1 Introduction

Microplastics are plastic debris with a diameter of < 5 mm that persist in the environment and have been identified as emerging contaminants (Arthur et al., 2009). They have been found in terrestrial, aquatic, and atmospheric systems around the world and are derived from small particles produced for specific applications, or from the breakdown of large plastics (Brahney et al., 2021; Evangeliou et al., 2020; Wang et al., 2021). The relatively large specific surface area of microplastics allows them to adsorb and transport heavy metals and other contaminants (Alimi et al., 2018; Godoy et al., 2019); they also generally contain colourants and plasticisers. Such characteristics can make them toxic to organisms (Horton et al., 2017; Wang et al., 2021). However, microplastics also reflect the technological progress of mankind; therefore, they can be considered the ‘technofossils’ of the Anthropocene when deposited in the geological record (Zalasiewicz et al., 2016). The Anthropocene is a new geological epoch, in which the dominant effect on geological processes originates from human agriculture, industry, and cities (Crutzen, 2002; Crutzen and Stoermer, 2000). Although generally debated, ~ 1900 was considered to be the beginning of the Anthropocene, marked by the extensive emergence of fly ash from the burning of coal (Lewis and Maslin, 2015; Snowball et al., 2014). Plastics were initially developed in the early twentieth century; therefore, this study focuses on that period. Hereafter, the term ‘Anthropocene’ refers to the post-1900 period.

Compared to other markers of the Anthropocene, including concrete, lead/aluminium, and other metal elements (Waters, 2016), plastics and microplastics are more abundant and extensively distributed (Wang et al., 2021). Plastics are precursors of microplastics and have been in use for more than 100 years; thus, the daily use of plastics is inevitable because of their multi-functionality (Fig. 1). Plastics were invented at the end of the nineteenth century, and several types of plastics were industrialised in the 1930s (British Plastics Federation, 2014). In 1950, the global output of plastics was only 1.5 million tons. Since then, global plastic production and consumption have increased at a rate of approximately 10% per year, resulting in plastics and microplastics continuously entering the environment. The global annual output of plastics reached 368 million tons in 2020 (Plastics Europe, 2008; Plastics Europe, 2020). Sediment has been considered a major sink for microplastics (Hidalgo-Ruz et al., 2012; Van Cauwenberghe et al., 2015). The abundance, type, and colour of microplastics in the sedimentary record change over time, with the variation of plastic production, type, and colour (Dahl et al., 2021). Although geoscientists have investigated the long-term fate of microplastics in the burial record and their interaction with geomorphology (Uddin et al., 2021; Zhong and Peng, 2021), the stratigraphic significance of microplastics has not been fully explored.

Fig. 1
figure 1

Appearance and development of major plastic types and products. The chronological years identify the year of invention of each plastic; years in brackets indicate its commercial release. The data were compiled from Braun (2001), British Europe Federation (2014), Crawford and Quinn (2016), and Zalasiewicz et al. (2016). PEEK, poly(aryl ether ether keton); PPSF, polyphenylethersulphone; PSU, polysulphone; PI, polyimide; PPE, polyphenylene ether; POM, polyformaldehyde; PC, polycarbonate; PP, polypropylene; ABS, acrylonitrile butadiene styrene; PET, polyethylene terephthalate; PTFE, polytetrafluoroethylene; PU, polyurethane; HDPE, high-density polyethylene; PA, polyamide; PCTFE, polychlorotrifluoroethylene; PCL, polycaprolactone; PE, polyethylene; LDPE, low-density polyethylene; PMMA, poly(methyl methacrylate); PS, polystyrene; PAN, polyacrylonitrile; PVC, polyvinyl chloride

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The type of microplastic provides some general information about its age, because different types of plastics and microplastics have emerged in a specific time sequence; this could form the basis of chronology studies of the Anthropocene. Thus, this study aims to evaluate the dating availability of microplastics and develop a new method for dating cores on a decade to century scale. We compiled and analysed dating method, variation of microplastic abundance, and microplastic polymers within selected dated cores. We established a chronological sequence of microplastics in the Anthropocene based on the development of plastic production and type, and verified this framework using published core data. The preservation and dating availability of microplastics in the Anthropocene is further discussed.

2 Background

2.1 Production and development of plastics

The first human-made plastic was a nitrocellulose called Parkesine, invented in 1862, and the first completely synthetic plastic was a phenolic resin with the trade name Bakelite, which was invented by Leo Baekeland in 1907 (Crawford and Quinn, 2016). Since then, the number of different types of available plastics has increased. Polyvinyl chloride (PVC), polystyrene (PS), polyethylene (PE), polyamide (PA), polyurethane (PU), and other commonly used plastics have been commercially produced since the 1930s, and polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), acrylonitrile butadiene styrene (ABS), and polypropylene (PP) have been mass-produced since the 1940s and 1950s. By the end of the 1950s, most of the commonly used plastics had been developed and were being commercially manufactured (Fig. 1). In the 1950s–1970s, the applications and colours of plastics were considerably enriched. Polyethylene terephthalate (PET) plastic beverage bottles were developed in 1973 and were rapidly commercialised. After the 1980s, various high-performance plastics were invented for specific purposes, and then gradually adopted into mainstream use. These include biodegradable plastics, linear low-density polyethylene, and composite plastic materials (British Plastics Federation, 2014). Between 2002 and 2004, the share of polymer resin was estimated to be dominated by PP (21%), low-density PE (20%), high-density PE (16%), PVC (12%), PET (10%), PU (8%), and PS (7.6%), while other types of plastic products accounted for only 4.9% (Geyer et al., 2017).

2.2 Transportation of microplastics

The presence of microplastics in the sedimentary record can reflect the production and use of plastics at a specific time. This is because abandoned plastics take a relatively short time to enter the sedimentary environment. In a terrestrial depositional environment, microplastics can be derived from plastics discarded and fragmented onsite, offsite emissions, or via airborne pathways (Fig. 2). Plastic mulching film used in farmlands (Büks and Kaupenjohann, 2020; Huang et al., 2020), plastics in landfill (Hou et al., 2021), and discarded plastics are deposited in situ and quickly enter the sediment. Additionally, microplastics can be transported before disposal. For example, a large amount of microplastics have been collected from sewage sludge and wastewater during treatment processes prior to agricultural use (Nizzetto et al., 2016) and vehicular tyre wear can be washed off roads by runoff (Evangeliou et al., 2020; Julien Boucher, 2017). Plastics from wastewater and road runoff can enter the sediment within a year as wastewater treatment generally takes no longer than a year (Luo et al., 2014) and road runoff occurs during rain events. As for atmospheric transportation, microplastics generally remain suspended in the air for 1‒3 h (Wright et al., 2020). River flow is a dominant vehicle for the movement of land-based plastics to freshwater and marine environments (Mai et al., 2020; Wang et al., 2019). Microplastics derived from riverine, coastal, and marine activities, together with airborne microplastics, can settle through the water column to be deposited in aquatic sediment (Zhang et al., 2019). Settling velocities can range from a few millimetres per second to hundreds of millimetres per second (Khatmullina and Isachenko, 2017; Kowalski et al., 2016); therefore, the time taken for microplastics to settle in an offshore seabed is generally within the annual scale.

Fig. 2
figure 2

Transportation of microplastics in the environment and their deposition in the sedimentary record. Black text indicates the source and transport pathway of microplastics. Text in parentheses represents the residence time of microplastics in the transportation process

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3 Materials and methods

3.1 Literature search and data collection

A search was conducted for the terms ‘microplastics’ and ‘core or record or profile’ within the titles, abstracts, and keywords of articles in the Web of Science and Scopus databases from before June 2022. From the respective databases, 456 and 351 papers were retrieved. The cores reported in these papers were further selected using the following criteria: (1) the research material comprised sedimentary core; (2) at least three layers of microplastic data were obtained from each core; (3) microplastic type data were reported for each layer, and (4) microplastic types were determined with the assistance of Fourier transform infrared spectroscopy or Raman spectroscopy. In total, 90 cores were selected for further analysis, including 39 cores with reported dating and 51 cores without dating (Fig. 3). The main reasons for exclusion were: (1) ice core or water column; (2) less than three layers of microplastic type data; (3) review articles without primary data; (4) indoor microplastic toxicology or migration experiments. Their core ID, sampling location and environment, and dating method information was compiled for the selected dated sedimentary cores, along with the variation of microplastic abundance and type with depth and sediment age.Footnote 1 Data presented graphically in articles was obtained with GetData Graph Digitizer 2.25. For undated sedimentary cores, microplastic types were recorded at respective depths. Moreover, representative plastics that were widespread in the core were then selected to build a plastic chronology for the sedimentary record. Microplastic dating was conducted on the selected sedimentary cores, and the results were verified against the reported ages obtained via traditional dating means.

Fig. 3
figure 3

Sampling location of collected cores selected for microplastic dating analysis. Blue triangles represent cores that have been reliably dated; yellow triangles represent cores without previous dating. The base map is derived from ArcGIS

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3.2 Establishment of microplastic chronology in the sedimentary record

Produced plastics are transported and fragmented in the environment and finally enter the geological record. However, not all types of plastics that are produced have been found in the sedimentary record. Therefore, in this study, a chronostratigraphic framework was established based on the first appearance datum and theoretical lowermost occurrence of the commonly found microplastics in the sedimentary record (Fig. 4). Lowermost occurrence is a biostratigraphy concept used to record the first occurrence of a species in geological records (Gong and Zhang, 2007). Although the appearance of plastics represents the invention of plastics and their earliest possible occurrence in the sedimentary record, microplastics were unlikely to occur in the sedimentary record until particular plastic types were commercially produced. Consequently, priority was given to the commencement of commercial plastic production as the theoretical lowermost occurrence in the sedimentary record, which showed good correlation with the position of plastics in a core. If the commercial production time of plastics was not precisely documented, the appearance time was used instead.

Fig. 4
figure 4

Microplastic chronological sequence in the sedimentary record. The theoretical lowermost occurrence of rayon, PVC, PS, PAN, PE, PA, and PP was determined based on the year of their commercial release, whereas that of PU, PAN, PET, PS foam, and PET fragments was based on the year of their appearance (invention)

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The type of plastic found in sediment can be used to determine the age of that sediment. The microplastic sequence contained four different assemblage zones of microplastic types (Fig. 4). The rayon zone consisted of mainly rayon and indicated a sediment age of later than 1910, as rayon was mass-produced after 1910. The microplastic composite zone included one or more of PVC, PS (non-foam), PU, PA, PE, PAN, or PET (non-fragment), but excluded PS foam, PP, or PET fragments, and the sedimentary age of the zone was attributed to the post-1930s period, as these types of microplastics were commercialised in the 1930s. Similarly, the PP and/or PS foam zone (including PP and/or PS foam but excluding PET fragments) and the PET fragments zone (containing PET fragments) indicated the post-1950s and post-1970s periods, respectively. PS foam and PP could be used as technofossils in sediments deposited after the 1950s, while PET fragments represent markers after 1973. PS foam and PET fragments were listed separately as they appeared much later than PS and PET with a distinctive shape that could be easily identified, making them the markers of the post-1950s period.

4 Results

4.1 Dating methods of microplastic research in sedimentary records

The 39 published cores were dated using 8 different methods; 53% of the sedimentary cores were dated using two or more methods to ensure a reliable result (Fig. 5). 210Pb, 137 Cs, palynological and radiogenic dating were conducted on Core HAMP1 for an accurate chronological framework (Turner et al., 2019). The 210Pb method is the most commonly used method for dating at the centennial scale, and it was used to analyse 37 (94%) of the dated cores. Of the cores dated with a single method, only one did not adopt 210Pb dating. 137Cs dating is often combined with 210Pb dating, and this combination was applied to 15 (38.5%) cores in our study. Other dating methods, such as AMS 14C, spheroidal carbonaceous particle, polychlorinated biphenyls, and palynological dating, could supplement 210Pb and 137Cs dating.

Fig. 5
figure 5

Dating method frequency in the sedimentary record

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4.2 Microplastic abundance in the sedimentary record

The microplastic abundance in the sedimentary record varies significantly between different places, and the difference in the surface layer can reach three orders of magnitude (Eo et al., 2022; Martin et al., 2020). The highest abundance of microplastics found in a lake environment was 7711 n/kg in the surface layer of Donghu Lake, China (Dong et al., 2020); in the coastal environment, the surface layer of Core AG.S on the Spanish Mediterranean coast had the highest microplastic abundance of 3784 n/kg (Dahl et al., 2021). However, the highest microplastic abundance in the marine environment occurred at a depth of 2.5 cm in Core EDP1 in the East Sea dumping site, Korea, with 123,616 n/kg (Eo et al., 2022).

Typically, the concentration of microplastics is highest in surface sediment and decreases gradually with an increase in depth or time, which is consistent with the global trend of plastics production (Fig. 6). In some cores, the peak concentration of microplastics appeared in sublayers beside the surface layer, generally due to region-specific factors. A Core in Hampstead Pond contained a small blue-fibre peak in the mid-1960s, which may be related to the popularity of specific-colour fabrics (Turner et al., 2019) (Fig. 6). However, the microplastic abundance of the subsurface layer was higher than that of the surface in some cores, such as Core Cn.21 in Tokyo Bay (Matsuguma et al., 2017) and cores in Jiaozhou Bay (Li et al., 2021). The earliest appearance of microplastic in the sedimentary record was dated to around 1870 in a core from Hamstead Pond in London (Turner et al., 2019).

Fig. 6
figure 6

Representative curves of microplastic abundance over time, in lake, coast, and marine sediments

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4.3 Characteristics of microplastic polymer in the sedimentary record

There are 34 kinds of microplastic polymer in the 39 sedimentary cores (Fig. 7). The microplastic types that appeared in more than half of the sedimentary cores are PP (22, 62.9%), PE (20, 57.1%), PET (19, 54.3%), PVC (19, 54.3%), and PS (18, 51.4%). The common occurrence of these microplastic polymers in the deposition record may be due to their high output and extensive use. Moreover, some copolymers or other less-common plastic types were also found in the sedimentary cores, such as PE-PP, poly(acrylate:styrene), and PS-b-PMMA. Core YDP2 in the Yellow Sea dumping site of Korea and Core EDP1 in the East Sea dumping site of Korea exhibited the most microplastic types, containing 18 and 15 plastic types, respectively (Eo et al., 2022). The surface layer of Core YDP2 was also the layer with the most diverse plastic types. Similarly, in most cores, the diversity of microplastics decreased with increasing depth (Table S1Footnote 2).

Fig. 7
figure 7

Occurrence frequency of different types of microplastic polymer in sedimentary record. PAK, polyacrylates; PES, polyester; EVA, ethylvinyl acetate

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Microplastics with a larger output and earlier commercial release time were found earlier in the sedimentary record. For example, PS and PU could be identified in sediment from the 1930s (Dahl et al., 2021; Martin et al., 2020) and PP was found in sediment from the 1950s (Eo et al., 2022). However, PE was not found until 1943 (Martin et al., 2020), possibly because its density is lower than that of water, limiting its ability to settle. Moreover, microplastic types with lower yield or a later invention time appeared later in the sedimentary core. The first appearance of PMMA and PTFE was in 2001 (Yellow Sea dumping site, Korea) and 2010 (Spanish Mediterranean coast), respectively (Dahl et al., 2021; Eo et al., 2022).

4.4 Verification of microplastic dating

To verify the validity of microplastic dating, the age range determined by microplastic types was compared to age reported for 39 published cores. This comparison is presented in Table S1 in detail. Microplastics were found to occur in the sedimentary record in a sequence that followed the appearance and commercialisation of the corresponding plastic types (Fig. 8). Moreover, the sediment age indicated by the microplastic types was consistently in agreement with the previous dating results, including 210Pb, 137Cs, and polychlorinated biphenyl (PCB) dating; that is, the dating results obtained by other dating methods always fell into the age range indicated by microplastics, except for Core 8 from the Arabian Gulf and the East Sea of Korea. This indicated that microplastics could sufficiently mark the age of sediments. Notably, in some areas, microplastics may appear in the sedimentary core considerably later than their global commercialisation time. This is because the occurrence and widespread utilisation of microplastics of a specific plastic type vary from place to place, as does the microplastic assemblage zone in the sedimentary record. For example, in a sedimentary core from Donghu Lake, PVC, PA, PE, and PET occurred only after the 1960s (Fig. 8), although they were mass-produced in the 1930s in developed countries. This is likely because the plastics industry in China remained under developed until the 1950s (Wang et al., 2018). Additionally, the age ranges of 51 published undated cores were determined based on microplastic type, and the results are shown in Table S2.Footnote 3

Fig. 8
figure 8

Application of microplastic type to determine the age of sediments in selected cores, and comparison with reported ages obtained via traditional means. The 6cores in the figure were selected from Allen et al. (2021), Chen et al. (2020), Dong et al. (2020), and Dahl et al. (2021), Li et al. (2022), and Martin et al. (2020). The age ranges indicated with grey, yellow, blue, and green lines represent the microplastic markers of the post-1910s, post-1930s, post-1950s, and post-1970s, respectively

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5 Discussion

5.1 Preservation of microplastics

The role of microplastics in Anthropocene research is dependent on their persistence and mobility. The persistence of microplastics is related to their physicochemical properties. Microplastics may take hundreds or thousands of years to degrade into harmless substances or CO2 and water in the environment (Ohtake et al., 1998). The degradation of microplastics depends on oxidation, ultraviolet radiation, or external forces, such as wind, water, and artificial wear (Shah et al., 2008). However, these effects are weakened when microplastics enter a sedimentary layer. Therefore, microplastics buried in sediments are considered highly preserved in strata and are unlikely to fade or degenerate into smaller particles (Gregory and Andrady, 2003). Moreover, microplastics can persist for longer periods under anoxic and anaerobic conditions (Zalasiewicz et al., 2016). The mobility of Anthropocene markers may result in them appearing in sedimentary layers earlier than when they were created, as observed for PCBs (Mai et al., 2005), which may reduce the applicability of some substances as chronological indicators. The hydrophobic properties of plastics can facilitate their easy combination with sediments (Frère et al., 2018). Moreover, continuous deposition can cause a decrease in the porosity of the sediment, a higher degree of compaction, and diagenesis (Berner, 1980). Ultimately, microplastics and sediments are cemented to each other, and the relative position of microplastics is fixed. Therefore, microplastics have limited mobility once buried.

5.2 Dating availability of microplastics in the Anthropocene

Microplastics are sufficiently preserved in the sedimentary record and have limited mobility once buried. Therefore, they should be regarded as ‘technofossils’ of the Anthropocene which can be used to determine the stratigraphic age of core layers. Such a method could be more convenient than other dating methods, and could be used to compare the stratigraphy of different sedimentary environments, such as lakes and oceans. Although centennial-scale dating methods, such as 210Pb and 137Cs, is various, most infer the age of each layer based on a constant deposition rate or a limited number of age-control points. For example, 137Cs dating is based on the maximum year of global nuclear weapons testing (1964) or the year of major nuclear events (Courtene-Jones et al., 2020). The deposition rate varies with time, and the dating error is relatively large in areas of erosion and deposition discontinuity, especially in coastal areas (Barsanti et al., 2020). This problem may be resolved with the assistance of microplastics because they are common in modern sedimentary layers and could offer more age control points for dating, particularly in the post-1910s period. Since 1910, new types of plastics have been created almost every two decades, resulting in the unique advantage of microplastics serving as markers of the Anthropocene in the sedimentary record. The application of microplastic types to sediment dating would be more feasible in places where plastics have been used for a long time. Moreover, reversed strata could be inferred if the abundance and diversity of microplastics increased abruptly in the deep layers of a core (Fig. 2). Therefore, microplastics could be an effective chronological indicator, as an alternative to traditional radionuclide dating methods.

Notably, microplastic types can be used to determine an age range, instead of an exact age of a sediment layer; therefore, this method could also be used to correct the results of other dating methods. The approximate age of a core could be quickly identified with microplastic dating before further dating is conducted. Compared with samples used for radionuclide dating methods (such as 210Pb dating), microplastic samples do not need to be kept for a period before they are dated (Zaborska et al., 2007). The detection speed of detecting microplastics with spectrometry can reach 3 s per sample (Tagg et al., 2015). The infrared spectra and microphotographs of common microplastic types are shown in Figure S1.Footnote 4 However, the age range determined by microplastic type may be much broader and earlier than the reported age determined by other methods, due to regional differences in plastic use; hence, establishing the microplastic chronological sequence on a regional scale could obtained a more accurate sediment and is conducive to applying microplastic dating in a specific area. Furthermore, our study found that in some cores, microplastics occurred in layers prior to their date of production, indicating environmental contamination or remobilisation (Martin et al., 2022). Therefore, a stricter quality control method, including pollution control in the sampling process and experimental process, should be established for a robust result, and cores should be assessed for sediment disturbance prior to obtaining the final dating result.

6 Conclusion and remarks

Undoubtedly, microplastics are omnipresent even in the sedimentary record. Microplastic abundance and diversity was found to decrease with increasing depth and sediment age, which is the typical pattern of microplastic distribution in the sedimentary record. Based on the ‘evolution’ of plastic types and the common microplastic types found in the sedimentary record, a microplastic chronological sequence in the sedimentary record was established and applied to collected cores. We defined four microplastic assemblage zones in the sedimentary archives, namely the rayon zone, the microplastic composite zone, the PP and/or PS foam zone, and the PET fragments zone. These zones indicated age ranges of post-1910s, post-1930s, post-1950s, and post-1970s, respectively. Moreover, the age range determined using microplastic type was highly consistent with the published ages of the collected sedimentary cores. Similar to fossils in biostratigraphy, different types of microplastics can be considered technofossils with specific successions in the sedimentary record, implying information about specific periods. The short migration time to the sedimentary record, good preservation, and limited mobility of microplastics could also provide a foundation for determining sediment age by microplastic types.