A mechanistic particle flux model applied to the oceanic phosphorus cycle

[摘要] The sinking and decomposition of particulate organic matter are criticalprocesses in the ocean's biological pump, but are poorly understood andcrudely represented in biogeochemical models. Here we present a mechanisticparticle remineralization and sinking model (PRiSM) that solves the evolutionof the particle size distribution with depth. The model can represent a widerange of particle flux profiles, depending on the surface particle sizedistribution, the relationships between particle size, mass and sinking velocity, andthe rate of particle mass loss during decomposition. The particle flux modelis embedded in a data-constrained ocean circulation and biogeochemical modelwith a simple P cycle. Surface particle size distributions are derived fromsatellite remote sensing, and the remaining uncertain parameters governingparticle dynamics are tuned to achieve an optimal fit to the globaldistribution of phosphate. The resolution of spatially variable particlesizes has a significant effect on modeled organic matter production rates,increasing production in oligotrophic regions and decreasing production ineutrophic regions compared to a model that assumes spatially uniform particlesizes and sinking speeds. The mechanistic particle model can reproduce globalnutrient distributions better than, and sediment trap fluxes as well as,other commonly used empirical formulas. However, these two independent dataconstraints cannot be simultaneously matched in a closed P budget commonlyassumed in ocean models. Through a systematic addition of model processes, weshow that the apparent discrepancy between particle flux and nutrient datacan be resolved through P burial, but only if that burial is associated witha slowly decaying component of organic matter such as might be achievedthrough protection by ballast minerals. Moreover, the model solution thatbest matches both data sets requires a larger rate of P burial (andcompensating inputs) than have been previously estimated. Our results implya marine P inventory with a residence time of a few thousand years, similar tothat of the dynamic N cycle.

[学科分类] 地球化学与岩石