Abstract

We analyzed coastal sediments of the Santa Barbara Basin, California, for historical changes in microplastic deposition using a box core that spanned 1834–2009. The sediment was visually sorted for plastic, and a subset was confirmed as plastic polymers via FTIR (Fourier transform infrared) spectroscopy. After correcting for contamination introduced during sample processing, we found an exponential increase in plastic deposition from 1945 to 2009 with a doubling time of 15 years. This increase correlated closely with worldwide plastic production and southern California coastal population increases over the same period. Increased plastic loading in sediments has unknown consequences for deposit-feeding benthic organisms. This increase in plastic deposition in the post–World War II years can be used as a geological proxy for the Great Acceleration of the Anthropocene in the sedimentary record.

INTRODUCTION

An estimated 4.8 million to 12.7 million metric tons of plastic waste enters the ocean every year (1). Larger populations produce more waste, and the world population is predicted to increase disproportionately in coastal regions (2). As coastal populations grow and synthetic clothing and plastic production increases, effluent-derived fibers are becoming a larger concern in nearshore areas (2). Previous studies focusing on buoyant plastics collected at the sea surface have shown a one– to two–order of magnitude increase in abundance of surface microplastic debris in the northeast Pacific from 1972–1987 to 1999–2010 (3, 4), but no significant increase in sea surface plastics in subtropical latitudes of the northeast Atlantic from 1986 to 2008 (5). There is a clear need for assessments of longer-term rates of accumulation in coastal ecosystems apart from surface waters. Here, we analyze microplastic particle deposition in coastal ocean sediments off southern California and demonstrate a pronounced and unabated increase in plastic deposition following World War II.

RESULTS

The sediment core

A box core that sampled Santa Barbara Basin sediments (fig. S1) showed a well-defined annual varved structure (Fig. 1), making it possible to establish a clear chronological sequence. Santa Barbara Basin was chosen because of its unique sediment structure. The high surface productivity in the Santa Barbara Channel, coupled with the restricted water movement due to the Santa Barbara Coastline in the north, the Channel Islands in the south, and the high eastern (230 m) and western (475 m) sill depths, creates an area of anaerobic bottom water, which minimizes bioturbation and allows the preservation of millimeter-scale seasonal laminae couplets, each couplet representing a year (Fig. 1) (6). Complete methods are found at the end of this paper.

Fig. 1 X-radiograph of sediment box core.

Chronology was assigned by enumerating individual varve couplets. A bacterial mat 1 to 2 cm thick at the top of the core indicates that surficial sediments were intact.

Plastics in the sediment core

Plastics were present and visually identified in every 0.5-cm transverse layer of the core (averaging 2.2 years per layer), including in the layers before 1945, before plastic polymers were produced in high quantities or widely used (1, 7, 8). The year 1945 was also the end of World War II, leading to many societal shifts in production and industry, and is the year denoting the beginning of the Great Acceleration of the Anthropocene (9, 10). Plastic particles were categorized as fibers, fragments, film pieces, and quasi-spherical particles (Fig. 2). Physical characteristics of every particle were recorded, including length, width, color, particle type, and amount of biofouling (fig. S2). The majority of plastics found in the core were fibers, which formed 77% of the particles (fig. S3). The contamination samples, from 1836 to 1945, were more dominated by fibers, at 89.1% of total particles (fig. S3). In post-1945 layers, 67.5% of the particles were fibers (fig. S3). Although previous literature has reported mostly bright-colored fibers and potentially overlooked many neutral-colored fibers (2), here the most common fiber color found was white (64.5%). The next most common particle category was fragments, at 14% of the overall particles, although there were many more in the post-1945 samples than in the pre-1945 samples (20.8% versus 5.8%, respectively) (fig. S3). A total of 9.7% of the post-1945 samples were film, compared to 4.9% of the pre-1945 samples. Almost no spherical plastic particles were found in the core (fig. S3).

Fig. 2 Plastic particles from box core.

Examples of (A) fibers, (B) fragments, (C) film, and (D) spherical particles.

Identification of plastics via FTIR spectroscopy

Fourier transform infrared (FTIR) identifications of plastic particles (Fig. 3) were sometimes difficult due to small particle size, particularly the small width of fibers, but 87.5% of visually identified plastic particles were definitively or likely plastic polymers, based on matching with standard plastic reference spectra (table S1). The plastic polymers that were identified in the core included polystyrene (PS), polyethylene (PE) including low-density polyethylene (LDPE), polyvinyl chloride (PVC), nylon (polyamide), polyester, PE terephthalate (PET), polypropylene (PP), and the box core liner (which had a distinctive FTIR signature).

Fig. 3 FTIR spectra of plastic standards and sediment samples.

PET, polyethylene terephthalate; LDPE, low-density PE; PS, polystyrene; PVC, polyvinyl chloride; HDPE, high-density PE; Unclear sediment sample, unidentified.

Plastic deposition rates

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 majority of pre-1945 plastic pieces were fibers of 500 to 1000 μm in length (fig. S2). The average contamination value of 7.8 particles 100 cm−2 year−1 for all pre-1945 samples was subtracted from all post-1945 samples, yielding the net change in plastic deposition rates over time since 1945 (Fig. 4A). Accordingly, plastic deposition rates in the Santa Barbara Basin from 1945 to 2009 increased exponentially, with an average doubling time of 15 years (Fig. 4A).

Fig. 4 The exponential increase in microplastic deposition in sediment is significantly correlated with the exponential increase in worldwide plastic production over the same time period (1945–2010).

(A) Total plastic deposition rate over time, corrected for contamination. All four plastic types combined, 1945–2009, with average value from 1836–1945 subtracted. (B) Plastic deposition rate in sediment compared to worldwide plastic production, 1950–2010. Worldwide plastic production numbers from PlasticsEurope (8).

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Fig. 4 The exponential increase in microplastic deposition in sediment is significantly correlated with the exponential increase in worldwide plastic production over the same time period (1945–2010).

(A) Total plastic deposition rate over time, corrected for contamination. All four plastic types combined, 1945–2009, with average value from 1836–1945 subtracted. (B) Plastic deposition rate in sediment compared to worldwide plastic production, 1950–2010. Worldwide plastic production numbers from PlasticsEurope (8).

Plastic deposition rates and environmental factors

Residuals of corrected plastic deposition rate of total plastics from the fitted exponential of Fig. 4A show that 1984, 1994, and 2002–2005 had anomalously high plastic deposition rates, and 1978, 1985, 1998, and 2007 were years of anomalously low deposition (fig. S5A). Attempts to relate these anomalies to years of anomalous rainfall, and thus coastal effluen