BIOLOGICAL PROCESSES AND APATITE FORMATION IN SEDIMENTARY ENVIRONMENTS

Much of the controversy in the microbial vs. non-microbial phosphorite debate results from a lack of rigor in defining the specific levels a t which microbial activity intervenes in phosphogenesis. A clear disti nction between the processes that control the sedimentary transformati ons of particulate phosphorus fractions and the processes that directl y participate in the apatite precipitation reaction seems to be necess ary in order to reconcile the conflicting views. Microbial processes a re the primary driving force behind the transformation of deposited pa rticulate phosphorus into dissolved phosphate, a critical first step i n the formation of apatite. The precipitation of apatite itself, howev er, is widely controlled by kinetic factors. Depending on the initial degree of supersaturation of pore solutions, different mechanisms of t he precipitation reaction may prevail in sediments. In most continenta l margin sediments, low to moderate concentrations of dissolved phosph ate may result in a direct nucleation of apatite crystals, leading to a slow formation of dispersed apatite in the sediment column. In phosp hogenic sediments, high localized rates of dissolved phosphate generat ion promote the fast nucleation of a metastable precursor of apatite. This creates a large number of crystallization sites in the sediment, and results in the rapid formation of phosphatic bodies. The transitor y fixation and release of phosphate by microbial communities may be re sponsible, in part, for the elevated concentrations of dissolved pore phosphate observed in close proximity to the water-sediment interface at sites of present-day phosphorite formation (e.g. sediments on the P eru-Chile shelf). Localized pulses of high supersaturation enhance the likelihood of rapid apatite deposition close to the water-sediment in terface. Microbial redox processes may further help focusing the rapid precipitation of apatite in the interface environment by telescoping chemical gradients and associated pH shifts in the surface sediment la yer. There is no convincing evidence of other important microbial effe cts being directly involved in the formation of apatite in marine sedi ments, for instance intra-cellular precipitation of calcium phosphate. A close association of benthic microbial activity and the formation o f apatite can be widely traced in ancient phosphogenic environments. T he organic matter of fossil phosphorites preserves a clear molecular s ignature of the intense degradation of sedimentary organic matter at t he seafloor, and points to liberation of phosphorus from organic compo unds as a predominant microbial process involved in apatite formation. Ultra-structure analysis of microbial fabrics preserved in phosphorit es permits the identification of abundant and diverse benthic microbia l communities. Extracellular precipitation of apatite was a common mec hanism of microbiota preservation, although no direct cellular control of apatite precipitation can be inferred. The development of localize d phosphatic bodies in many ancient phosphorites is likely to reflect the spatial association between the activity of individualized benthic microbial communities and the precipitation of apatite. Stromatolitic phosphorites provide an extreme example of such an association.