Candidate sulphide-oxidising microbial mats in a Mississippian Black Shale

Candidate Sulphide-oxidising Microbial Mats in a Mississippian Black Shale J. Emmings1,2, S. Davies2, S. Poulton3, J. Hennissen1, M. Stephenson1, C. Vane1, M. Leng4,5, V. Moss-Hayes1, A. Lamb4, A. Boom2 1 British Geological Survey, Keyworth, Nottingham, UK josmin65@bgs.ac.uk 2 School of Geography, Geology and the Environment, University of Leicester, Leicester, UK 3 School of Earth and Environment, University of Leeds, Leeds, UK 4 NERC Isotopes Geosciences Facilities, British Geological Survey, Keyworth, Nottingham, UK 5 Centre for Environmental Geochemistry, University of Nottingham, Nottingham, UK Microbial mats are markers for compressed redox gradients at seabed, biostabilisers of sediment and are important mediators in cycling of S and C. Yet ancient examples are rare. Here we report candidate sulphide-oxidising microbial mats in the Mississippian Bowland Shale Formation (Craven Basin, UK). The Bowland Shale represents a heterogeneous, mud dominated succession that was remotely linked to a tropical delta system. Sedimentological observations coupled with palaeoredox proxies, such as Fe-speciation and redox-sensitive trace metal analysis, suggest deposition under dominantly anoxic and at least intermittently sulphidic bottom water conditions. The palaeoredox proxies indicate bottom waters became progressively ventilated during basin infill. ‘Facies G’ mudstones span this transition from anoxic to oxic bottom water conditions. Facies G consists of gravel-sized, flattened mud clasts and wispy organic-rich interlaminae exhibiting ‘roll-up’ structures. Facies G also contains early diagenetic pyrite nodules and pyritised micro-burrows, and exhibits high δ34Spy relative to muds deposited under anoxic conditions. This suggests highly sulphidic, closed system conditions persisted (at least initially) in porewaters near the relatively ventilated bottom waters. The extracted palynological fraction is dominated by large, sheet-like amorphous organic matter. Composite rip-up clasts of Facies G, exhibiting ‘frayed edges’, are present in down-dip hybrid event beds. Organic S analysis (via Fe-speciation) and flash pyrolysis indicates abundant organically bound S in Facies G. ‘Roll up’ and frayed edge textures are suggestive of biostabilisation. Taken together, these observations suggest intermittently high redox gradients developed at seabed, and that these conditions were utilised by sulphide-oxidising mats. This suggests C and S cycling in Late Palaeozoic seaways included a pool of marine, benthic microbial mats likely as chemoheterotrophs