Upper Pliensbachian-Lower Toarcian methane cold seeps interpreted from geochemical and mineralogical characteristics of celestine concretions (South Iberian palaeo-margin)
Abundant sulfate mineralizations are recorded in the Upper Pliensbachian-Lower Toarcian marly-limestones and marls of La Cerradura section (Betic Cordillera, South Spain). These mineralizations are celestine concretions (6–41 cm in diameter), sulfated wood coal remains and yellow-powder patches composed mainly of natrojarosite, native sulfur and gypsum. These deposits are mostly recorded in the base of Serpentinum Zone (Lower Toarcian) in the interval with the highest TOC values (0.4 wt%) and a negative carbon isotopic excursion, correlative with the Toarcian Oceanic Anoxic Event. The 87Sr/86Sr and δ18O values of celestine are within the values of the Early Toarcian seawater but δ34S values are anomalously high. The isotopic composition of these sulfates is congruent with their precipitation from fluids that have been submitted to variable degrees of microbial sulfate-reduction combined with anaerobic oxidation of methane, as reported from recent examples. The record of thin laminated crusts composed of celestine with calcite as the nucleus of concretions containing fossil filaments (observed under scanning electron microscopy) is compatible with the activity of sulfate-reducing bacteria. Therefore, celestine concretions have been related to methane cold seeps or high methane concentrations in the sediment. The source of the methane, which moved within the pore-space of marine sediment via diffusion or as bubbles, was the microbial methanogenesis of the organic-rich marls. The very high δ34S values indicate that celestine crystals formed in a semi-closed system below the sediment-water interface, which favored the efficient microbial sulfate-reduction that preferentially removed the light 32S isotope from pore-water. The celestine crystals grew from mixing of this 34S-enriched sulfate pool with reducing and Sr-rich seep fluids advecting with the methane hydrates. The precipitation of celestine in the dark marls of the Serpentinum Zone occurred in some cases over wood coal remains. Fossil chimneys related to the cold seeps have not been recorded in the studied outcrop indicating that the amount of methane hydrates reaching the sea-bottom was not enough for building this kind of structures. The growth of each celestine concretions ended when the source of methane, the organic matter, was spent and the activity of methanogenic microbes and sulfate-reducing bacteria decreased. New celestine concretions developed when new methane bubbles were channeled upwards in different layers.