Diffusion-limited retention of porous particles at density interfaces

Abstract

Downward carbon flux in the ocean is largely governed by particle settling. Most marine particles settle at low Reynolds numbers and are highly porous, yet the fluid dynamics of this regime have remained unexplored. We present results of an experimental investigation of porous particles settling through a density interface at Reynolds numbers between 0.1 and 1. We tracked 100 to 500 μm hydrogel spheres with 95.5% porosity and negligible permeability. We found that a small negative initial excess density Δρ p relative to the lower (denser) fluid layer, a common scenario in the ocean, results in long retention times of particles at the interface. We hypothesized that the retention time was determined by the diffusive exchange of the stratifying agent between interstitial and ambient fluid, which increases excess density of particles that have stalled at the interface, enabling their settling to resume. This hypothesis was confirmed by observations, which revealed a quadratic dependence of retention time on particle size, consistent with diffusive exchange. These results demonstrate that porosity can control retention times and therefore accumulation of particles at density interfaces, a mechanism that could underpin the formation of particle layers frequently observed at pycnoclines in the ocean. We estimate retention times of 3 min to 3.3 d for the characteristic size range of marine particles. This enhancement in retention time can affect carbon transformation through increased microbial colonization and utilization of particles and release of dissolved organics. The observed size dependence of the retention time could further contribute to improve quantifications of vertical carbon flux.marine snow | stratification | biological pump T he settling of particulate organic matter in the ocean is the main vector of carbon export from surface waters toward the deep sea (1, 2). Besides its key role in the oceanic carbon cycle, this particle flux supplies resources to waters below the surface layer, impacting ocean productivity and trophic dynamics (3, 4). The sinking speed of marine particles and their residence time in the epipelagic control the rate at which particulate organic matter is exported from the upper ocean or consumed and remineralized by microbes.Marine snow particles are 0.5 to 20 mm amorphous aggregates of various origins, both organic and inorganic, bound together by exopolymeric material (5, 6). Their small excess density relative to ambient water [typically <O(1 kg·m )] (7) results in small sinking speeds (U = 0.01 − 1 mm·s −1 ) (4). Their Reynolds number, Re = aU/ν (where a is the particle radius and ν the kinematic viscosity of seawater), is O(0.1 − 1), and sinking is thus primarily controlled by viscous, rather than inertial forces.Marine snow often accumulates at density interfaces (pycnoclines), resulting in layers of highly elevated particle abundance that can persist for days (5,8,9). Despite the important effect that this process has on particle sinking rates and local trophic dynamics, the ...

Sediment–Water Interface Flow with the Multiparticle Collision Dynamics

“…Also, due to the possibility of modelling spatial changes in permeability of the model by varying λ in space, it is possible to simulate various types and shapes of porous structures, i.e. transport through rippled sea bed (Huettel et al 1996;Kharab 1990) layered porous media or porous particles (Kindler et al 2010) which we confirmed in the flow over porous sediment scenario. …”

Effects of attached bacteria on organic aggregate settling velocity in seawater

“…Eqs. (2−4) can be rewritten as follows: (5) This model assumes that the aggregates are nonpermeable (there is no advective flow through the aggregates), which is consistent with the suggestion that marine aggregates have very low permeability (Ploug & Passow 2007, Kindler et al 2010. Clearly, this assumption does not hold for all aggregate types in marine environments.…”

“…We also note that changes in the porosity of aggregates due to attached bacterial action have implications for the regulation of fluid exchange between interstitial spaces and the ambient water in aggregates. Kindler et al (2010) suggested that more porous aggregates can retain water for longer at density interfaces because of diffusion-limited fluid entrainment in the interstitial spaces of aggregates. Bacterial enhancement of aggregate porosity can be an important mechanism that promotes aggregate accumulation at density interfaces, a prominent phenomenon widely observed in oceanic environments (Alldredge & Gotschalk 1988, MacIntyre et al 1995.…”

Dynamic solute release from marine aggregates

“…Both of these dynamic processes of solute exchange occur at density discontinuities (pycnoclines), where aggregates are retained and form thin layers in the ocean (MacIntyre et al 1995). This process has been demonstrated in laboratory experiments on real aggregates (Li et al 2003) and porous hydrogel spheres (Kindler et al 2010), where typical marine aggregates were found to have retention times of many days at pycnoclines (Li et al 2003). During this time, the dissolved organic matter inside aggregates increases due to the lack of advection.…”

“…Thus, the increase of DOC within the aggregates is mainly caused by the lack of advection during the residence time within a pycnocline. According to recent studies (Li et al 2003;Kindler et al 2010), these residence times (t R ) were found to follow the diffusion relaxation time as…”