Award: OCE-1260403

The most numerous plastic debris in the marine environment are microplastics, particles smaller than 5 mm in size that have largely resulted from the fragmentation of larger plastic items upon exposure to sunlight. The "age", or time of exposure, and time scale of fragmentation to sizes observed in the open ocean have not yet been determined, thus the "lifetime" and ultimate fate of plastics in the marine environment is presently unknown. From plastics samples collected using surface-towing plankton nets in the North Atlantic and North and South Pacific Oceans over more than 30 years, as well as laboratory and field exposure experiments, we identified physical characteristics and chemical markers associated with plastics degradation, which vary according to polymer type and by exposure conditions such as temperature, and air versus seawater exposures. From visual analysis and polymer identification using Raman spectroscopy of more than 1100 individual particles collected at sea, we determined a set of visual characteristics that can be under to identify, with good confidence, polymer type of particles larger than ~1 mm using low-power microscopy alone, without the use of expensive spectroscopy equipment. An analysis of polymer type found a surprisingly large proportion of polyethylene (PE) compared to polypropylene (PP) considering the amounts of these polymers produced, used and disposed of as consumer waste. Physical characteristics suggest that PP fragments into small particles more readily than PE. Analysis of particle size of more than 5000 particles, combined with particle form (pellet, fragment, film, foam, line) and polymer type, were used to evaluate the relative "age" of sets of particles collected in plankton net tows in different geographies and time periods. The presence of film and foam, and a higher proportion of PP, suggest a relatively "younger" collective age, typically in samples measured closer to land. Using FTIR spectroscopy we determined that a chemical metric commonly used to assess weathering age, the carbonyl index, is not a reliable measure of exposure when assessing the surface of field-collected samples because of inorganic deposits that can alter the signal. However, cross-sectional analysis suggests that the presence of carbonyl groups indicates degradation into the interior of a particle. Further, scanning electron microscopy and elemental analysis suggest that cracks that initially form at the particle surface allow oxygenated seawater to penetrate the particle, causing oxidation and further weakening in the interior that may ultimately result in particle breakage into large daughter particles. This mechanism acts in parallel to surface weakening resulting from relatively shallow (10s of microns thick) penetration of UV radiation causing photodegradation. Thus, we propose a revised model of particle fragmentation that includes both thin particle shedding at the exposed surface, together with bulk fragmentation facilitated by penetrating cracks. Further work to analyze particle size distribution resulting from degradation and fragmentation is recommended. Analysis of mechanical degradation (using tensile strength tests) of plastics exposed to accelerated weathering conditions shows variation according to polymer type, with degradation rates fastest for oxodegradable LDPE, followed by high density polyethylene (HDPE), PP and low density polyethylene (LDPE). The ratios of PE to PP in ocean-collected microplastics are consistent with this finding, although further analysis is required to distinguish between HDPE and LDPE in field samples. This project has supported extensive training of a full-time research technician, several undergraduate students and four high school students, among others, in the analysis of field-collected microplastics, conducting accelerated weathering experiments in the laboratory, and assessment of mechanical and chemical degradation of plastics. Extensive communication of experimental results, as well as the topic of ocean plastics pollution more generally, has been carried out throughout the duration of the project to audiences including K-12 students, the general public, academic and scientific audiences, government agencies, international bodies, and industry and product manufacturers. Not only have these audiences become better informed about the reality of plastics pollution (moving away from misconceptions of floating islands of trash and towards understanding of the transformation of everyday consumer products into microplastics, with associated risks to wildlife), but the science we have accomplished has informed policymakers and other stakeholders who are engaged in developing solutions to reduce ocean plastics pollution. Last Modified: 09/28/2018 Submitted by: Kara L Law