Plastic deposition rates
This protocol is extracted from research article:
Multidecadal increase in plastic particles in coastal ocean sediments
Sci Adv, Sep 4, 2019; DOI: 10.1126/sciadv.aax0587

The box core had a surface area of approximately 174 cm2. To calculate plastic deposition rates, the number of plastic particle pieces in a 0.5-cm transverse layer was divided by the time interval represented by the transverse layer and normalized to 100 cm2 of seafloor during 1 year (No. particles*100 cm−2 year−1). The plastic deposition rate (Particles*100 cm−2 year−1) was calculated for the four individual particle types, from 1836 to 2009 (fig. S4). Pieces of core liner, identified via FTIR, were treated separately from other fragments. Fibers dominated the fluxes (fig. S4B).

The deposition rate of particles in layers corresponding to 1836–1945 was averaged, as an indication of baseline contamination of plastic particles. The contamination values before 1945 were relatively consistent throughout the 1836–1945 region (left of the gray line in fig. S4), and thus, an averaging is appropriate here. Any plastic before 1945 was treated as contamination due to the low amounts of plastic in production at that time (7), and the fact that some plastics (PET, polyester, polypropylene, etc.) had not yet been invented. The bottom sample of the core, corresponding to 1834, was removed from analysis due to previous knowledge that it had high levels of contamination from contact with the bottom of the box core liner during processing. The average from 1836 to 1945 was then subtracted from the deposition rate of all other samples to correct for baseline contamination.

Baseline-corrected deposition rates from 1945 to 2009 were plotted against time, and an exponential was fitted. Residuals of plastic deposition rates were calculated from the exponential function (Fig. 4A). Deposition rates were calculated for fragments, film, and spherical particles (fig. S6A) separate of fibers (fig. S6B), and both had an exponential increase, with no significant difference between them (ANCOVA, P > 0.05).

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