PLASTICS AND THE ANTHROPOCENE

Plastics are anthropogenic compounds that are mass produced globally. The life cycle of a plastic product is controlled by humans, from: 1) removal of the source petroleum, 2) production, 3) transportation to industries and retailers, 4) purchasing and use, and 5) disposal in landfills, incinerators, or as litter in the environment. Once in the environment, these products degrade chemically, mechanically and biologically to form microplastics (MPs) and nanoplastics (NPs). These tiny particles are easily transported by wind, precipitation, and flowing water, and are either ingested by organisms or are deposited in natural and anthropogenic sinks. Some sinks are transient, but plastics can also become buried beneath sediment or beneath other waste items in landfills. Key natural sinks are low-energy settings like lagoons, sheltered embayments, deep offshore ocean zones, and lake centers. Recent studies have shown that MPs in dated lake sediment cores can be used as proxies for fluctuations in macroeconomic conditions, population growth, and plastic production. Microplastic abundances in these studies were as high as 488 particles per gram of dry weight sediment, and MPs had been accumulating in the sediment for up to 72 years. Key anthropogenic sinks are both nonregulated and regulated landfills, which have been considered to represent a novel sedimentary environment in which future technofossils will form. Cross sections of landfill deposits and near-surface sediment display accumulation of plastic debris items along specific soil horizons, much like natural chronohorizons containing distinct fossils (from mass extinction), impact spherules (from bolide impact), and bentonite (from volcanic eruption). Burial delays the degradation of plastic debris, which increases its potential to be preserved in the future geological record.

Although the majority of plastic products are low density compared to water, numerous studies have shown that MPs composed of polyethylene (PE) and polypropylene (PP) are abundant in bottom sediment of rivers, lakes, and oceans. Settling of plastic particles depends primarily on their density, but other factors, such as shape, size, and degree of degradation impact the floating capacity of plastics. In addition, once a plastic particle gloms onto denser particles in the environment, its propensity to sink is enhanced. Most anthropogenic materials contain some natural elemental component, for example, slag, gyprock, concrete, bricks, and metal-based tools. Plastic, however, is unnatural. Although it is developed from a natural substance, it is a purely synthetic material, and that perhaps makes it the ideal technofossil for the informal anthropocene epoch.