pubs.acs.org/est Article Can a Sediment Core Reveal the Plastic Age? Microplastic Preservation in a Coastal Sedimentary Record Laura Simon-Sánchez,* Michaël Grelaud, Claudia Lorenz, Jordi Garcia-Orellana, Alvise Vianello, Fan Liu, Jes Vollertsen, and Patrizia Ziveri Cite This: Environ. Sci. Technol. 2022, 56, 16780−16788 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: The seafloor is the major sink for microplastic (MP) pollutants. However, there is a lack of robust data on the historical evolution of MP pollution in the sediment compartment, particularly the sequestration and burial rate of small MPs. By combining a palaeoceanographic approach and state-of-the-art analytical methods for MP identification down to 11 μm in size, we present the first high-resolution reconstruction of MP pollution from an undisturbed sediment core collected in the NW Mediterranean Sea. Furthermore, we investigate the fate of MPs once buried in the sediments by evaluating the changes in the size distribution of the MPs and the weathering status of the polyolefins, polyethylene, and polypropylene. Our results indicate that the MP mass sequestered in the sediment compartment mimics the global plastic production from 1965 to 2016. We observed an increase in the weathering status of the polyolefins as the size decreased. However, the variability in the size and weathering status of the MPs throughout the sedimentary record indicated that these pollutants, once incorporated into sediments, remain preserved with no further degradation under conditions lacking remobilization. KEYWORDS: microplastics, sediments, weathering, carbonyl index, accumulation 1. INTRODUCTION and be transported by deeper currents.13 Furthermore, The proposed Anthropocene epoch frames the geological time physicochemical changes caused by fragmentation, weathering, when intensified anthropogenic activities induced mounting and biological interaction, such as ingestion−egestion, aggregation with organic and inorganic matter, and biofilm changes in Earth-system processes.1 This period has been formation, might facilitate the export of MPs to the ocean characterized by manufacturing new materials that are floor.14 Once on the seafloor, the fate, depositional trends, and indispensable in our societies. Among these materials, plastics environmental degradation of MPs, especially the smaller are unique. Not only does the global plastic mass outstrip the fraction (<300 μm), remain undetermined.15 living animal biomass on Earth,2 but the rate of production and MPs buried in sediments can interact with benthic biota.16,17 disposal of these materials already exceeds the planetary These pollutants potentially can also be used as a chronological boundary.3 Moreover, the durability of plastics favors their tracer of sedimentary records.18 As such, MPs may be effective preservation as potential long-lasting distinctive markers in as a temporal tracer; however, their preservation and sedimentary records.4 The increasing presence of plastics in degradation in sediments have not been explored. Depositional our oceans raises concerns about the harm they represent to environments, such as river prodeltas, offer high-resolution the functioning of ecosystems, from alterations to the marine stratigraphy over recent decades, making them suitable to track carbon cycle5 to individual ecotoxicological damage.6 None- the evolution of MP pollution. Moreover, rivers play an theless, elucidating the dispersion and accumulation of essential role as a source of MPs to the open sea and hold a microplastics (MPs; 1−5000 μm7) and the elusive nanoplastics relevant storage capacity for these pollutants.19 In this study, (NPs; <1 μm) remain challenging as fluxes, fate, and residence time are still poorly understood, partly due to the constraints of available analytical methods.8,9 The growing body of data on Received: June 15, 2022 the presence of MPs in the marine environment points to the Revised: October 24, 2022 seabed as a significant sink for these pollutants.10,11 The Accepted: October 25, 2022 mechanisms for reaching this deep environment are primarily Published: November 14, 2022 related to the density of the MPs, as buoyant MPs are expected to float at sea. However, MPs can sink in the water column12 © 2022 The Authors. Published by American Chemical Society https://doi.org/10.1021/acs.est.2c04264 16780 Environ. Sci. Technol. 2022, 56, 16780−16788 Environmental Science & Technology pubs.acs.org/est Article Figure 1. (A) Geographical location of the sampling station. (B) Abundance of mass-microplastic registered in the sediment core (y-axis right, units MP mass: mg kg−1), against the global plastic production (y-axis left) from 1965 to 2016. (C) Abundance of microplastic (number) classified as fragments and fibers, from 1965 to 2016. we investigate MP accumulation over time as well as the fate of aliquot (3−5 g) was separated from each section for these pollutants once buried in sediments. We combined geochronological analysis. different analytical approaches to reconstruct the accumulation 2.2. Age Model for ST17_MUC2. Dried sediment of small MPs (11−1000 μm), including sedimentary geo- samples were analyzed to determine 210Pb specific activities chronological methods and state-of-the-art FPA-μFTIR-Imag- by α-spectrometry through the analysis of its granddaughter ing (Focal Plane Array-Fourier Transform Infrared-Imaging- 210 Po at the Grup de Recerca en Radioactivitat Ambiental de Micro-Spectroscopy). The radiometric analysis (210Pb,137Cs) Barcelona (GRAB) at the Universitat Autò n oma de provides robust chronologies for the Plastic Age (i.e., post- Barcelona�following the method described in ref 24. After 1950s) with years to decades resolution,18 while FPA-μFTIR- adding 209Po as an internal tracer, 200−300 mg of sediment Imaging spectroscopy allows reliable detection of particles aliquots were totally digested in acid media using an analytical down to 11 μm in size, avoiding analyst’s bias.20 In addition, microwave oven, and Po isotopes were plated on silver discs in we explore the degradation status of these pollutants once HCl 1 N at 70 °C while stirring for 8 h. Alpha emissions of buried in the continental shelf by quantifying the weathered 210 Po and 209Po were measured using Passivated Implanted status of the polyolefins, polyethylene (PE), and polypropylene Planar Silicon detectors (PIPS; CANBERRA, Mod. PD-450.18 (PP). The investigated polymers represented, as of 2015, A.M.). 50.3% of the global plastic waste generation,21 and despite The age model derived from 210Pb was obtained using the their positive buoyancy, PE and PP are the most abundant Constant Supply: Constant Flux (CS:CF) model.25 The mean polymers reported in sedimentary environments.22,23 We mass accumulation rate (MAR; g cm−2 year−1) was obtained discuss our findings in the context of previous MP by performing a non-linear least-square fit between the excess observations, the variability of MP properties over time, and 210 Pb (Bq kg−1) and the cumulative dry mass (g cm−2) of the the environmental factors leading to the sequestration and sediment core. The excess 210Pb (210Pbxs) was determined by preservation of MPs in the seabed. subtracting the constant 210Pb supported, which was estimated as the average 210Pb concentration of the deeper sediment 2. METHODS layers analyzed, wherein 210Pb activities reached constant 2.1. Core Material. A total of five sediment cores (K/C values. To calculate the sedimentation rate (SR; cm year−1), Denmark Multi Corer) were extracted in the Balearic Sea (NW the MAR was divided by the dry bulk density (DBD) of the Mediterranean) during the MERS_BI cruise in November section, where the DBD of the section is obtained by dividing 2019 onboard the R/V Sarmiento de Gamboa. The cores were the DW mass (g) of the slice by its volume (cm3). age-profiled as described below and the core showing the most The activities of 137Cs were measured using an HPGe γ- constant and continuous sediment accumulation rate was spectroscopy detector (CANBERRA, Mod. GCW3523 ad chosen for further analysis, assuming that this core was the one Mod.SAGe Well). Aliquots of dry sediments (1−3 g) were least disturbed by bioturbation and seismic activities. The placed into PE counting vials using calibrated geometries and selected sediment core ST17_MUC2 (40.7726°N, 1.1643°E; counted for around 186,000 s. 104 m water depth; 37 cm in length; Figure 1A) was sectioned 2.3. Microplastic Analysis. The upper 10 cm of the into 1 cm intervals. The samples were transferred to tared low- sediment core, corresponding to deposits since the beginning density polyethylene (LDPE) zip lock bags and stored at 4 °C. of the plastic era, was cut up at 1 cm resolution and analyzed In the laboratory, each section was homogenized and dried at for MP content following the protocol described in Liu et al.26 50 °C until a constant Dry Weight (DW) was reached. An and the Supplementary Material. Unfortunately, the first 16781 https://doi.org/10.1021/acs.est.2c04264 Environ. Sci. Technol. 2022, 56, 16780−16788 Environmental Science & Technology pubs.acs.org/est Article section (9−10 cm) was lost in the sample preparation, and account for potential contamination during sampling. We therefore the data present 1965 and onward. The sediment assumed that the MP recovered in this section represents the mass processed for MP analysis varied between slices, ranging potential contamination during the coring and storing as the between 22.8 and 59.5 g (Table S1). Briefly, the sample matrix existence of plastic materials in these sections would was removed using a multi-step sample treatment, including correspond to the 19th century, according to the age model. pre-oxidation, density separation, buffered multi-enzymatic 2.5. Data Analysis. For each sample, the results of each treatment, and catalyzed oxidation. The resulting isolated ZnSe window were combined and corrected for contami- particles were transferred into a glass 10 mL headspace vial nation. The contamination recorded for the sampling and in with HPLC grade 50% ethanol, and the solvent was evaporated the analytical blanks were subtracted from the samples, using an evaporator (TurboVap LV, Biotage). Finally, a fixed considering the size class and polymer composition. The volume (3 mL of HPLC grade 50% ethanol) was used to re- total amount of MPs (items kg−1 DW and mg kg−1 DW), MP mobilize the particles. To minimize data extrapolation, the fluxes (items m−2 year−1 and mg m−2 year−1), and polymer total volume of each sample was deposited onto multiple diversity was then calculated. The shape of particles was windows (four to five per sample) and analyzed. For that, the classified as fragments or fibers, according to Vianello et al.,31 sample was homogenized (vortex), and multiple aliquots of the while size classes were adopted from Lorenz et al.32 MP burial suspension were deposited with a capillary glass pipette rates (items m−2 year−1) were calculated by multiplying the (micro-classic, Brand GmbH, Germany) onto an area of 78.5 MP concentration by the MAR of the core and standardized to mm2 (⌀10 mm) of a zinc selenide window (ZnSe�⌀13 mm, 2 m2. Polymer diversity across the sediment core was assessed mm thickness, Crystran LTD, UK) held by a compression cell using the Shannon−Wiener diversity index (H′) and Pielou’s (Pike Technologies, USA). The deposited sample was dried evenness index (J′). The data normality of the dataset was overnight at 55 °C prior to analysis. tested using the Shapiro−Wilk test. Non-parametric tests Sample analysis was carried out using FPA-μFTIR-Imaging (Kruskal−Wallis, followed by a post hoc Wilcoxon−Mann− spectroscopy. Measurements were performed using an Agilent Whitney) were applied to reveal differences. Data analysis and 620 FTIR microscope equipped with a 128 × 128 pixel MCT- figures were produced in QGIS Desktop 3.12 ’Bucureşti’33 and FPA detector (Mercury Cadmium Telluride�Focal Plane R-4.1.1,34 using ggplot35 and cowplot36 packages. The level of Array) coupled with a Cary 670 FTIR spectrometer (Agilent statistical significance was set at p < 0.05. Technologies, Santa Clara, CA, USA). Optical images of the 2.6. Carbonyl Index. The degradation status of polyolefins ZnSe window were determined with a 15× objective. The IR (PE, PP) was investigated using the carbonyl index (CI), map was collected in transmission mode in the range of 3750 which allows the identification of the chemical changes in to 850 cm−1, using a 15× IR Cassegrain objective-condenser several polymeric materials by targeting the specific absorption system with a spectral resolution of 8 cm−1, 30 co-added scans band of the carbonyl species produced mainly during thermo- for the sample, and 120 for the background. This setup allowed and photo-oxidation.37,38 The CI was computed adopting the measuring the whole area of the ZnSe window (⌀10 mm, 78.5 Specified Area Under Band (SUAB) methodology,39 applying mm2) with a pixel resolution of 5.5 × 5.5 μm. the equation: The resulting hyperspectral images were analyzed for 1 systematic automated MP identification with the software integrated area under band 1850 1650 cm CI = 1 siMPle.27 After converting the recorded spectra from % integrated area under band 1500 1420 cm (1) Transmittance (%T) to Absorbance (Abs) and performing baseline correction,28 the software chemically identifies the The spectra of the particles were recorded in transmission particles on the sample by comparing every pixel of the IR map mode through the entire particle’s thickness, including the with a custom-built library containing 441 spectra of inorganic spectral contribution from the particle’s inner core. All the and organic materials. The resulting scores are then used to spectra belonging to PE and PP particles were automatically provide a material-based map of the sample, quantitative data exported from each sample dataset using a custom-design on particle abundance, and detailed physicochemical informa- feature in siMPle. The exported spectra were loaded into tion for each particle (polymer composition, two-dimensional SpectraGryph 1.2.15 software;40 the integrated areas under the size, estimated volume, and mass). The mass of an MP is selected bands were calculated using the peak analysis tool and estimated by equivalenting the MP by an ellipsoid and used to compute the CI measurements. applying the density of the determined polymer type as described in references 29, 30. 3. RESULTS AND DISCUSSION 2.4. Contamination. The sample preparation was 3.1. Sediment Archive. The 210Pbxs profile of the conducted inside a laminar flow bench (Telstar AV-100), sediment core showed an almost constant and continuous and cotton lab coats were worn. All the reagents (e.g., H2O2, sediment accumulation rate, indicating an undisturbed ZnCl2, NaOH, FeSO4, and enzymatic buffers) were filtered condition of the core (Figure S1). The application of the through a glass fiber filter (0.7 μm, Whatman) prior to use. CF−CS dating model resulted in an SR of 0.12 ± 0.01 cm Only glassware or stainless-steel materials were used whenever year−1. The age model determined the beginning of the Plastic possible, except for the density separation step, where silicon Age (50s) around 10−11 cm (1951 ± 3 year). The artificial tubes and polytetrafluoroethylene (PTFE) stopcocks were radionuclide 137Cs used to validate the geochronology did not unavoidable. All the materials were carefully rinsed three times provide discernible activities. This limitation can be related to with MilliQ and immediately covered. In addition to these the limited sediment mass analyzed, the decay of 137Cs, the low measures, one analytical blank was run alongside each set of concentration in sediments, and its potential mobility.41 samples to assess contamination. Nevertheless, in its absence, historical events can corroborate A section of the core below the Plastic Age (pre-1950s, 34− the 210Pb geochronology. The Ebro River discharge records 35 cm) was selected and analyzed along with the samples to showed two marked trends after 1951, and the maximum 16782 https://doi.org/10.1021/acs.est.2c04264 Environ. Sci. Technol. 2022, 56, 16780−16788 Environmental Science & Technology pubs.acs.org/est Article recorded in 1959, which were followed by a significant considering all size classes, in the Mediterranean Sea from decrease in the annual contribution within the following years 2006 to 2016. They estimated that the sinking plastic fluxes (Figure S2). Remarkably, the Ribaroja and Mequinenza dams from the Algerian to the Spanish coast ranged from 0.1 to 1.0 g located around 100 km upstream, whose constructions dated km−2 day−1. Our results showed that in 2016 ± 1, the sinking from 1958 to 1966, heavily impacted the Ebro River’s water mass of small MPs almost doubled their higher estimated value and sediment flow balance.42 The anomaly of 210Pbxs observed (1.89 g km−2 day−1). The relevance of the study area should be in the general exponential trend between 8.5 and 10.5 cm noted as river deltas are vulnerable systems subjected to corresponds to the years 1965 ± 2 and 1951 ± 3, coinciding upstream anthropogenic stressors and are recognized accumu- with the years of construction of the dams, a fact that adds lation areas for several pollutants.19,46 confidence to the chronological model obtained. 3.4. Relevance of Mass Unit on Microplastic Seques- 3.2. Microplastic Concentration. MPs were successfully tration. In contrast to previous studies,15 no significant extracted and analyzed in 9 out of the 11 samples, except for correlation was found between the abundance of MPs (items two of them (4−5 cm and 9−10 cm) that were lost during the kg−1) and the sediment depth (Pearson’s, r = −0.52, p = 0.19; sample preparation. MPs were found in all the investigated Figure 1C). Formerly, the MP fluxes reaching and accumulat- sections of the sediment core and control samples. The ing in the sediment compartment have been reported to procedural blanks contained 7 and 13 MPs, revealing polyester directly correlate with global plastic production, growing (PET), PP, and polystyrene (PS) contamination. In the pre- population,47 and landscape changes by using plastic materials Plastic Age section (34−35 cm), seven MPs were recovered, (e.g., greenhouses48). Reconstructing the MP export (number showing contamination by PET, PE, and PP (Table S1). A of particles) to the benthic environment based on the plastic total of 902 MPs (1.39 × 102 μg) were recovered, considering production and waste generation requires the assumption that the blank corrections (Table S2). Figure 1 shows the MP MP deposition is spatially homogeneous, constant, and concentration in the sediment core recovered from the Ebro increasing exponentially over time. In general, MP studies prodelta. The total MP abundance ranged from 706 to 6939 reported patchiness in the spatial occurrence of these items kg−1 DW. The mass concentration ranged from 0.05 to pollutants across different environmental compartments.49−51 1.43 mg kg−1 DW. The highest and lowest concentrations were When comparing the MP abundances at the sea surface and found at the surface of the core (0−1 cm) and in section 7−8 the sediments lying beneath, the concentrations and polymer cm, respectively. The highest mass concentration was recorded composition significantly differed.32 The assumption of in section 1−2 cm of the core, whereas the lowest was in the constant deposition of MPs oversimplifies the complex and 8−9 cm. diverse compounds that MPs are. MPs comprise a wide size To our knowledge, there are no previous observations in the range (1−5000 μm), morphologies, specific densities, and literature of MP abundance in sedimentary records combining multifaceted chemical compositions7,52 that undoubtedly affect palaeoecological approaches (i.e., radiometric analysis, varve their behavior and fate under natural environmental con- counting) with measurements of imaging μFTIR for MP ditions.53 Despite an incontestable increasing trend, the MP identification. However, this method has previously been sequestration (number of particles) of our study showed intra- applied to characterize the MP presence in marine sediment variability over the last 54 years (Figure 1C). This variability samples, where the upper 5 cm of the seabed were retrieved was likely driven by the synergistic combination of the using sediment corers22,43 or a Van-Veen grab sampler.32 We heterogeneous distribution of MPs along the surface waters calculated the MP abundance for the top 4 cm (3122 items of the Mediterranean Sea54 and the different mechanisms, kg−1) of our sediment core for comparison. In the remoteness which are still poorly understood, leading the MP export from of the Kamchatka trench, northwest Pacific Ocean, MP the surface to the benthic environment14 (e.g., marine snow, concentrations were one to two orders of magnitude lower biofilm formation, aggregates, ingestion−egestion, vertical (14−209 items kg−1)22 than our findings. Similarly, lower migration of species, deep-ocean currents, ocean turbulence). concentrations (3−1189 items kg−1) were reported in the In our sediment record, the median MP size (58.7 μm, Q1−Q3: southern part of the North Sea.32 In contrast, higher MP 41−91 μm) agrees with small MPs’ prevalent dominance in the concentrations (42−6595 items kg−1) were reported at the sediments.55 The evidence of small MPs in the surface water Fram Strait, west of Svalbard.43 In the Mediterranean Sea, the compartment is scarce, which may be explained (i) by the closer analytical methods can be attributed to Vianello et al.,44 constraints on the current predominant sampling methods who found relatively lower values (672−2175 items kg−1) in (i.e., Neuston nets) and (ii) by the vertical transport of MPs. the sediments of the Lagoon of Venice, Italy. These In the literature, there has been reported a mismatch in the observations agree with our previous results19 that despite size-abundance distribution of MPs floating in the surface the Ebro River being a critical system for understanding the waters (empirical data) and the expected concentrations MP fluxes entering the Mediterranean Sea and being under the derived from fragmentation models, showing a dearth of influence of the Gulf of Lion current, the MP pollution levels small MPs in this compartment.56,57 Emerging studies in this system are intermediate to low. investigating the MP occurrence along the water column 3.3. Microplastic Sequestration and Burial Rate. The showed a higher relative abundance of small MPs as depth accumulated MP inventory since 1965 ± 2 was 1.44 × 106 increased,58−60 highlighting the need for further MP knowl- items m−3 (0.22 g m−3). MP burial rate ranged from 865 m−2 edge along this vast compartment of the ocean to identify year−1 in 1973 ± 2 to 8507 m−2 year−1 in 2016 ± 1. The MP processes leading the vertical transport and to understand if burial rate has increased by 973% since 1965, with an average these mechanisms are polymer- and size-selective by which the standardized rate of 18% per year. The mass of MPs MP composition sequestered in the sediments may be sequestered in the sediments ranged from 0.061 mg m−2 influenced. Nonetheless, to compare and assess the plastic year−1 in 1965 ± 2 to 1.76 mg m−2 year−1 in 2012 ± 1. sinking fluxes and sequestration in the sediments regarding the Kaandorp et al.45 calculated the sinking flux of plastics, global mass plastic produced, the unit MP mass concentration 16783 https://doi.org/10.1021/acs.est.2c04264 Environ. Sci. Technol. 2022, 56, 16780−16788 Environmental Science & Technology pubs.acs.org/est Article Figure 2. (A) Microplastic polymer composition in the sediment profile. (B) MP mass (mg kg−1) in the sediment core according to polymer composition from 1965 to 2016. Figure 3. Illustrative example of the calculation of the carbonyl index for a polypropylene particle. From left to right, the image on the visualization of a particle, every pixel represents a collected spectrum. Spectra were exported from the siMPle software and treated in SpectraGryph 1.2.15 where the area below the band was computed. is more appropriate than the number of particles. The protocols are essential to prevent contamination during estimated MP mass sequestered in our sediment core over sampling and sample preparation. time showed a significant trend with sediment depth A total of 16 different synthetic polymers were identified in (Pearson’s, r = −0.79, p < 0.05), as well a similar exponential the sediment core (Figure 2). The most abundant were PS trend was observed between the MP mass sequestered in the (28.4%), followed by PP (22.4%), PET (15.9%), PE (12.6%), sedimentary records of Ebro prodelta and the global mass polyvinyl chloride (PVC; 8.5%), acrylonitrile butadiene styrene plastic production (Figure 1B). (ABS; 5.9%); polyurethane (PU; 1.9%), polyamide (PA; 3.5. Microplastic Characterization. Most MPs found in 1.7%), alkyds (1.1%), and others (2.5%). The number of the sediment core were fragments (Figure 1C), accounting for different synthetic polymers in each section ranged between 5 83.3% of the total particles. Similar findings were reported in and 14 (Figure 2A). The Shannon−Wiener (H′) values ranged sediment cores collected in the North and Celtic Sea, where from 1.0 in section 7−8 cm to 2.1 in section 5−6 cm, corresponding to 1973 ± 2 and 1987 ± 1, respectively. No the MP identification was done by the Nile red method.61 In significant trend was found between the polymer diversity and contrast, fibers were exclusively found in the sediment records sediment depth (Pearson’s, r = −0.04, p = 0.912). The recovered in the Donghu urban lake, China.62 Similarly, fibers synthetic polymer diversity evenness (J’) was equal (J’ = 0.8) were the predominant MP morphology in the sedimentary in all the sections investigated, except for section 7−8 cm (J’ = records investigated in the Santa Barbara Basin47 (77.0%) and 0.6). Notably, the MP mass sequestered in the sediments in the Rockall Trough,63 North Atlantic Ocean (89.0%). sorted by the polymer (Figure 2 B) showed a similar Noteworthy, the methods described in these studies involved exponential growth by the polymer group until early 2000. visual presorting and isolation of the potential plastic particles After 2006 ± 1, the major mass contribution is driven by the for spectroscopy analysis. This visual approach is prone to sequestration of PP (43.6%), PET (35.8%), and PS (14.0%). analyst bias as fibers are easier to recognize than smaller MP 3.6. Carbonyl Index. FTIR spectroscopy is one of the fragments.64 Moreover, fibers are one of the primary sources of most common techniques for MP characterization.9 The contamination in MP analysis.65 Thus, robust and quantitative potential of this technique was broadened to assess the aging of 16784 https://doi.org/10.1021/acs.est.2c04264 Environ. Sci. Technol. 2022, 56, 16780−16788 Environmental Science & Technology pubs.acs.org/est Article Figure 4. Summary of the MP weathering status and size variability in the sedimentary record A. Boxplot of the carbonyl index in the sediment profile classified as polyethylene (dark blue) and polypropylene (light blue). (B) Boxplot of the size variability (y-axis logarithmic scale) in the sediment core. C. Increase of the oxidation status of the polymers, measured as carbonyl index, as the particle size decreases (y-axis logarithmic scale). the polymers under natural environmental conditions38,66 and degradation in seawater occurs much slower than when accelerated weathering in the laboratory.67 The measurements exposed to sunlight in dry conditions.37 are generally gathered with attenuated total reflection (ATR)- Along with the depth profile (Figure 4A), no significant FTIR recording the changes occurring at a single point of the differences were found for PP (Kruskal−Wallis, chi-squared = particle’s surface (limited to particles >300 μm in size). 11.379, df = 6, p = 0.077). In contrast, significant differences Notably, the degradation of the particle may not occur evenly were found for the CI of PE (Kruskal−Wallis, chi-squared = across the MP. This study collected spectra of small MPs (11− 15.732, df = 6, p < 0.05), between sections 5−6 cm and 7−8 1000 μm) in the transmission mode. Under this setup, the cm (p < 0.05). Several uncertainties limit the explanation of limitation of the measurement is defined by the thickness of the CI variability over time. First, although MP oxidation may the particle as the IR light passes through the sample, occur in benthic environments under aerobic conditions, the recording the particle’s inner core as well as its surface. oxygen-rich products that allow CI estimation mainly occur However, by using hyperspectral images, where several spectra under light-exposure conditions. Furthermore, this process is are measured per particle (Figure 3), it is possible to severely retarded in seawater and further impaired by biofilm characterize the weathering status across the whole dimension formation.37 Second, the tendency for polyolefins to be of an MP in contrast to ATR-FTIR measurement. affected by photooxidation is defined by the original chemical Furthermore, the method shows the potential of transmission composition that might vary highly depending on the additives measurements for computing the CI of small MPs in a (plasticizers, retardants, antioxidants, stabilizers) used during standardized and automatic manner. A total of 246 polyolefin particles were used to compute the CI, 83 PE, and 172 PP their manufacturing. In this context, the interpretation of CI particles. These particles yielded a total of 11,337 and 21,712 results is dependent on the uncertainties of the initial chemical spectra, respectively. Due to non-normal distribution, the composition, the distance to the sources, and the exposure median value was computed to describe the CI per particle. time to natural conditions before its sequestration into the The overall median CI for PE was 1.05 (Q1−Q3: 0.90−1.25) sediments. and for PP was 0.48 (Q1−Q3: 0.23−0.83). The SUAB method Linear regression using the CI of PE and PP, and the major produces significantly higher CI values than previous methods, dimension of these polyolefins (logarithmically transformed) which statistically rejects the results’ intercomparison.39 No indicated a tendency that smaller MPs were more oxidized other study has previously used this method to investigate (Figure 4C). However, no significant correlation was found MP’s weathering under natural environmental conditions. (PE: R2 = 0.003 and PP: R2 = 0.160). Overall, this observation Potrykus et al.68 applied it to characterize the chemical agrees with the conclusion that MP degradation leads to the modifications on a PP plastic sample after five years of embrittlement of a particle, which favors the fragmentation of degradation in a landfill. The authors reported a CI ranging these pollutants.37 Our results indicate that this process occurs from 0.37 to 1.29. However, it should be noted that plastic prior to sequestration into the sedimentary compartment, 16785 https://doi.org/10.1021/acs.est.2c04264 Environ. Sci. Technol. 2022, 56, 16780−16788 Environmental Science & Technology pubs.acs.org/est Article where MPs are accumulating with no signal of further physical Polymer and size class of the microplastic recorded in degradation. the blanks; microplastic concentration and blank 3.7. Is MP Degradation an Active Process in the correction; microplastic extraction protocol; 210Pbxs Sediment Compartment? In our sediment core, the MP specific activity profile; and Ebro river discharge records size, measured as the particle’s major dimension, presented a from 1951 to 2016 (PDF) non-normal distribution (Shapiro−Wilk normality test, W = 0.52413, p < 0.001), ranging from 13 to 983 μm. Other MP measures, such as the area the particle took up in its imaged two-dimensional projection, could have been used to ■ AUTHOR INFORMATION Corresponding Author Laura Simon-Sánchez − Institute of Environmental Science characterize MP size. We chose the major dimension (its and Technology (ICTA), Autonomous University of length) to follow the criteria suggested by Hartman et Barcelona (UAB), Bellaterra 08193, Spain; orcid.org/ al.52Figure 4 displays the MP size along with the sediment 0000-0003-3252-6253; Email: laura.simon@uab.cat core. Significant differences in the MP size were found between the sections investigated (Kruskal−Wallis, chi-squared = 76.54, Authors df = 7, p < 0.001). The subsequent post hoc Wilcoxon− Michaël Grelaud − Institute of Environmental Science and Mann−Whitney test revealed that the MP size in section 2−3 Technology (ICTA), Autonomous University of Barcelona cm differs from the rest of the core (all p < 0.001) and section (UAB), Bellaterra 08193, Spain; orcid.org/0000-0001- 6−7 cm differs from the top part of the core (section 0−1 cm 8649-9743 to 5−6 cm, all p < 0.05). Overall, the variability in MP size Claudia Lorenz − Department of the Built Environment, with no significant differences between the oldest and most Aalborg University, Aalborg Øst 9220, Denmark recent MPs sequestered in the sediment column suggests that Jordi Garcia-Orellana − Institute of Environmental Science MPs have not been subjected to physical degradation. and Technology (ICTA), Autonomous University of Furthermore, the general decrease in MP occurrence with Barcelona (UAB), Bellaterra 08193, Spain; Departament de sediment depth and constant sediment accumulation in steady- Física, Universitat Autònoma de Barcelona, Autonomous state conditions showed by the 210Pb profile rules out vertical University of Barcelona (UAB), Bellaterra 08193, Spain remobilization of the MPs along the investigated sediment Alvise Vianello − Department of the Built Environment, core. In addition, our results on the CI indicate the Aalborg University, Aalborg Øst 9220, Denmark preservation of MPs, most likely due to a slowdown of Fan Liu − Department of the Built Environment, Aalborg weathering effects, particularly photooxidation and hydrolysis. University, Aalborg Øst 9220, Denmark Lastly, these observations support the feasibility of MPs as Jes Vollertsen − Department of the Built Environment, long-lasting chronostratigraphic markers, as previously sug- Aalborg University, Aalborg Øst 9220, Denmark gested in the literature.18,69,70 Limitations should be consid- Patrizia Ziveri − Institute of Environmental Science and ered. The presence of MPs in sediment records can be used to Technology (ICTA), Autonomous University of Barcelona corroborate geochronologies when the archive shows an (UAB), Bellaterra 08193, Spain; Catalan Institution for undisturbed nature. Additionally, the reliable characterization Research and Advanced Studies (ICREA), Barcelona 08010, of MPs requires targeted analytical methods and prevention of Spain cross-contamination that otherwise might mislead the Complete contact information is available at: interpretation of the historical records. https://pubs.acs.org/10.1021/acs.est.2c04264 4. IMPLICATIONS Notes In considering the limitations of this study, we acknowledge The authors declare no competing financial interest. that coastal benthic environments are complex and highly dynamic, with exposure to disturbing natural and anthro- pogenic events that can alter the MP accumulation in these ■ ACKNOWLEDGMENTS We thank the captain and crew of the Spanish R/V Sarmiento systems. The main objectives of this study were to investigate de Gamboa and the researchers part of the MERS_BI cruise the sequestration and long-term fate of small MPs (11−1000 for supporting the sampling of this study. We thank Joan μm) buried in marine sediments. To comply with these Manuel Bruach for his support in the analysis of 210Pb. We also research questions, selecting an undisturbed sediment core thank Roberta Johnson for the proofreading and editing of the with a high SR from a relatively high MP-polluted area was English. This research contributes to the ICTA-UAB “Unit of indispensable to provide high-resolution data on the fate of Excellence” (Spanish Ministry of Science, Innovation and buried MPs on a sub-decadal scale. The application of the Universities, CEX2019-000940-M) and MERS (2017 SGR- state-of-the-art FPA-μFTIR-Imaging method for MP character- 1588; Generalitat de Catalunya). L.S.-S. acknowledges financial ization and CI computation described in this study provided support from the FI-2018 fellowships of the Generalitat de robust results that indicated that (i) the MP mass sequestered Catalunya (2018_FI_B 00918). M.G. and P.Z. acknowledge in marine sediments increased exponentially from 1965 to the i-plastic project (JPI-Oceans�Grant PCI2020-112059) for 2019 and (ii) MP properties do not vary over time, suggesting financial support. preservation of these pollutants within the sedimentary record. ■ ASSOCIATED CONTENT ■ REFERENCES (1) Crutzen, P. J.The “Anthropocene”. Earth System Science in the Anthropocene; Springer: Berlin, Heidelberg, 2006, 13 −18. * Supporting Information sı (2) Elhacham, E.; Ben-Uri, L.; Grozovski, J.; Bar-On, Y. M.; Milo, R. The Supporting Information is available free of charge at Global Human-Made Mass Exceeds All Living Biomass. Nature 2020, https://pubs.acs.org/doi/10.1021/acs.est.2c04264. 588, 442. 16786 https://doi.org/10.1021/acs.est.2c04264 Environ. Sci. Technol. 2022, 56, 16780−16788 Environmental Science & Technology pubs.acs.org/est Article (3) Persson, L.; Carney Almroth, B. M.; Collins, C. D.; Cornell, S.; (18) Bancone, C. E. P.; Turner, S. 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