Tag Archive for: carbon export

New guidelines for the application of Stokes’ models to the sinking velocity of marine aggregates

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Numerical simulations of ocean biogeochemical cycles need to adequately represent particle sinking velocities (SV). For decades, Stokes’ Law estimating particle SV from density and size has been widely used. But while Stokes’ Law holds for small, smooth, and rigid spheres settling at low Reynolds number, it fails when applied to marine aggregates complex in shape, structure, and composition. Minerals and zooplankton can alter phytoplankton aggregates in ways that change their SV, potentially improving the applicability of Stokes’ models. Using rolling cylinders, we experimentally produced diatom aggregates in the presence and absence of minerals and/or microzooplankton. Minerals and to a lesser extent microzooplankton decreased aggregate size and roughness and increased their sphericity and compactness. Stokes’ Law parameterized with a fractal porosity modeled adequately size‐SV relationships for mineral‐loaded aggregates. Phytoplankton‐only aggregates and those exposed to microzooplankton followed the general Navier‐Stokes drag equation suggesting an indiscernible effect of microzooplankton and a drag coefficient too complex to be calculated with a Stokes’ assumption. We compared our results with a larger data set of ballasted and nonballasted marine aggregates. This confirmed that the size‐SV relationships for ballasted aggregates can be simulated by Stokes’ models with an adequate fractal porosity parameterization. Given the importance of mineral ballasting in the ocean, our findings could ease biogeochemical model parameterization for a significant pool of particles in the ocean and especially in the mesopelagic zone where the particulate organic matter : mineral ratio decreases. Our results also reinforce the importance of accounting for porosity as a decisive predictor of marine aggregate SV.

Sinking velocities vs. ESD (equivalent spherical diameter) for aggregates formed in each tank of the four treatments and comparison with theoretical expectations from different parameterizations of Stokes’ Law and the general Navier‐Stokes’ drag equation. P: phytoplankton; PZ: phytoplankton + microzooplankton (rotifers); PM: phytoplankton + mineral (calcite); PMZ: phytoplankton + mineral + microzooplankton. (a) Model 1, Stokes’ Law with constant porosities of 0% (dashed lines) and 99% (solid lines). (b) Model 2, general Navier‐Stokes’ drag law with constant drags of 1 (dashed lines) and 5 (solid lines), and a constant porosity of 99%. (c) Model 3, Stokes’ Law with a porosity scaled on a fractal geometry with coefficient a = 0.03 and D3 = 1.4 (dashed lines) and D3 = 1.8 (solid lines). (d) Model 4, general Navier‐Stokes’ drag law with a porosity scaled on a fractal geometry with coefficient a = 0.03 and D3 = 1.4 (solid lines) and 1.8 (dashed lines). See the text for details on drag calculation

Reference

Laurenceau-Cornec, E.C., Le Moigne, F.A.C., Gallinari, M., Moriceau, B., Toullec, J., Iversen, M.H., Engel, A., and De La Rocha, C.L. 2020. New guidelines for the application of Stokes’ models to the sinking velocity of marine aggregates. Limnol. Oceanogr. doi:10.1002/lno.11388.
Link to the journal:

Silica and the silicon cycle

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This ambitious objective requires identifying and quantifying silicon sources and sinks at the interfaces and describing the internal dynamics of the cycle in different ecosystems and on a global scale. At LEMAR we are particularly interested in the roles of different silicifiers not only for the silicon cycle, but also in other major biogeochemical cycles and in the functioning of ecosystems.

The silicon cycle is a historical theme of LEMAR. Our objective is to understand the oceanic cycle of silicon and the interactions with other biogeochemical cycles such as carbon, nitrogen, phosphorus and even iron and sulphur. This ambitious objective requires identifying and quantifying silicon sources and sinks at the interfaces and describing the internal dynamics of the cycle in different ecosystems and on a global scale. We have developed a transdisciplinary approach, including chemistry, biogeochemistry, biochemistry, physiology and biology, and use several experimental and modelling tools and multi-scale approaches from laboratory experiments to better understand the processes influencing the cycle to major natural environment observation campaigns. We have recently created an international “silicon school” bringing together a consortium of universities and organizations that offer higher-level teaching and research opportunities and an online learning course (under development) on the theme “Silica: from stellar dust to the living world”. The Silica School consortium currently includes 23 marine research institutes from 11 countries and continues to grow.

Silicifiers are living organisms that take advantage of the abundance of silicon (silicon is the second most abundant element in the earth’s crust) to build silicified architectures (in biogenic silica) from silica dissolved in water (orthosilicic acid or silicates). Their biogenic silica skeletons can help improve their physical strength, protect them from predators, improve their motility or help light and nutrients penetrate cells. In the marine domain, diatoms play a key role in the food webs of the most productive coastal or ocean ecosystems, as well as in the production of oxygen on which we depend and in the transfer of CO2 from the surface to the interior of the oceans (the biological carbon pump). The physiology and biochemistry of pelagic diatoms have been studied in depth, but there are still many gaps in the mechanisms by which they can biosynthesize biogenic silica under natural conditions far from those required for glass production in industry. Their role in the biological carbon pump and more generally the link between the Si and C cycles must also be reassessed.

In addition, recent meetings between international silicon specialists initiated by LEMAR (SILICAMICS and SILICAMICS 2) have shown that silicifiers other than pelagic diatoms can no longer be neglected. We have therefore expanded our research to include benthic diatoms, ice diatoms, sponges, picocyanobacteria, and some rhizaria that contribute to the dynamics of the silicon cycle and the functioning of many ecosystems more significantly than previously thought.