Origin of carbonate concretions from mud mounds in the Gulf of Cadiz (SW Iberian Peninsula) ; Origen de las concreciones carbonatadas de los montículos de fango en el Golfo de Cádiz (SO Península Ibérica)

Abstract

© 2015, Instituto Geologico y Minero de Espana. All rights reserved. The Gulf of Cadiz displays a number of structures that are associated with fluid circulation (mud volcanoes, mud mounds and pockmarks).This area has been used as natural laboratory for the sedimentological, biological and biogeochemical studies of these environments. Analysis of the associated authigenic carbonates has been widely used as a proxy to yield insights into the circulation and chemical composition of these fluids. A study of carbonate concretions from the Ibérico, Cornide and Arcos mud mounds in the Diasom Field was undertaken to better understand the origin and type of fluids from which these concretions precipitated. The concretions display varying morphologies, some of which correspond to bioturbation traces. X-ray diffractions revealed that these carbonate concretions are mainly composed of dolomite, Fe-rich dolomite, high magnesium calcite (HMC) and ankerite. The δ¹³C values of carbonate minerals ranged between -48.3 and-10.9 ‰ V-PDB, which suggests that the main processes involved in their genesis are organic matter oxidation, bacterial sulphate-reduction (BSR) and anaerobic methane oxidation (AOM). The origin of the methane is mainly thermogenic, and only few concretions yielded δ¹³C values lower than -40 ‰ V-PDB, suggesting oxidation of microbial methane. Fluids involved in the carbonate precipitation are interpreted as being related to gas hydrate destabilisation (δ¹⁸Ofluid-V-SMOW values higher than +2 ‰) and, to a lesser extent, modified seawater enriched in ¹⁸O due to rock-water interaction. Nevertheless, the highest δ¹⁸Ofluid-V-SMOW values suggest that the influence of other deep-seated fluids due to clay-mineral dehydration cannot be ruled out.