Microplastics in Antarctica: potential pathways and influences

‍Introduction

The detrimental consequences of plastic pollution have been the subject of research since the late 1960s when the first intensive studies on marine litter were conducted.^1,2 Since then, plastic production has increased exponentially with growing concern for its worldwide presence in marine, terrestrial and freshwater ecosystems.

In the environment, chemical and physical processes fragment this durable material into plastic particles, known as secondary micro(nano)plastics (MnPs).^3,4 Those specifically made to be ~5 mm are known as primary MnPs, typically manufactured as a raw material to make rigid plastic packaging and cosmetic products.^5,6,7

Microplastic (1-5000 µm) and nano plastic (<1 µm) research has gained widespread attention in the past decade, due to the ubiquitous presence and hazardous impact of plastics on human and ecosystem health. The majority of MnP research is conducted on marine environments, with less exploration but growing interest in the transboundary issue of airborne MnPs.⁸ Observational data for atmospheric MnPs is currently limited and uncertainties are amplified by the lack of inter comparability between research methods. The nature of ocean-atmosphere-terrestrial exchanges, in relation to MnP transport, demonstrate highly complex processes that require additional focus and harmonised studies.⁹

Transportation of these particles is influenced by wind speed, directions, convection lift, drafts and turbulence, allowing MnP transportation to reach remote, vulnerable environments without human activity. Recent observations have highlighted MnP accumulation from atmospheric deposition in the Pyrenees, the Alps and even the polar regions.^10,11,12

Antarctica and the surrounding Southern Ocean region were originally thought as pristine environments, relatively free from contaminants and protected by the Circumpolar Current.¹³ Global plastics production has been one of the many anthropogenic activities threatening Antarctica’s unspoilt ecosystem, with plastic debris leaching into the environment at every stage of its life cycle.^14,15 The continent’s unique characteristics, including a constant desert of ice, snow and rock, and a rich marine and nutrient life, are under threat from MnP contamination - an industry that is predicted to increase waste threefold by 2060.^16,17 Initially, it was thought that MnPs were only harmful to marine life and not the entire ecosystem. However, subsequent research has shown that plastic pollution affects even the most isolated areas.¹⁸ An example is airborne, marine plastics scattering incoming solar radiation, potentially contributing to temperature changes in the water column by impacting feedback cycles. In the cryosphere, darker-coloured MnPs can induce snow and ice melting by absorbing light and lowering the albedo.¹⁹ Accelerated melting events may reduce snow cover and, in turn, alter the regional carbon cycle.²⁰ In Antarctica, the extent of contamination is largely unknown, and the pathways misunderstood, therefore it is difficult to quantify the risks and impact of MnP in Antarctica.

Understanding the pathways of MnPs to remote regions offers a new perspective on the consequences of plastic production, highlighting air-sea-land interactions as a major player in their fate. Earth’s systems interlink and there are no boundaries to prevent transport of MnPs within the constant flow of biogeochemical exchanges.²¹ The fate of MnPs is also governed by their characteristics, which determine their capabilities to transport various distances in the environment and deposit in remote regions. Deepening our understanding of the complex fate and transport of MnPs is fundamental to mitigating risks and developing effective strategies in order to maintain the protection of this fragile climate.

Microplastics in remote regions

Plastics vary in size, shape and surface properties depending on their age, environment, and primary source. It is understood that plastic can enter the environment at all stages of the life cycle, including production, manufacture, consumption and waste.²² As MnPs enter the environment, each particle is subject to thermal, chemical, physical, and biological changes, influencing their composition and mobility.²³ The changing characteristics of these MnPs play a key factor in determining their transportation pathway and deposition, such as their aerodynamic capabilities which are affected by shape.

The emerging concern for MnPs in remote regions has been influenced by studies such as snow pit samples collected in Tibetan’s glacier, Demula, where the average MnP load was 9.55 ± 0.9 items per litre.²⁴ Evidence of MnPs were also found in Arctic ice floes, accumulating a relatively low average of 0 to 14.4 × 10³ items L⁻¹,²⁵ and in the Italian Alps where 74.4 ± 28.3 (mean ± standard error) items per kilogram were observed in sediment samples.²⁶ These studies, in conjunction with others, suggest that MnP accumulation in remote regions, separate to waterways and human activity, tend to occur via atmospheric pathways that allow for long-range, efficient travel. Additionally, oceanic and atmospheric fluxes and reservoirs are interlinked mechanisms that play an important role in the ability of MnPs to transport to remote regions. With the aid of precipitation, prevailing winds, aerosols and ocean currents, certain MnPs can transport to uninhabited environments. Although little is known about the primary and secondary sources of remote MnPs, identifying the characterisation and location of contamination demonstrates their potential pathways.²⁷

‍Antarctic plastic pollution

Antarctica consists of the Antarctic Peninsula in close proximity to Chile, and East and West Antarctica divided by the Transantarctic Mountains (Fig. 1). East Antarctica is known as a polar desert, spanning the Plateau and its four coastal regions, Wilkes Land Coast (70–150°E), Weddell Sea Coast(60–15°W), Victoria Land and Ross Sea (150–170°E), and Dronning Maud Land Coast (15°W–70E).²⁸ This dry, and record cold climate makes up 73% of Antarctica’s main landmass on a continental shield, neighbouring Africa to Eastern Australia.²⁹ It is surrounded by the west-east circumpolar current, with coastal currents flowing in the opposite direction and thus generating two oceanic gyres - the Weddell Gyre and Ross Gyre.

Antarctic pollution via anthropogenic activities can originate from both global and local sources. The first signs of anthropogenic pollution in Antarctica were evident in paleoclimate data when ice cores > 700 years old presented an increase in black carbon emissions over time. Similarly, lead isotope ratios in ice cores have also been observed across the continent, with an increase seen from ~0.6 pictogram per gram (pg/g⁻¹) to ~1.8 pg/g⁻¹,spanning the years 1650-1885. As a sound environmental monitor, lead concentrations in Antarctica indicate atmospheric transport that predate human exploration, and can be traced back to early mining activities.^30,31

Although Antarctica is considered one of the last pristine regions on Earth, it is evidently facing advanced signs of pollution due to increasing industry, human presence and the growing production of plastic.³² Persistent organic pollutants, for example, have been found in the fat tissue of penguins and other seabirds in West Antarctica, indicating that these toxic chemicals are present in the food chain.³³ According to Kennicutt et al.,³⁴ there is greater human presence and thus, pollution levels in West Antarctica compared to East Antarctica, primarily due to the concentration of research stations, tourism and fishing activities in the region.³⁴ Compared to Eastern Antarctica, Western Antarctica is a popular destination for tourists and fishing enthusiasts, since there is close proximity to South America and an added ease of travel through the Drake Passage.³⁵ In contrast, continental Eastern Antarctica is relatively secluded and contains most of the world's freshwater resources in the form of ice sheets and lakes which are well-preserved.³⁶

‍Atmospheric deposition

Atmospheric MnPs have been observed in remote regions of the world, and back-trajectory and dispersion modelling has suggested long-range transportation, making it possible for MnPs to reach Antarctica and the Southern Ocean. When MnPs were observed in the French Pyrenees for example, they were deposited at an altitude of 1425 m above sea level and with a daily MnP loading of ~365 particles per metre.² It was estimated the MnPs transported over 95 km and similar studies in Greenland, the Arctic and Antarctica have suggested that atmospheric transport is a major pathway for global MnPs, enabling them to travel hundreds to thousands of km.^37,38 With supporting evidence from air mass or particle trajectory modelling, atmospheric transport is generally viewed as a more efficient pathway for MnPs than terrestrial ones.

Aerosols also play an important role in transporting MnPs to Antarctica from neighbouring countries, since MnPs have been reported in numerous dust and aerosol samples.^39,40,41 Coagulation between aerosols and MnPs may increase the size of particles, which can affect their settling rates and their ability to reach the surface. Although MnPs can travel with aerosols, their abundance is relatively small in comparison to overall aerosol loadings, with one study reporting less than 1% of total particulate matter in the Arctic.⁴²

Typically, MnPs accumulate on the sea surface and can act similarly to dust aerosols, becoming airborne, with the aid of turbulent and grasshopping processes,⁴³ the latter referring to the geochemical process whereby air pollutants transport from warmer to colder climates, bearing more than one atmospheric cycle in the process.⁴⁴ Once airborne, a variety of mechanisms can support the long-range transport of MnPs to East Antarctica via movement of air masses into the upper atmosphere. Atmospheric mixing also creates pathways to cross the Polar Front as MnPs become entrained with different air masses and particles.^45,46 Westerly winds tend to migrate dust particles and similar aerosols from Southern South America to Antarctica, as the high-pressure cyclonic system is coupled with Antarctica’s anti-cyclonic movements in the upper troposphere.

Observed in Patagonia, large volumes of aerosols are suspended into the atmosphere and transported by cyclonic dynamics, due to extreme dryness in the air. As one cyclone dissipates and another forms, the cyclone's southern limit crosses 80°S over the Weddell Sea.⁴⁷ Extra-Andean Patagonia is recognised as a reliable source for atmospheric aerosols, and its close proximity to the Antarctic Peninsula allows for transportation to Antarctica's continental surface via anticyclonic conditions that sinks aerosols with diverging air.⁴⁸ Patagonia dust sources have been identified as being transported to East Antarctica within a 7-day window and it has been proposed that Southern South America was the main source for dust particles. However, others have argued that New Zealand, Australia and South Africa are equally dominant dust source areas.^49,50

MnP snow samples found in remote regions generally indicate atmospheric transport. With East Antarctica’s isolated location, MnPs would need to travel approximately 4,000 km from neighbouring countries (New Zealand, Australia, and South Africa) to deposit in Antarctica. A study conducted by Trainic et al.³⁸ found that MnP can remain in the atmosphere from 5 minutes to ~2 days, depending on the particle’s settling velocity, potentially enabling them to be transported thousands of kilometres.³⁹ Similarly, Aves et al.⁵¹ observed MnPs in West Antarctic snow samples, estimating that if long-range atmospheric transport occurred, MnP may have been transported 6,000 km with a suspension time of 6.5 days.⁵¹ However, MnPs identified in that study also may have originated from the local scientific bases.

Deposition pathways from atmospheric to terrestrial or aquatic environments are part of a dynamic source-sink relationship, whereby pathways can impact the flux and retention of MnPs.⁵² In snowfall events, correlations have been observed between MnP count and meteorology, indicating that snowfall events have a positive correlation with MnP fallout in Antarctica. Scavenging represents an important vector for MnP accumulation in snow and ice, along with wind speed, which appears to increase dry deposition velocities of particles.^53,54

The process by which atmospheric MnPs are transported and deposited is multifaceted and recent studies suggest that whilst suspended, MnPs could alter cloud microphysics and optical properties by acting as cloud condensation nuclei or ice nuclei.^55,56,57 This largely depends on their size, shape and surface properties, with smaller, irregular shaped and hydrophilic particles more likely to act as cloud condensation nuclei and larger, smooth particles more likely to act as ice nucleating particles.^58,59 Once MnPs are incorporated into droplets or ice crystals, their settling rates may be affected, as well as their ability to transport over distances. However, more research is required in this area.⁶⁰‍

Local activity

Exploration of the Southern Ocean region has been popularised by humans since the 18th century, with the primary motivation being the exploitation of seals, whales and penguins for fur, food and cosmetics. Attempted explorations to the South Pole influenced the construction of research stations, which were further established after the Antarctic Treaty was signed in 1959.⁶¹ Since then, scientific research and the exploitation of marine mammals introduced a wave of human presence in Antarctica, now home to 70 scientific research stations and, more recently, commercial tourism.

The intensity of human activity continues to increase in the Antarctic and Southern Ocean, exposing the region's natural environment to anthropogenic contamination. The Antarctic Treaty governs all practices south of 60°S latitude, with objectives to minimise the environmental impact of waste disposal, marine pollution and plastic pollution. It has recognised a 'lack of plastics monitoring data’ that would otherwise help inform decision making to reduce the use of plastic material in construction, protective clothing, packaging and fieldwork. Tourism in East Antarctica makes up only 2% of touristic voyages in Antarctica, with 98% in West Antarctica, because of harsher climates and longer distances.^62,63

Synthetic microfibres (polyethylene terephthalate and cellulose polyamide) were found in 27% of whelk samples in the Ross Sea region. Microfibres matched the chemical composition of technical outdoor clothing commonly worn near Terra Nova Bay.⁶⁴ Similarly, Aves et al.⁵¹ identified average concentrations of 29 particles L⁻¹ per snow sample in the Ross Island region, with polymers (polyethylene terephthalate) compatible with clothing used in nearby stations. Taxa in both the shallow and deep waters of the Southern Ocean and pelagic food webs are subject to the bioaccumulation of these marine MnPs and associated toxicological effects.

It is recognised that humans ingest and inhale MnPs from various environmental conditions, evidenced in deep lung tissue and faecal samples. Using infrared and Raman spectroscopic methods, studies have identified 20 MnPs (50-500 μm) per 10g of human stool from 8 samples and 9different polymer types.⁶⁵ The duration of time for MnPs to remain in the human system is unclear, but research indicates it ranges  from 3 weeks to more than a year.⁶⁶ With > 1,000 personnel in McMurdo station alone, there is the possibility for the introduction of MnPs to Antarctica via wastewater, albeit in small quantities.

Wastewater from research stations is managed with biological methods to dissolve substances, rather than using harsh chemical treatments. Failing this, disposal methods resort to sewage and domestic liquid wastes discharged directly into the sea.⁶⁷ The assimilative capacity of the marine environment is considered prior to discharge but the impact of MnPs on ecosystems is still uncertain and may not be quantified. The conditions for dispersal must adhere to initial dilution, with large quantities of waste treated by maceration. Whilst this would break down plastics, MnPs can exist in the waste stream.

Oceanic pathways

The notoriety of MnPs in the Southern Ocean region has been highlighted in recent years as a result of several observations. In the 1990s, researchers found polystyrene pieces and fishing-related fragments on the surface waters of the Southern Ocean, whilst a number of reports noted the increase in fishing debris on fur seals.^68,69,70 Observations of entanglement debris and sea surface MnPs have been closely followed by deep-sea sediment and thermocline samples.

Antarctica’s isolated ecosystem is maintained by the Polar Front, which causes a distinct change in oceanic temperature and depth. A distinct biogeographical difference exists between the benthic taxa in the North and South of the Antarctic Circumpolar Current as a result of evolutionary endemism, protected by water exchange barrier.⁷¹ It is widely understood the Polar Front barrier blocks migration towards the Southern Ocean, with the exception of seabirds and marine mammals. However, observations have challenged this view since the 1960s. Samples of pumice, driftwood and fishing materials were observed crossing the Antarctic Circumpolar Current and MnPs were found to have been ingested by snow petrel species that remain south of the Polar Front year-round.^72,73,74 It is increasingly clear that Antarctica is not isolated from MnP contamination via oceanic transport, and floating and submerged materials can reach the southernmost regions of Earth.^75,76

^‍Physical properties such as density and buoyancy influence the ability of MnPs to cross the Polar Front. Sampling at vertical distributions in the North Atlantic Gyre indicated that buoyant MnPs remain in the uppermost layer of the ocean.⁷⁷ Isobe et al.⁷⁸ also observed a higher concentration of MnPs close to Antarctica than those at offshore stations, further suggesting that MnPs are likely trapped South of the Antarctic Circumpolar Current, within the upper layer, once transported. More than half of plastics produced are buoyant, increasing the probability of MnPs with a lower density in seawater. As buoyant MnPs exist in the upper water column, Stokes drift plays an important role in their long-range transport, along with wind-driven movement.

Vertical distribution of MnPs in the Southern Ocean is largely dependent on density, with high density MnPs accumulating in the marine benthic environment.⁷⁸ Advective transport and vertical mixing processes act askey forces for different dispersion results of particles but larger MnPs are less influenced by turbulent mixing. High density MnPs eventually deposit indeep-sea sediment and their non-buoyant nature increases sink velocities. The distinctive shape of MnPs influences their sinking velocity, as seen by Ballentet al., who observed a higher sinking rate of polyamide (1.14 g cm⁻³) than polyvinyl chloride (1.56 g cm⁻³).^79,80

Biofouling has also been known to enhance sinking rates, despite MnP density and hydrophobicity, along with attachment to zooplankton faecal pellets⁸¹ and through particle feeding.⁸² Entering the food chain is one of the many pathways for MnP benthic sinking, although it can similarly preserve MnP availability in the Southern Ocean upper layer. This has been demonstrated in correlations between MnP and chlorophyll abundance in East Antarctic fast ice (a form of sea ice attached to the coastline or continental shelf), suggesting microorganisms excrete extracellular polymeric substances (EPS) that bind with MnP particles as they would with trace metals.

Sea ice could play an important role in harbouring MnPs, along with other biological materials, presenting its bioavailability to diatoms, bacteria and protozoans.⁸³ Once stored in sea ice, redistribution is typically determined by drift patterns and sea-ice trends that alter local entrainment and long-distance transport of MnPs, much of which is controlled by atmospheric dynamics and winds.⁸⁴ Whilst sea ice can accommodate particles, snow and ice eventually act as temporal sinks, as the temperature rises and melts ice forms, releasing MnPs into the Southern Ocean.⁸⁵

Research demonstrates that MnPs will eventually deposit in Antarctic benthos,^86,87,88 particularly when transported by thermohaline circulation (deep-ocean current caused by differences in temperature and salinity), creating MnP hot spots on the seafloor where biodiversity abundance is seen.⁸⁹ Global sea surface accumulation of MnPs, on the other hand, represents just 1% of the marine plastic budget. This correlates with the study by Melvin et al. that found over 90% of Antarctica’s coastline to be plastic free, when surveying 3,284 Antarctic shoreline sites.⁹⁰ Wind and waves can influence the downward mixing of particles from the ocean’s surface layer, which in turn decreases the density of MnP accumulation at the sea surface. However, traditional sea surface observations of MnPs have been disputed as underestimating the true upper ocean concentrations. Kukulka et al.⁹² observed that high wind condition estimates of MnP were up to 27 times the surface observations, generating an overall 2.5 increase in observations. It has since been advised that measurements of MnPs should take into account wind datasets, as it is likely research has underestimated the quantification of marine plastics in the past.^91,92

MnPs can be ejected from the ocean via sea spray and sea foam mechanisms, forcing them into the atmosphere with other sea surface aerosols. Although there are considerable uncertainties as to the quantification of oceanic contributions to atmospheric MnPs, it is believed that sea spray aerosols are a significant source.⁹³ Allen et al.⁴⁶ indicated an ocean-atmosphere MnP exchange when an average of 0.159 µg/m³, with an average diameter of 25 micrometres (µm) in sea spray mist events on the French Atlantic coast were observed. Particle ejection appeared significantly lower on rainy days, further suggesting that MnP are either washed out from the atmosphere in precipitation events or rained out within the cloud droplets - similarly to airborne aerosols.⁴⁶

Discussion

This article has highlighted various modes of transport for MnPs to reach Antarctica, including atmospheric, oceanic and human-induced pathways, by examining existing literature and recent data on the subject. It is suggested that long-range atmospheric transport is likely an important contributor to MnP accumulation in Antarctica. The fate of MnPs is, however, largely determined by a multitude of mechanisms in Earth’s systems that interlink and, as a result, govern the trajectory of MnPs.

Precipitation, prevailing winds, eddies, biofouling and marine snow are just some of the examples of environmental conditions that can not only impact transport pathways but also alter the physicochemistry of MnPs. Each process can influence the velocity and density of MnPs, which in turn can modify their capability to remain suspended in the atmosphere, settle in sea ice, traverse the sea surface or sink to oceanic zones and marine sediment.

The literature shows that the ultimate fate of global MnPs is most likely entrainment in deep sea sediment, before undergoing degradation into smaller particles - with the possibility to resuspend via mixing processes. Substantial data are required to quantify the distribution and deposition rates of MnPs in Antarctica and the Southern Ocean region and understand the accumulation of MnPs in remote regions. 

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