Coastal marine sediments as a major sink for small microplastics: Evidence from a ~100-year varved sediment record

To gain a better understanding of the fate of the ocean's "missing plastics" and the onset of the Anthropocene through historical trends in the abundance of small microplastics (SMPs; 20-300 μm), varved marine sediments were analyzed to reconstruct records of sedimentary SMP concentrations, particle deposition fluxes, and mass deposition fluxes over the past ~100 years. Using a rigorously contamination-controlled analytical protocol, SMPs were first detected in sediments dating to 1951, after which their concentrations increased with large fluctuations. This first appearance is concordant with the 1952 onset of the Anthropocene Epoch proposed by the Anthropocene Working Group. The mean particle flux of SMPs after 1980 was 125 particles cm^-2 yr^-1, which represented a fourfold increase relative to earlier periods and was approximately 9700 times greater than the flux of large microplastics (LMPs; 300-5000 μm). The mean mass flux of SMPs was 0.017 mg cm^-2 yr^-1, approximately 27 times higher than that of LMPs. These results indicate the rapid transfer of SMPs to the seafloor, while many LMPs may be transported laterally out of the system, indicating that coastal marine sediments may represent a major sink for SMPs, and partly explaining the missing plastics phenomenon in the ocean. Because peaks in SMP deposition flux did not coincide with those of LMPs, their depositional processes likely differ. Multivariate analysis incorporating biological community proxies and meteorological and oceanographic parameters revealed a significant positive correlation between the SMP deposition flux and lutein, suggesting the attachment to and aggregation with prasinophytes, small phytoplankton with cell sizes of a few micrometers. These results indicate size-dependent sinking processes in coastal marine environments, whereby SMPs are rapidly transferred to the seafloor through flocculation/aggregation with fine marine particles and subsequent particle scavenging, whereas LMPs may be associated with longer-term processes such as biofouling before sinking.