoenviromental 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 0.70993±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±0.00081). 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.

Arrecifes de corales de aguas frías como posibles indicadores de emisiones de hidrocarburos en el Golfo de Cádiz

El presente documento trata de exponer la relación existente entre los corales de aguas frías encontrados en el Golfo de Cádiz (Atlántico NE) con los procesos tectónicos y las estructuras relacionadas con la migración y la emisión de fluidos ricos en hidrocarburos (principalmente metano). Para ello, desde el año 2000 hasta la actualidad, se han llevado a cabo diversas campañas oceanográficas en las cuales se han obtenido datos y muestras relacionadas con las emisiones de hidrocarburos al fondo marino. Gracias a los datos y las muestras recogidas durante las campañas se han creado una base de datos, en la que se reúnen todos los datos significativos relacionados con las campañas oceanográficas y con el objeto de este trabajo. Para finalizar, tras la creación de la base de datos, y con las muestras proporcionadas por el IGME recogidas durante la campaña oceanográfica MVSEIS08, se han podido llevar a cabo diversos estudios de laboratorio como la racemización de aminoácidos, análisis de biomarcadores así como la datación de los esqueletos aragoníticos de los corales de aguas frías mediante la técnica de U-Th en un futuro, con el objetivo de esclarecer la relación existente entre estas emisiones de fluidos a través de diversas estructuras y los corales de aguas frías, su edad, y en última instancia determinar los cambios producidos en la composición química de la masa de agua atlántica así como la variación de las corrientes en el Golfo de Cádiz durante el último periodo glaciar. ABSTRACT This project attempts to explain the relationship between cold-water coral found in the Gulf of Cadiz (NE Atlantic) with tectonic processes and structures related to migration and hydrocarbon-rich fluids emissions (mainly methane). Since 2000 until present, have conducted various oceanographic cruises in which data were obtained and samples related to hydrocarbon emissions to the seabed. Thanks to data and samples collected during the cruises, I have created a database in which all gather meaningful data related to oceanographic and the object of this work. Finally, after the creation of the database, and the samples provided by the IGME collected during the oceanographic cruise MVSEIS08 have been able to carry out laboratory studies of racemization of aminoacids, biomarkers analysis and dating of aragonitic skeletons of cold-water corals in a future by U-Th data , in order to clarify the relationship between these fluids emissions through various structures, ages cold-water corals, and ultimately determine changes in the chemical composition of Atlantic water mass and the change of currents in the Gulf of Cadiz during the last glacial period.