Deep-ocean seafloor islands of plastics

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Science  05 Jun 2020:
Vol. 368, Issue 6495, pp. 1055
DOI: 10.1126/science.abc1510

One of the most striking images of ocean pollution are the patches or islands of floating plastic debris, concentrated in open-ocean gyres (1) and large enough to be seen from space. These concentrations of plastics on the ocean surface were first recognized in the early 1970s (2). Even with the impressive size of these patches, mass-balance estimates for ocean-borne plastics pointed toward a sink. That sink has recently been shown to be deposition on the deep seafloor (3). On page 1140 of this issue, Kane et al. document the occurrence of enriched zones or islands of plastic debris that accumulate on the seafloor of the deep ocean (4).

Very different styles of plastics transport are associated with sea-surface and seafloor pollution. These styles are well understood for sediment transport on Earth's surface, which in turn are linked to surface evolution (5). Kane et al. document that the accumulation of microplastics on the Mediterranean seafloor is not simply passive vertical settling through the water column as has been commonly assumed, but rather represents reworking by deep-sea currents. These concentrate microplastics into patches or islands at predictable locations if the microplastics are treated as sediment that can be eroded, transported, and deposited by a deep-ocean flow field. The same processes that control patterns of erosion and deposition on terrestrial landscapes and shallow-marine shelves also pertain to plastics in the deeper ocean. The authors add much needed confirmation that concepts and methods developed from easier-to-access environments can be applied with confidence to remote undersea inquiries (5).

Although the primary focus of Kane et al. is to demonstrate the fate and focusing of pollutants in the deep sea, the authors also identify a likelihood for colocated hotspots in microplastic concentration and deep-ocean biodiversity. The same currents driving the enrichment of microplastics are efficient conveyers of nutrients and dissolved oxygen to the seafloor. Positioning of the colocated hotspots is affected by the submarine topography that guides the deep-ocean currents. This linkage is analogous to those already identified between surface transport and food webs on terrestrial landscapes (5, 6). This observation opens an opportunity to connect the spatial structure of deep-water ecosystems and pollutants to the spatial structure of surrounding submarine landscapes. Understanding the connections between seafloor pollution by microplastics and deep-sea ecology requires a unified science of Earth's surface dynamics that remains one of the great integrating challenges of environmental studies.

Determining the fate of plastic debris transported from land to sea (7) contributes to quantifying mass fluxes defining Earth's source-to-sink system. Research on sediment-routing systems (8, 9) has already identified many of the complications and pitfalls inherent to tracking particles across the environment. Sediment-flux signals can be phase-shifted, lagged, and buffered by the internal dynamics of the transport system. These same dynamics will confound analyses of routing signals for plastics across Earth's surface and into the deep-ocean sink. Kane et al. demonstrate that microplastics deposited in this deep-sea environment are still subject to later erosion, transport, and redeposition due to time and space variations in the near-bed velocity fields of thermohaline or contour currents. Understanding the ultimate fate of these microplastics requires high-resolution submarine topography or bathymetry because the deep-sea currents interact with this topography to produce the spatial changes in velocity that set patterns of sediment and microplastic erosion, transport, and accumulation. Unfortunately, high-resolution bathymetric data simply do not exist at the global scale. The most widely used seafloor dataset is the General Bathymetric Chart of the Oceans, available on a 15–arc sec world grid (∼463-m resolution at the equator). It is still surprising that this worldwide ocean product exists at a resolution poorer than that of the 200-m digital elevation model available for the entire surface of Mars (10). Until ocean-basin scale models improve, our detailed understanding of solids transport at and near the seafloor will be restricted to local or regional studies connected with higher-resolution bathymetric models.

Kane et al. identify extremely high concentrations for microplastics in deep-sea sediment drift deposits (11). This style of deposit has been the focus of previous studies because relatively rapidly accumulating drift deposits constitute an excellent archive of proxy measures for past Earth states (12). This correlation of highest plastic concentrations with high-fidelity paleoenvironmental records suggests the potential for a Global Boundary Stratotype Section and Point (13), if the Anthropocene is formally recognized as a geologic epoch (14). The first occurrence of microplastic in core from drift deposits could serve as a “golden spike” recording the lower boundary or beginning of the proposed epoch (14, 15). In less than 80 years, plastic debris has been effectively distributed across Earth's surface. Kane et al. demonstrate that even in the deep sea, its motion and accumulation is dominated by the feedbacks between fluid flow, sediment transport, and topography that are an overarching hallmark of Earth's surface system.

References and Notes

Acknowledgments: Thanks to N. Tull, P. Passalacqua, J. Hariharan, K. Wilson, H. Hassenruck-Gudipati, K. Wright, T. Jarriel, E. Prokocki, J. Swartz, and S. Rahman for brainstorming on plastics transport.
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