SCIENCE ADVANCES | RESEARCH ARTICLE ENVIRONMENTAL STUDIES Copyright © 2020 The Authors, some Exponential increase of plastic burial in mangrove rights reserved; exclusive licensee sediments as a major plastic sink American Association for the Advancement of Science. No claim to C. Martin1*, F. Baalkhuyur1, L. Valluzzi1, V. Saderne1, M. Cusack1, H. Almahasheer2, original U.S. Government P. K. Krishnakumar3, L. Rabaoui3, M.A. Qurban3,4, A. Arias-Ortiz5, P. Masqué6,7,8, C. M. Duarte1,9 Works. Distributed under a Creative Sequestration of plastics in sediments is considered the ultimate sink of marine plastic pollution that would justify Commons Attribution unexpectedly low loads found in surface waters. Here, we demonstrate that mangroves, generally supporting NonCommercial high sediment accretion rates, efficiently sequester plastics in their sediments. To this end, we extracted micro- License 4.0 (CC BY-NC). plastics from dated sediment cores of the Red Sea and Arabian Gulf mangrove (Avicennia marina) forests along the Saudi Arabian coast. We found that microplastics <0.5 mm dominated in mangrove sediments, helping ex- plain their scarcity, in surface waters. We estimate that 50 ± 30 and 110 ± 80 metric tons of plastic may have been buried since the 1930s in mangrove sediments across the Red Sea and the Arabian Gulf, respectively. We observed an exponential increase in the plastic burial rate (8.5 ± 1.2% year−1) since the 1950s in line with the global plastic production increase, confirming mangrove sediments as long-term sinks for plastics. INTRODUCTION (22). Persistent and substantial stocks of microplastics are therefore The mass production of plastic, starting in the 1950s and growing expected in habitats supporting high sediment accumulation rates, at 8.4% per year (1), coupled with inefficient disposal and recy- such as vegetated coastal habitats, including seagrasses, salt marshes, cling systems, has generated increasingly high loads of waste enter- and mangroves (23–28). These are termed blue carbon habitats ing the marine environment (2). The inherent durability of plastic because of their remarkable capacity to sequester and accumulate material makes it extremely persistent in seawater (3), where it frag- large stocks of carbon in their sediments at millenary time scales ments but does not degrade (4), and its buoyancy facilitates its (23). Attenuation of wave action and currents by plant structures transport by currents and winds to the most remote locations (5–7). promotes sedimentation and reduces resuspension, contributing to As a result, microplastics, i.e., particles <5 mm in diameter (8), are the high rates of carbon sequestration in blue carbon habitats (23). continuously accumulating across vast areas of the oceans (9–11), Particle retention in blue carbon habitats by these mechanisms may with concentrations increasing over time (12). Yet, global assess- also be effective in supporting the retention of microplastic parti- ments indicate that only 1% of plastic entering the marine envi- cles since their shoots, roots, and pneumatophores interact with the ronment remains floating in surface waters (9, 13), with these plastic top layers of the water column, where microplastics are typically found stocks particularly depleted in items <1 mm (11). These findings have (18). led some to hypothesize the existence of a range of size-dependent The hypothesis that blue carbon sediments may be important sinks removal processes, such as ingestion by fauna (14–16), sinking to for plastics is further supported by recent findings that Red Sea deeper layers of the water column (3, 17, 18), and sequestration in mangrove forests sustain higher loads of plastic than adjacent bare sediments (19). shores, by retaining large plastic items within their mesh of pneu- Burial in sediments is thought to be the major sink of plastic in matophores (29). Moreover, the Red Sea has been reported to hold the marine environment (20). Negatively buoyant plastic material, unexpectedly low concentrations of plastics in its surface waters often ballasted by biofouling (17, 21), sinks and is deposited in sedi- (30). Similarly, low concentrations of plastic in surface waters and ments, entering long-term sequestration. The signature of plastics high loads in mangrove forests were reported in the Arabian Gulf in the sediment record provides stratigraphic evidence that, since (29, 31). The low plastic concentration in the surface waters is un- the mid-20th century, we have entered a new epoch, the Anthropocene expected given that both the Red Sea and the Arabian Gulf are char- acterized by an inverse estuarine circulation (for 8 months and all 1 year long, respectively), which should lead to the retention of plas- Red Sea Research Center and Computational Bioscience Research Center, King tic litter advected in the two seas by surface currents from the Indi- Abdullah University of Science and Technology, Thuwal 23955, Saudi Arabia. 2De- partment of Biology, College of Science, Imam Abdulrahman Bin Faisal University an Ocean (32–34) and of those entering the Red Sea and the Arabian (IAU), Dammam 31441-1982, Saudi Arabia. 3Center for Environment and Water, Gulf from land and ships. Therefore, the low stocks of floating Research Institute, King Fahd University of Petroleum and Minerals (KFUPM), plastics suggest the presence of significant removal processes. Given Dhahran 31261, Saudi Arabia. 4Geosciences Department, College of Petroleum Engineering and Geosciences, King Fahd University of Petroleum and Minerals the extensive coverage of blue carbon habitats, especially man- (KFUPM), Dhahran 31261, Saudi Arabia. 5 Ecosystem Science Division, Depart- groves, along the Red Sea and the Arabian Gulf (35, 36), sequestra- ment of Environmental Science, Policy and Management, University of California, tion in blue carbon sediments is likely to be an important removal Berkeley, Berkeley, CA 94720, USA. 6Institut de Ciència i Tecnologia Ambientals and Departament de Física, Universitat Autònoma de Barcelona, Bellaterra, Barcelona process for plastics in the region. To test this hypothesis, we collect- 08193, Spain. 7School of Science and Centre for Marine Ecosystems Research, Edith ed and extracted plastics from dated sediment cores of mangrove Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia. 8International forests of the Red Sea and of the Arabian Gulf and estimated the Atomic Energy, 4a Quai Antoine 1er, Principality of Monaco 98000, Monaco. 9Arctic plastic burial rates in mangrove sediments. Given that plastic Research Centre, Department of Biology, Aarhus University, Ole Worms Allé 1, DK-8000 Aarhus, Denmark. production and input to the ocean have increased exponentially *Corresponding author. Email: cecilia.martin@kaust.edu.sa since the onset of mass production of plastics in the 1950s (1, 12), Martin et al., Sci. Adv. 2020; 6 : eaaz5593 28 October 2020 1 of 8 SCIENCE ADVANCES | RESEARCH ARTICLE we further expect an exponential increase in plastic burial rates in the of plastic deposition and sequestration in sediments, with a par- Red Sea and Arabian Gulf mangrove sediments during the past 70 years. ticular attention to the key milestones of the plastic production his- tory, such as year 1907, the date of invention of plastic polymers; the 1930s, onset of industrial plastic production; and the 1950s, RESULTS AND DISCUSSION the start of mass plastics production (39). As expected from the Small-sized plastics dominate in sediments chronology of plastic production, plastic particles were not found Plastic particles were present in all the nine cores sampled in seven in sediments dated older than 1907. mangrove forests of the Red Sea and the Arabian Gulf. We found a The estimated abundance of plastic, excluding fibers, sequestered total of 126 plastic items, excluding fibers (fig. S1 and data S1), in sediments accumulated since 1930 (mean depth of 21.0 ± 3.1 cm) averaging 14 ± 3 plastic particles per core (means ± SE). was 7840 ± 1630 items m−2 in the Red Sea and Arabian Gulf sam- Microplastic items smaller than 0.5 mm, which are reported as pled mangroves. Specifically, we calculated that half of the total missing from surface water inventories in the Red Sea and the abundance, 3920 ± 940 items m−2, was located in the upper 5 cm of Arabian Gulf (30, 31), dominated plastic stocks in mangrove sedi- surface sediments. ments (fig. S1A). Specifically, plastic items in the mangrove sediments The average stock of plastic buried since 1930 in the mangrove sampled were significantly smaller than plastic items found in the sediments sampled from the Red Sea and the Arabian Gulf is Red Sea surface waters (fig. S1A; two-sample Kolmogorov-Smirnov 510 ± 290 × 103 g km−2, with no significant difference between the test; D = 0.232, P value = 1.5 × 10−4) (30). This tendency is likely due two basins (Fig. 1; see Materials and Methods). In the Red Sea, we to the higher susceptibility of small plastic items to be entrained by estimated the plastic stock accumulated in sampled mangrove sed- vertical mixing and sink (17, 18, 21). Fragmentation of plastic debris iments since 1930 (top 21.5 ± 6.2 cm of sediment) as 340 (±200) × following burial is unlikely since the size distribution of fragments 103 g km−2 (Fig. 1). The reported mean concentration of plastic in and films in sediments remained uniform with time (fig. S1B). This Red Sea surface waters (top 15 cm) is 1.1 ± 0.3 g km−2 (30). Hence, suggests that plastic sequestration in sediments is size dependent, which the plastic stock in 1 km2 of surface water is equivalent to the plastic helps explain why small microplastics are less frequent than expected stock accumulated in 3 m2 (from 1.5 to 10 m2) of mangrove sedi- in the surface waters of the Red Sea and the Arabian Gulf (30, 31), which ments. In the Arabian Gulf, the estimated plastic stock accumulated is consistent with the depletion of plastic stocks <1 mm in surface in sampled mangrove sediments since 1930 (top 20.5 ± 3.3 cm of waters globally (11). The overall shape, color, and polymer composi- sediment) is 660 (±510) × 103 g plastic km−2 (Fig. 1). Estimates of tion of plastics in the sediments sampled did not differ from the plastics floating plastic stocks in Arabian Gulf surface waters are limited to reported for the Red Sea and Arabian Gulf surface water (excluding regional surveys. For example, 7.8 ± 5.8 g plastic km−2, excluding fibers), i.e., majority of blue and white fragments and films made fibers, are estimated to be floating in the top 50 cm of Qatar surface of polyethylene and polypropylene (fig. S1, C to E) (30, 31). waters (Fig. 1; see Materials and Methods) (31). Hence, the plastic stock in 1 km2 of Qatar surface water is equivalent to the stock of Concentration of plastic in mangrove sediments along plastics in 12 m2 (from 7 to 53 m2) of mangrove sediment. The much a depth profile higher loads of plastics in mangrove sediments compared to surface We first calculated the concentration of plastic as the number of waters point to the efficiency of mangrove habitats in sequestering plastic items per kilogram of dry sediment in the upper 2 and 5 cm and accumulating plastics in their sediments. of the mangrove sediments sampled in the Arabian Gulf to allow We can provide with an estimate of the magnitude of the role of comparisons with results from previous studies conducted in the same mangrove forests of the Red Sea and the Arabian Gulf in sequestering region that analyzed plastics in surface sediments. Our estimate of plastic in their sediments. Extrapolating the calculated plastic plastic concentration in Arabian Gulf mangrove sediments was higher stocks to the whole area of mangroves in each basin (132 km2 in the than the concentration of plastics (excluding fibers) in the top 2 Red Sea and 165 km2 in the Arabian Gulf; Fig. 1) (35, 36), we calculate and 5 cm of intertidal beach sediments reported for the same that a total of 50 ± 30 and 110 ± 80 metric tons of plastic may have region (Table 1) (31, 37, 38). The difference in plastic item concen- been buried since 1930 in the upper approximately 20 cm of sedi- tration between sediments of vegetated and unvegetated coastal ments in the mangrove forests across the Red Sea and the Arabian habitats in the Arabian Gulf suggests that mangrove forests might Gulf, respectively. This estimate is, despite uncertainties, remark- play a key role in the accumulation of plastics in their sediments. able when considering that mangroves comprise only 0.03% of the To account for the size and mass of the plastic items, we then area of the Red Sea and 0.07% of the area of the Arabian Gulf. This calculated the concentration as milligrams of plastic items per kilogram suggests that mangroves’ habitats may indeed play an important of sediment. We observed that, generally, the concentration of plastic role as a relevant plastic sink at the basin scale and should encour- decreased exponentially, going from the surface to the deeper layers age efforts to derive more robust estimates. Doing so requires a of the sampled mangrove sediments (fig. S2A). However, one sedi- stratified sampling effort to extend the estimates reported here to ment core sampled in the Arabian Gulf (core no. 8) presented a other regions in the Red Sea and the Arabian Gulf, representing very large plastic concentration at approximately 13 cm in depth, different mangrove stands and oceanographic settings, and to nearby much higher than the concentration in the upper sediment layers of unvegetated control sites. that site (fig. S2B and data S1). Plastic burial rates in mangrove sediments Accumulation of plastic litter in mangrove Plastic burial rates increased exponentially from the beginning of sediments (1930– 2015) the 20th century in six of seven sampled mangroves (fig. S3). The Cores were sliced in 1-cm-thick samples, and each sample was dated large burial rate value in one of the Arabian Gulf sites, correspond- (see Materials and Methods), allowing us to obtain the chronology ing to the 1920s (data S1) and derived from the high concentration Martin et al., Sci. Adv. 2020; 6 : eaaz5593 28 October 2020 2 of 8 SCIENCE ADVANCES | RESEARCH ARTICLE Table 1. Estimates of plastic abundance (excluding fibers) in the upper 2 and 5 cm of sediments in coastal environments (mangroves and beaches) of the Arabian Gulf. NA, not available. Plastic stock in the top Plastic stock in the top Site Habitat 2 cm, n items kg−1, 5 cm, n items kg−1, n of samples Reference means ± SE (median) means ± SE (median) Saudi Arabia (Arabian Mangroves 84 ± 35 (58) 83 ± 29 (74) 5 cores This study Gulf coast) Qatar Beach (intertidal) 8±7 NA 24 Abayomi et al. (31) Iran (Arabian Gulf coast) Beach (intertidal) NA 9±2 15 Naji et al. (37) Iran (Arabian Gulf coast) Beach (intertidal) 15 ± 5 NA 12 Naji et al. (38) Fig. 1. Plastic stocks in mangrove sediments. Plastic stocks (in grams of plastic per square kilometer accumulated since 1930) in each of the nine cores sampled in seven mangrove forests of Saudi Arabia (four on the Red Sea coast, framed in red, and three on the Arabian Gulf coast, framed in blue). The sand-colored cylinders on the aerial images indicate the core sampling point in the mangrove forests and the plastic stocks. The bar graph shows the means ± SE of the plastic stocks in the mangrove cores from the Red Sea (in red) and the Arabian Gulf (in blue), which are not significantly different between the two basins (Wilcoxon rank sum test; W = 12 and P = 0.73). The map reports, in green, the area of mangrove forests in the Red Sea (35) and in the Arabian Gulf (36); in blue, the extension of Red Sea waters and Arabian Gulf waters; and in a striped pattern, the areas where the concentration of plastics in surface waters has been assessed for the Saudi Red Sea (30) and Qatari Arabian Gulf (31) waters. Aerial images were printed from Bing Maps (Microsoft product screenshots reprinted with permission from Microsoft Corporation). value in core no. 8 (fig. S2B), leads to the exponential increase being 6.2 ± 2.2% year−1 (Arabian Gulf; Fig. 2), 80 (30 to 160) metric tons and rejected for that site (fig. S3). Such unexpected anomaly may be due 70 (10 to 220) metric tons of plastic will be buried by year 2050 in the to the activity of burrowing animals that may transport recently mangrove sediments of the Red Sea and the Arabian Gulf. As previ- deposited microplastics deeper into the sediments. Plastic burial ously remarked, these estimates are indicative of the role of mangrove rates exponentially increased since 1950 in all sampled mangroves habitats, which should lead to efforts to derive robust basin-scale of the Red Sea and the Arabian Gulf (Fig. 2), at a combined rate estimates of plastic burial in the Red Sea and Arabian Gulf mangrove of 8.5 ± 1.2% year−1 (n = 7; Fig. 2), consistent with the 8.4% annual sediments, as well as mangrove sediments elsewhere. increase in global plastic production since then (1). The current (2001–2020) burial rate of plastic in the sampled Mangroves are efficient sinks of plastic mangroves of the two basins averaged 29 ± 15 and 64 ± 4 mg plastic In conclusion, our results show that mangrove habitats in the Red m−2 year−1. Extrapolating to the whole Red Sea and Gulf mangrove Sea and the Arabian Gulf are efficient sinks of marine plastic pollu- area, we calculate the recent (2001–2020) plastic burial rates of 4 ± 2 tion. As shown by previous studies, small plastic particles undergo and 11 ± 7 metric tons of plastic per year in the mangrove sediments in rapid changes in their buoyancy (faster than for larger items) when the Red Sea and the Arabian Gulf, respectively. In a “business-as-usual” biofouled (17, 21). Especially in blue carbon habitats, they can be scenario, where plastic burial in mangrove sediments continues to readily deposited in seafloor sediments, leading to a size-dependent increase exponentially at rates of 10.2 ± 0.8% year−1 (Red Sea) and removal process that is consistent with the depletion of floating Martin et al., Sci. Adv. 2020; 6 : eaaz5593 28 October 2020 3 of 8 SCIENCE ADVANCES | RESEARCH ARTICLE A B Fig. 2. Plastic burial rates in mangrove sediments since the 1950s. Historical increases in (A) standardized (to the maximum burial rate of each site) plastic burial rates in Red Sea and Arabian Gulf mangrove sites and in each (B) of the four Red Sea and three Arabian Gulf studied mangrove sites, from the 1950s to date (see details in data S1 and in Materials and Methods). Solid lines represent the exponential fit, for which the fitted equation including the SE of the slope, R2, and P value are reported. Present 1950 1907 Fig. 3. Conceptual model of plastic burial in mangrove sediments. Mangrove forests, through the mesh created by their pneumatophores, enhance the deposition of plastic particles on their sediments. Since sinking of plastics is size dependent (17), mostly small plastic particles reach the bottom, while larger items may remain sus- pended in surface waters longer. Once deposited, plastics are sequestered for decades undisturbed. Therefore, sediments are an archive of the chronology of plastic consumption, as demonstrated by the absence of plastic particles in sediments dated older than 1907 (year of the invention of the first fully synthetic polymer) and by the exponential increase of plastic particles toward the most recent sediment layer, as a consequence of the mass production that began in the 1950s. Martin et al., Sci. Adv. 2020; 6 : eaaz5593 28 October 2020 4 of 8 SCIENCE ADVANCES | RESEARCH ARTICLE Table 2. List of cores from the Red Sea and the Arabian Gulf processed for plastic extraction. We report coordinates (latitude and longitude) of sampled mangrove cores, the name of the site, and MARs (in grams per square centimeter per year) calculated for each core. Core ID Sea Latitude (N) Longitude (E) Site MAR (g cm−2 year−1) 1 Red Sea 22.972008 38.848878 Khor Al-Kharrar 0.52 ± 0.09 2 Red Sea 22.938872 38.878769 Khor Al-Kharrar 0.021 ± 0.002 3 Red Sea 22.752567 38.997394 Khor Al-Baqila 0.29 ± 0.12 4 Red Sea 22.282150 39.084325 Al Taweelah 0.20 ± 0.06 5 Arabian Gulf 27.28603 49.56685 Abu-Ali 0.25 ± 0.02 6 Arabian Gulf 27.28283 49.56523 Abu-Ali 0.45 ± 0.05 7 Arabian Gulf 26.71674 50.02111 Ras Tanura–Safwa 1 0.50 ± 0.05 8 Arabian Gulf 26.64246 50.01462 Ras Tanura–Safwa 2 0.28 ± 0.09 9 Arabian Gulf 26.63699 50.01067 Ras Tanura–Safwa 2 0.44 ± 0.06 microplastic stocks if plastic particles are <0.5 mm (Fig. 3) (11, 35) lation pattern occurs all year long in the Arabian Gulf (34), the Red and supported by our findings that plastics <0.5 mm dominate in Sea supports this circulation for 8 months a year, from October to mangrove sediments. Once settled and sequestered, plastics can re- May. From June to September, the main surface circulation in the main undegraded in sediments for decades, as demonstrated by southern half of the Red Sea reverses and a three-layer current (an their presence in layers dated from the 1930s, and likely over inflowing current sandwiched between a surface and deep layer of more extended time scales, given the stability of mangrove sedi- outflowing currents) flows through the strait (32). Mangrove stands ments reflected in carbon sequestration at the millenary time scale cover 132 and 165 km2 in the Red Sea and the Arabian Gulf, re- (23), but may be released again if the mangrove cover is disturbed. spectively, and, while in the Arabian Gulf, the mangrove coverage is The plastic loads in mangrove sediments thus provide an archive of decreasing following the global trend, in the Red Sea, it had an un- historical plastic usage, reflecting the exponential growth in pro- expected 12% increase during the last 40 years (35, 36). Given the duction and subsequent input to the oceans since the emergence of high temperatures and salinity, the oligotrophic waters, and the lack mass plastic production in the 1950s (Fig. 3). of permanent riverine inputs in both basins, the mangrove stands Our results reveal very large burial rates of plastics in mangrove of the Arabian regions are often small patches characterized by sediments, which are sufficiently high as to help account for the dwarf trees (42). low concentrations of plastic (excluding fibers) found in the surface Saudi Arabia stretches for approximately 2000 and 560 km on waters of the Red Sea (30) and of the Arabian Gulf (32) and possibly the Red Sea and Arabian Gulf coast and has approximately 50 and in other tropical regions where fringing mangroves are abundant. 10 km2 of mangrove forests on the two coasts, respectively. We Yet, plastics are also sequestered in sediments beyond mangroves sampled a total of nine cores in four mangrove forests of the Red (e.g., seagrass, salt marshes, and non–blue carbon sediments) and Sea (four cores collected in 2015) and three forests of the Arabian structures [e.g., coral reefs (40, 41)] across the region. Hence, the Gulf (five cores collected in 2016) along the coast of Saudi Arabia removal of floating plastics from the Red Sea and Arabian Gulf sur- (Table 2 and Fig. 1). Sampled mangrove forests are monospecific face waters into sediments and benthic habitats is likely faster than stands of Avicennia marina. that reflected in sequestration in mangrove sediments alone. Our results identify mangrove forests as hot spots for plastic Sample collection and dating sequestration in their sediments. Hence, blue carbon strategies to Sediment cores were collected with polyvinyl chloride (PVC) tubes conserve and restore mangrove habitats are not only effective to with a sharp-angled end, carefully hammered into the sediment and mitigate and adapt to climate change (23) but are also critically im- retrieved manually or with the help of a winch. White corers (1.7 m portant to prevent the remobilization of plastic litter accumulated long, 6.26 cm in inner diameter) were used to collect sediments in in the sediments. Whether the accumulation of plastic litter in the Red Sea and gray corers (1.4 m long, 7 cm in inner diameter) in mangrove sediments affects the associated benthic fauna or man- the Arabian Gulf. The upper extremity of the core was tapped to groves themselves is unknown and should be studied to assess the allow vacuum and avoid sediment collapse during retrieval from the impacts of plastic burial on mangrove ecosystems. soil. After collection, cores were stored at 4°C before further pro- cessing. Cores were opened using a circular saw, and the length of the sediment retrieved was recorded to later calculate the compression MATERIALS AND METHODS effects against the depth of the core introduced in the soil. Cores were Study area and design cut in 1-cm slices using a ceramic knife, and each slice (from now on The Red Sea and the Arabian Gulf are semienclosed basins connected referred also as “sample”) was dried at 60°C. Dry weight was measured, to the Indian Ocean by relatively narrow straits (the Bab-el-Mandeb and dry bulk density (g cm−3) was calculated. A compression correc- strait in the Red Sea and the strait of Hormuz in the Arabian Gulf). tion was applied following Serrano et al. (43). Both seas are characterized by an inverse estuarine circulation, sup- Sediment cores were dated as described by Saderne et al. (44). ported by high evaporation rates, involving an inflow of surface Specifically, a 5- to 10-g subsample of the top five slices and every currents and an outflow of deeper denser waters. While this circu- second slice from 5- to 19-cm depth of each core were grinded by Martin et al., Sci. Adv. 2020; 6 : eaaz5593 28 October 2020 5 of 8 SCIENCE ADVANCES | RESEARCH ARTICLE using a pestle and sieved through a 125-m mesh and sent to Edith red spectroscopy (FTIR). Particularly, we used Nicolet 10 FTIR for Cowan University (Australia) for 210Pb dating. 210Pb was analyzed particles ≥250 m and Nicolet 6700 FTIR for particles <250 m. via the determination of its granddaughter 210Po. Two hundred to Obtained spectra were matched with the OMNIC spectrum library, 300 mg of aliquots of sediment from each sample were spiked with and polymer type was assessed. Extracted and confirmed particles a known amount of 209Po, acid was digested using an analytical were assigned to shape categories: fragments (hard pieces from broken microwave, and the polonium isotopes were plated onto silver disks. objects), films (bags, wrappings, or pieces of them), foams (expanded The emissions of the polonium isotopes were measured by alpha cellular plastics), and threads (filaments from fishing lines, ropes, spectrometry using passivated implanted planar silicon detectors. and nets), following the same classification of Martin et al. (33). Fibers The concentrations of excess 210Pb used to obtain the age models (i.e., thin filaments normally originating from textiles) were excluded were calculated as the difference between total 210Pb and supported from the analysis since no appropriate blanks were set during the 210 Pb, continuously produced by decay of 226Ra, the latter measured whole processing. Plastic particles were weighed, and, if weight was by low-background liquid scintillation counting (Wallac 1220 <0.01 mg (detection limit of the scale used), then the weight was Quantulus) or by gamma spectrometry. The constant flux:constant inferred as the product of volume (obtained from image analyses in sedimentation model (45, 46) was used to estimate the average soil ImageJ) and density (obtained from the FTIR characterization mass accumulation rates (MARs; in grams of dry weight per square of polymers). As a conservative measure to prevent contamina- centimeter per year; Table 2) for each core and then divided by the tion during sampling and core opening procedures, we excluded dry bulk density to obtain sediment accretion rates (in centimeters all plastic particles that could have originated from the PVC cor- per year). Dates of each sample were then obtained by dividing the ers. Particularly, we excluded all PVC particles (except threads) of depth of the sample (as accumulated mass) by the core MAR. white color from the cores collected in the Red Sea and those of gray color from the cores collected in the Arabian Gulf (see data S1). Plastic extraction Since the SMI unit is also made of plastic material, it could have The top 1 cm and other 4 to 12 samples per core were processed for been a source of contamination, too. However, no transparent plastic extraction (data S1). The number of samples processed per PMMA particles (the material of the tubes in the unit) were en- core depended on the dating results: More and deeper samples were countered in any of the samples, and gray PVC particles (the mate- processed for cores with higher sedimentation rates and vice versa rial of the ball valve in the unit) were excluded from the Red Sea since the focus was mainly on samples dated after plastic invention samples, too. Therefore, the number of plastics in the dataset com- (1907). We processed a total of 88 samples dated from mid-19th piled from this study (data S1) should be considered conservative. century to 2015. We included five samples dated before the invention Since all the white PVC pieces excluded from the Red Sea samples of plastic in 1907 as a control. Extraction of plastic was implemented were clearly attributable to the corers (they were bright white, not using the method described by Coppock et al. (47), being cost effi- degraded nor fouled) and since we excluded all gray PVC pieces cient and accurate. Particularly, a sediment-microplastic isolation from the samples of both basins, the results from the two seas are (SMI) unit was manufactured, as per the design proposed by comparable despite the use of corers of two different colors. Coppock et al. (47) consisting of a column made by two transpar- Recovery rates for the three test samples spiked with green poly- ent tubes 14 cm long [we used polymethyl methacrylate (PMMA) ethylene beads were 100% in each sample, showing that the ex- instead of PVC] connected by a ball valve (gray, in PVC). We traction of plastic at the SMI unit is efficient for all sizes tested changed the fixed base for a removable one to ease wash of the column (50 to 1000 m). Instead, the recovery was 70 ± 8% in the three after the processing of each sample and to avoid cross-contamination samples spiked with irregular biofouled fragments, which were (fig. S4). To test the method, we spiked three clean sediment samples harder to detect at the microscope than green beads, indicating that (~50 g) each with 20 green polyethylene beads of four size classes the limitation of the method is the visual inspection. Pieces of plas- (53 to 63, 212 to 250, 425 to 500, and 850 to 1000 m) and other tic that were not recovered were mainly of the smallest size class three sediment samples each with 20 irregular fragments of bio- (approximately 50 m), while recovery of larger particles was fouled polyethylene of the same size classes but various colors. To 91 ± 9%, making the method reliable for items >200 m. extract plastics from the test sediment samples and the samples se- lected from each core for the analysis, we added each dry sediment Data analyses sample (<60 g) to the SMI with the ball valve open, together with a The size distribution of plastic items recovered (n = 126) was ob- magnetic stirring bar and a 700-ml solution of zinc chloride (ZnCl2) tained, assigning the sorted plastic pieces, according to their Feret (1.5 g cm−3). The top of the unit was closed with aluminum foil, and diameter, to four size classes with increasing bin size following a the solution was stirred on a magnetic stirring plate for 5 min at logarithmic scale base 5. Abundance of items in each size class was 500 rpm to allow the complete resuspension of plastic trapped into the normalized per width of the size class (in millimeters). Similarly, sediment. After mixing, the sediment was allowed to decant, and the floating plastics from Red Sea surface waters [data from Martí et al. ball valve was closed. The supernatant, containing particles less dense (30)] were distributed in the same size classes. The cumulative dis- than 1.5 g cm−3, was vacuum filtered through a 25-m nylon mesh, tributions (probability that objects with a size X are smaller than a and the material retained in the mesh was then inspected under a dissect- given size x, F(x) = P[X ≤ x]) in size classes of items retrieved from ing microscope (Carl Zeiss, Stemi 2000) in search of plastics. The mesh the Red Sea and Arabian Gulf sediments and from the Red Sea surface was thoroughly screened two times at two different magnifications. waters were compared using a two-sample Kolmogorov-Smirnov test. Putative plastic particles were picked, photographed to be measured Moreover, we questioned whether plastic items buried in sediments with an image analysis software (ImageJ; http://imagej.nih.gov), fragment (and thus become smaller) through time, by testing the null and rinsed with hydrogen peroxide to eliminate organic fouling and hypothesis that plastic size is independent of the estimated date of ease the successive characterization at the Fourier transform infra- accumulation, by fitting a linear model to the data. However, we Martin et al., Sci. Adv. 2020; 6 : eaaz5593 28 October 2020 6 of 8 SCIENCE ADVANCES | RESEARCH ARTICLE included in this analysis only the main shapes of regression plastic year) and the concentration of plastic (in milligrams of plastic per (fragments, n = 57, and films, n = 35), which we analyzed separately gram of sediment) in each processed sample. We then plotted a since their different shape may imply a different fragmentation pathway. 20-year moving average of burial rates at the y axis against the mid- To allow comparison with the existing literature covering the date of the corresponding 20-year window at the x axis, starting from Arabian Gulf region, where sediment samples are often collected in the oldest processed sample dated >1950 of each core until reaching the top 2 and 5 cm, we calculated the average abundance of plastic the most recent processed sample (Fig. 2). Year 1950 was chosen as the per kilogram of sediment in the top 2 and 5 cm of Arabian Gulf cores. baseline because it marks the start of the exponential increase in Samples in the top 1 and 2 cm of the cores were all processed, while plastic production. However, we also report the 20-year moving one to three samples per core were processed between 2 and 5 cm. average of plastic burial rates for data before the 1950s for compari- Since abundance is not informative of the loads of plastic in sediments, son (fig. S3). We used a 20-year window, as the density of plastic which depend on the size of the plastic items, we calculated the concentra- particles, constrained by sample size (core diameter), could not be tion of plastic as milligrams of plastic per kilogram of sediment, too. reliably resolved at smaller time intervals. When replicate cores of We then calculated plastic abundance and stocks in mangrove a site were available (see Table 2), these were considered together sediments of each basin (Red Sea and Arabian Gulf) as the number when averaging the burial rate, and a single plot was given for each and milligrams of plastic per square meter accumulated since year site. We then fitted an exponential curve for each plot to test the 1930, respectively. Considering that plastic production increased hypothesis of exponential increase in plastic burial rates and to ob- exponentially since the 1950s and that there is always an error asso- tain the annual rate of increase for each site. The current burial rate ciated with the dating, we conservatively selected year 1930 as a base- (average burial rate of plastic deposited in the most recent 20-year line to calculate the stocks. Moreover, year 1930 marks the industrial window, from 2001 to 2020) was multiplied by the area covered by onset of plastic production (39). Specifically, stock values were cal- mangroves in the two basins to obtain the contemporary rates of culated for each core as the total plastic load in samples from total plastic mass buried in each of the Red Sea and Arabian Gulf 1930 to present, normalized per surface unit area, considering that mangrove sediments. the area sampled by each core is 30.78 and 38.48 cm2 for Red Sea Statistical analyses were performed in RStudio 1.1.383. Com- and Arabian Gulf cores, respectively (see data S1 for details on cal- parisons are considered significantly different when P value is <0.05. culations). The depth corresponding to year 1930 varied across sites All values are reported as means ± SE. according to sedimentation rates. Particularly, in the processed cores, the year 1930 corresponded to samples ranging from 5.6 to 31 cm in sediment depth (means ± SE; 21.0 ± 3.1 cm; fig. S5). 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