Abstract
Abstract. Various functions have been suggested and applied to represent the sedimentation and remineralisation of particulate organic matter (POM) in numerical ocean models. Here we investigate some representations commonly used in large-scale biogeochemical models: a constant sinking speed, a sinking speed increasing with depth, a spectrum of particles with different size and different size-dependent sinking velocities, and a model that assumes a power law particle size distribution everywhere in the water column. The analysis is carried out for an idealised one-dimensional water column, under stationary boundary conditions for surface POM. It focuses on the intrinsic assumptions of the respective sedimentation function and their effect on POM mass, mass flux, and remineralisation profiles. A constant and uniform sinking speed does not appear appropriate for simulations exceeding a few decades, as the sedimentation profile is not consistent with observed profiles. A spectrum of size classes, together with size-dependent sinking and constant remineralisation, causes the sinking speed of total POM to increase with depth. This increase is not strictly linear with depth. Its particular form will further depend on the size distribution of the POM ensemble at the surface. Assuming a power law particle size spectrum at the surface, this model results in unimodal size distributions in the ocean interior. For the size-dependent sinking model, we present an analytic integral over depth and size that can explain regional variations of remineralisation length scales in response to regional patterns in trophodynamic state.
Highlights
The sinking and remineralisation of particulate organic matter (POM) in the ocean creates vertical gradients in dissolved inorganic tracers, and affects the air-sea gas exchange of CO2 and O2 between the ocean and the atmosphere
Besides the obvious finding that the parameterisation of vertically increasing sinking speed can have a strong effect on the vertical distribution of biogeochemical tracers, the results presented so far suggest, that (1) the particle size distribution in the ocean interior might be a unimodal function of diameter, even if the upper boundary conditions were characterised by a power law and (2) that the vertical distribution of tracers and fluxes might depend on the surface size distribution
We examined two different descriptions of POM fluxes – namely the Martin curve and constant particle sinking speed – which have been often applied especially in threedimensional large-scale models, for their intrinsic assumptions and ability to fit observed profiles of normalised sedimentation
Summary
The sinking and remineralisation of particulate organic matter (POM) in the ocean creates vertical gradients in dissolved inorganic tracers, and affects the air-sea gas exchange of CO2 and O2 between the ocean and the atmosphere. Given the sensitivity of biogeochemical model results to the parameterisation of sinking speed (Heinze et al, 2003; Howard et al, 2006; Gehlen et al, 2006), in this paper we investigate two functions commonly applied (the Martin function and constant sinking speed) and their effect on the representation of POM profiles, sedimentation, and remineralisation. We do this by means of analytic solutions for the above mentioned functions, assuming stationary and seasonally varying boundary conditions for POM sinking out of the surface layer. To what extent, we can predict deep sedimentation from the size distribution of POM in the surface layer
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