Ediacaran cap carbonates with microbial build-ups capping barite-bearing methane seep networks in the Kaarta Mountains, Taoudeni Basin, Mali

Abstract

Defining the variability and distribution of methane seeps and microbial activity in the aftermath of the Marinoan glaciation is a long-standing challenge in the field of Snowball models. Early diagenetic barite is commonly linked to tepee structures and associated breccias lacking microbial textures and fabrics, giving the impression that chemosynthetic microbes, or at least methane-tolerant microbes, did not participate in the carbonate production of their cap carbonate host. This apparent paradox has been an outstanding question in the lowermost Ediacaran cap carbonates of the Taoudeni Basin, NW Africa. In the Kaarta Mountains of Mali these carbonates exhibit, over short distances (<10 km), sharp facies-related environmental modifications with quiescent-dominated seafloor conditions, episodically interrupted by metre-scale disrupted substrates. The latter comprises fissure and fracture networks, occluded with tabular- and rosette-shaped barite cements, and sealed by decimetre-scale stromatolitic build-ups exhibiting intergrowths with barite needles. The strongly 13C-depleted carbon isotope values of the microbial carbonates (δ13C as low as −43.2 ‰ PDB) suggest the influence of methane, also preserved as fluid inclusions in barite crystals (documented with RAMAN spectroscopy) derived from a gas reservoir below the cap carbonate. Th of other fluid inclusions (Linkam microthermometry), ranging from 174 °C to 222 °C, provides minimum entrapment temperatures for barite precipitation. The microbially induced oxidation of methane and input of Ba-rich fluids were coupled to reduction of sulphate derived from seawater. The Sr/S isotope ratio and barite shape and size point to diagenetic barite crystals. The biomarkers yielded by the cap carbonate reflect a C29-dominant steroidal signature characteristic of stigmastanoid algal blooms. Although present-day microbial build-ups related to methane sources commonly occur in deep substrates and under anoxic bottom waters, the cap carbonate of the Kaarta Mountains is representative of shallower substrates, whereas its biomarkers point to deposition under episodic non-oxidising conditions.