PHOSPHORITES AND THE MARINE PHOSPHORUS CYCLE

The role that phosphorite formation plays in the marine phosphorus (P) cycle has long been debated. A shift has occurred from early models that evoked strikingly different oceanic P cycling during times of widespread phosphorite deposition to current thinking that phosphorite deposits may be lucky survivors of a series of inter-related tectonic, geochemical, sedimentological, and oceanic conditions. This paradigm shift has been facilitated by careful examination of the geochemistry and accumulation processes of P in the ocean, as well as an awareness of the widespread nature of phosphatization—the authigenic formation of authigenic P-bearing minerals that contributes to phosphorite formation. This process occurs not just in continental margin sediments, but in deep sea oozes as well, and helps to clarify the driving forces behind phosphorite formation and links to the marine P cycle.

The accumulation rate of P is highest along continental margins, due to focused productivity and rapid transit of organic and biogenic material (main carriers of P) through the water column/active sediment regions. The concentrations of P, however, are nearly identical between continental margin sediments and deep sea sediments, due to substantial terrigenous dilution on margins. How then do these marginal deposits alone become so concentrated in P? Two processes seem to come into play to make phosphorite deposits: physical dynamism, and chemical dynamism. Physical dynamism involves the reworking or sedimentary capping of P-rich sediments, which can either concentrate the relatively heavy and insoluble disseminated P-bearing minerals or provide an episodic change in sedimentology to concentrate chemically mobilized P. Both processes can results from along-margin current dynamics and/or sea level variations. Chemical dynamism involves the diagenetic release and subsequent concentration of P-bearing minerals in particularly horizons, controlled either by sedimentology or geochemical fronts. Interestingly, net P accumulation rates are highest (i.e., the P removal pump is most efficient) when phosphorites are NOT forming. Both physical and chemical pathways involve processes not dominant in deep sea environments, and contribute to the formation of a marginal phosphorite deposit unique on sedimentological grounds but not in terms of the marine P cycle.