Seafloor microplastic hotspots controlled by deep-sea circulation
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
REPORTS
Cite as: I. A. Kane et al., Science
10.1126/science.aba5899 (2020).
Seafloor microplastic hotspots controlled by deep-sea
circulation
Ian A. Kane1*, Michael A. Clare2, Elda Miramontes3,4, Roy Wogelius1, James J. Rothwell5,
Pierre Garreau6, Florian Pohl7
1School of Earth and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK. 2National Oceanography Centre, University of Southampton Waterfront
Campus, Southampton SO14 3ZH, UK. 3Faculty of Geosciences, University of Bremen, 28359 Bremen, Germany. 4MARUM-Center for Marine Environmental Sciences,
University of Bremen, 28359 Bremen, Germany. 5Department of Geography, University of Manchester, Manchester M13 9PL, UK. 6IFREMER, Univ. Brest, CNRS UMR 6523,
IRD, Laboratoire d’Océanographie Physique et Spatiale (LOPS), IUEM, 29280, Plouzané, France. 7Department of Earth Sciences, Durham University, Durham DH1 3LE, UK.
*Corresponding author. Email: ian.kane@manchester.ac.uk
While microplastics are known to pervade the global seafloor, the processes that control their dispersal and
Downloaded from http://science.sciencemag.org/ on December 23, 2020
concentration in the deep sea remain largely unknown. Here we show that thermohaline-driven currents,
which build extensive seafloor sediment accumulations, can control the distribution of microplastics and
create hotspots of up to 1.9 million pieces m−2. This is the highest reported value for any seafloor setting,
globally. Previous studies propose that microplastics are transported to the seafloor by vertical settling
from surface accumulations; here we demonstrate that the spatial distribution and ultimate fate of
microplastics is strongly controlled by near-bed thermohaline currents (bottom currents). These currents
are known to supply oxygen and nutrients to deep sea benthos, suggesting that deep sea biodiversity
hotspots are also likely to be microplastic hotspots.
Plastic pollution has been observed in nearly all environ- Microplastics on the deep seafloor are preferentially fo-
ments on Earth (1) and across all of its oceans (2–4). The ef- cused within distinct physiographic settings, rather than cor-
fects of plastic pollution on marine ecosystems and responding to the extent of overlying surface garbage patches
implications for human health are of growing concern, as (7, 19). Submarine canyons and deep ocean trenches, which
more than ten million tonnes of plastic enter the global ocean are foci for episodic, yet powerful, gravity flows, appear to be
each year (4–6). Converging surface currents in oceanic gyres microplastic hotspots (7, 20, 25–27) (Fig. 2A). This physio-
are responsible for the global distribution of plastics on the graphic bias suggests that the transfer of microplastics to and
ocean surface (2, 3). These gyres effectively concentrate posi- across the deep seafloor is therefore not solely explained by
tively buoyant plastics into the now infamous “garbage vertical settling from surface gyres. In the same manner as
patches” (2, 3). However, sea surface accumulations only ac- surface currents disperse and concentrate microplastics, it is
count for approximately 1% of the estimated global marine likely that deep sea currents play a similar role (26, 28), yet a
plastic budget (3, 4, 7, 8). Most of the missing 99% of plastic paucity of contextual data (e.g., bathymetric, oceanographic,
ends up in the deep sea (7–9) (Fig. 1A). A considerable pro- and sedimentological) hinders the linkage of physical
portion [estimated at 13.5% (8)] of the marine plastic budget transport processes to the distribution and ultimate fate of
occurs as microplastics: small (
resolution seafloor and ocean circulation data afford the spa- seafloor as determined from hydrodynamic modeling (Fig. tial and temporal context to investigate our key questions. We 3B). These variations in shear stress explain the localized sea- analyze data from the Tyrrhenian Sea, where ocean water cir- floor distribution of microplastic particles, which typically culation is driven by the East Corsican Current (ECC) and its have lower densities than silt- and sand-forming minerals return branch (Figs. 1C and 2E) which reaches local velocities and therefore are more easily entrained (39), accounting for of >0.4 m s−1 near the surface and >0.2 m s−1 near the seafloor depleted levels of microplastic within certain physiographic (30). The strongest bottom currents generally occur between domains and concentration within others (Figs. 2D and 3C). 600 and 900 m water depth, where they actively sculpt exten- The lowest concentrations of microplastics are found within sive muddy contourite drifts [Fig. 1D;
more widely the Mediterranean Sea, circulating contour cur- REFERENCES AND NOTES
rents are likely to preferentially accumulate microplastics
1. M. L. Taylor, C. Gwinnett, L. F. Robinson, L. C. Woodall, Plastic microfibre ingestion
within contourite drifts. On open continental slopes, contour by deep-sea organisms. Sci. Rep. 6, 33997 (2016). doi:10.1038/srep33997
currents may disperse rather than concentrate microplastics. Medline
In such settings, they may play a key role in their spatial seg- 2. M. Eriksen, L. C. M. Lebreton, H. S. Carson, M. Thiel, C. J. Moore, J. C. Borerro, F.
regation into hotspots. Galgani, P. G. Ryan, J. Reisser, Plastic pollution in the world’s oceans: More than 5
trillion plastic pieces weighing over 250,000 tons afloat at sea. PLOS ONE 9,
We have shown that the overall pattern of bottom cur- e111913 (2014). doi:10.1371/journal.pone.0111913 Medline
rents controls the distribution of microplastics at seafloor. 3. E. van Sebille, C. Wilcox, L. Lebreton, N. Maximenko, B. D. Hardesty, J. A. van
Numerical modeling and direct measurements in the Tyrrhe- Franeker, M. Eriksen, D. Siegel, F. Galgani, K. L. Law, A global inventory of small
nian Sea reveal a pronounced seasonal variation in bottom floating plastic debris. Environ. Res. Lett. 10, 124006 (2015). doi:10.1088/1748-
9326/10/12/124006
current velocities, with bottom currents being most intense 4. J. R. Jambeck, R. Geyer, C. Wilcox, T. R. Siegler, M. Perryman, A. Andrady, R.
in the winter (31). Modeling of microplastic transport shows Narayan, K. L. Law, Plastic waste inputs from land into the ocean. Science 347,
that some of the near-bed bottom current shear stresses are 768–771 (2015). doi:10.1126/science.1260352 Medline
close to, or in excess of, that required to entrain both fibers 5. R. Geyer, J. R. Jambeck, K. L. Law, Production, use, and fate of all plastics ever
made. Sci. Adv. 3, e1700782 (2017). doi:10.1126/sciadv.1700782 Medline
Downloaded from http://science.sciencemag.org/ on December 23, 2020
and fragments (Figs. 3A and 4). While low intensity currents 6. L. C. M. Lebreton, J. van der Zwet, J.-W. Damsteeg, B. Slat, A. Andrady, J. Reisser,
in the summer may allow for the accumulation of microplas- River plastic emissions to the world’s oceans. Nat. Commun. 8, 15611 (2017).
tics at seafloor, at some locations (in particular contourite doi:10.1038/ncomms15611 Medline
moats) previously deposited microplastics may be re-ex- 7. R. C. Thompson, Y. Olsen, R. P. Mitchell, A. Davis, S. J. Rowland, A. W. G. John, D.
McGonigle, A. E. Russell, Lost at sea: Where is all the plastic? Science 304, 838–
humed as shear stresses exceed the critical boundary for re- 838 (2004). doi:10.1126/science.1094559 Medline
mobilization and suspension (Fig. 3A). More powerful, but 8. A. A. Koelmans, M. Kooi, K. L. Law, E. van Sebille, All is not lost: Deriving a top-down
ephemeral seafloor flows such as gravity currents that have mass budget of plastic at sea. Environ. Res. Lett. 12, 114028 (2017).
been recorded in deep-sea submarine canyons, can reach ve- doi:10.1088/1748-9326/aa9500
9. C. A. Choy, B. H. Robison, T. O. Gagne, B. Erwin, E. Firl, R. U. Halden, J. A. Hamilton,
locities two order of magnitude greater than the bottom cur- K. Katija, S. E. Lisin, C. Rolsky, K. S. Van Houtan, The vertical distribution and
rents in the Tyrrhenian Sea (up to 20 ms−1) and can last for biological transport of marine microplastics across the epipelagic and
several days (40–42). These powerful events will undoubtedly mesopelagic water column. Sci. Rep. 9, 7843 (2019). doi:10.1038/s41598-019-
flush accumulated microplastics either further down-slope 44117-2 Medline
10. L. Van Cauwenberghe, L. Devriese, F. Galgani, J. Robbens, C. R. Janssen,
(43) or loft them for recirculation by thermohaline bottom Microplastics in sediments: A review of techniques, occurrence and effects. Mar.
currents (Fig. 5). Such gravity flows play a globally important Environ. Res. 111, 5–17 (2015). doi:10.1016/j.marenvres.2015.06.007 Medline
role in the lateral transport of lightweight particulate matter 11. A. Vianello, A. Boldrin, P. Guerriero, V. Moschino, R. Rella, A. Sturaro, L. Da Ros,
such as organic carbon (41, 44); hence, it stands to reason Microplastic particles in sediments of Lagoon of Venice, Italy: First observations
on occurrence, spatial patterns and identification. Estuar. Coast. Shelf Sci. 130,
that they should also be important for microplastics 54–61 (2013). doi:10.1016/j.ecss.2013.03.022
transport. Episodic flushing of submarine canyons (41, 42, 45) 12. V. Zitko, M. Hanlon, Another source of pollution by plastics: Skin cleaners with
suggests that canyons may only be temporary microplastic plastic scrubbers. Mar. Pollut. Bull. 22, 41–42 (1991). doi:10.1016/0025-
storage sites (26). This is analogous to rivers where flooding 326X(91)90444-W
13. S. A. Mason, D. Garneau, R. Sutton, Y. Chu, K. Ehmann, J. Barnes, P. Fink, D.
can flush high levels of microplastics downstream (46). Papazissimos, D. L. Rogers, Microplastic pollution is widely detected in US
Bottom currents are efficient conveyors of nutrients and municipal wastewater treatment plant effluent. Environ. Pollut. 218, 1045–1054
oxygen, and consequently dictate the location of important (2016). doi:10.1016/j.envpol.2016.08.056 Medline
biodiversity hotspots (41, 47–49). Unfortunately, we show 14. M. A. Browne, P. Crump, S. J. Niven, E. Teuten, A. Tonkin, T. Galloway, R.
Thompson, Accumulation of microplastic on shorelines worldwide: Sources and
that the same seafloor currents can also transport and em- sinks. Environ. Sci. Technol. 45, 9175–9179 (2011). doi:10.1021/es201811s
place microplastics. The highest concentrations of microplas- Medline
tics on the seafloor occur in contourite drifts formed by 15. A. L. Andrady, Microplastics in the marine environment. Mar. Pollut. Bull. 62, 1596–
bottom currents, and their distribution is controlled by spa- 1605 (2011). doi:10.1016/j.marpolbul.2011.05.030 Medline
16. X. Tubau, M. Canals, G. Lastras, X. Rayo, J. Rivera, D. Amblas, Marine litter on the
tial variations in current intensity. How effectively microplas- floor of deep submarine canyons of the Northwestern Mediterranean Sea: The role
tics are buried, or become re-exhumed (and hence become of hydrodynamic processes. Prog. Oceanogr. 134, 379–403 (2015).
more available for trophic transfer) depends on temporal doi:10.1016/j.pocean.2015.03.013
fluctuations in current intensity. Although there are ongoing 17. E. D. Goldberg, Plasticizing the seafloor: An overview. Environ. Technol. 18, 195–
201 (1997). doi:10.1080/09593331808616527
efforts to reduce the release of plastics into the environment, 18. A. Ballent, A. Purser, P. de Jesus Mendes, S. Pando, L. Thomsen, Physical transport
our oceans will continue to be impacted by the legacy of past properties of marine microplastic pollution. Biogeosciences Discuss. 9, 18755–
waste mismanagement (4, 5, 8, 50). Seafloor currents will play 18798 (2012). doi:10.5194/bgd-9-18755-2012
a crucial role in the future transfer and storage of microplas- 19. C. K. Pham, E. Ramirez-Llodra, C. H. S. Alt, T. Amaro, M. Bergmann, M. Canals, J.
B. Company, J. Davies, G. Duineveld, F. Galgani, K. L. Howell, V. A. I. Huvenne, E.
tics in the deep ocean. Isidro, D. O. B. Jones, G. Lastras, T. Morato, J. N. Gomes-Pereira, A. Purser, H.
Stewart, I. Tojeira, X. Tubau, D. Van Rooij, P. A. Tyler, Marine litter distribution and
First release: 30 April 2020 www.sciencemag.org (Page numbers not final at time of first release) 3
density in European seas, from the shelves to deep basins. PLOS ONE 9, e95839 38. A. Sanchez-Vidal, R. C. Thompson, M. Canals, W. P. de Haan, The imprint of
(2014). doi:10.1371/journal.pone.0095839 Medline microfibres in southern European deep seas. PLOS ONE 13, e0207033 (2018).
20. L. C. Woodall, A. Sanchez-Vidal, M. Canals, G. L. Paterson, R. Coppock, V. Sleight, doi:10.1371/journal.pone.0207033 Medline
A. Calafat, A. D. Rogers, B. E. Narayanaswamy, R. C. Thompson, The deep sea is a 39. I. A. Kane, M. A. Clare, Dispersion, accumulation, and the ultimate fate of
major sink for microplastic debris. R. Soc. Open Sci. 1, 140317 (2014). microplastics in deep-marine environments: A review and future directions. Front.
doi:10.1098/rsos.140317 Medline Earth Sci. 7, 80 (2019). doi:10.3389/feart.2019.00080
21. V. Fischer, N. O. Elsner, N. Brenke, E. Schwabe, A. Brandt, Plastic pollution of the 40. D. J. Piper, P. Cochonat, M. L. Morrison, The sequence of events around the
Kuril–Kamchatka Trench area (NW pacific). Deep Sea Res. Part II Top. Stud. epicentre of the 1929 Grand Banks earthquake: Initiation of debris flows and
Oceanogr. 111, 399–405 (2015). doi:10.1016/j.dsr2.2014.08.012 turbidity current inferred from sidescan sonar. Sedimentology 46, 79–97 (1999).
22. W. Courtene-Jones, B. Quinn, S. F. Gary, A. O. M. Mogg, B. E. Narayanaswamy, doi:10.1046/j.1365-3091.1999.00204.x
Microplastic pollution identified in deep-sea water and ingested by benthic 41. M. Azpiroz-Zabala, M. J. B. Cartigny, P. J. Talling, D. R. Parsons, E. J. Sumner, M. A.
invertebrates in the Rockall Trough, North Atlantic Ocean. Environ. Pollut. 231, Clare, S. M. Simmons, C. Cooper, E. L. Pope, Newly recognized turbidity current
271–280 (2017). doi:10.1016/j.envpol.2017.08.026 Medline structure can explain prolonged flushing of submarine canyons. Sci. Adv. 3,
23. A. J. Underwood, M. G. Chapman, M. A. Browne, Some problems and practicalities e1700200 (2017). doi:10.1126/sciadv.1700200 Medline
in design and interpretation of samples of microplastic waste. Anal. Methods 9, 42. J. J. Mountjoy, J. D. Howarth, A. R. Orpin, P. M. Barnes, D. A. Bowden, A. A. Rowden,
1332–1345 (2017). doi:10.1039/C6AY02641A A. C. G. Schimel, C. Holden, H. J. Horgan, S. D. Nodder, J. R. Patton, G. Lamarche,
24. A. R. Thurber, A. K. Sweetman, B. E. Narayanaswamy, D. O. B. Jones, J. Ingels, R. M. Gerstenberger, A. Micallef, A. Pallentin, T. Kane, Earthquakes drive large-scale
L. Hansman, Ecosystem function and services provided by the deep sea. submarine canyon development and sediment supply to deep-ocean basins. Sci.
Downloaded from http://science.sciencemag.org/ on December 23, 2020
Biogeosciences 11, 3941–3963 (2014). doi:10.5194/bg-11-3941-2014 Adv. 4, eaar3748 (2018). doi:10.1126/sciadv.aar3748 Medline
25. D. K. Barnes, F. Galgani, R. C. Thompson, M. Barlaz, Accumulation and 43. F. Pohl, J. T. Eggenhuisen, I. A. Kane, M. A. Clare, Transport and burial of
fragmentation of plastic debris in global environments. Philos. Trans. R. Soc. microplastics in deep-marine sediments by turbidity currents. Environ. Sci.
London Ser. B 364, 1985–1998 (2009). doi:10.1098/rstb.2008.0205 Medline Technol. 54, 4180–4189 (2020). doi:10.1021/acs.est.9b07527 Medline
26. A. Ballent, S. Pando, A. Purser, M. F. Juliano, L. Thomsen, Modelled transport of 44. V. Galy, C. France-Lanord, O. Beyssac, P. Faure, H. Kudrass, F. Palhol, Efficient
benthic marine microplastic pollution in the Nazaré Canyon. Biogeosciences 10, organic carbon burial in the Bengal fan sustained by the Himalayan erosional
7957–7970 (2013). doi:10.5194/bg-10-7957-2013 system. Nature 450, 407–410 (2007). doi:10.1038/nature06273 Medline
27. X. Peng, M. Chen, S. Chen, S. Dasgupta, H. Xu, K. Ta, M. Du, J. Li, Z. Guo, S. Bai, 45. M. Canals, P. Puig, X. D. de Madron, S. Heussner, A. Palanques, J. Fabres, Flushing
Microplastics contaminate the deepest part of the world’s ocean. Geochem. submarine canyons. Nature 444, 354–357 (2006). doi:10.1038/nature05271
Perspect. Lett. 9, 1–5 (2018). doi:10.7185/geochemlet.1829 Medline
28. I. E. Chubarenko, I. E. Esiukova, A. Bagaev, I. Isachenko, N. Demchenko, M. Zobkov, 46. R. Hurley, J. Woodward, J. J. Rothwell, Microplastic contamination of river beds
I. Efimova, M. Bagaeva, L. Khatmullina, “Behavior of microplastics in coastal significantly reduced by catchment-wide flooding. Nat. Geosci. 11, 251–257
zones” in Microplastic Contamination in Aquatic Environments (Elsevier, 2018), (2018). doi:10.1038/s41561-018-0080-1
pp. 175–223. 47. S. J. Hall, The continental shelf benthic ecosystem: Current status, agents for
29. M. Rebesco, F. J. Hernández-Molina, D. Van Rooij, A. Wåhlin, Contourites and change and future prospects. Environ. Conserv. 29, 350–374 (2002).
associated sediments controlled by deep-water circulation processes: State-of- doi:10.1017/S0376892902000243
the-art and future considerations. Mar. Geol. 352, 111–154 (2014). 48. C. Treignier, S. Derenne, A. Saliot, Terrestrial and marine n-alcohol inputs and
doi:10.1016/j.margeo.2014.03.011 degradation processes relating to a sudden turbidity current in the Zaire canyon.
30. S. Vignudelli, P. Cipollini, M. Astraldi, G. P. Gasparini, G. Manzella, Integrated use Org. Geochem. 37, 1170–1184 (2006). doi:10.1016/j.orggeochem.2006.03.010
of altimeter and in situ data for understanding the water exchanges between the 49. A. J. Davies, G. C. A. Duineveld, M. S. S. Lavaleye, M. J. N. Bergman, H. van Haren,
Tyrrhenian and Ligurian Seas. J. Geophys. Res. Oceans 105, 19649–19663 J. M. Roberts, Downwelling and deep‐water bottom currents as food supply
(2000). doi:10.1029/2000JC900083 mechanisms to the cold‐water coral Lophelia pertusa (Scleractinia) at the
31. E. Miramontes, P. Garreau, M. Caillaud, G. Jouet, R. Pellen, F. J. Hernández-Molina, Mingulay Reef Complex. Limnol. Oceanogr. 54, 620–629 (2009).
M. A. Clare, A. Cattaneo, Contourite distribution and bottom currents in the NW doi:10.4319/lo.2009.54.2.0620
Mediterranean Sea: Coupling seafloor geomorphology and hydrodynamic 50. D. Xanthos, T. R. Walker, International policies to reduce plastic marine pollution
modeling. Geomorphology 333, 43–60 (2019). from single-use plastics (plastic bags and microbeads): A review. Mar. Pollut. Bull.
doi:10.1016/j.geomorph.2019.02.030 118, 17–26 (2017). doi:10.1016/j.marpolbul.2017.02.048 Medline
32. G. Dalla Valle, F. Gamberi, Slope channel formation, evolution and backfilling in a 51. A. Shields, “Anwendung der Aehnlichkeitsmechanik und der Turbulenzforschung
wide shelf, passive continental margin (Northeastern Sardinia slope, Central auf die Geschiebebewegung,” thesis, Technische Universität Berlin (Preussischen
Tyrrhenian Sea). Mar. Geol. 286, 95–105 (2011). Versuchsanstalt für Wasserbau, 1936).
doi:10.1016/j.margeo.2011.06.005 52. L. C. van Rijn, Sediment transport, part II: Suspended load transport. J. Hydraul.
33. I. N. McCave, Formation of sediment waves by turbidity currents and geostrophic Eng. 110, 1613–1641 (1984). doi:10.1061/(ASCE)0733-9429(1984)110:11(1613)
flows: A discussion. Mar. Geol. 390, 89–93 (2017). 53. Y. Niño, F. Lopez, M. Garcia, Threshold for particle entrainment into suspension.
doi:10.1016/j.margeo.2017.05.003 Sedimentology 50, 247–263 (2003). doi:10.1046/j.1365-3091.2003.00551.x
34. S. Liubartseva, G. Coppini, R. Lecci, E. Clementi, Tracking plastics in the 54. I. Kane, M. A. Clare, E. Miramontes, R. Wogelius, J. J. Rothwell, P. Garreau, F. Pohl,
Mediterranean: 2D Lagrangian model. Mar. Pollut. Bull. 129, 151–162 (2018). Seafloor microplastic hotspots controlled by deep-sea circulation, Dryad (2020);
doi:10.1016/j.marpolbul.2018.02.019 Medline https://doi.org/10.5061/dryad.tht76hdwf.
35. C. Cencini, Physical processes and human activities in the evolution of the Po 55. M. Claessens, S. De Meester, L. Van Landuyt, K. De Clerck, C. R. Janssen,
delta, Italy. J. Coast. Res. 14, 774–793 (1998). Occurrence and distribution of microplastics in marine sediments along the
36. D. A. Kroodsma, J. Mayorga, T. Hochberg, N. A. Miller, K. Boerder, F. Ferretti, A. Belgian coast. Mar. Pollut. Bull. 62, 2199–2204 (2011).
Wilson, B. Bergman, T. D. White, B. A. Block, P. Woods, B. Sullivan, C. Costello, B. doi:10.1016/j.marpolbul.2011.06.030 Medline
Worm, Tracking the global footprint of fisheries. Science 359, 904–908 (2018). 56. L. Van Cauwenberghe, A. Vanreusel, J. Mees, C. R. Janssen, Microplastic pollution
doi:10.1126/science.aao5646 Medline in deep-sea sediments. Environ. Pollut. 182, 495–499 (2013).
37. M. Bergmann, V. Wirzberger, T. Krumpen, C. Lorenz, S. Primpke, M. B. Tekman, G. doi:10.1016/j.envpol.2013.08.013 Medline
Gerdts, High quantities of microplastic in arctic deep-sea sediments from the 57. J. H. Dekiff, D. Remy, J. Klasmeier, E. Fries, Occurrence and spatial distribution of
HAUSGARTEN observatory. Environ. Sci. Technol. 51, 11000–11010 (2017). microplastics in sediments from Norderney. Environ. Pollut. 186, 248–256
doi:10.1021/acs.est.7b03331 Medline (2014). doi:10.1016/j.envpol.2013.11.019 Medline
First release: 30 April 2020 www.sciencemag.org (Page numbers not final at time of first release) 4
58. V. Hidalgo-Ruz, L. Gutow, R. C. Thompson, M. Thiel, Microplastics in the marine seafloor circulation patterns. F.P. integrated microplastic and sediment
environment: A review of the methods used for identification and quantification. transport modeling work, R.W. analyzed FT-IR spectra. All authors contributed to
Environ. Sci. Technol. 46, 3060–3075 (2012). doi:10.1021/es2031505 Medline writing the manuscript. Competing interests: The authors declare no competing
59. R. L. Coppock, M. Cole, P. K. Lindeque, A. M. Queirós, T. S. Galloway, A small-scale, interests. Data and materials availability: The data that support the findings of
portable method for extracting microplastics from marine sediments. Environ. this study are available from https://doi.org/10.5061/dryad.tht76hdwf (54).
Pollut. 230, 829–837 (2017). doi:10.1016/j.envpol.2017.07.017 Medline
60. B. De Witte, L. Devriese, K. Bekaert, S. Hoffman, G. Vandermeersch, K. Cooreman, SUPPLEMENTARY MATERIALS
J. Robbens, Quality assessment of the blue mussel (Mytilus edulis): Comparison science.sciencemag.org/cgi/content/full/science.aba5899/DC1
between commercial and wild types. Mar. Pollut. Bull. 85, 146–155 (2014). Materials and Methods
doi:10.1016/j.marpolbul.2014.06.006 Medline Figs. S1 to S5
61. P. L. Corcoran, M. C. Biesinger, M. Grifi, Plastics and beaches: A degrading Table S1
relationship. Mar. Pollut. Bull. 58, 80–84 (2009). References (55–72)
doi:10.1016/j.marpolbul.2008.08.022 Medline Data S1
62. M. R. Jung, F. D. Horgen, S. V. Orski, V. Rodriguez C., K. L. Beers, G. H. Balazs, T.
T. Jones, T. M. Work, K. C. Brignac, S.-J. Royer, K. D. Hyrenbach, B. A. Jensen, J. 16 December 2019; accepted 9 April 2020
M. Lynch, Validation of ATR FT-IR to identify polymers of plastic marine debris, Published online 30 April 2020
including those ingested by marine organisms. Mar. Pollut. Bull. 127, 704–716 10.1126/science.aba5899
(2018). doi:10.1016/j.marpolbul.2017.12.061 Medline
Downloaded from http://science.sciencemag.org/ on December 23, 2020
63. N. P. Edwards, H. E. Barden, B. E. van Dongen, P. L. Manning, P. L. Larson, U.
Bergmann, W. I. Sellers, R. A. Wogelius, Infrared mapping resolves soft tissue
preservation in 50 million year-old reptile skin. Proc. Biol. Sci. 278, 3209–3218
(2011). doi:10.1098/rspb.2011.0135 Medline
64. I. N. McCave, B. Manighetti, S. G. Robinson, Sortable silt and fine sediment
size/composition slicing: Parameters for palaeocurrent speed and
palaeoceanograpy. Paleoceanography 10, 593–610 (1995).
doi:10.1029/94PA03039
65. B. Graca, K. Szewc, D. Zakrzewska, A. Dołęga, M. Szczerbowska-Boruchowska,
Sources and fate of microplastics in marine and beach sediments of the Southern
Baltic Sea—a preliminary study. Environ. Sci. Pollut. Res. Int. 24, 7650–7661
(2017). doi:10.1007/s11356-017-8419-5 Medline
66. H. A. Leslie, S. H. Brandsma, M. J. M. van Velzen, A. D. Vethaak, Microplastics en
route: Field measurements in the Dutch river delta and Amsterdam canals,
wastewater treatment plants, North Sea sediments and biota. Environ. Int. 101,
133–142 (2017). doi:10.1016/j.envint.2017.01.018 Medline
67. J. Martin, A. Lusher, R. C. Thompson, A. Morley, The deposition and accumulation
of microplastics in marine sediments and bottom water from the Irish continental
shelf. Sci. Rep. 7, 10772 (2017). doi:10.1038/s41598-017-11079-2 Medline
68. L. D. K. Kanhai, C. Johansson, J. P. G. L. Frias, K. Gardfeldt, R. C. Thompson, I.
O’Connor, Deep sea sediments of the Arctic Central Basin: A potential sink for
microplastics. Deep Sea Res. Part I Oceanogr. Res. Pap. 145, 137–142 (2019).
doi:10.1016/j.dsr.2019.03.003
69. P. Lazure, F. Dumas, An external–internal mode coupling for a 3D hydrodynamical
model for applications at regional scale (MARS). Adv. Water Resour. 31, 233–250
(2008). doi:10.1016/j.advwatres.2007.06.010
70. T. Duhaut, M. Honnorat, L. Debreu, Développements numériques pour le modele
MARS. Contrat Prévimer–Ref 9, 211 (2008).
71. F. Léger, C. Lebeaupin Brossier, H. Giordani, T. Arsouze, J. Beuvier, M.-N. Bouin, É.
Bresson, V. Ducrocq, N. Fourrié, M. Nuret, Dense water formation in the north‐
western Mediterranean area during HyMeX‐SOP2 in 1/36° ocean simulations:
Sensitivity to initial conditions. J. Geophys. Res. Oceans 121, 5549–5569 (2016).
doi:10.1002/2015JC011542
72. H. Schlichting, K. Gersten, Boundary-Layer Theory (Springer, 2016).
ACKNOWLEDGMENTS
We sincerely thank Thomas Bishop and John Moore in the Department of Geography
at The University of Manchester for their help with a range of analyses. We thank
the staff at the British Ocean Sediment Core Research Facility (BOSCORF) for
access to sediment cores. We thank GALSI PROJECT for access to survey data.
The constructive comments of David Piper and two anonymous reviewers, and
the editorial work of Jesse Smith, are gratefully acknowledged. Funding: M.A.C.
was supported by the CLASS program (NERC Grant No. NE/R015953/1).
Author contributions: I.A.K. and M.A.C. conceived and designed the study, and
sub-sampled core samples. I.A.K. carried out the microplastic extraction and
analysis. M.A.C. and I.A.K. analyzed seafloor, subsurface data and integrated
microplastics concentrations with modeling outputs. E.M. and P.G. modeled the
First release: 30 April 2020 www.sciencemag.org (Page numbers not final at time of first release) 5Downloaded from http://science.sciencemag.org/ on December 23, 2020 First release: 30 April 2020 www.sciencemag.org (Page numbers not final at time of first release) 6
Fig. 1. Plastic sources and ocean circulation affecting the Tyrrhenian Sea. (A) Location of
study area in the Tyrrhenian Sea, annotated with published terrestrial (~80%) and maritime
(fishing and shipping; ~20%) plastic sources (34). Terrestrial input sources shown as circles
and diamonds. Vessel traffic and shipping lanes shown as dashed black lines. Modeled (and
hence inferred) plastic debris fluxes on the Mediterranean seafloor illustrated as a blue-
green-red backdrop. The period modeled was for 2013–2017, assuming vertical settling
from surface distributions using a 2D Lagrangian model (34). Inferred values in the North
Tyrrhenian Sea areDownloaded from http://science.sciencemag.org/ on December 23, 2020
Fig. 2. Global and local abundance of seafloor microplastics.
(A) Comparison of microfiber abundance in different deep-sea settings
worldwide (see references in supplementary material), demonstrating the
high concentrations observed in the contourite drift deposits in the
Tyrrhenian Sea. Results from the Tyrrhenian sea illustrating: (B) how
microplastic abundance does not decrease with distance from terrestrial
input sources; (C) microplastics appear to be concentrated within a depth
range of c. 600 to 900 m; (D) microplastic concentration is biased toward
physiographic domains, in particular enhanced within mounded drifts, and
depleted within moats. (E) A 3D rendering (perspective view looking toward
the south-west) of the regional EMODNet bathymetry (greyscale) and AUV
bathymetry (colored) shows the relationship of sediment samples to the
local seafloor currents (10 × vertical exaggeration). Horizontal distance
from bottom left core (red) to top left (green) is approximately 100 km,
compare to map view in Fig. 1C. Inset microscope photographs show
representative examples of fibers and fragments extracted from the
sediment.
First release: 30 April 2020 www.sciencemag.org (Page numbers not final at time of first release) 8Downloaded from http://science.sciencemag.org/ on December 23, 2020
Fig. 3. Influence of bottom currents on the distribution of seafloor
microplastics. (A) As near bed-shear stresses initially increase, so does the
concentration of microplastics; however, when a threshold (c. 0.04 N m−2)
is exceeded, there is a sudden reduction in the abundance of microplastics.
This corresponds to the modeled threshold of motion based on empirical
approaches (Fig. 4 and fig. S6). (B) Hydrodynamic modeling of bottom
current circulation shows the formation of seafloor gyres, with corridors of
enhanced bed-shear stress along the continental slope and particularly
focused within contourite moats and on the flanks of a prominent
seamount. 90th percentile for bed shear stress and mean near-bed (bottom
current) velocity are shown (see supplementary material). This focusing of
bottom current intensity explains the limited abundance of microplastics on
the shelf, upper continental slope and within contourite moats (C). Zones of
lower shear-stresses adjacent to these corridors of elevated currents
(where mounded contourite drifts form) feature the highest concentrations
of microplastics.
First release: 30 April 2020 www.sciencemag.org (Page numbers not final at time of first release) 9Fig. 4. Relating microplastics to seafloor shear
stress. Flow regime diagram to show that the critical
shear stress required to move particles [i.e., above
the dashed grey line (51)] falls between 0.03 and
0.04 N m−2, and that suspension, i.e., within or above
the grey filled area (52, 53), may occur >0.1 N m−2.
Two densities of microplastics are assumed (nylon
and polyethylene) based on the results of FTIR
analysis. Three particle sizes for microplastics are
shown, to represent the general range observed
through visual microscopic examination [0.1 mm
(red symbols), 0.5 mm (blue) and 1 mm (green)
width]. The upper and lower bound measured grain
size (D90) for the host sediment are also shown by
grey symbols.
Downloaded from http://science.sciencemag.org/ on December 23, 2020
Fig. 5. Bottom currents control the deep-sea fate of microplastics.
Schematic diagram illustrating the role of near bed currents in the transfer,
concentration and storage of microplastic in the deep sea. Along shelf
currents disperse microplastics, powerful gravity flows effectively flush
microplastic to the deep sea, while thermohaline-drive bottom currents
segregate microplastics into localized hotspots of high concentration; the
effectiveness of their long-term sequestration depends upon the intensity of
subsequent bottom current activity and rate of burial.
First release: 30 April 2020 www.sciencemag.org (Page numbers not final at time of first release) 10Seafloor microplastic hotspots controlled by deep-sea circulation
Ian A. Kane, Michael A. Clare, Elda Miramontes, Roy Wogelius, James J. Rothwell, Pierre Garreau and Florian Pohl
published online April 30, 2020
Downloaded from http://science.sciencemag.org/ on December 23, 2020
ARTICLE TOOLS http://science.sciencemag.org/content/early/2020/04/29/science.aba5899
SUPPLEMENTARY http://science.sciencemag.org/content/suppl/2020/04/29/science.aba5899.DC1
MATERIALS
RELATED http://science.sciencemag.org/content/sci/368/6495/1055.full
CONTENT
REFERENCES This article cites 68 articles, 6 of which you can access for free
http://science.sciencemag.org/content/early/2020/04/29/science.aba5899#BIBL
PERMISSIONS http://www.sciencemag.org/help/reprints-and-permissions
Use of this article is subject to the Terms of Service
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published by the American Association for the Advancement of
Science, 1200 New York Avenue NW, Washington, DC 20005. The title Science is a registered trademark of AAAS.
Copyright © 2020, American Association for the Advancement of ScienceYou can also read
