Research Projects

Dynamic Oxygen Uptake by Sinking Marine Aggregate

Sinking marine snow aggregates play an important role in oxygen uptake and carbon production in the ocean. During their descent, particulate organic carbon is gradually turned over by microbial respiration and solubilization, while oxygen is constantly taken up from the ambient water. However, when the flow or reaction undergoes a rapid change, this steady-state scenario is interrupted. We investigate numerically the flow and concentration in and around a porous spherical aggregate. We propose a new relation for the dynamic uptake of oxygen by sinking, porous aggregates in terms of the average Sherwood number. From this relation, the times and sinking depths to reach 99% of oxygen uptake can be readily obtained as a function of aggregate size and excess density.
Figure 1. Time series of diffusion dominated solute release from a porous sphere (radius a = 0.45 mm) with sinking velocity ws = 1.9 m d-1 (Re = 0.01), D = 1.4 × 10-9 m2 s-1 (Sc = 700), and = 5 × 10-5 (saturated concentration c0 = 1 at t = 0). A–D — Solute release from an aggregate with high permeability, k = 2.0 × 10-10 m2 (Da = 10-3). E–H — Solute release from an aggregate with weak permeability, k = 2.0 × 10-13 m2 (Da = 10-6).

Liu, B. , K. Kindler, A. Khalili. (2012). Dynamic solute release from marine aggregates. Limnology and Oceanography: Fluids and Environments, 2.doi:10.1215/21573689-2016772

Porosity variation in microbial mats and biofilms

The exchange of concentration, energy and momentum in the vicinity of water-sediment interface depends to a large extent on the correct quantification of porosity. Usually constant bulk porosity is taken, however, we know that the volume fraction of the solid matrix near the interface region differs from that in the core. This indicates that the near-sediment surface porosity is not constant which plays a crucial role in all interfacial exchange processes. Within this study, we investigate the porosity variation below the water-sediment interface for granular and non-granular porous beds.
Figure 2. The impact of depth-dependent porosity is exemplified by the diffusion of a solute from a fluid layer into a biofilm substrate with a constant sink. The predictions show that concentration profiles are significantly different (up to approx. 37%) and the solute migrates much deeper into the biofilm when taking porosity variation into account.

Khalili, A., Morad, M., Matyka, M., Liu, B., Malekmohammadi, R., Weise, J., Kuypers, M. M. M. (2014). Porosity variation below a fluid–porous interface. Chemical Engineering Science. 107: 311-316.

Modeling of oxygen consumption by marine aggregates

The measurement of oxygen concentration in the vicinity of the sinking aggregate is the key to obtain their oxygen respiration rate. Usually the oxygen profile is measured either along the downstream or along the equatorial axis of the aggregate. The total flux is then quantified by multiplication of the concentration gradient and surface area of the aggregate, assuming a uniform distribution of the flux along the whole surface. We study numerically the velocity and the concentration field around and within an aggregate. The aggregate is held fixed by an upward flow from below to compensate its sinking speed. The model results help us to evaluate existing experimental data available for quantification of oxygen consumption rates by marine aggregates.
Figure 3. Left image: Physical model for numerical simulation. The aggregate is considered as a porous sphere with a flow from below having a constant far-field velocity vs; right image: Concentration distribution around and within an aggregate. Due to the symmetry, only the half of the aggregate is shown (black semi-circle)

Gas seep-mediated downward migration of organic matter in the sediment

We study the downward migration of dye concentration mediated by the air bubble emergence through the sediment column. The experimental setup is a rectangular container half-filled with a saturated sediment column with a dyed water layer on top. Air is injected centrally from the bottom of the container. Further, the downward dye migration will be numerically simulated. The results contribute to provide improved quantification of the downward flux of organic and particulate matter in the vicinity of gas seeps. In the first step we visualize the gas emergence from the sediment surface. The knowledge of gas emergence modi is the key to explain dynamics of bubble rise within a permeable sediments.
Figure 4. Gas seeps occur in muddy fine-grained sea beds. Single bubble (mushroom-like) in the water layer emerges from the fine sediment bed surface (black region below).
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