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

Defining the variability and distribution ofmethane seeps and microbial activity in the aftermath of theMarinoan glaciation is a long-standing challenge in the field of Snowballmodels. 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 environmentalmodifications 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 C-13-depleted carbon isotope values of the microbial carbonates (delta C-13 as low as -43.2 % PDB) suggest the influence of methane, also preserved as fluid inclusions in barite crystals (documentedwith RAMAN spectroscopy) derived froma gas reservoir belowthe cap carbonate. Th of other fluid inclusions (Linkam microthermometry), ranging from174 degrees C to 222 degrees 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 C-29-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 KaartaMountains is representative of shallower substrates, whereas its biomarkers point to deposition under episodic non-oxidising conditions. (c) 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

The study focuses on the carbonates in the Kaarta Mountains of Mali and their formation under episodic non-oxidising conditions. The presence of methane oxidation by microbes and the input of barium-rich fluids were observed, challenging previous understanding of carbonates formation. This research provides valuable insights into the variability and distribution of methane seeps and microbial activity in the aftermath of glaciation.