Phosphogenesis, aragonite fan formation and seafloor environments following the Marinoan glaciation

Abstract

Carbonates in the Sete Lagoas Formation (São Francisco craton, Brazil) preserve a record of chemical, biological, and oceanographic changes that occurred during the Ediacaran Period. The base of this formation constitutes a post-glacial cap carbonate, which contains seafloor precipitates (carbonate and barite crystal fans) as well as various authigenic and diagenetic minerals (apatite, pyrite, and barite). Here, we present petrographic and geochemical data on this unit, and discuss the significance of its mineral association for marine environments following the Marinoan (‘Snowball Earth’) glaciation. For the first time, we report well-developed apatitic cements in a Neoproterozoic cap carbonate. Isopachous and intergranular void-filling cements encrust and surround seafloor-precipitated fan crystals that precipitated as aragonite. We propose a model for the origin of this mineral association, which relates phosphogenesis and aragonite fan formation to a single set of environmental conditions. According to this model, the boundary between oxic and anoxic conditions was located at or just below the sediment-water interface. Burial of iron (oxyhydr)oxides below this boundary liberated phosphate to pore water and provided fuel for iron reduction. Iron reduction released Fe2+, which inhibited nucleation of carbonate and allowed for aragonite growth on the seafloor. Concurrently, ‘iron-pumping’ shuttled phosphate from the water column to the sediment, and perhaps in conjunction with organic phosphorus remineralization via anaerobic microbial pathways, created conditions conducive to phosphate mineralization. This model corroborates the hypotheses that aragonite crystal fan formation requires the presence of an inhibitor to carbonate nucleation, in addition to high alkalinity, and that Fe2+ serves as this inhibitor. Overall, our work documents a close association between aragonite crystal fan formation and phosphogenesis at the beginning of the Ediacaran, illuminates the paleoenvironments of cap carbonates with seafloor precipitates, and contributes to understanding of phosphogenesis following low latitude glaciations.