Pyrite framboids as biomarkers for iron-sulfur systems

The role of microorganisms in the formation of pyrite framboids has been suggested, but remains unresolved. We identified subsurface habitats from a sulfidic aquifer present in carbonate rock in Romania, where abundant pyrite framboids were found associated with microbial mats. Three types of subsurface microbial communities were studied: submerged mats overlaying clay-rich sediments, floating mats developed at the water-gas (sulfide/carbon dioxide) interface, and detrital FeS-rich sediments associated with floating microbial mats. All three habitats are anoxic, low temperature, low salinity, saturated with bicarbonate, and contain hydrogen sulfide, thiosulphate, and elemental sulfur. Sulfur stable isotope fractionation suggests that pyrite framboids from the floating microbial mats have a biogenic origin. Pyrite framboids characteristics, such as nucleation, growth, and spheroidicity, depend on conditions within the microbial mats and the inheritance of shape during the replacement of spheroidal greigite. We hypothesize that biogenic pyrite is formed in these microbial communities via a sulfide/ ferrous iron pyritization mechanism. Some of the signatures of this type of biogenic pyritization are a similarity in size distribution between FeS spheroids and FeS2 framboids, the presence of microorganisms using sulfide and ferrous iron as a source of energy, reversed iron sulfide bilayers, a narrow size distribution of the pyrite framboids, a small numbers of large microcrystallites within the framboids, a S-fractionation up to 11.14 between H2S and FeS and up to 16.24 between H2S and FeS2. These signatures are relevant for the study of the biologically controlled pyritization and thus they are valuable tools for geomicrobiology, paleo-microbiology and exobiology.