Plastic as a sediment in riverbeds

Plastic has become an almost ubiquitous component of Earth's sedimentary systems being recorded from diverse environments ranging from desert plains through to ocean trenches. It is both abundant and durable over hundreds of years (perhaps even millennia), so we need to consider plastic as part of the sedimentary cycle. To date, many investigations have recorded occurrence, type and concentration of plastic in different environmental settings, though there has been little focus on contextualising plastic distribution within the sedimentary cycle. This is not surprising because plastic is a highly diverse sedimentary material. Plastic varies in size from nano-and micron-sized particles through to meter-sized macro plastic. It has an array of compositions and can be composite with metal, rubber and other anthropogenic materials. It can be formed in almost any shape imaginable, from thin sheets through to robust spheres. As such, understanding plastic within a sedimentological framework and examining the interactions of plastics within the sedimentary cycle is highly challenging. Here we outline a scheme that provides a conceptual and unifying framework for future studies and indicates that plastic can be considered as a 'new' sedimentary component of our planet. Our scheme is supported through evidence collected globally from the modern environment and experimental data, footage of which will be presented. Our data demonstrate that whilst plastic is highly heterogeneous in many of its characteristics, there are common transport mechanisms and depositional behaviours that occur across the spectrum. This information can be used for practical application such as understanding and predicting where plastic builds up in the natural environment in order to inform where environmental clean up may be necessary and most effective.