Integrated manufacturing of REciclable multi-material COmposites for the TRANSport sector

An estimated 8.3 billion tons of non-biodegradable plastic has been produced over the last 65 years. Much of this is not recycled and is disposed into the natural environment, has a long environmental residence time and accumulates in sedimentary systems worldwide, posing a threat to important ecosystems and potentially human health. We synthesize existing knowledge of seafloor microplastic distribution, and integrate this with process-based sedimentological models of particle transport, to provide new insights, and critically, to identify future research challenges. Compilation of published data shows that microplastics pervade the global seafloor, from abyssal plains to submarine canyons and deep-sea trenches (where they are most concentrated). However, few studies relate microplastic accumulation to sediment transport and deposition. Microplastics may enter directly into the sea as marine litter from shipping and fishing, or indirectly via fluvial and aeolian systems from terrestrial environments. The nature of the entry-point is critical to how terrestrially sourced microplastics are transferred to offshore sedimentary systems. We present models for physiographic shelf connection types related to the tectono-sedimentary regime of the margin. Beyond the shelf, the principal agents for microplastic transport are: (i) gravity-driven transport in sediment-laden flows; (ii) settling, or conveyance through biological processes, of material that was formerly floating on the surface or suspended in the water column; (iii) transport by thermohaline currents, either during settling or by reworking of deposited microplastics. We compare microplastic settling velocities to natural sediments to understand how appropriate existing sediment transport models are for explaining microplastic dispersal. Based on this analysis, and the relatively well-known behavior of deep-marine flow types, we explore the expected distribution of microplastic particles, both in individual sedimentary event deposits and within deep-marine depositional systems. Residence time within certain deposit types and depositional environments is anticipated to be variable, which has implications for the likelihood of ingestion and incorporation into the food chain, further transport, or deeper burial. We conclude that the integration of process-based sedimentological and stratigraphic knowledge with insights from modern sedimentary systems, and biological activity within them, will provide essential constraints on the transfer of microplastics to deep-marine environments, their distribution and ultimate fate, and the implications that these have for benthic ecosystems. The dispersal of anthropogenic across the sedimentary systems that cover Earth’s surface has important societal and economic implications. Sedimentologists have a key, but as-yet underplayed, role in addressing, and mitigating this globally significant issue.

Introduction: What Are Microplastics and Why Do We Care?

Plastic is an incredibly versatile and inexpensive material, which is ubiquitous in modern life. Since mass-produced plastics appeared in the 1950s, production has increased exponentially (Andrady and Neal, 2009; Andrady, 2011). It has been estimated that 8.3 billion tons of plastic has been produced over the last 65 years; 6.3 billion tons of which is now predicted to be waste (Geyer et al., 2017). In 2012 alone, it is estimated that 288 million tons of plastic was manufactured (Plastics Europe, 2013). Between 4.8 and 12.7 million tons of plastic entered the Earth’s oceans in 2010, with this figure estimated to rise by one order of magnitude by 2025 (Jambeck et al., 2015; Geyer et al., 2017). At least 5.25 trillion pieces of plastic are estimated to be afloat in the world’s oceans (Eriksen et al., 2014).

Microplastics are small plastic particles and fibers, which are found in the present and recent anthropogenically modified environment (Figure 1). Microplastics have been defined as ranging from <5 mm to 250 μm in diameter (Arthur et al., 2009; and many others), however, here we follow Browne et al. (2011) and Claessens et al. (2011), and other subsequent prominent investigations of microplastics (e.g., Van Cauwenberghe et al., 2013, 2015; Vianello et al., 2013; Dekiff et al., 2014) who suggested that <1 mm is more logical as this size class predominates in marine environments, and ‘micro’ generally refers to micrometer size range. Microfibers typically have lengths of 50 μm up to a few mm, and a diameter of <10 μm. Primary microplastic particles are either manufactured (e.g., microbeads in cosmetics, blasting media, and other industrial applications; Zitko and Hanlon, 1991; United States Environmental Protection Agency [USEPA], 1992; Fendall and Sewell, 2009; Mason et al., 2016), or secondary, when derived from the breakdown of larger plastic debris (e.g., Andrady, 2011; Cole et al., 2011; ter Halle et al., 2016). Microfibers are derived from synthetic textiles and are typically discharged from sewage plants (e.g., Browne et al., 2011; Dubaish and Liebezeit, 2013). As an illustration of the numbers of microfibers released, Browne et al. (2011) showed that up to 1,900 microplastic fibers can be shed from a single garment during one wash cycle.

FIGURE 1

Figure 1. (A) Microplastic fibers and (B) microplastic fragments; both from seafloor cores, c. 800 m water depth, Tyrrhenian Sea.

Despite being documented since the 1970s (e.g., Buchanan, 1971; Carpenter and Smith, 1972; Colton et al., 1974; Gregory, 1978), plastic waste in the marine environment did not attract significant scientific or societal attention until later, when it became clear that plastic waste was having a deleterious effect on marine wildlife, particularly larger fauna such as dolphins and turtles (Barnes et al., 2009; Gall and Thompson, 2015). Microplastics were documented as early as 1972 on the surface of the Sargasso Sea (Carpenter et al., 1972); however, concern for the potential consequences for ocean life has only recently been raised. These small and light plastic particles are readily available to many organisms throughout the marine food-web. Furthermore, microplastics are preferential sites for the adhesion of organic pollutants, while their degradation can release toxic compounds (Teuten et al., 2009; Cole et al., 2011). Ongoing research is therefore required to quantify the risks posed to marine life (including fishing stocks), and potential knock on effects to human health (Van Cauwenberghe and Janssen, 2014; Galloway, 2015; Sharma and Chatterjee, 2017; Barboza et al., 2018).

Given their high mobility and long residence times, microplastics are found globally; from the beaches of isolated oceanic islands (Costa and Barletta, 2015; Lusher, 2015), within Antarctic currents (Lusher, 2015), to the seafloor of the Arctic (Bergmann and Klages, 2012; Bergmann et al., 2017; Kanhai et al., 2019) and the sea ice above it (Bergmann et al., 2017). In short, there appears to be no environment on Earth that has escaped microplastic pollution (Taylor et al., 2016). However, our knowledge of the locations of microplastic accumulation in the marine realm is presently incomplete, and in particular the distribution on the seafloor is poorly constrained (Thompson et al., 2004; Barnes et al., 2009; Ballent et al., 2013; Woodall et al., 2014; Martin et al., 2017). This is significant as it is estimated that approximately half of all plastics have a density greater than seawater (United States Environmental Protection Agency [USEPA], 1992; Morét-Ferguson et al., 2010). The seafloor is therefore considered a sink for global plastics, which could account for much of the ‘missing’ microplastic in global budgets (Goldberg, 1997; Thompson et al., 2004; Ballent et al., 2013; Van Cauwenberghe et al., 2013; Pham et al., 2014; Woodall et al., 2014; Fischer et al., 2015; Courtene-Jones et al., 2017; Hardesty et al., 2017; Underwood et al., 2017).

Challenges

Goldberg (1997) suggested that to better understand plastic accumulation on the seafloor required standardized monitoring to assess whether or not seafloor plastic contamination is increasing and whether or not it is affecting marine ecology. However, whilst seafloor microplastics have since been documented in an increasing number of studies, this has been done on a largely ad hoc basis, using existing cores and samples from older studies. Attention has been paid to ingestion of microplastics by seafloor organisms, however, there has been extremely limited attention paid to the physical mechanisms that control how microplastics reach the seafloor, how they are distributed and what governs their ultimate fate (e.g., Gregory, 2009; Corcoran et al., 2017; Graca et al., 2017; Horton and Dixon, 2017). Process-based sedimentological studies routinely relate sediment and other particulate accumulations to the processes that transport, deposit, and bury them. The present lack of characterization and quantification of the processes that control the influx, distribution, and ultimate burial of microplastics in the oceans, provides an opportunity for the application of process-based sedimentology to assess this globally significant issue (Hodgson et al., 2018a).

Aims

Here, we aim to synthesize existing knowledge on seafloor microplastic distribution, and integrate that with a process-based understanding of how particles are transported, and the known sedimentology of deep-marine systems. We do this in order to provide new insights from recent research and to identify future research challenges. We specifically address the following questions:

(1) What types of microplastics are found on the seafloor and where do they come from (see section “Where Do Microplastics Come From and What Types Are Found on the Seafloor?”)?

(2) How are onshore microplastic transport pathways linked to offshore pathways? (see section “How Might Terrestrial Microplastics Be Introduced to Deep-Sea Environments?”) This is currently a missing part of the microplastic cycle (Zalasiewicz et al., 2016). More specifically, are those pathways direct, or more complex with staggered transport and filtering mechanisms? Because of the dominance of river emissions in contributing microplastics to the world’s oceans (Lebreton et al., 2017), here, we focus primarily on clastic systems, but many of our discussions will also relate to carbonate systems.

(3) Based on existing knowledge, where do microplastics accumulate on the seafloor, and is there disproportionality in where they are found (see section “In Which Physiographic Domains Have Microplastics Been Documented to Accumulate at Seafloor?”)? How important for example are different physiographic domains such as submarine canyons, which are known conduits for sediment and organic carbon transport and nutrients, compared to, open continental slopes?

(4) Based on what we know of microplastic density, size and shape, what physical processes might be responsible for their transport and deposition on the seafloor (see sections “Settling Velocities of Microplastic Particles,” “Enhanced Suspension Fall-Out Due to Reversing Buoyancy and Biological Modifications,” “Inhibited Settling due to Thermohaline Stratification and Influence of Near-Bed Ocean Currents,” and “Modified Settling in Sediment-Laden Fluids and the Importance of Sediment Gravity Flows”)? What are the unique aspects of microplastics transport compared to natural sediments? Do microplastics accumulate within certain grain size ranges as a product of the environment in which they are found?

(5) Where should we expect microplastics to be deposited within individual deep-sea environments (see section “Where Should We Expect Microplastics to Be Deposited Within Individual Deep-Sea Deposits?”), i.e., do they have an affinity for different sedimentary facies, and how might they vary across and down systems such as submarine channels, levees and lobes? What are the implications of these predicted vertical and lateral distributions for deep-sea ecosystems?

(6) What is likely to be the ultimate fate of microplastics (see section “Implications for the Long-Term Distribution of Microplastics Within Depositional Settings and Their Ultimate Fate”)? We explore the implications of deep-sea sediment transport that may initially result in preservation of microplastic-bearing deposits over short timescales, but over longer time-scales may be subject to repeated re-exhumation and remobilization (e.g., canyon filling and flushing). We address this by comparing recent repeat seafloor surveys that span periods of days to decades in active settings, to consider the local residence time and ultimate fate of microplastics within the depositional record over anthropogenic timescales.

Objectives and Datasets

In order to address the questions outlined above, we synthesize the following datasets:

(1) Observations of microplastic distributions from various published studies.

(2) Process-based observations and models developed for distribution of lightweight, highly mobile particles in deep-sea systems such as organic carbon and pollutants.

(3) Experimental analysis of settling rates and settling behavior of microplastics.

(4) Measurements of sediment transport processes using recent direct monitoring technology (e.g., of river plumes, turbidity currents, internal tides).

(5) High resolution repeat seafloor surveys to understand the dynamic nature of active deep-sea sediment transport systems.

(6) Geological archives that demonstrate what is ultimately preserved in the depositional record, including sediment cores from modern systems and ancient outcrops that form the basis for the development of system-wide ecological models.

Where Do Microplastics Come From and What Types Are Found on the Seafloor?

The global production of plastic increased from approximately 30 million tons in the 1960s, to >140 million tons by the turn of the 21st Century (Goldberg, 1997; Thompson et al., 2004). It has proven challenging, however, to quantify the input rate of plastics to the oceans as there are only poor constraints on degradation rates in different environments, and plastic age does not necessarily reflect the age it was deposited (Ryan et al., 2009). In addition, there is a multitude of pathways for plastics to reach the seafloor and these may be heavily modulated by the effects of both surface and water-column currents (e.g., thermohaline currents) (Ryan et al., 2009; Cole et al., 2011; Doyle et al., 2011). A general trend of increasing macroplastic pollution has been observed in long term monitoring studies (e.g., Chiba et al., 2018; Maes et al., 2018); however, an encouraging decline in the occurrence of plastic bags has been noted in the North Sea, suggesting that legislation can have a positive impact (Maes et al., 2018). Only in the last 5 years have microplastics been identified in the deep and abyssal oceans; the largest marine habitat on the planet (Woodall et al., 2014) (Figure 2). This new identification may in part be explained by advances in analytical approaches that enable microplastic identification, coupled with a growing societal concern to understand the global significance of plastic pollution; however, it is highly likely that the deep-sea is now experiencing the legacy of the exponential increase in microplastic production over the past five decades (Thompson et al., 2004).

FIGURE 2

Figure 2. Global distribution of studies which have identified microplastics in deep-marine sediment samples. Positions marked are approximate.

Microplastics documented on the seafloor are dominated by fibers. The main source of microplastic particles is thought to be the breakdown of primary plastics (e.g., Andrady, 2011; Cole et al., 2011; ter Halle et al., 2016). These primary plastics are typically those which are not recycled and undergo breakdown in the terrestrial realm, e.g., on land and in rivers, and are transported to the marine realm either as microplastics or larger pieces which may degrade on the sea surface or seafloor (e.g., Willis et al., 2017; Hurley et al., 2018; Pierdomenico et al., 2019) (see section “Enhanced Suspension Fall-Out due to Reversing Buoyancy and Biological Modifications” on reversing buoyancy), as well those from fishing boats and shipping (Pham et al., 2014). Microfibers are derived from synthetic textiles and are typically derived from sewage plants where they are not retained, and from fishing gear (e.g., Browne et al., 2011; Dubaish and Liebezeit, 2013). The distribution and dynamic behavior of microplastics in the water column is poorly constrained but is known to be affected by dredging, trawling, tidal currents, and other processes which affect turbulence in the water column (e.g., Browne et al., 2010, 2011; Claessens et al., 2011; Van Cauwenberghe et al., 2015; Alomar et al., 2016; Moreira et al., 2016).

How Might Terrestrial Microplastics Be Introduced to Deep-Sea Environments?

Sediment, including macroplastic and microplastic, is transported to coastal zones by rivers, wind and ice. Rivers, in particular, are key agents in the transport of microplastics to the coast (e.g., Moore et al., 2011; Klein et al., 2015; Mani et al., 2015; Horton et al., 2017; Lebreton et al., 2017; Willis et al., 2017; Hurley et al., 2018; Pierdomenico et al., 2019). Other contributors of microplastics to the coastal zone include wastewater from treatment plants, shipyards, harbors, and other industries (e.g., Stolte et al., 2015), and urban run-off (e.g., Patters and Bratton, 2016) (Figure 3). When rivers reach the coast, the sediment within them is either sequestered into shallow marine sediment deposits, where it is prone to reworking by coastal processes including longshore drift, or it is fed into a submarine canyon head (e.g., Zalasiewicz et al., 2016; Blum et al., 2018; Pierdomenico et al., 2019) (Figure 4). Recent studies have shown that microplastics in beach sands can also be derived from oceanic waste transported by landward-directed surface currents and that this in some cases can dominate over delivery of river-derived microplastics (Chubarenko et al., 2018).

FIGURE 3

Figure 3. Microplastic input, transport vectors, and sinks. Green boxes represent primary input, blue boxes range boxes represent temporary and permanent sinks, white boxes represent transport mechanisms and arrows represent transport vectors. Insets show the potential distribution and transport vectors of microplastics in (A) a channel-levee system and (B) a bottom current moat and drift system, respectively. Modified and extended (to include marine realm) from Horton and Dixon (2017). WWT, waste water treatment.

FIGURE 4

Figure 4. Compilation of microplastic and microfiber distribution in deep-marine sediments and their gross depositional environments (data compiled from: Van Cauwenberghe et al., 2013; Woodall et al., 2014; Fischer et al., 2015; Bergmann et al., 2017; Graca et al., 2017; Leslie et al., 2017; Martin et al., 2017; Peng et al., 2018; Sanchez-Vidal et al., 2018; Kanhai et al., 2019). As a comparison data from marine litter distributions collected by Pham et al. (2014) are shown.

The duration of sediment storage in the onshore realm generally depends on the relief of the margin, which is related to the tectonic regime. Steep, tectonically active margins tend to have minimal onshore storage (e.g., Romans et al., 2016), and hence sediment and microplastics have a short residence time onshore; the opposite being true for mature, passive margins (e.g., the Mississippi River feeding into the Gulf of Mexico – Galloway et al., 2013). Longer duration of onshore storage will allow more time for macroplastics to degrade and for larger fragments to break into smaller fragments (Figure 3). The downstream transfer of sediment and microplastics in many systems may be staggered, particularly on passive margins with extensive drainage systems. Plastics may undergo temporary storage during periods of relatively low discharge, but be re-exhumed during flood events wherein they are flushed seawards (Hurley et al., 2018). The tectonic configuration of the continental margin also controls the transfer pathways of sediment and microplastics from rivers to deep-sea sediment routing systems, such as submarine canyons. As well as featuring steep and short onshore catchments, active margins are characterized by narrow continental shelves, and steep continental slopes, typically incised by submarine canyons (e.g., Cascadia and California margins, NW United States). Passive margins are characterized by long and relatively lower relief catchments with gentler slopes and wider shelves; hence there is often a much greater distance between rivers and the continental slope (with the exception of infrequent instances where submarine canyons cut back into the continental shelf; e.g., Congo Canyon, West Africa; Babonneau et al., 2002). The role of the continental shelf as both a filter and a conveyor of sediment is critical and especially so in today’s highstand sea-level conditions where slope conduits may be isolated from a feeder system (e.g., Cosgrove et al., 2018). In such detached scenarios, areas of broad continental shelf may provide loci for long-term storage or along-contour redistribution of microplastics (as is also the case for organic carbon), depending on the vigor of along-shelf currents and other oceanographic perturbations, such as storm waves and surges (e.g., Aller and Blair, 2006).

We now consider a range of shelf and slope configurations that may explain variability in the efficiency of microplastics transfer from onshore to the deep-sea (Figure 4). At one end of the spectrum are situations where a river debouches directly into an offshore canyon head, in which the efficiency of transfer for denser microplastics is likely to be high (Figure 5A). This is particularly the case where rivers with sufficiently high concentrations enter the sea, and lead to plunging of dense sediment-laden water (termed ‘hyperpycnal flow’) that initiates a turbidity current (e.g., Gaoping Canyon, Taiwan; Var Canyon, NW Mediterranean; Mulder et al., 2003; Khripounoff et al., 2009; Carter et al., 2012) (Figure 5). High outflows in such settings may also lead to rapid sediment accumulation in the canyon head, setting up slope failures, or settling from homopycnal river plumes, that can also trigger turbidity currents (Carter et al., 2012; Pope et al., 2017; Hizzett et al., 2018). The Messina Strait canyons of the Mediterranean are subject to flash-flood induced hyperpycnal flows, and these have been shown to transport huge volumes of anthropogenic waste to over 1,000 m water depth (Pierdomenico et al., 2019). These systems are termed ‘reactive’ with the source sediment supply conditions being relatively well-recorded in the deposits of the sink (Figure 6); an example is the La Jolla canyon-channel system (Romans et al., 2016).In such high efficiency transfer zones, the direct connection between terrestrial outflow and submarine canyon would be expected to result in a concentration of microplastics; in particular the larger size fractions but potentially also microfibers. If a river lacks a direct connection to a submarine canyon (Figures 5B–D), then along-shelf currents and wave action can redistribute and disperse sediments, thus reducing the efficiency of the river to slope connectivity (Mulder et al., 2012; Eidam et al., 2019). These systems may be termed ‘buffered,’ with the input conditions being less faithfully recorded in the sink (Figure 6); an example being the Indus River – Indus Submarine Fan (Romans et al., 2016). It has been demonstrated that longshore drift acts as a grain size segregator, with the finer and/or hydrodynamically lighter grains being more-readily transported along the shelf (Aller and Blair, 2006). Sediment will be transported along the shelf, until the load is diminished through wave and storm action, or until it meets an intersecting canyon head (e.g., La Jolla or Monterey Canyons, California; Xu et al., 2002; Covault et al., 2007) (Figures 5C,D). Thus, microfibers and the lightest microplastic particles will be more-readily dispersed in low-efficiency and buffered margin transfer zones (e.g., disconnection between river and canyon/continental slope, wide shelf dominated by currents), becoming preferentially transported along the coast and shelf. The general advection of fine grained particles over the shelf edge during longshore drift is anticipated to be widespread away from zones of fluvial input, both due to shelfal processes and hydrodynamic aspects, some of which are unique to microplastics. We now discuss some of the key properties of microplastics, and how they compare to sand and mud particles that are more routinely characterized and modeled in sediment transport studies.

FIGURE 5

Figure 5. The efficiency of transfer of microplastics and larger primary plastic items from the terrestrial to deep-marine realm is dependent on the duration of onshore sediment storage and the proximity of the canyon head to the principal terrestrial sources of sediment, e.g., river mouths, which is influenced primarily by the tectonic style of the margin. (A) Direct fluvial input to the canyon head, (B) delayed fluvial input as sediment is temporarily stored in the canyon head, (C) indirect fluvial input due to the canyon being offset from the river mouth, and (D) no direct fluvial feeder, sediment is sourced by longshore drift. Longshore currents as well as the volume and density of fluvial outflow will also have a strong influence on the likelihood of plastics reaching the canyon head. Modified from Sømme et al. (2009).

FIGURE 6

Figure 6. Reactive versus buffered sediment delivery systems. Longer residence periods for sediment in the buffered systems suggests that plastics may be more degraded, both physically and chemically, suggesting that plastics delivered in these systems will be more degraded and smaller. In the buffered example, the seafloor expression of surges in plastic production may significantly lag behind their introduction to the sedimentary environment. Modified from Romans et al. (2016).

In Which Physiographic Domains Have Microplastics Been Documented to Accumulate at Seafloor?

Despite the relative infancy of marine microplastics research, a considerable, and steadily growing, number of publications now provide compelling evidence of the pervasive nature of microplastics across the seafloor worldwide. Many studies show that deep-sea microplastics occur in similar (or even higher) concentrations as in intertidal and shallow sub-tidal sediments, with microplastic particles being distributed primarily, but not solely, around input points such as submarine canyons (e.g., Woodall et al., 2014; Taylor et al., 2016; Bergmann et al., 2017; Hurley et al., 2018) (Figures 2, 3 and Table 1). Analysis of previous deep-sea studies that sampled seafloor sediments reveals that submarine canyons and ocean trenches are the physiographic domains with the highest density of microplastics (Table 1 and Figure 4). These two environments feature almost double the microplastic density at seafloor compared to other deep-sea settings, such as continental shelves, open continental slopes, abyssal plains and seamounts (Figure 4). While a relatively wide range of settings have been sampled, it should be noted that there is some geographic bias to the existing sampling for microplastics (e.g., Atlantic, Mediterranean, and Pacific focus). Future efforts should ensure a wider geographic, as well as physiographic coverage.

TABLE 1

Table 1. Compilation of published studies documenting microplastics in deep-marine seafloor sediments.

Submarine canyons have previously been shown to be marine litter hotspots, especially where they occur in close proximity to industrial and densely populated coastal areas (Mordecai et al., 2011; Pham et al., 2014; Tubau et al., 2015; Buhl-Mortensen and Buhl-Mortensen, 2017). Sites offshore from popular tourism centers can be inundated with large quantities of litter that is transported offshore and to deeper water (over 1,000 m water depth) following storms or during seasonal cascades of dense water (Tubau et al., 2015; Pierdomenico et al., 2019). A study of the Lisbon, Blanes, Guilvinec, and Setubal canyons (NE Atlantic) found litter at all sites and all water depths (from 35 to 4,500 m), with a higher density than from all other physiographic settings; reaching an average of 9.3 ± 2.9 items ha⁻¹ (Pham et al., 2014). Perhaps unsurprisingly, there is a close match between the recorded distribution of marine litter and microplastic fragments in the ocean (Figure 4).

Levels of microfiber contamination in bottom waters have been shown to be considerably higher than in surface waters, for example, 11 microfibers per liter of water were sampled in the Mariana trench, western Pacific Ocean, compared to a few pieces per liter in surface waters and 200–2,200 per liter of sediment (Peng et al., 2018). This may partially explain the relative abundance of microplastics in ocean trenches, with the presence of larger plastics and marine litter in that setting being attributed mostly to fishing and shipping activities (Peng et al., 2018).

While previous studies have provided valuable information on the type and abundance of microplastics in seafloor sediments, they have not yet included any detailed sedimentological data that can explain these physiographic biases for microplastic distribution, nor to enable prediction of how microplastics may be distributed within different system types (e.g., across the full extent of a deep-sea submarine channel system). As an example, the lack of microplastic particles identified at the deep-sea Congo Fan was viewed as anomalous due to the presence of major industrial cities on the Congo River (Van Cauwenberghe et al., 2013), but no information is provided about whether the part of the fan sampled was active (i.e., subject to recent turbidity current activity or near-bed oceanographic currents), nor regarding the grain size of the host sediment. Previous studies have demonstrated that, while much of the Congo canyon, deep-sea channel and fan system is a highly active conduit for sediment and organic carbon transport in the present day (Khripounoff et al., 2003; Azpiroz-Zabala et al., 2017), not all of the deep sea distributary networks are active (Picot et al., 2019). Thus, without any detailed information on the seafloor sediments and specific location within the submarine channel-fan system, it is challenging to determine how representative one core location is for an entire system. Recent studies in fluvial and shallow marine/tidal environments have started to address these issues, including the spatial, sedimentological, and temporal controls on microplastic distributions (e.g., Van Cauwenberghe et al., 2015; Hurley et al., 2018). We suggest that there is a pressing need to provide more detailed contextual information to link microplastics with the transport processes responsible for their accumulations in order to explain their distributions across different deep-sea depositional environments (Table 1 and Figure 4). If we ever hope to link microplastics to the processes that control their distribution in the deep-sea, we must first understand how they are introduced to marine environments. The following section therefore first outlines the processes that transfer microplastics to the ocean, and then discusses how different tectonic, physiographic and oceanographic configurations may result in the wider dispersal or localized concentrations of microplastics.

Properties of Microplastics and Physical Controls on Their Suspension, Transport and Deposition

Microplastics span a wide range of densities; from very low density, such as polystyrene (40 kgm⁻³) to the densest, e.g., polytetrafluoroethylene (2,020 kgm⁻³). In contrast, mineral sediment with a grain size larger than clay, i.e., silt and sand, delivered by rivers is dominated by quartz (2,650 kgm⁻³), feldspar (2,560 kgm⁻³), and mica (2,750 kgm⁻³). Therefore, most of our understanding of sediment gravity flows on the seafloor is dominated by sand and mud transport, with less known about the behavior of lighter particles including microplastics, organic material such as plants, leaves and woody material (e.g., Zavala et al., 2012; Yamada et al., 2013). It is known that plant material can reach the distal parts of submarine fans, e.g., in Permian strata of Tanqua Karoo, Hodgson (2009) reported that plant material had been transported at least 150 km from the contemporaneous shoreline (where it was likely sourced). As well as particle density, grain shape plays an important role in the settling of grains from suspension, with platy particles, such as leaf fragments and mica (despite its high density), or those with intricate shapes, e.g., shells, typically settling more slowly than spherical particles (e.g., McNown and Malaika, 1950; Dietrich, 1982; Oehmig, 1993). Previous work has suggested the hydraulic ‘equivalence’ of organic material with ‘platy’ grains such as mica (Stanley, 1982), such that these particles often settle at lower velocities than slightly finer-grained siliciclastic sand and develop micaceous and organic rich caps to turbidite sand beds (e.g., Hodgson, 2009; Zavala et al., 2012) (Figure 7). Microplastics may exhibit a similar distribution and this would mean they are prone to erosion from subsequent flows, gradual down-system reworking, as well as erosion and resuspension by bottom-currents which may flow obliquely to the ‘primary’ depositional system (Figure 3).

FIGURE 7

Figure 7. (A) Organic-rich material incorporated within turbidite laminae, and (B) concentrated on the bed top; both from Bute Inlet, British Columbia (courtesy of Maarten Heijnen). (C) Organic material distributed within a hybrid bed and in the uppermost dark-colored division (cored interval, Palaeogene, Gulf of Mexico; Kane and Pontén, 2012). (D) Plant fragments in the upper part of a hybrid bed (Permian, Tanqua Karoo; Hodgson, 2009). (E) Thin-section photomicrograph showing small plant fragments within a hybrid bed (Permian, Tanqua Karoo; Kane et al., 2017). Microplastic fragments and fibers might be anticipated to have similar distributions in deep-water deposits.

The principal candidates for the transport of microplastics to and across the deep seafloor are: (i) settling, or conveyance through biological processes, of material that was formerly floating on the surface or suspended in the water column; (ii) gravity-driven transport in sediment-laden flows, such as turbidity currents; (iii) reworking and transport by thermohaline currents; and (iv) internal tides (i.e., topographically steered internal waves that exhibit tidal frequencies; Shepard, 1975). We will now discuss the settling velocity of particles, and then consider the implications of these transport processes.

Settling Velocities of Microplastic Particles

The paucity of contextual sedimentological data in existing microplastics studies (Table 1) inhibits a detailed investigation of transport processes at present. Information on the specific depositional environment (e.g., a canyon axis that features regular turbidity currents versus an adjacent flank elevated above the zone of flow interaction) and critically the grain size of the host sediments are omitted in most of the existing studies. One of the few studies to provide grain size information (Maes et al., 2017) does not provide water depths or other environmental information for the samples collected. Despite the lack of observational data to link microplastics to transport process, a number of recent laboratory studies have made measurements of the settling velocities of microplastics such that we can investigate the anticipated range of processes and environments that control their transport, dispersal and/or concentration. Laboratory experiments to measure settling velocities for a range of plastic particles (Kowalski et al., 2016; Khatmullina and Isachenko, 2017) have demonstrated the expected deviation from theoretical values (e.g., following Dietrich, 1982). Settling velocities (w_s) for spherical particles within a turbulent flow can be estimated, following Ferguson and Church (2004), using w_s = RgD2C1v+0.75C2RgD3√RgD2C1v+0.75C2RgD3 where R is the relative submerged density of the particles, g is gravity, D is the particle diameter, C₁ and C₂ are constants, 18 and 1, respectively. Relative submerged density (R) is given by (ρs − ρf)/ρf where ρs is the density of sediment and ρf is the density of fluid, in this case seawater, at 1,026.2 kg m⁻³. Settling velocities estimated for various microplastic particles are lower than the main ‘sand-forming’ minerals, which indicates that for a given size, plastic particles will be deposited later than sand grains (Figure 8). This means, for example, that a 0.5 mm diameter spherical polyurethane pellet would settle at approximately the same rate as a 0.15 mm diameter quartz grain (fine sand), or a 5 mm diameter pellet at the same rate as a 0.75 mm diameter quartz grain (coarse sand). Both natural and plastic particles have a range of shapes and surface roughness so these theoretical values (for spheres) typically over-estimate settling velocity.

FIGURE 8

Figure 8. Settling velocities estimated for a range of spherical particles in a turbulent flow; as an example a 1 mm polyurethane sphere would settle at similar velocity to a 0.25 mm quartz grain. Natural particles have a wide range of shapes and surface roughness which will affect their settling, and microfibers will diverge significantly from this model.

Sediment particle size sorting occurs during deposition and resuspension according to the relationship of w_s to the fluid shear stress (τ) (either turbidity current or bottom current). In general the fluid shear stress for non-cohesive sediment deposition (τ_d), is lower than that required for erosion (τ_e), which is lower still than that required for suspension (τ_s), such that τ_d < τ_e < τ_s (e.g., McCave et al., 2017). Plastic particle flocculation is poorly understood but it is assumed that most microplastics behave in a non-cohesive manner, however, they may be incorporated into larger aggregates when mixed with clay (Galgani et al., 2015) or when surfaces have accumulated biofilms (e.g., Lobelle and Cunliffe, 2011). Owing to their wide range of shapes, microfibers are anticipated to behave substantially differently to spherical (e.g., microbeads) or fragmented microplastics (e.g., Högberg et al., 2010). While there may be analogs with plankton fallout (Ptacnik et al., 2003; Peterson et al., 2005), plastic microfiber settling remains an area for future research (Kowalski et al., 2016; Khatmullina and Isachenko, 2017).

We now discuss some of the ways in which the preceding estimates of settling velocity may be modified, due to interactions of microplastics with the ambient marine environment.

Enhanced Suspension Fall-Out Due to Reversing Buoyancy and Biological Modifications

While many plastic particles start out with a given low density (and hence slow settling velocity), this can change over time as particles can: (i) accumulate biofilms (biofouling; e.g., Lobelle and Cunliffe, 2011; Muthukumar et al., 2011; Long et al., 2015; Cole et al., 2016; Fazey and Ryan, 2016; Kaiser et al., 2017); (ii) break down through UV light degradation (photodegradation; Shah et al., 2008); (iii) act as focal points for precipitation of chemicals and minerals on particle surfaces (Mato et al., 2001; Corcoran et al., 2015); (iv) undergo leaching of additives (Van Cauwenberghe et al., 2013, 2014 and (v) form aggregates of marine sediments (Galgani et al., 2015). Biofouling may explain the apparent lack of plastics on the sea surface; recorded levels of plastic at the surface are at least two orders of magnitude lower than anticipated (Cózar et al., 2014). Consumption of microplastic particles by organisms, such as polychaete worms, mysid shrimps and copepods can lead to the expulsion of microplastic-bearing fecal pellets with a greater density than init

» Author: Kane, Ian A.; Clare, Michael A.

» Reference: Kane, Ian A.; Clare, Michael A.. 2019 Dispersion, accumulation, and the ultimate fate of microplastics in deep-marine environments: a review and future directions. Frontiers in Earth Science, 7. https://doi.org/10.3389/feart.2019.00080

» Publication Date: 23/05/2019

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