EAGER: Could plastics pollution impact natural marine tracer systems?

Plastics have been found in all aquatic realms, from groundwater to the most remote areas of the global ocean, but estimates of their abundance, depth variability, longevity, and impact on marine biogeochemical cycles are unknown. The goal of this project is to determine how the presence of microplastics in the ocean may impact the distribution of naturally-occurring radioactive isotopes.

These isotopes are commonly used to track processes associated with natural particles like sediments and biological materials. Plastics have different chemical and physical properties than natural particles. These differences may affect how isotopes “stick” to particles, and change how oceanographers use the isotopes to calculate things like how fast particles sink through the ocean.

The team will use a combination of laboratory experiments and field work to investigate these questions. The project will facilitate the training of graduate and undergraduate researchers from the University of Rochester and incorporate unique educational experiences for undergraduates and local high school students. One graduate student’s PhD work and two undergraduate senior theses will focus on aspects of this project.

The investigators will also engage with high school students from Rochester City School District (RCSD) in two workshops focused on “the radioactivity around us” and how radioisotopes are used to answer big questions in earth sciences and beyond. The workshops will be offered through the Upward Bound program, which has a proven track record of increasing college admission rates for RCSD students.

Persistent, buoyant, and metals-scavenging plastics have the potential to impact tracer distributions, especially for longer-lived isotopes, as a lateral source and/or as a standing stock at depths could obscure or contribute to natural variations. While the ultimate goal is to know what tracers stick to (i.e., to plastics or to associated biochemical materials), the most pressing question is to what degree do particle-reactive radioisotopes associate with microplastics (MPs) in the marine environment and does this behavior differ from the typical associations observed for the average ocean particle?

Specifically, do MPs have comparable partition coefficients (Kds) to bulk particles for key radioisotope tracers, and how different are Kds with location or varying biogeochemical conditions? The project will address three objectives. Objective 1 is to quantify the association (Kds) of select particle-reactive radioisotopes with plastics in controlled environments through laboratory experiments.

Objective 2 is to quantify the association (relative Kds) of select particle-reactive radioisotopes with plastics at a coastal and open ocean location to determine whether a gradient could exist. Finally, Objective 3 is to assess whether there is a potential for ‘inherited’ radioisotope signals to come from plastics with terrestrial origins or introductions, through collection of sediment cores from beaches and estuaries.

If the proposed work shows that MPs have some potential for impacting radioisotope distributions (hypothesis 1: plastics Kds ≥ typical particle Kds) or that MPs have the greatest potential for impacting radioisotope distributions (hypothesis 2: plastics Kds ≥ typical particle Kds and observed gradients suggest inherited tracer signals could be transported in a manner unique to plastics), observational radiochemists and modelers will have a baseline to account for the ‘plastics effect’. Collectively, the field of chemical oceanography can begin to study the significance of plastics for various elemental cycles and incorporate the ‘plastics effect’ into future efforts focused on key marine tracers.

Currently, this type of proposal, and marine plastics studies in general, sit on the edge of several existing programs. The proposed work can be used directly for future decision making on paired plastics-radioisotopes and general tracer studies by NSF Chemical Oceanography and other agencies. If, after the proposed work is carried out, there is little to no predicted impact of microplastics on radioisotope tracer distribution in the oceans (the ‘null’ hypothesis), the ongoing debate of the effect of plastics will be closed but radioisotopes will be established as tracers of plastics.

In this case, the proposed measurements will produce upper ocean plastics fluxes and residence times.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.