On the vertical distribution of microplastics particles in sea ice cores

Sea ice is shown to be heavily contaminated by microplastics (MPs, <5 mm), but its vertical distribution is not consistent. Previous investigations suggested that contamination by MPs is size-specific and time variable. To attain the level of contamination and vertical distribution of MPs in the ice of the Novik bay (Peter the Great Bay, Sea of Japan), three ice cores (ca. 100 m apart from one another, thickness ~40 cm) were taken from the one-year sea ice on 8 March 2020. One core was used to obtain the salinity profile within the ice, while two other cores were segmented into 5-cm thick layers and analyzed for MPs content using different methods: µ-FTIR analysis for particles in the size range of 25-300 µm (S-MPs), and visual identification followed by Raman spectroscopy for 300 µm – 5 mm particles (L-MPs).MPs particles of both size ranges were found in all layers of the cores. Contamination in upper\lower layers was up to 3 times larger than in the middle of the cores (observed in both size ranges). The majority (45%) of L-MPs are 1-2 mm in size. Preliminary, the level of contamination by MPs in the ice of the Novik bay is of the order of 200-300 items per liter of melted water (in each of the size ranges), which confirms a certain deficit of smaller MPs within the ice. In shape, fibers (53%) followed by fragments (44%) are prevalent types of L-MPs, while among S-MPs more than 60% are films. Ice salinity (averaged over 5 cm depth) generally increases with depth (from 4 to 8 psu), however has a minimum (3 psu) in the layer 10-15 cm. Minima of MPs content are in the layers 20-25 cm (for S-MPs) and 5-15 cm (for L-MPs). Environmental conditions are analyzed and physical processes are discussed, which might have led to the observed MPs distribution.Investigations are supported by RSF via grant No 19-17-0041.