Stratigraphic records of microplastic deposition in Puerto Rico coastal sediments: a focus on hurricane impacts

Abstract Microplastic in the coastal environment is a massive global problem that has only recently been identified, even though widespread plastic usage and subsequent disposal in the environment have occurred since ~1950 CE. Unfortunately, the vast majority of studies on microplastic focus on surficial sediments rather than investigating microplastic concentrations in three dimensions. We remedy the paucity of studies on this understudied topic by examining two sediment cores from the southern and northern coasts of Puerto Rico. The Guayama study site is a protected lagoon in a rural, arid region, whereas the Levittown site is an unprotected tidal lagoon in an urban/suburban area. Despite their differences, both sites contained microplastic in quantifiable amounts, though relatively low compared to other global sites. Compared with Levittown, Guayama contained a lower total amount (48 vs 72), a lower concentration (79 MP/kg vs 116 MP/kg), and less variety of morphotypes (1 vs 4). Radiometric dating on both cores confirmed an increase in microplastic deposition in recent sediments. Only the Guayama site contained sediment that was deposited prior to 1950 CE, and microplastic was found below this sediment horizon, suggesting post-depositional transport likely via bioturbation. Additionally, hurricane event layers were identified using sedimentary and geochemical data (particularly terrestrial signals) which corresponded with increased microplastic concentration, suggesting that hurricane-induced terrestrial flooding is likely responsible for the transport and deposition of high quantities of microplastic into the coastal region. This study is the first to investigate sedimentary microplastic in three dimensions in Puerto Rico, providing information on multiple novel aspects of microplastic transport and deposition in coastal sediments.