213-3 Early Permian Microbial Biomarkers and Foraminifera Characteristic of Methane Seeps and Surrounding Putrid Mud: Extremophiles or Conservatives

Session: Cushman Symposium: Microfossils of Extremophiles: Living in the Danger Zone

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Among the best-preserved methane-seep deposits in the Upper Paleozoic are carbonate seep mounds found in the most restricted shallow-marine parts of the earliest Permian East Gondwana Interior Rift (Irwin Basin, Western Australia). These were associated with the waning of the vast ice sheets that covered the region during much of the Pennsylvanian. The authigenic seep-carbonate deposits have complex multiphase cement fabrics, negative δ13C values (mostly, -25 – -45‰ VPDB), and a seep biota that includes a framework of tubeworms, less common algal-like Tubiphytes, and problematica (? algae or sponges). Also present are microbial mats, the monostromatic macrophyte Litostroma, foraminifers, sponges with siliceous monaxon spicules, small rostroconchs, microgastropods, ostracods, and mobile polychaetes (represented by scolecodonts). The seeps were oases of high organic productivity on an otherwise barren muddy seafloor. Carbonate cementation was facilitated by sulfate-driven anaerobic oxidation of methane (SD-AOM), with extremely ¹³C-depleted archaeal and bacterial lipid biomarkers present. A dominance of ANME-2 (anaerobic methanotrophic archaea) is suggested, with high contents of crocetane and scarcity of biphytanes indicating high seepage intensity and high rates of SD-AOM. High hydrogen sulfide production was probably utilized by chemosymbiotic biota (e.g. tubeworms) and may have affected surrounding sea-floor muds. Foraminifera entrapped in seep carbonates include trochamminoid and Tolypammina organic-cemented agglutinated types and encrusting and free coiled tubular porcelaneous Miliolata. In the immediately surrounding muds, only single chambered siliceous agglutinated types were present consisting of elongate tubular Sansabaina, Kechenotiske, Hyperammina and possible Pseudohyperammina, coiled tubular Glomospirella and Ammodiscus, and spherical or sac-shaped Thuramminoides, Thurammina, Sacculinella, Placentammina, and Lagenammina. Further away in mud that includes carbonate-cemented mud nodules (possibly the result of methanogenesis) a more diverse assemblage of siliceous agglutinates, Miliolata, Nodosariata, and rare Fusulinata (very rare endothyrids and Tetrataxis) is present. The seep foraminifers are not endemic but occur in mud in other parts of the estuarine-like interior sea. Most were cosmopolitan and have very long stratigraphic ranges. This suggests very conservative evolution. In structure and general composition, the Permian seeps were like modern seeps. This conservatism suggesting slow evolution and the wide biogeographic ranges displayed by most of the foraminifers in these “extreme” discontinuous seep and estuarine-like mud environments raise questions on faunal migration pathways and provide a distinct contrast to contemporaneous punctuated evolution patterns in supposed more “normal” marine environments.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5350

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