Frazil Ice Formation Causes Divergent Levels of Microplastic and Nanoplastic Accumulation in Sea Ice

Plastic pollution is found in polar sea ice with concentrations of microplastics (MPs, 1 μm–5 mm) 2 to 4 orders of magnitude higher than in the underlying seawater. Such accumulation is often attributed to the scavenging of MPs by rising frazil ice crystals, but there is no direct evidence of such a process. Furthermore, nanoplastics (NPs, 1 nm–1 μm) are suspected to be present, but their fate is unknown due to the lack of field measurements. Here micro- and nanoplastics’ (MNPs) enrichment by frazil ice was quantified using model particles and an experimental setup which generated realistic frazil ice. Particle size had a significant impact on behavior: on one hand, high-density MPs concentrations were 2.97 ± 1.13 times higher in the ice than in the underlying water, which is consistent with field observations. On the other hand, NPs were depleted to the same extent as sea salts, by a factor of 0.32 ± 0.13. Like salts, NPs may be locally enriched in the brine channels and pockets of sea ice where microalgae thrive. Overall, this work shows that frazil ice plays an important role in the cycling of MNPs, as it can concentrate MPs in sea ice and concentrate NPs locally in brine channels and pockets.