Science - USA (2020-06-05)

SCIENCE sciencemag.org

By David Mohrig

O

ne of the most striking images of

ocean pollution are the patches or is-

lands of floating plastic debris, con-

centrated 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 impres-

sive 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 sea-

floor pollution. These styles are well un-

derstood 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 pas-

sive vertical settling through the water col-

umn as has been commonly assumed, but

rather represents reworking by deep-sea

currents. These concentrate microplastics

into patches or islands at predictable lo-

cations 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 ap-

plied 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 driv-

ing the enrichment of microplastics are ef-

ficient conveyers of nutrients and dissolved

oxygen to the seafloor. Positioning of the

colocated hotspots is affected by the sub-

marine topography that guides the deep-

ocean currents. This linkage is analogous

to those already identified between surface

transport and food webs on terrestrial land-

scapes (5, 6). This observation opens an

opportunity to connect the spatial struc-

ture of deep-water ecosystems and pollut-

ants 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 dynam-

ics that remains one of the great integrating

challenges of environmental studies.

Determining the fate of plastic debris

transported from land to sea ( 7 ) contrib-

utes 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 sys-

tem. 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 en-

vironment 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 sub-

marine topography or bathymetry because

the deep-sea currents interact with this to-

pography to produce the spatial changes in

velocity that set patterns of sediment and

microplastic erosion, transport, and accu-

mulation. 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 con-

centrations for microplastics in deep-sea

sediment drift deposits ( 11 ). This style of

deposit has been the focus of previous stud-

ies because relatively rapidly accumulat-

ing 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 bound-

ary 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 accumula-

tion is dominated by the feedbacks between

fluid flow, sediment transport, and topog-

raphy that are an overarching hallmark of

Earth’s surface system. j

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4. I. A. Kane et al., Science 368 , 1140 (2020).


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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.

10.1126/science.abc1510

OCEANOGRAPHY

Deep-ocean seafloor islands of plastics

The processes controlling sediment transport also concentrate microplastics

Jackson School of Geosciences, The University of Texas at

Austin, Austin, TX USA. Email: [email protected]

“In less than 80 years, plastic

debris has been...distributed

across Earth’s surface.”

5 JUNE 2020 • VOL 368 ISSUE 6495 1055

Published by AAAS