From iron cores to estuaries: the interaction of environmental flows with their transported particles

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• Dr Quentin Kriaa, University of Twente
• Monday 11 November 2024, 13:00-14:00
• MR3, CMS.

If you have a question about this talk, please contact Kasia Warburton.

The difficulty to resolve large-scale environmental flows down to the size of the small particles they transport imposes to use simplified models to predict them. In two contexts, this seminar offers observations of subtle yet impactful effects of the dynamics of particles that should be incorporated to refine such models.

We will start with ‘iron snow’ on Ganymede, a natural satellite of Jupiter. The solidification of its core leads to the formation of pure iron crystals at the core periphery, that sink deep in the core due to gravity. The settling dynamics of a cloud of crystals is modelled experimentally with particle clouds of sub-millimetric glass spheres settling in water. These clouds grow with depth due to the entrainment of ambient water into the clouds. The canonical model of entrainment by Morton et al. 1956 would predict their growth rate to be unaffected by the particles’ size. Yet, their growth rate is maximum for a specific particle size. This optimum originates from the partial decoupling between the settling particles and the flow, as explained with complementary numerical simulations.

When iron crystals reach very large temperatures in Ganymede’s core, they remelt. Their dense molten snow drives a compositional convection that is assumed vigorous enough to power a magnetic field through dynamo. To test this assumption, experiments are conducted where sugar grains (aka the iron snow flakes) are continuously sieved above a water tank (aka the deep convective core). The size of grains controls particle-scale interactions with the flow, with a critical influence on the length scales and velocity scale of convection, on the laminar/turbulent nature of the flow, and on the depth where sugar grains fully dissolve – with paramount implications for the emergence of dynamo in Ganymede.

We will finally focus on the dispersal of microplastics (MP) in the ocean. Although most plastics have a density lower than sea water and are thus expected to float, recent observations show that the sea floor is an important sink of MP. To understand why, I analyse the transport of light-density MP by turbidity currents driven by avalanching glass spheres (aka sediments) in the lab. Unexpectedly, MP deposit during experiments. In-situ observations reveal that glass spheres attach to the plastics. This aggregation massively affects the transport of MP by enabling their deposition, by delaying the detrainment of rising MP, and by enhancing the transport of aggregates farther than the runout distance of sediments. These experiments suggest that buoyant MP entering the ocean from estuaries may be deposited along the continental margins when carried to the abyss by turbidity currents.

This talk is part of the Quantitative Climate and Environmental Science Seminars series.

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