Palaeozoic cold seep carbonates from Europe and North Africa—an integrated isotopic and geochemical approach

Abstract

In this paper, we report the highest and lowest carbon isotope values known from Palaeozoic carbonate rocks. These unusual δ13C values (−50 to +23.5‰) are due to microbial methanogenesis and methanotrophy in Silurian to Carboniferous carbonates. Trace elements were used to decipher the primary mineralogy of the carbonate cements. Very high Sr values and low amounts of Mg, Fe and Mn point toward aragonite precursors, whereas high Fe and Mn values are indicative of primary calcites and allow reconstruction of the redox conditions. Four carbonate deposits are described from the Meseta and the Antiatlas of Morocco, the Pyrenees (France) and the Harz mountains (Germany). The highest δ13C values in concretion below the uppermost Silurian Spinatrypa Mound (Moroccan Meseta) give evidence, that CO2 was produced during methanogenesis. δ13C values between −10 and −32‰indicate that the formation of microbial carbonates and cements in the Middle Devonian Hollard Mound (Antiatlas) and in the Lower Carboniferous sediments of the Iberg (Harz) formed at thermogenetic methane or petroleum seeps. The Late Bashkirian carbonate mound of the High Pyrenees (Tantes Mound) is the first Palaeozoic carbonate with seepage fluids being dominated by biogenic methane. Matrix carbonates exhibit δ13C values as low as −34‰. In some parts, voids make up more than 50 vol% of the mound. They are filled with several generations of cement. The earliest void filling is isopachous fibrous cement, which represents former aragonite. Most negative δ13C values of −50‰were measured in these isopachous fibrous cements. The difference of 55‰in δ13C values between normal sediments and early aragonite cements can only be explained by the contribution of CO2 from anaerobic oxidation of biogenic methane in a cold seep setting.