Putative Analogs of Pyrite Suns Forming in Proglacial Alaska Mudflats

Disc-shaped pyrite suns of the Pennsylvanian age Anna Shale are thought to have formed where pressure restricted pyrite crystal growth to a flattened disc shape during diagenesis at the Anna Shale and the underlying Herrin coal boundary. Others have proposed syndepositional involvement of sulfate-reducing bacteria in the depositional environment. We hypothesize that the first steps in pyrite sun formation occur in mudflats of proglacial Alaska, with cyanobacterial mats trapping glacier silts within extracellular polymeric substances where microbial communities interact with allochthonous hydrocarbons, sulfur, and iron to precipitate amorphous iron sulfide minerals. We compared pyrite sun morphology with pyrite sun precursor formations, used 16S rRNA amplicon sequencing to investigate putative pyrite-forming bacteria, and determined iron content and mineralogy using XRD and sequential iron extraction of Matanuska Glacier mudflats sampled in June 2023. Recovered 16S rRNA sequences include EPS-generating cyanobacteria (Aphanizomenon NIES81), sulfur cycling bacteria (e.g., Thiobacillus, Sulfuritalea, Desulfovibrio), and iron cycling bacteria (e.g., Rhodoferax, Geobacter). In our proposed model, methane and hydrogen sulfide generated within anoxic mud form gas domes in benthic silt resting in a disc shape at the air-water interface. Iron sulfides precipitate below the surface of the cyanobacterial mats and are later buried and transformed into pyrite crystals with diagenesis across the disc shape. This combination of high organic carbon availability with sulfur and iron cycling and resulting iron sulfide mineral precipitation across a sharp redox gradient in depositional silt is a close match to the ancient depositional environment of the pyrite sun containing unit within the Anna Shale.