Oceanic Transport and Source Inference of Nanoplastics

Plastic pollution has become a pervasive environmental issue, with plastics ultimately accumulating in the ocean. Marine plastic can harm marine life through ingestion and entanglement, transport pathogens, and potentially introduce toxic chemicals into the food web. Understanding the sources, fate, and transport of plastic in the ocean is crucial for assessing its environmental impact and informing policies to limit single-use plastic production. However, the complex physical processes controlling the dispersal and fate of plastics make it challenging to pinpoint pollution origins and determine accumulation rates of plastic in the marine environment. This thesis advances our understanding of plastic origins, transport, and fate using numerical Lagrangian simulations. Lagrangian simulations compute the trajectories of virtual particles in the ocean, which allow for the reconstruction of possible plastic pathways in the real ocean. In this thesis, we use Lagrangian simulations to perform three studies aimed at understanding and unraveling the transport mechanisms and pathways of plastics in the ocean. The first study explores strategies for generating ensemble-like variability in single-member Lagrangian simulations of the Gulf Stream. Ensemble simulations aim to capture the full range of particle dispersal outcomes, while single-member simulations only provide a subset of these outcomes. Our study tests spatially and temporally varying particle release strategies in single-member runs, using ensemble simulations as a reference. We implement an information theory approach to define and compare the variability in the single-member strategies with the ensemble. Our findings improve trajectory variability representation in particle trajectories and define a framework for uncertainty quantification in Lagrangian ocean analysis. The second study develops a Bayesian probabilistic framework to attribute riverine sources of floating plastic found in the South Atlantic Ocean. By combining prior estimates of plastic emitted from rivers with Lagrangian simulations, our framework constructs spatially resolved probability maps of plastic origins. Our approach enables analysis of source probabilities as a function of particle age and beaching location, providing valuable insights for targeted pollution reduction efforts. The third study investigates whether fragmentation can explain the presence of nanoplastics sampled in the abyssal South Atlantic Ocean. Lagrangian simulations incorporating an idealized fragmentation scheme suggest the sampled nanoplastics likely originated from the breakdown and fragmentation of larger microplastics during their descent from the ocean surface, rather than directly entering the ocean as nanoplastics. Our results highlight the importance of considering fragmentation processes in modeling the vertical transport of plastic particles and show that fragmentation timescales can significantly influence drift patterns and transit times of plastic particles into the deep sea. Through these novel methodologies and insights, this thesis contributes to a better understanding of the origins, transport pathways, and ultimate fate of plastic debris in the ocean. The developed tools and techniques have broad applicability for identifying pollution sources, predicting realistic particle trajectories, and optimizing numerical simulation strategies. Our findings highlight the importance of considering multiple physical processes, such as fragmentation, in modeling the journey of plastic particles from source to sink.