Vertical re-distribution of microplastics particles in sea ice due to cooling/warming cycles: A laboratory experiment

Microplastics (MPs) accumulate in natural sea ice, as shown by field observations. However, the distribution of MPs particles in vertical is rather "chaotic". Since the mechanisms of interaction of MPs with sea ice are still poorly known, and sea ice per se is a complex medium, the laboratory experiments are the most effective tool in unravelling the processes behind the observed pattern. A series of laboratory experiments were performed to attend the processes of penetration from the ice surface and the bottom of the container into the ice body of polystyrene fragments (density 1.05 g/cm³, size range 0.2-0.5 mm), and their re-distribution in vertical when the ice core is transferred into permeable regime due to cooling/warming cycles. All four identified MPs re-location mechanisms - sinking with newly-formed brine, floating with gas bubbles, adjusting to the brine density within the channels, and following convective circulations in the mushy layer - led in our experiments to the displacement of the center of mass of plastics in ice cores during the first 2-3 cooling/warming cycles. Remarkably, the order of magnitude of both upward and downward advancement of the center of mass of MPs through the ice is merely the same - units of mm per week. Even though very approximate, this value might be useful for better understanding of the observed field patterns of natural ice contamination with MPs and for its modelling. Possible spatiotemporal variability of the buoyancy of MPs particles in natural sea ice brine is highlighted for the first time.