Microplastics in entire sea ice cores

To quantify the MP concentration and composition we obtained ice cores during expeditions of the German research ice breaker Polarstern in spring 2014 and summer 2015 in the Fram Strait and Central Arctic (Fig. 1a, Table 1). The highest MP particle concentration ((1.2 ± 1.4) × 107 N m−3) was detected in an ice core taken in the pack ice of Fram Strait (core B; Fig. 1b). The MP concentrations of all sea ice cores were highly variable with the second highest MP concentration found in the land-fast ice of the Fram Strait (core A; (4.1 ± 2.0) × 106 N m−3). The MP load of core C and E, collected north of Svalbard and in the Nansen Basin respectively, varied between (2.9 ± 2.4) × 106 N m−3 and (2.4 ± 1.0) × 106 N m−3. The lowest concentration was found in core D (1.1 ± 0.8) × 106 N m−3; Fig. 1b) from north of Svalbard. The values recorded in this study are two to three orders of magnitude higher than in a previous study from the Central Arctic2 (1.3–9.6) × 104 N m−3, values exclude rayon, for further details see method section), which can largely be explained by the different methodology used. In the previous study2, the filter area was first inspected by light microscopy and suspected MP particles were then analyzed individually by Fourier-transform infrared (FTIR) microscopy. In contrast, we used Imaging FTIR27,28, where entire areas were scanned. This excluded the human bias introduced by visual selection of particles (Fig. 2a). Imaging FTIR includes the far more informative infrared region of the spectrum from the very onset of the analysis and enables the detection of very small particles (down to 11 µm), which are most likely overlooked by visual inspection and therefore not included in the majority of the previously published studies. By using this approach, we were able to show that most of the MP particles identified in the sea ice cores were smaller than 50 µm. On average 67% of the particles were within the currently smallest detectable size class of 11 µm (Fig. 3). Such small particles were not considered in the previous study on MP in sea ice by Obbard et al.2. Concerning the extremely high error values calculated for the individual samples it should be noted, that they result from the analyses of three different areas per filter indicating an unequal particle distribution (Fig. 2b). In contrast to our previous studies30,31, the samples were not macerated beforehand and only H 2 O 2 was applied after filtration to remove natural organic residues. MP particles tend to form hetero-aggregates with microalgae or natural organic matter32 and therefore it is likely that these aggregates were printed on the membranes by filtration. Thereafter the organic matrix was removed by wet oxidation before the Imaging FTIR measurement. However, it should be noted that, to the best of our knowledge, our study is the first to indicate and document an uneven MP particle distribution on filters overall. With respect to the early days of direct bacterial counting, where similar observations were made, future MP studies should address this problem by improving the general sample preparation or by applying a statistically valid recording approach as used in direct bacterial counting33. We provide our measurements as particle count per volume for consistency with previous studies. However, we suggest that future studies also consider polymer-specific MP mass per volume data34 to allow for calculation of fluxes or total load of synthetic polymers (independently of the degree of fragmentation).

Fig. 1 Pathway and microplastic content of sea ice cores in the Central Arctic. a Sampling position of sea ice cores (A–E) obtained during three Polarstern expeditions overlaid with the sea ice concentration (June 2014) and a schematic view of the major cold and warm water currents. Blue arrows indicate the inflow of Pacific water. For comparison, previously sampled sea ice cores2 are included (Ha–Hd); b Total microplastic (MP) particle load m−3 of the various sea ice cores (this study) and data reproduced from Fig. 2 of Obbard et al.2*; c Average % composition of polymers (polyethylene (PE), varnish (including polyurethanes and polyacrylates), polyamide (PA), ethylene vinyl acetate (EVA), cellulose acetate (CE-Alkylated), polyester (PES) and polypropylene (PP) and others) from the entire core (this study) and digitized data of figure two from Obbard et al.2*, acrylic equals varnish (others include acrylonitrile butadiene, chlorinated polyethylene, nitrile rubber, polycaprolactone, polycarbonate, polylactic acid, polyimide, polystyrene, polyvinyl chloride, rubber); d Drift trajectories of sea ice cores, except for land-fast ice station of Greenland (A) and the sample originating from the Chukchi Shelf Ha. The map was created using ArcGIS 10.3 and based on the General Bathymetric Chart of the Oceans (GEBCO)-08 grid, version 20100927, http://www.gebco.net, with permission from the British Oceanographic Data Centre (BODC). *The polymer rayon was excluded Full size image

Table 1 Sea ice core sampling and accompanying information table Full size table

Fig. 2 Images of the microplastic analysis. a Overview image collected by the fourier-transform infrared imaging (FTIR) microscope prior to measurement. b Polymer dependent false-colour image of an exemplary measurement field after FTIR measurement and automated analysis Full size image

Fig. 3 Size classes of observed microplastic particles. Box and whiskers plot of percentage (%) shares of MP numbers in different size classes in all sea ice cores. The boundary of the box closest to zero indicates the 25th percentile, the line within the box marks the median and the boundary of the box farthest from zero indicates the 75th percentile. Whiskers (error bars) above and below the box indicate the 90th and 10th percentiles. Blue lines indicate the mean and black circles indicate outliers Full size image

In total, 17 different polymer types were identified (Supplementary Fig. 1; Supplementary Table 1), with polyethylene (PE), varnish (including polyurethanes and polyacrylates), polyamide (PA) also called nylon, ethylene vinyl acetate (EVA), cellulose acetate (CE alkylated), polyester (PES) and polypropylene (PP) contributing on average between 48% (PE) and 1.65% (PP) to the total measured MP composition. The following were on average below 1%: nitrile rubber, rubber, polystyrene (PS), polylactic acid (PCA), polyvinyl chloride (PVC), chlorinated polyethylene (PE-Cl), polycarbonate (PC), polycaprolactone (PCL), acrylonitrile butadiene (AB) and polyimide (PI) (summarized as others in Figs. 1 and 4). Overall, the MP composition of the sampled sea ice cores was variable with PE being almost exclusively found in the upper horizons of core B sampled in the Fram Strait (above 90%; Fig. 1c). PE, which is among the economically most important polymers35, also dominated the other core from the Fram Strait (A) and the cores retrieved north of Svalbard (core C, D; Fig. 1c). In these cores, PA, which is usually associated with fishing gear35, accounted for 6 and 22% of the MPs. Both cores also contained varnish, which dominated core E taken in the Nansen Basin (Fig. 1c) and was present in the land-fast sea ice (core A, Fig. 1c). The polymer type varnish includes the previously described acrylic polymer type, known to account for up to 10% of the MP in marine systems35. Core A and E also shared a relatively high proportion of EVA (up to 10%). The latter core was further characterized by almost 9% of CE-Alkylated (Fig. 1c), which is indicative of cigarette filters and commonly found in ocean debris22. Except for PE, overall the sea ice cores only partly reflect the composition of the globally produced polymers, which are dominated by PE, followed by PP, PVC, PS, PUR and polyethylene terephthalate (PET)35.

Fig. 4 Vertical distribution of ancillary data in sea ice cores. a Refers to the salinity (PSU), b refers to temperature (°C), c refers to particulate organic carbon content (POC; mg×L−1) and d refers to chlorophyll a concentration (µg×L−1) for each core. Steps indicate the sampling horizons taken for each core Full size image

Sea ice trajectories and MP comparison to a previous study

Large portions of sea ice are formed on the Siberian shelves9. Depending on individual ice floe drift patterns, they pass through different regions of the Central Arctic Ocean but are eventually carried to the Fram Strait via the Transpolar Drift9,36. To determine drift trajectories and source areas of sampled sea ice we tracked sampled sea ice backwards in time using low-resolution ice drift and concentration products from passive microwave satellites17. This back-tracking approach17 showed that the sea ice samples originated from different source areas, namely the Amerasian and Eurasian Basins (Fig. 1d; Table 1). In particular core B can be retraced to the Makarov Basin, while the cores within the Eurasian Basin originated from the Laptev Sea (core E), near Franz Josef Land (core D) and the deeper Nansen Basin (core C). Except for the land-fast ice, east of Greenland (core A), all ice cores encountered the main path of the Transpolar Drift pack ice9,36. To enable a comparisons with Obbard et al.2, we applied the back-tracking approach to the four sea ice cores from that study (Fig. 1d). The origin of the sea ice cores from the earlier study2 only overlap in the region of the Laptev Sea (core Hd) with one of our cores (E), while the other cores can be related to the Beaufort Gyre (core Hc), the Chukchi Shelf (core Ha) and the East Siberian Sea (Fig. 1d). The strikingly high contribution of varnish (58%) in the sea ice cores originating from the Laptev Sea in our study (core E; Fig. 1c) did not feature at all in the earlier core Hd. However, even in the previous study, acrylic (similar type as varnish) was present in one core (Hb) and the contribution was in the same order of magnitude as described in other studies35. PA seemed to be a common compound observed in eight out of the nine investigated cores in both studies (Fig.1c) and was particularly dominant in the core from the Chukchi Shelf (core Ha). Ice cores analyzed by Obbard et al.2 also showed high concentrations of PET (Fig. 1c), which might be partly reflected in our polymer type PES. The large difference between both studies might be related to our exclusion of fibres. Alternatively, it might reflect the increased effort to recycle this particular compound37.

Ancillary variables and MP along sea ice cores

Since the formation and growth of sea ice is a process in space and time, it is most likely, that the MP composition of the seawater–ice interface reflects the respective water body, which is in contact with the drifting sea ice floes as shown by the drifting path of the sampled ice floes (Fig. 1d; Supplementary Fig. 2). Given our current lack of knowledge about MP contamination of surface waters from the Central Arctic the mere consideration of the complete core is probably not sufficient with respect to the spatiotemporal history of the sea ice floe. In order to consider this spatiotemporal history we studied the vertical distribution of MP in the context of environmental and biological variables (ancillary variables) in each horizon (~20–30 cm sections) of the sea ice cores (Figs. 4 and 5). Temperature and salinity measurements suggest that the sampled sea ice consisted of first and second year ice (Fig. 4). Second year ice was indicated by a low salinity in the surface (core C & E). With a maximum of 1.67 m (core E), sea ice thickness was very low for spring conditions, but is in agreement with the observed reduction of sea ice in the Fram Strait36 and Central Arctic38. The highest concentrations of ice algal biomass, as indicated by chlorophyll a, were mostly found in the lowest core horizons (Fig. 4; max. 6.64 µg L−1), with a general L-shaped distribution pattern. Only the land-fast-ice core (A) had a maximum concentration at the top. Particulate organic carbon (POC) had a maximum of 1.34 mg L−1 and also showed an L-shaped distribution pattern (Fig. 4). Only the land-fast ice core (A) and one core sampled north of Svalbard (D) had their POC maximum in the middle of the core.

Fig. 5 Vertical distribution of microplastic in sea ice cores. a refers to the concentration of microplastic particles (in N×L−1) for each core. b refers to the polymer composition for each core: polyethylene (PE), varnish (including polyurethane and polyacrylate), polyamide (PA), ethylene vinyl acetate (EVA), cellulose acetate (CE-Alkylated), polyester (PES) and polypropylene (PP) and others. c refers to polymer richness (N), and d refers to the Shannon–Wiener index (H'). Steps indicate the sampling horizons taken for each core Full size image

In contrast to these generally L-shaped vertical profiles of biological variables, MP quantities in the various sea ice core horizons were extremely variable with concentrations ranging between 33 and 75,143 L−1 (Fig. 5, units are adjusted to environmental data (L−1)). The latter concentration was found in the surface horizon of core B originating from the Makarov Basin (Fig. 1d). Another high surface value with 6421 L−1 was observed in core E, while all other cores contained relative low MP concentration in the top 20 cm (max. of 992 MP L−1). The core originating from the Laptev Sea (E) displayed another MP maximum in the bottom horizon at the seawater–ice interface (4437 L−1), while core C was characterized by a sub-surface maximum (Fig. 5). All other cores were characterized by variable concentrations reaching a maximum concentration of particles throughout the middle of the ice core, between 4159 and 13,794 L−1 (Fig. 5). The synthetic polymer composition of the individual horizons showed strong differences within a single sea ice core (Fig. 5). PE dominated the synthetic polymers in the top 100 cm of the sea ice core B originating from the Makarov Basin (Fig. 5). PE was also present in all other cores but with highly variable contributions in the different horizons. In the core originating from the Makarov Basin (B), a change in salinity was associated with a drop in MP numbers and a shift to a more complex polymer composition, with high proportion of varnish in the bottom horizon (Figs. 4 and 5).

However, only few significant correlations between MP numbers, MP derived diversity indices (N, H', Fig. 5), MP sizes and ancillary variables were found (Supplementary Table 2). Highest Spearman rank correlations (negatively correlated) were found between chlorophyll a, total MP numbers, numbers of PE particles and the numbers of particles in the six lower size classes (covering the sizes 11–125 µm). This underscores that sea ice does not have a uniform MP imprint throughout the whole ice body and that the MP numbers or the polymer composition cannot be explained by ancillary variables recorded in parallel in the different horizons. This finding might point to a patchy distribution of MP in Arctic waters where local MP populations of the respective waterbodies are archived when in contact with the ice–seawater interface. This is evident for sea ice cores C and D, which had extremely different polymer distributions even though both sampling locations were in close proximity north of Svalbard. Indeed, the back trajectories of these two cores revealed different sea ice origins and pathways along the Transpolar Drift (Fig. 1d). The core originating from Franz Josef Land (core D), showed two horizons dominated by PE polymers, with varnish and PA distributed along the core, particularly in the two lowest horizons. In contrast, the core originating from the deeper Nansen Basin (C) showed a strong variability of polymer distributions in all horizons with a higher proportion of PE and varnish and with the occurrence of PA, EVA and PES (Fig. 5). The highest contribution of varnish in all sea ice core horizons was evident in the core originating from the Laptev Sea (E). The middle part of this sea ice core was further characterized by various proportions of CE-Alkylated. This compound was also found in one horizon of core B, together with PP, which was also present in few horizons from core C (Fig. 5). Overall results from the 1D sea ice growth model highlight localized polymer entrainment (Supplementary Fig. 3), whereby, e.g., PE is present in high concentrations associated with the Atlantic and Pacific inflow in the Central Arctic, while e.g. varnish and EVA are more concentrated in the eastern region of the Eurasian basin (for details see Supplementary Fig. 3).

Cluster analyses of MP numbers, MP sizes and ancillary variables resulted in contrasting sample groups. No clear groups of the different cores or horizons were obtained for MP numbers and sizes (Supplementary Figs. 4, 5 and 6). In contrast, the ancillary variables grouped cores A, B and E and C and D (Supplementary Fig. 5). These results were also supported by the SIMPROF analysis. The clear separation found in the ancillary data might be related to the drift trajectories of the floes, where cores A, B and E follow the main path of the Transpolar Drift9, while cores C and D drifted nearer the Atlantic water inflow10. Cores C & D (SIMPROF group c) displayed significantly higher Chl a concentrations, salinities and C/N-ratios compared to A, B and E (SIMPROF group d). Still, POC, PON and temperature were significantly higher in cores A, B and E (Supplementary Table 2). However, when superimposing the SIMPROF grouping of the ancillary variables on the polymer-specific MP numbers or MP derived diversity indices in the different core horizons, with the exception of nitrile rubber and PVC-particles there was no significant difference displayed in average between the two groups. This was also the fact for the MP numbers in all size classes (Supplementary Table 3 and Supplementary Data 1 and 2). Overall, it can be assumed that all environmental and biological variables are characterized by a strong seasonality39,40, while MP particles once they are incorporated into the sea ice during sea ice growth seem to be more stationary in the ice core matrix.