Bacterial calcification as a possible trigger for francolite precipitation under sulfidic conditions

Francolite (carbonate-fluor-apatite) is known to form in sediments where intense organic-matter decay occurs, but under oxic-to-suboxic conditions, because the alkalinity rise accompanying bacterial sulfate reduction (anoxic conditions) increases francolite solubility, hence preventing its supersaturation and precipitation. However, lagoonal, organic matter-rich, cryptalgal carbonates of Jurassic age located in the French Jura Mountains contain francolite that formed during early diagenesis, under anoxic-sulfidic and highly alkaline conditions. To explain this paradoxical situation, we propose the following “chain of reactions”: the presence of abundant biomass in the sediment would supply the “raw material” to the reactional system, i.e., the initial P budget, released through organic decay. The development of cryptalgal, bacterial mats at the sediment–water interface would have limited exchanges between the water column and the pore space, hampering pore water renewal and favoring the early onset of sulfate-reducing reactions. The onset of sulfate reduction would increase pore water alkalinity, potentially preventing francolite precipitation. In addition, the presence of sulfide ions would induce organic matter sulfurization, resulting in a relative pH rise. Alkalinity and pH rises would trigger the development of conditions leading to carbonate supersaturation, inducing bacterial-structure calcification. The sudden fall in alkalinity induced by early calcification would allow francolite precipitation, despite sulfidic conditions.