The Plastic Cycle – An Unknown Branch of the Carbon Cycle

Using known frameworks to elucidate the plastic cycleWe can adopt the terminology of biogeochemical cycles (i.e., reservoirs, sources, sinks, fluxes, and mean residence times) 7 to better describe the fate of plastics in the environment. When particles of a certain substance accumulate in a place of storage, that place is deemed a "reservoir". "Sources" are reservoirs that release more particles than they accumulate. "Sinks" absorb more particles than they release. Disturbances may cause a sink to become a source, and vice versa. A "flux" is the amount of particles moving from one location to another per unit surface area per unit time. The mean residence time is the ratio of particle mass in a reservoir to the sum of either its input or output fluxes, and represents the average time a particle remains within a particular reservoir 7 .In the case of carbon, major reservoirs include the terrestrial biosphere, the ocean, and fossil carbon ( Figure 1). Due to anthropogenic disturbance, fossil carbon -which was once a neutral reservoirhas now transformed into a carbon source. At this point, while pH values permit, the ocean is a carbon sink. But with decreasing alkalinity levels due to increasing greenhouse gas emissions, at some point carbon uptake into the ocean will falter and the ocean may become a carbon source 7 . Carbon fluxes, including ocean-terrestrial, ocean-atmosphere, and terrestrial-atmosphere fluxes, link This is a provisional file, not the final typeset article the reservoirs together. Likewise for plastic pollution, there exist reservoirs, sources, sinks, and fluxes of plastic. So far, the terrestrial environment is a major source of plastic due to the numerous activities -e.g. washing laundry, littering, wastewater effluent -that emit plastic pollution into the environment. While there are many suspected reservoirs of plastic pollution in the environment, it is not well-known whether they are temporary or permanent. Plastic particles are thought to be transported via a variety of means between reservoirs such as wind currents through the atmosphere, advection in rivers and streams, waves and surface currents on the ocean surface, and subsurface currents and animal migration below the ocean surface; a name has yet to be assigned to each of these fluxes of plastic. The ocean bottom is suspected to be a sink of plastic since more plastic accumulates there than leaves. But what is very likely is that since there is no upper threshold to how much plastic the environment can hold, the majority of the environment may continue to absorb the plastic we produce indefinitely as one big sink unless we find ways to mitigate our input.Potentially by accident, as described above, plastic pollution researchers are already using this terminology to describe plastics. From examining the vertical "flux" of plastic to the deep ocean 8 , to predicting the deep sea as a major plastic "sink" 9 , researchers have adopted these terms to described observations. To this day, the names and locations of numerous plastic reservoirs have been hypothesized from the deep ocean 9,10 to animals 10 to coastlines 11 , but they need to be better quantified globally -and although the ocean surface reservoir has been extensively modelled [12][13][14][15] , the range of estimates can still be narrowed and improved. Likewise for plastic fluxes, much remains to be understood. It is also important to make sure that these terms are used accurately: for instance, researchers should use caution when labelling a resting place as a "sink" when it is more of a temporary storage place or reservoir, especially when there is not enough research available yet to support this claim. This is the case for animals and the ocean column, but is also true for the deep ocean and coastlines. Moving forward, we should adopt these biogeochemical cycling terms formally and use them with purpose to better quantify and characterize the reservoirs, sinks, fluxes, sources and other cycling parameters of plastic within its global cycle.Plastic as an unknown branch of the carbon cycle Plastic is carbon. More specifically, almost all plastic is fossil carbon locked up in polymer form [16][17][18][19] . We know that fossil fuels hold 10,000 gigatonnes (Gt) of carbon and that 9 Gt of carbon is emitted into the atmosphere every year, but it is often forgotten that roughly 7 Gt of that fossil carbon is now in the form of plastic 20 . In fact, since plastics account for 6% of global oil consumption 16 , hypothetically if all 10,000 Gt of fossil fuels were extracted, 600 Gt of plastic would be produced (assuming business as usual). The sheer amount of plastic produced each year places plastic on the same scale as global carbon fluxes, and thus makes plastic a fundamental part of the carbon cycleas a net flux out of the fossil carbon reservoir, which is currently overlooked. Plastic and greenhouse gas production are intimately connected. We spend so much time asking questions about how the use of fossil fuels increase greenhouse gas emissions and impacts our climate, but maybe now is the time to ask questions about how the increasing use of plastic is impacting our climate as well.We can think about how plastic cycles, but we can also think about how plastic cycles carbon. Carbon moves through the cycle through various processes. Carbon is incorporated into living cells via photosynthesis, respired into the atmosphere as carbon dioxide, dissolved into the ocean as bicarbonate, buried in the deep ocean as calcium carbonate, etc. 7 Since plastic is carbon, the movement of plastic through its global cycle inevitably transports carbon from one reservoir to another. Examples include the incorporation of plastic into sedimentary records 21 , the assimilation of plastic carbon into biomass 22 , and the suspension and deposition of airborne plastic particles as part of atmospheric fluxes 6 . Interestingly, plastic settling to the bottom of the ocean is also a flux of carbon to the bottom, thereby working in synergy with the biological pump. Furthermore, inorganic carbon continuously leaches from plastic 23 and plastic may release greenhouse gases as a result of UV exposure 24 or open burning 25 . In all of these cases, the carbon within plastic is being chemically transformed or transported to different places; plastic is a vector of carbon. Consequently, it is crucial to better understand just how much carbon is moving between reservoirs in the form of plastic, and how plastic contributes to carbon cycling overall.In this discussion of plastic cycling and carbon cycling, there is a common theme of anthropogenic interference. While the carbon cycle has become altered as a result of anthropogenic activities, anthropogenic activities have catapulted the existence of the entire global plastic cycle itself. Since we have influenced these cycles, it is crucial that we better understand the mechanisms behind how they work and consistently evaluate the impact of such changes on these cycles.In the words of Thompson et al. 26 : "Lost at sea, where is all the plastic?". Using a known framework, maybe we can begin to tackle this question globally. To better understand the plastic cycle, we need to put values to various components of the plastic cycle both locally and globally. How much plastic resides in each reservoir? What are the fluxes of plastic in the atmosphere, in the ocean, and on land? What are the fragmentation, degradation, and mineralization rates of plastic in various environments? As we attempt to answer these questions, we also need to take into consideration how the magnitudes of the reservoirs and fluxes vary through space and time. Furthermore, we should think deeply about the interconnections between plastic and the carbon cycle and consider plastic within the bigger picture of the carbon cycle, by asking ourselves the following questions: how much carbon is moved by plastic locally and globally? This must include through the atmosphere, terrestrial soils, aquatic environments, and via biotic transport. I believe both plastic cycling itself, as well as the contribution of plastic to carbon cycling, need to be better understood to help us assess the impact that plastic has on our lives and our planet. Now, we must dedicate our time to filling in the gaps in these cycles (Figure 1). *4-12 MT plastic/year is for the year 2010 and taken from Jambeck et al. 27 .**0.1-0.2 MT plastic is for microplastics only and taken from van Sebille et al. 15 .