A model for microbial phosphorus cycling in bioturbated marine sediments: Significance for phosphorus burial in the early Paleozoic

A diagenetic model is used to simulate the diagenesis and burial of particulate organic carbon (C-org) and phosphorus (P) in marine sediments underlying anoxic versus oxic bottom waters. The latter are physically mixed by animals moving through the surface sediment (bioturbation) and ventilated by burrowing, tube-dwelling organisms (bioirrigation). The model is constrained using an empirical database including burial ratios of C-org with respect to organic P (C-org: P-org) and total reactive P (C-org: P-reac), burial efficiencies of C-org and P-org, and inorganic carbon-to-phosphorus regeneration ratios. If P-org is preferentially mineralized relative to C-org during aerobic respiration, as many previous studies suggest, then the simulated P-org pool is found to be completely depleted. A modified model that incorporates the redox-dependent microbial synthesis of polyphosphates and P-org (termed the microbial P pump) allows preferential mineralization of the bulk P-org pool relative to C-org during both aerobic and anaerobic respiration and is consistent with the database. Results with this model show that P burial is strongly enhanced in sediments hosting fauna. Animals mix highly labile P-org away from the aerobic sediment layers where mineralization rates are highest, thereby mitigating diffusive PO43- fluxes to the bottom water. They also expand the redox niche where microbial P uptake occurs. The model was applied to a hypothetical shelf setting in the early Paleozoic; a time of the first radiation of benthic fauna. Results show that even shallow bioturbation at that time may have had a significant impact on P burial. Our model provides support for a recent study that proposed that faunal radiation in ocean sediments led to enhanced P burial and, possibly, a stabilization of atmospheric O-2 levels. The results also help to explain C-org: P-org ratios in the geological record and the persistence of P-org in ancient marine sediments. (C) 2016 Elsevier Ltd. All rights reserved.