The Arctic Ocean as a dead end for floating plastics in the North Atlantic branch of the Thermohaline Circulation

Abstract

The subtropical ocean gyres are recognized as great marine accummulation zones of floating plastic debris; however, the possibility of plastic accumulation at polar latitudes has been overlooked because of the lack of nearby pollution sources. In the present study, the Arctic Ocean was extensively sampled for floating plastic debris from the Tara Oceans circumpolar expedition. Although plastic debris was scarce or absent in most of the Arctic waters, it reached high concentrations (hundreds of thousands of pieces per square kilometer) in the northernmost and easternmost areas of the Greenland and Barents seas. The fragmentation and typology of the plastic suggested an abundant presence of aged debris that originated from distant sources. This hypothesis was corroborated by the relatively high ratios of marine surface plastic to local pollution sources. Surface circulation models and field data showed that the poleward branch of the Thermohaline Circulation transfers floating debris from the North Atlantic to the Greenland and Barents seas, which would be a dead end for this plastic conveyor belt. Given the limited surface transport of the plastic that accumulated here and the mechanisms acting for the downward transport, the seafloor beneath this Arctic sector is hypothesized as an important sink of plastic debris.

Keywords: Arctic waters; North Atlantic; floating plastic debris; thermohaline circulation.

Fig. 1. Environmental conditions and concentrations of floating plastic debris in the Arctic Ocean.

(A) At-sea vessel density. (B) Human population and Arctic sea ice extent. Annual minimum and maximum ice extents correspond to the monthly mean of September and March, respectively. Historic data account for the 1981 to 2010 median. (C) Remote-sensed sea surface salinity for August 2013 at the mid-sampling period. The seasonal cycle is shown in fig. S1. psu, practical salinity units. (D) Locations and plastic concentrations of the sites sampled. The summer extension of the polar ice cap in August 2013 is shown in white area, and the classical schematic drawing of the North Atlantic SOG and the THC poleward branch is indicated by green curves (8). The northern passage from Barents Sea to Kara Sea is zoomed in, with contour lines describing salinity measured at a depth of 5 m. (E) Plastic concentrations as total weight (upper graph) and abundances per plastic type (lower graph) along the circumpolar track, from the Greenland Sea to the Labrador Sea, as indicated in the left map by the black line connecting the sampling sites. Salinity at depths of 5 and 20 m is also shown in the upper graph, and two dashed lines are used as reference for 34.5 salinity and the median of plastic concentrations measured in the inner accumulation zones of SOGs [175 g·km⁻² (2)]. The correlation between plastic and salinity is shown in fig. S2. Note that the plastic fibers, presented in the lower graph as a dotted line, were excluded from all our analyses, including total plastic concentrations and load estimates in both weight and number. The Arctic Polar Circle (66.34°N) is marked in all maps.

Fig. 2. Typology and size distribution of the floating plastic debris collected in the Arctic Ocean compared to the plastic accumulation zones in the SOGs and the Mediterranean Sea.

In the pie charts, the percentages of plastic types are shown in relation to weight (charts showing the number of items and surface area are presented in fig. S3). The size distributions are presented in the lower graph. Horizontal axes indicate both log-transformed and nontransformed size limits of the bins. Plastics in the interval from 0.32 to 0.50 mm are graphed using open circles because these abundances are possibly underestimated for the Arctic due to the sampling net with combined 0.5- and 0.33-mm meshes for the body and cod end, respectively. Sample collections for the SOGs (4173 items) and the Mediterranean (3854 items) are described in previous reports (2, 7). The total number of items used for the analyses in the Arctic was 796; absolute abundances for each size bin are provided in table S1. The number of large items (>12.6 mm) in relation to the total was 2.1% (17 items), 3.2% (134 items), and 4.4% (170 items) for the Arctic Ocean, the SOGs, and the Mediterranean Sea, respectively.

Fig. 3. Oceanic pathway of the plastic accumulations in the Greenland Sea (upper maps) and the Barents Sea (lower maps) obtained by simulations backward in time.

Tracers were released at the locations of maximal plastic concentrations in Greenland and Barents seas (red circles), and their surface transport was modeled for the previous years (1 to 3 years). Units are expressed as percentage of tracers in each pixel. The background image for the ocean shows the vessel density in gray scale. Note the close agreement between the modeled plastic pathway and the THC route described in the literature (Fig. 1). Likewise, the tracers released in the Northeastern Barents Sea were placed 1 year before into a zone where high plastic concentrations were also measured.