The Stable Carbon Isotopic Compositions of n-Alkanes in Sediments of the Bohai and North Yellow Seas: Implications for Sources of Sedimentary Organic Matter

DANG Tianxiang, CAO Yunyun, and XING Lei, *

The Stable Carbon Isotopic Compositions of-Alkanes in Sediments of the Bohai and North Yellow Seas: Implications for Sources of Sedimentary Organic Matter

DANG Tianxiang1), 2), CAO Yunyun1), 2), and XING Lei1), 2), *

1) Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China,Qingdao 266100, China 2) Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China

Stable carbon isotopic compositions of-alkanes in surface sediments of the Bohai and North Yellow Seas were investigated to elucidate sources of sedimentary organic matter in these seas. The long-chain-alkanes in surface sediments are predominantly long-chainC27, C29, and C31types, with obvious odd carbon predominance. The δ13C values of long-chain-C27,-C29, and-C31alkanes are −30.8%±0.5‰, −31.9%±0.6‰, and −32.1%±1.0‰, respectively, within the range of-alkanes of C3terrestrial higher plants. This suggests that sedimentary-alkanes are derived mainly from terrestrial higher plants. Compound-specific carbon isotopic analysis of long-chain-alkanes indicates that C3terrestrial higher plants predominate (64%–79%), with angiosperms being the main contributors.The-alkane δ13C values indicate that mid-chain-alkanes in sediments are derived mainly from aquatic emergent macrophytes, with significant petroleum pollution and bacterial degradation sources for short-chain-alkanes.

biomarker; carbon isotopes;-alkanes; Bohai Sea; North Yellow Sea

1 Introduction

The Bohai Sea (BS) and Yellow Sea (YS) are typical semi-enclosed marginal shallow seas of China, with complex hydrodynamics and large inputs of land-based materials. The annual organic carbon flux from the Yellow River and other rivers to the YS and BS is (1210 ± 240) × 104ton (t) yr−1, accounting for 79% of all organic carbon entering these seas (Liu., 2015a), while atmospheric deposition contributes < 2% (Qiao., 2017). In addition, the Yellow Sea Warm Current (YSWC) transports about 106tyr−1of Yangtze River sediments to the North Yellow Sea (NYS) (Gao., 1996). The tracing of sedi- mentary organic matter (SOM) sources improves understanding of the organic matter cycle in aquatic environments (Hedges., 1997). SOM is a complex mixture of marine and terrestrial organic compounds, and it is difficult to quantitatively distinguish its sources at the edge of the continental shelf. Normal (-) alkanes are commonly applied as biomarkers given their widespread occurrence in marine and terrestrial environments, and can be preserved in marine sediments. Compositions and distributions of-alkanes from different biological sources are generally different. In addition, compared with fatty acids and alkanols, structure of-alkanes is relatively stable and has strong anti-degradation ability (Mead, 2005). Previous studies have shown that- alkanes may be effective in characterizing sources of SOM in coastal marine systems (Xing., 2011; Wang., 2013). Long-chain-alkanes derived from terrestrial higher plants are most abundant in C27, C29, and C31-alkanes (Bray and Evans, 1961). The freshwater and marine non-emergent macrophytes and sphagnum mosses are enriched in mid-chain-alkanes (Pancost, 2002; Mead, 2005; Mügler, 2008; Bush and Mc- Inerney, 2013). Short-chain-alkanes with odd carbon predominance such as C17-alkane are generally considered to be derived from aquatic algae and photosynthetic bacteria (Meyers and Ishiwatari, 1993; Silliman and Schelske, 2003; Liu, 2012). Additionally, petroleum-derived hydrocarbons also contribute short-chain-alkanes, with no obvious odd/even predominance (Hos- tettler, 1999). The long-chain (≥C24)-alkanes have clear odd carbon-number predominance in the NYS, indicating predominant input of terrestrial higher plant material (Lu., 2011). Analyses of-alkanes in sediments of the Yellow River Estuary indicate that SOM originates mainly from terrigenous inputs, while marine microorganisms contribute to short-chain (C12–C22)-al- kanes offshore (Wang., 2018). Compositional analysis of-alkanes in surface sediments of the central South Yellow Sea (SYS) indicates that SOM is derived mainly from terrestrial higher plant input from the modern and old Yellow rivers, with the contribution of herbaceous to woody plants is comparable (Zhang., 2014).

Similar chemical-alkane compositions have been found in different types of organisms, which may confuse their interpretation (Ficken., 2000; Mead., 2005; Sikes., 2009). However, stable carbon isotopic com- positions of individual-alkanes from different sources in marine sediments are generally distinctive and may therefore constrain their sources (Hayes., 1990; Mead., 2005; Ankit., 2017), with-alkane com- positions and stable carbon isotopic characteristics together having been used to identify SOM sources (Eg- linton, 1969; Jeng and Huh, 2008; Hu., 2013). Previous studies have shown that stable carbon isotopic compositions of long-chain-alkanes in surface soils of eastern China can be used as an indicator of C3/C4plant proportions in overlying vegetation (Rao., 2008). The analysis of long-chain-alkanes and their stable carbon isotopic compositions in sediments of Qinghai Lake indicates that δ13C values of C31-alkanes are consistent with those of modern land plants around the lake, and can therefore be used as a reliable tracer of C3/C4compositions of terrestrial vegetation (Liu., 2015b). A recent study found that δ13C values of organic matter indicate that terrestrial organic carbon from the Yellow River accumulates mainly at the river mouth and in two muddy areas around it (Sun., 2018a).

Until now, little has been known of the spatial distribution of-alkane stable carbon isotopes in sediments of the BS and NYS. The aim of this study was to elucidate the stable carbon isotopic composition of SOM-alkanes and the sources of-alkanes in surface sediments of the BS and NYS.

2 Study Area and Methods

2.1 Study Area

The BS is a shallow, semi-enclosed epicontinental sea. About 90% of its sediment input is supplied by the surrounding rivers, especially the Yellow River. The YS is a shallow, semi-enclosed, continental margin West Pacific sea, with an area of 400,000 km2and an average water depth of 44 m, joining the BS in the north and the East China Sea in the south. The YS is divided into southern and northern parts by the Shandong and Korean peninsulas. The overall topography of the NYS seabed is inclined southward. Water depths in the BS and NYS are generally < 60m. In the study area, surface currents include coastal currents and the northwestward YSWC (Fig.1). The YSWC is a branch of the Tsushima Current with warm and saline water. There is no major direct local riverine input to the YS although, over time, fine-grained riverine sediment can be resuspended and transported from the BS to the YS by coastal currents. Analyses of sediment sources indicate that fine-grained sediments within the NYS originate mainly in the modern and old Yellow Rivers (Alexander., 1991; Lim., 2007). From 1976 to 2005, runoff and sediments from the Yellow River averaged 140.36×108m3yr−1and 3.31×108tyr−1, respectively (Cui and Li, 2011), with most sedimentary materials being deposited in front edge of the delta and estuary, and finer grained sediment transported to the coastal and shelf areas outside the Yellow River estuary. Muddy sediments along the north coast of Shandong Peninsula are considered to be directly and indirectly from the Yellow River (Yang and Liu, 2007). In addition to riverine input, coastal erosion from the old Yellow River Delta also contributes to sediments (Hu., 1998).

Fig.1 Sampling sites and surface currents in the BS and NYS. BSCC, Bohai Sea Coastal Current; SDCC, Shandong Coastal Current; YSCC, Yellow Sea Coastal Current; YSWC, Yellow Sea Warm Current; LDCC, Liaodong Peninsula Coastal Current. Blocks represent the sites. Shading indicates the muddy areas.

2.2 Samples

Surface sediments (0–3cm depth) were collected from 23 sites in the BS and NYS using a box corer deployed fromduring a cruise sponsored by the National Natural Science Foundation of China (NSFC) in June 2011 (Fig.1). Sediment samples were wrapped in aluminum foil and stored at −20℃ until analysis.

2.3 Analytical Methods

2.3.1 Lipid extraction and purification of-alkanes

Freeze-dried, powdered, and homogenized sediment samples were extracted four times with dichloromethane/ methanol (DCM/MeOH; 3:1, v/v) with ultrasonication (15min each time), after adding internal standards containingC24D50. Extracts of the samples were dried in a N2stream and hydrolyzed with 6% KOH in MeOH. Non- polar fractions containing-alkanes were separated using activated silica gel column chromatography with elution by-hexane, anddried in a N2stream.

To accurately measure the δ13C values of individual-alkanes,-alkanes need to be further purified. Zeolite molecular sieve is a commonly used and high recovery method for-alkanes purification. The extracted-alkanes were transferred to a columnwith AgNO3-Silica gel and molecular sieve, and then eluted with-hexane to ensure the components of the inner wall of the AgNO3- Silica gel glass column were completely transferred into the molecular sieve. After elution of about 1.5mL, the upper AgNO3-Silica gel column was removed and the molecular sieve column was baked in an oven at 40℃ for >12h. Subsequently, the zeolite power was transferred to a 10mL Teflon bottle for digestion with HF to release-alkanes. A preheating Pasteur column (6mm, i.d.×2cm) filled with Na2SO4was used to remove residual HF before the-alkanes were extracted with-hexane (four times) and then was dried in a gentle N2stream pending instrumental analyses. The average recovery rate of long- chain (≥C26), mid-chain (C21–C25), short-chain(C15–C20)-alkanes in all samples was 63%, 53%, 57%, respectively.

2.3.2-alkanes analysis

The-alkane compositions were determined by an Agi- lent 6890N gas chromatography, with chromatographic separation on an HP-1 capillary column (50 m ×0.32mmi.d.× 0.17μmfilm thickness, J&W Scientific) using H2as a carrier gas (1.2mLmin−1). Samples were injected in splitless mode with an injector temperature of 300℃. Oven temperature was programmed from 80℃ to 200℃ at 25℃ min−1, 200℃ to 250℃ at 3℃min−1, 250℃ to 300℃ at 1.8℃min−1,300 to 310℃at 5℃min−1, and holding at 310℃ for 5min. Quantification of compounds was performed by peak area integration in FID GC (Agilent 6890N) relative to the internal standards. The average relative standard deviation in concentrations was <10%.

The average chain length (ACL; Cranwell, 1987), the terrigenous/aquatic ratio (TAR; Bourbonniere and Meyers, 1996), the Pmar-aq(odd mid-chain alkanes/odd mid- and long-chain alkanes; Ficken, 2000; Mead, 2005) of-alkanes were calculated as follows:

2.3.3 Stable carbon isotopic composition (δ13C) analysis

Gas chromatography isotope ratio mass spectrometry (GC-IRMS; on an HP 6890 GC coupled with a Thermo Delta-V system.) was used to measure stable carbon isotopic compositions of-alkanes. Chromatographic separation was achieved using a DB-1MS capillary column (60m×0.32mmi.d.×0.25μm film thickness, J & W Scientific). The GC oven temperature was programmed from 60℃ to200℃ at 15℃min–1, 200℃ to 250℃ at 4℃min–1, 250℃ to 300℃ at 1.8℃min–1, 300 to310℃ at 5℃min–1, and holding at 310℃ for 5min. The authentic standard was analyzed under the same conditions after every seven samples. The standard deviation for duplicate analysis of the standard was 0.3‰. Isotopic ratios were expressed as δ13C values (per mil) relative to the Vienna Pee Dee Be- lemnite (VPDB).

3 Results

3.1 Composition of n-Alkanes and the Hydrocarbon Indices

The GC-FID chromatograms of-alkanes showed that-alkaneswereeffectively purified after using the molecular sieve (Fig.2). Total-alkane contents (SC15–35) ranged from 456 to 3837ngg−1(average=1897ngg−1). The contents of long-chain-alkanes (SC25–35) ranged from 267 to 2826ngg−1(average=1300ngg−1). In addition, the average percentage of long-chain, mid-chain, short-chain-alkanes in samples was 58%, 27%, 14%, re- spectively. Furthermore, the total-alkane contents of samples from muddy areas (average = 2666ngg−1) were significantly higher than those from non-muddy areas (average=1683ngg–1). The ACL values varied between 26.1 and 28.9(Fig.4a). The values of TAR and Pmar-aqranged from 3.4 to 25.7,from 0.2 to 0.7 (Figs.4b, 4c), res- pectively.

Fig.2 GC-FID chromatograms for n-alkanes of surface sediments (site B28): (a), Before purification; (b), After purification.

3.2 δ13C Values of n-Alkanes in Surface Sediments

Compound-specific average δ13C values of-alkanes in surface sediments were shown in Fig.3, with average individual values for C17–C31of −30.1‰±0.5‰, −28.7‰± 0.4‰, −29.8‰±0.6‰, −28.4‰±0.3‰, −29.9‰±0.5‰, −29.4‰ ±0.6‰, −30.4‰±0.3‰, −29.9‰±0.5‰, −30.2‰±0.5‰, −30.1‰±0.5‰, −30.8‰±0.5‰, −30.8‰±0.8‰, −31.9‰±0.6‰, −31.7‰±1.1‰, and −32.1‰±1.0‰, respectively. In both the BS and NYS, δ13C values of mid-chain-alkanes (C21–C23) varied within a narrow range, while those of short- and long-chain-alkanes were more variable. Furthermore, for short- and mid-chain-alkanes, δ13C values of even-carbon-numbered cases were more positive than those of odd-carbon-numbered cases.

Fig.3 Compound-specific average δ13C values for the individual n-alkanes (C17–C31) from 23 BS and NYS samples.

4 Discussion

4.1 Long-Chain n-Alkanes

The contents of long-chain-alkanes were relatively high and exhibited a strong odd carbon predominance in C27, C29, and C31homologues (Fig.3), consistent with terrestrial higher plant sources. The ACL describes the average number of carbon atoms in odd carbon-alkanes in higher plants (Cranwell, 1987). The ACL values of BS and NYS surface sediments ranged from 26.1 to 28.9 (average = 27.5). ACL value of about 29 in sediments near the Yellow River estuary suggests an origin of terrestrial higher plants (Fig.4a). The relative contribution of terrestrial-alkanes to marine sediments can be assessed using the TAR index. TAR values of BS and NYS surface sediments ranged from 3.4 to 25.7, with an average value of 14.1 (Fig.4b). This indicates a predominance of terrigenous-alkanes input (Ankit., 2017). Furthermore, compositional analysis of-alkanes in surface sediments of the BS and NYS also indicates that long- chain-alkanes are derived mainly from terrestrial higher plant input (Cao,, 2018). Hence, Long-chain-al- kanes in the study areas were thus mainly derived from such plants.

The δ13C values of long-chain-alkanes produced by C3and C4plants typically range from −31.0‰ to −39.0‰ and −18.0‰ to −25.0‰, respectively (Collister., 1994; Schefuß., 2003). Modern terrestrial higher plants from eastern China are characterized by-alkane δ13C values of −21.9‰ to −34.8‰, −25.3‰ to −36.1‰, and −22.9‰ to −36.7‰ for C27, C29, and C31components (Rao., 2008), consistent with our corresponding average δ13C values of −30.8%±0.5‰, −31.9%±0.6‰, and −32.1%±1.0‰ (Fig.3), respectively, and indicating that long-chain-alkanes are mainly derived from terrigenous sources. Generally, odd-carbon-numbered long-chain- alkanes are somewhat13C-enriched than those of even- carbon-numbered long-chain-alkanes in terrestrial higher plants (Chikaraishi and Naraoka, 2003). However, our results showed δ13C values of even-carbon-numberedlong-chain-alkanes (C26–30) were more positive than those of odd-carbon-numbered long-chain-alkanes (C27–31) in the study area (Fig.3). This implies there may be different sources of even-carbon-numbered long-chain-alkanes. A previous study reported14C ages forC29+31alkanes (Δ14C = −288‰ to −612‰) of 2670 to 755014C yr, which differ markedly from those of strongly14C- depletedC26+28+30+32alkanes (Δ14C = −700‰ to −961‰) ages of 9600 to 2605014C yr for Yellow River suspended particulate matter, implying ancient organic carbon inputs (Tao., 2015). This may indicate that even-carbon- numbered long-chain-alkanes in the BS and NYS are derived from ancient organic carbon.

Fig.4 Spatial distribution of n-alkane indices: (a), ACL; (b), TAR; (c), Pmar-aq and (d), C3plants contribution to n-alkanes and C3/C4 ratio in surface sediments, based on the end-member modeling of compound-specific δ13C values in the study area.

Weighted mean average δ13C of long-chain-alkanes from sediment samples were determined to calculate the changes in biomass of C3and C4plants in historical periods (Kuang., 2013). A binaryend-member mixing model was used to estimate the relative contributions of long-chain-alkanes from C3and C4plants(Garcin., 2014), with δ13C values of −36.0‰ and −21.0‰ being used as end-members for these plants, respectively (Col- lister., 1994; Zhang., 2003). Calculations were performed as follows:

= (δ13C27× C27+ δ13C29× C29+ δ13C31× C31)/( C27+ C29+ C31) = (−36.0‰) ×+ (−21.0‰) × (100% −), (4)

whereis the weighted mean average δ13C value of long-chain-alkanes, andis the C3contribution (%).

End-member estimations for the BS and NYS indicated that terrestrial C3plants were dominant-alkane sources, with relative contributions of 64%–79% (Fig.4d).This is consistent with the predominance of C3plants in north China, with a previous study having shown that δ13C values of-alkanes in aerosols near the north China coast have terrestrial C3plant origins with the C4contribution being negligible (Guo., 2006).Moreover, soil organic matterδ13C values in a N–S section (34–52˚N) through central and eastern Asia indicate that vegetation in the area comprises mainly C3plants (Feng., 2008). Records of δ13Cvalues in surface soils of northeast China indicate that the abundance of C4plants is relatively high in warm periods and almost exclusively C3plants exist in cold periods (Sun., 2018b). Previous studies have shown thatδ13C values of dominant C3plants in the Chinese Loess Plateau range from −30.7‰ to −22.6‰, with average value of 27.2‰ (Zheng and Shangguan, 2007) and −27.1%±2.4‰ (=39; Liu., 2005). Both δ13C values of total organic carbon and long-chain-alkanesderived from terrestrial higher plants show minor variations among surface soil samples from northern China,indicating the major contributor is from local grasses with a uniform C3photosynthetic pathway (Rao., 2011). It is likely, therefore, that long-chain-alkanes in BS and NYS surface sediments are mainly derived from terrestrial higher plants, particularly C3plants.

Furthermore, recent studies have also shown thatδ13C valuesof-alkanes in gymnosperms are heavier than those in angiosperms (Diefendorf., 2011; Lane, 2017; Zhao., 2018). And angiospermδ13C values generally decrease with increasing chain length of-alkanes, while gymnosperm values increase (Bush and McInerney, 2009).It is clear here that δ13C values of long-chain-alkanes decrease with increasing chain length (Fig.3). Average δ13Cvalues of C29and C31-alkanes are −31.9%±0.6‰ and −32.1%±1.0‰, respectively, similar to values for herbaceous plants in the modern Yellow River drainage basin (−31.1‰ to −31.5‰ for C29-alkanes, and −31.3‰ to −32.6‰ for C31-alkanes in dust episode periods,Guo., 2006).This suggests that the contribution of C3angiosperms to the sedimentary long-chain-alkanes is greater. This is consistent with the predominance of angiosperms in the last glacial period and Holocene on the Chinese Loess Plateau (Li., 2016).

4.2 Mid-Chain n-Alkanes

C21, C23, and C25-alkanes are mainly contributed by aquatic plants. Previous studies have shown that theδ13C values of mid-chain-alkanes in aquatic emergent macro- phytes range from −28.6‰to −31.2‰ (Chikaraishi and Naraoka, 2003; Mead., 2005). Although non-emer- gent marine macrophytes can also produce mid-chain-alkanes, their δ13C values are relatively heavy, ranging from −13.0‰ to −22.0‰ (Ficken., 2000; Jaffé., 2001). In the NYS, there was little difference between stable carbon isotopic compositions of samples from muddy and non-muddy areas: average δ13C values of mid-chain-alkanes (C21, C23, and C25) in non-muddy areas were −29.8‰, −30.3‰, and −30.2‰, respectively, and those in muddy areas were −29.7‰, −30.3‰, and −30.2‰, respectively. This also applied to the BS, where average δ13C values were −29.7‰, −30.4‰, and −30.2‰, respectively, indicating that stable carbon isotope compositions of mid-chain-alkanes in the BS and NYS were similar. The narrow range of these values may be due to there being a common source for BS and NYS sediments, namely the Yellow River (Bi., 2010). The δ13C values of C21, C23, and C25-alkanes fell within the range of values for the corresponding-alkanes in aquatic emergent macrophytes, with sediment mid-chain-alkanes in the study area thus being mainly derived from such plants. Furthermore, the Pmar-aq index provides a measure of the relative contributions of aquatic non-emergent/emergent plants and terrestrial vegetation, with values of <0.25 corresponding to terrigenous plants, 0.3–0.6 to aqua- tic emergent plants, and >0.6 to aquatic non-emergent macrophytes in coastal marine environments (Ficken, 2000; Mead, 2005). The Pmar-aq values ranged from 0.2 to 0.7 (average = 0.4) in the study area (Fig.4c). We concluded, therefore, that mid-chain-al- kanes were mainly derived from aquatic emergent macro- phytes in the BS and NYS.

4.3 Short-Chain n-Alkanes

Short-chain-alkanes are generally considered as being derived from microorganisms and marine algae. Those produced by marine planktonic algae are mainly C15, C17, and C19-alkanes with odd carbon predominance, while even-carbon-numbered short-chain-alka- nes (C16, C18,and C20) are derived from marine bacteria or petroleum hydrocarbons (Gogou., 2000; Wang and Fingas, 2006). Short-chain-alkanes in marine sediments are predominantly C17, indicating the major contribution of algae and photosynthetic bacteria (Han and Calvin, 1969), while even-carbon-numbered (C16–22)-alkanes in marine sediments are mainly attributable to non-photo- synthetic bacteria(Jeng and Huh, 2008). Most of sediments in the BS and NYS exhibited an even-carbon- number preference in the range of-C16to-C22(Fig.3), indicating that these short-chain-alkanes could be from non-photosynthetic bacterial sources. Thevalues of- C18/-C17can be used to compare the relative contributions of-alkanes from petroleum-derived-alkanes and natural-alkanes from algae and photosynthetic bacteria. Here, the calculated-C18/-C17values of surface sediments are higher than 1 at all stations, indicating that short-chain-alkanes are affected by petroleum pollution to some degree. Extremely depleted △14C values (−932‰ to −979‰) for short-chain-alkanes (C16and C18) were found in BS and YS sediments, suggesting a predominant input from sedimentary rocks (organic carbon) or petroleum products(Tao., 2016). The average δ13C value of short-chain-alkanes, δ13C17, δ13C18, and δ13C19, is −30.1%±0.5‰, −28.7%±0.4‰, and −29.8%±0.6‰, respectively (Fig.3). Previous studies show that the δ13CC17values of cyanobacteria vary from −34.0‰ to −36.0‰ (Kristen., 2010), while δ13C17and δ13C19values of petroleum hydrocarbons are about −30.6‰ and −31.0‰, respectively (Li., 2009). The average δ13C value of algae in Laizhou Bay is −20.5‰ (Cai and Cai, 1993). Our results showed δ13C values of short-chain-alkanes were relatively lighter than those of algae, possibly due to biodegradation of bacteria and input of petroleum hydrocarbons or other sources.

5 Conclusions

The relative inputs of terrestrial and marine organic matter were assessed using-alkane. Terrigenous plants are the main source of-alkanes in BS and NYS sediments. Long-chain-alkanes in sediments were mostly derived from terrestrial sources with some contribution from biogenic and/or petroleum sources.The average δ13C values of long-chain-C27,-C29, and-C31alkanes are −30.8% ± 0.5‰, −31.9% ± 0.6‰, and −32.1% ± 1.0‰, respectively, within the range of-alkanes δ13C values of terrestrial C3plants. A hydrocarbon source distribution derived using a binary end-number mixing model based on δ13C values of long-chain-alkanes indicates that organic matter in BS and NYS sediments is mainly sourced from C3plants, particularly angiosperms. The relative contribution of C3plants decreases from estuary to ocean. δ13C values of mid-chain-alkanes in surface sediments indicate that mid-chain-alkanes are mainly of aquatic emergent macrophyte origin. δ13C17, δ13C18and δ13C19values,-C18/-C17ratios indicate that short-chain-alkanes in BS and NYS sediments have complex sources including petroleum pollution and bacterial action.

Acknowledgements

This work was financially supported by the Ministry of Science and Technology of People’s Republic of China (No. 2016YFA0600904), and the National Natural Science Foundation of China (No. 41476058).

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September 23, 2020;

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© Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2021

. E-mail: xinglei@ouc.edu.cn

(Edited by Ji Dechun)