Pyrite formation in an anoxic estuarine basin

In a shallow, fjord-like estuary, the Upper and Lower Basins of the Pettaquamscutt River (Rhode Island) each contain water columns stratified into oxic and anoxic plus sulfidic layers. Both water columns contain iron monosulfides and pyrite from just below the oxic-anoxic interface down to the sediment-water interface. Concentrations of iron monosulfides increase with increasing depth in the water columns, but concentrations of pyrite are relatively constant with depth. Suspended pyrite is present only as framboids, whereas the underlying sediments contain dominantly framboids but also euhedral grains and infilled framboids. Based on pyrite textures, S-isotopic compositions of water-column and sediment sulfides, and settling rate calculations, about 70 percent of the pyrite burial nux to the Pettaquamscutt sediments is accounted for by framboids forming at the oxic anoxic interface and settling to the sediment-water interface. The remaining pyrite burial nux is secondary pyrite nucleation and growth within the sediments represented by euhedral grains and infilled framboids. The S-isotopic composition of pyrite in the Pettaquamscutt sediments, and presumably of pyrite in the sediments of other modern anoxic basins and analogous ancient sediments, therefore, contains a component originating near the oxic-anoxic interface and a component added below the sediment-water interface. Pyrite framboids form in the water column where there is high supersaturation with respect to pyrite and slight undersaturation with respect to the iron monosulfides. We propose that framboid nucleation and growth occurs in the water column, adjacent to, and below, the oxic-anoxic interface, where supplies of ferrous species, sulfide species, and suitable electron accepters are available. Infilled and overgrown framboids form in the sediments as a result of surface nucleation and continued pyrite growth on framboids that originate above the sediment-water interface.