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Quantifying microplastic mass in deep-sea sediment along a transect in the north pacific
Summary
Researchers collected sediment cores at six sites along an 800-km east-west transect offshore from San Diego and used pyrolysis-GC/MS targeting ten polymers to quantify microplastic mass fractions, finding higher polymer detection rates and concentrations at sites closer to shore compared to offshore locations.
Plastics have been found in sediments across all ocean basins; however, the rate at which sedimentation happens and the ultimate fate of plastics have yet to be well understood. Sediment cores are an apt tool to improve our understanding of these topics, as they allow for both spatial and temporal analysis. We collected sediment cores at six sites along an east/west transect that spanned approximately 800 km offshore from San Diego, California (USA) in December 2022. At each location, two cores were collected from a multicorer: one used for microplastic analysis, the other for radiometric (Pb210) dating. In both cases, each core was sectioned from top to bottom into ten, 1-cm sections followed by a single 5-cm section. To analyze microplastic concentrations, the top layer of the core was run through a novel density separation process, photographed with a microscope under a 15x objective, and analyzed on pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), targeting ten plastic polymers. Shipboard sampling blanks, as well as procedural blanks in the lab, were collected to evaluate avenues of contamination. Results from the top layer showed that more polymers were detected at sites closer to shore than offshore. Seven polymers were detected across samples, but not all at concentrations above the limit of quantification. Of those that were above the limit of quantification, the three sites closer to shore had higher polymer mass fractions than the three sites further offshore. As the distance from the shore correlated with depth along the sample transect, it is difficult to determine if the distance from the surface or shore drives this trend. This study applied standardized and accurate methods to measure microplastics in deep-sea sediments. These are vital for comparing samples across oceans and gaining a more holistic understanding of the open ocean plastic sedimentation rates. Also see: https://micro2024.sciencesconf.org/559036/document
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