Sinking flux of particulate organic matter in the oceans: Sensitivity to particle characteristics

Abstract

The sinking of organic particles produced in the upper sunlit layers of the ocean forms an important limb of the oceanic biological pump, which impacts the sequestration of carbon and resupply of nutrients in the mesopelagic ocean. Particles raining out from the upper ocean undergo remineralization by bacteria colonized on their surface and interior, leading to an attenuation in the sinking flux of organic matter with depth. Here, we formulate a mechanistic model for the depth-dependent, sinking, particulate mass flux constituted by a range of sinking, remineralizing particles. Like previous studies, we find that the model does not achieve the characteristic 'Martin curve' flux profile with a single type of particle, but instead requires a distribution of particle sizes and/or properties. We consider various functional forms of remineralization appropriate for solid/compact particles, and aggregates with an anoxic or oxic interior. We explore the sensitivity of the shape of the flux vs. depth profile to the choice of remineralization function, relative particle density, particle size distribution, and water column density stratification, and find that neither a power-law nor exponential function provides a definitively superior fit to the modeled profiles. The profiles are also sensitive to the time history of the particle source. Varying surface particle size distribution (via the slope of the particle number spectrum) over 3 days to represent a transient phytoplankton bloom results in transient subsurface maxima or pulses in the sinking mass flux. This work contributes to a growing body of mechanistic export flux models that offer scope to incorporate underlying dynamical and biological processes into global carbon cycle models. The oceans absorb about 2.5 × 10 12 kg of carbon from the atmosphere annually, amounting to a net uptake of about 40% of the anthropogenically produced CO 2 since industrialization 1. Understanding the controls on the oceanic uptake of atmospheric CO 2 is important for predicting how the uptake rate might change in the future. The oceanic biological pump 2 plays a significant role in this process: Phytoplankton produce organic matter by taking up dissolved inorganic carbon through photosynthesis in the sunlit ocean. While a major portion of this production is recycled, a small fraction (typically, less than 20%) rains out to depth, carrying with it organic carbon, nutrients and minerals. Such particles are formed through a variety of mechanisms: cells age, form cysts, are ingested and egested by zooplankton, aggregate, coagulate, and gradually sink as 'marine snow'. The sinking material varies in shape, size and character, ranging from individual cells to pellets and aggregates, most of which is rapidly colonized and consumed by heterotrophic bacteria, contributing to the attenuation of the sinking flux with depth (Fig. 1). The shape of the depth-diminishing flux profile has traditionally been characterized by empirical fits to data obtained from particle intercepting sediment traps 3 o...

Size‐Differentiated Export Flux in Different Dynamical Regimes in the Ocean

“…Traditionally, POC export is thought to occur through gravitational sinking and one‐dimensional models have been used to describe the sinking POC flux with depth (Armstrong et al., 2001; DeVries et al., 2014; Jackson et al., 1997; Omand et al., 2020). Particles produced through primary and secondary production in the surface layer that are relatively large and fast sinking tend to sink out of the upper surface layer on timescales shorter than the timescale on which the particles get remineralized.…”

Marine snow morphology illuminates the evolution of phytoplankton blooms and determines their subsequent vertical export

“…ne key vector of the biological carbon pump is marine snow, representing large (>500 µm) particles of detritus (including zooplankton fecal pellets), organic matter (remnants and exudates of plankton), inorganic matter, and/or aggregated mixtures of those [1][2][3][4][5] . Changes in both phytoplankton and zooplankton community structure directly influence the quantity and composition of marine snow 6,7 and thus the efficiency of the biological carbon pump 8,9 .…”

“…Changes in both phytoplankton and zooplankton community structure directly influence the quantity and composition of marine snow 6,7 and thus the efficiency of the biological carbon pump 8,9 . Many previous biogeochemical models 5,10,11 and calculations estimating carbon export, based on in situ observations [12][13][14] , assumed a fixed mass to volume relationship and simple geometry of particles, such as spheres or a defined fractal dimension. This simplification was necessary because marine snow is difficult to study due to its fragile nature and complex morphological characteristics.…”

Influence of pycnocline on settling behaviour of non-spherical particle and wake evolution

“…Studies on spheres and amorphous aggregates have provided physical explanation for the prolonged residence times of particles in density transitions 13,20,23 . Nonetheless, these findings have not been yet incorporated to widely used ocean sedimentation and biogeochemical models, which may be partially responsible, beside ignoring some biochemical processes, for unreliable estimation of particulate flux [41][42][43] . The oversimplifications are in two aspects: (1) physical conditions in a water column and (2) physical characteristics of particles.…”