Plastic as a Sediment – A Universal and Objective practical solution to growing ambiguity in plastic litter classification schemes

There is a universal and growing challenge of ambiguity in plastic classification schemes, which affects the predictability of plastic accumulation in all environments world-wide. Plastic pollution is an ever-growing global issue, and understanding plastic items and their sedimentological relationship is a solution to this increasing concern. Definitions of micro- meso- and macro- plastic is inconsistent between studies, as are categories of plastic, and the properties recorded. This is understandable because every project has a different objective, but the consequence is that different studies are not laterally relatable. It is widely agreed that as a community, we need a system that has room for specialism of study but has an objective basis that can allow for inter-project and inter-disciplinary collaborations. By considering plastic as a sediment, we can outline an objective and quantifiable classification scheme that builds on the principles of sedimentology for use in plastic studies, such that we can better understand why plastics accumulate where they do. This is importantly not just another classification scheme, but a philosophically grounded solution to a long-standing challenge that is set to be of increasing significance. Additionally, whilst these advances may be of immediate usefulness to the scientist interested in plastic transport and accumulation, the environmental scientist or biologist will find that these philosophies and classification scheme will aid to quantitatively support and compliment aligning data. Through this, our new plastic and sediment environment can be further understood both spatially and temporally, and connected to other studies. We outline the key philosophies of sedimentology and use these to: i) unify and define plastic size classification from nano-scale to mega- and introduce giga- scale; ii) we outline a shape classification that can tackle simple through to complex shapes; iii) we discuss and demonstrate the importance of total density over polymer density; and iv) we discuss the importance of material properties. In using this classification scheme, we can relate any plastic item to any other item, and to itself over time. This manuscript contains a summary and worksheet that can be used in the field or in a laboratory to utilise this scheme and present objectively comparable results. We are confident that the philosophies presented here will be of use to the plastic research community, such that we can integrate plastic studies with longstanding and deeply understood sedimentological knowledge, thereby enhancing our understanding of plastic routing and accumulation in the environment.