Distribution, sources and burial fl ux of sedimentary organic matter in the East China Sea*

WU Xiaodan , , WU Bin , JIANG Mingyu , , CHANG Fengming , , , NAN Qingyun , , YU Xinke , , Saren Gaowa ,

1 CAS Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China

2 Key Laboratory of Marine Geology and Metallogeny, First Institute of Oceanography, Ministry of Natural Resources, Qingdao 266061, China.

3 Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China 4 Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China

Abstract Distribution of total organic carbon (TOC), total nitrogen (TN), stable isotope ratio of organic carbon and nitrogen (δ 13 C and δ 15 N) in 31 surface sediments were examined to quantitatively discriminate the source and burial of sedimentary organic matter (SOM) in the East China Sea (ECS). Results show that high content of TOC and TN occurred in the southern inner shelf and the northeast area due to the transport of organic matter (OM) from the Changjiang (Yangtze) River southwards by the Zhejiang Fujian Coastal Current (ZFCC) and the delivery from the old Huanghe (Yellow) River estuary by the Subei Coastal Current (SbCC), respectively. The signifi cant relationship of TOC to clay provided evidence that the fi ne-grained sediment was the primary carrier for SOM from the inner shelf to open sea. The TN varied proportionally to the TOC with the neglected intercept, suggesting that most of nitrogen measured was related to the SOM and the infl uence of the sorption of inorganic nitrogen was insignifi cant. The seaward enrichment of δ 13 C and seaward depletion of C/N and δ 15 N illustrated the decrease of terrestrial OM (TOM) and the increase of marine OM (MOM). The close of average C/N ratio to the Redfi eld ratio indicated the minor role of terrestrial plant debris in the SOM. Distribution of bulk OM properties was ascribed to the terrestrial and marine input by the shelf circulation and phytoplankton with the contributions of 76.1% MOM and 23.9% TOM to SOM, respectively. The burial fl ux of OC ranged within 0.34-7.56 mg/(cm 2·a) (averaged 2.80 mg/(cm 2·a)) and enriched along the shore and in the fi ne-grained area, manifesting the signifi cant input of the land-based sources and the eff ect of shelf mud depositional process on the fate of SOM. The sharp seaward decrease of burial fl uxes of terrestrial and marine OC gave evidences that the majority of organic carbon were settled in the inner shelf with the small fraction of them further transported eastwards.

Keyword: distribution; sedimentary organic matter; sources; burial; East China Sea (ECS)

1 INTRODUCTION

Distribution, sources, and burial fl ux of marine sedimentary organic matter (SOM) have great eff ects on the global cycling of carbon (C) and nitrogen (N) (Blair and Aller, 2012). With the large riverine input, the intensifi ed human activity and the complex hydrodynamic process, the coastal area, as the vital sink and active interface, receives ～80% of organic carbon (OC) from the land-derived materials (Berner, 1982; Bianchi and Allison, 2009; Hu et al., 2012). The SOM in the coastal region mainly originates from the river runoff , terrigenous and anthropogenic materials as the allochthonous input while the phytoplankton primary productivity as the autochthonous input (Lamb et al., 2006). Generally, stable isotope ratio of the organic carbon and nitrogen (δ13C and δ15N) and the C/N ratio in the sediment can identify the organic matter (OM) origin in the coastal environment based on the diff erence of various sources (Kubo and Kanda, 2017; Yu and Zhang, 2017). Low δ13C value (-27.94‰- -26.05‰) indicates the terrestrial input from the higher plant, high δ15N value (10‰-14‰) suggests the recent river borne particulate and the C/N ratio ranged 15.13-29.69 signals the mixed contribution of the marine OM (MOM) and the terrestrial OM (TOM) (Voβ and Struck, 1997; Kumar et al., 2017). The combined eff ect of the rapid sedimentary accumulation, the active continent-sea interaction and the large biological production complicates the provenance of OM in the coast, with the amount of OM deposited and preserved ultimately relied on the origin and bulk property of sediment (Belicka et al., 2002; O’Reilly et al., 2014; Zhang et al., 2014; Carreira et al., 2016). The TOM is mainly from the terrestrial vascular plant and transported from the continent to the ocean by the fl uvial input instead of the aeolian transport (Hedges et al., 1997). And the MOM is predominantly from the aquatic phytoplankton, zooplankton and aquatic macrophyte, with the fl ux generally dependent on the primary production.

The East China Sea (ECS) is a one of the largest river-dominated marginal sea around the globe and a vital carbon sink for terrestrial OC due to the massive input of nutrient and terrestrial material from the Changjiang (Yangtze) River and the accumulation of autochthonous and allochthonous materials by the hydrodynamic fl ow (Suk, 1989; Milliman and Syvitski, 1992). Compared with the global average burial effi ciency of riverine input (20%) and MOM (0.8%), they were much higher in the ECS, with 38% and 5.5%, respectively (Deng et al., 2006). TOM, the critical component in the mud region of the Changjiang River estuary, provides valuable insight on the distribution, abundance and burial of SOM in the dynamic environment (Zhang et al., 2007). The hydrodynamic condition substantially aff ects the redistribution of SOM in the shore of the ECS (Li et al., 2012). The eutrophication induced by the rising freshwater plankton, intensifi ed human activities and climate change signifi cantly drives the cycling of C in the Changjiang River estuary (Li et al., 2014). Previous studies focused on the qualitative identifi cation of the combined eff ect of SOM and the dynamics of the overlying water rather than comprehensive quantifi cation (Xing et al., 2011; Zhu et al., 2011; Hu et al., 2012, 2014). The occurrence, distribution and abundance of sedimentary total organic carbon (TOC), total nitrogen (TN), δ13C, δ15N, and C/N ratio in the ECS were thus comprehensively examined to better discriminate the source and the infl uence factors on the accumulation of SOM in the dynamic coastal region, to quantitatively evaluate the contribution of TOM and MOM to SOM, and to preliminarily estimate the burial fl uxes of OC in the ECS.

2 MATERIAL AND METHOD

2.1 Study area and sample collection

As one of the widest marginal sea between the Eurasia and the Pacifi c, the ECS covers an area of 0.74×106km2, with an average depth of 300 m (Wong et al., 2000). Our investigations focus on the key area of the ECS with various input rivers, complex ocean circulations and large economically developed cities (Fig.1). Cruise on R/V Kexue 3 ( Science in Chinese) was carried out in the region (26.5°N-37°N, 122.5°E-125.5°E) during 6 to 25 April 2011, with 31 stations designated for the surface sediment (Fig.1). The depth of the water varied 10-150 m (51 m in average), with an increase from the coast to the open sea. Position of sampling sites was recorded by a global positioning system (GPS). Surface sediments were collected using a box sampler and the top 2-cm sediments were subsampled into the pre-cleaned polyethylene bags. Immediately after collection, samples were refrigerated on board and stored in the dark at -20°C in the laboratory until the analysis in 2016. Data were on dry weight basis in this study.

2.2 Analytical method

Sediment grain size was measured after being treated by H2O2(10%) and HCl (1 mol/L) for wet samples by a Laser Particle Size Analyzer (Cilas 940L) in the CAS Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences. After removing carbonate with HCl (1 mol/L) for dried samples, TOC and TN were determined by Elemental Analyzer (Thermo NE1112), and δ13C and δ15N were examined using a Thermo Finnigan Delta plus AD mass spectrometer in the Key Laboratory of Submarine Geosciences of Ministry of Natural Resources, with the precision of δ13C and δ15N of 0.2‰. TOC and TN were reported in weight percentage (%), and δ13C and δ15N in standard δ-notation as per mil (‰) relative to Vienna Pee Dee Belemnite (VPDB) standard and air, respectively. The temperature and salinity of water in each station were measured or calculated by a CTD profi ler (SeaBird-911).

Fig.1 Monitoring sites, various input rivers and major hydrodynamic fl ow patterns in the ECS

3 RESULT

3.1 Hydrography

Distribution of SOM in the dynamic coastal margin is complicated under the complex hydrological and geological settings (Gordon and Goñi, 2003). The deposition, resuspension and remobilization of sediments induced by the coastal hydrodynamic conditions have great eff ects on the accumulation of SOM in the dynamic depositional environments (Hu et al., 2012). The river discharge and upwelling play key roles in the burial of SOM in the marginal seas as well (Deng et al., 2006). As the predominant source of runoff and terrestrial sediment to the ECS, the Changjiang River discharge ～5×108t terrestrial particulate matter and 12×106t OM annually (Wang et al., 2008).

The changes of riverine input, estuarine processes, currents, and shelf circulation refl ected by the precise and detailed oceanographic and geochemical observation of temperature and salinity were exhibited in Fig.2. Temperature varied within 9.51-20.6°C (13.4°C in average) in the surface water and decreased from the south to the north. The circulation of South Yellow Sea (SYS) contributed to the low temperature in the northeast. The high temperature in the south was ascribed to the incursion of Taiwan Warm Current (TWC) and Kuroshio Current (KC). The decrease of temperature in the bottom water (9.48-18.1°C, averaged 13.5°C) from the shore to the open sea suggested the dilution of riverine water to the seawater in the off -shore. In surface water, the salinity (26.3-34.5, averaged 31.9) exhibited the bidirectional extension of Changjiang Diluted Water (CDW): one fl owed toward north or northeast to the southwest of SYS and the other toward south or southeast to the ECS (Wang, 1998). Salinity in the bottom water ranged within 28.5-34.6 (averaged 32.9), with the high value reached as far as the coastal area owing to the incursion of KC.

Fig.2 Distribution of temperature and salinity in surface and bottom water of the ECS

3.2 Sediment characterization

The hydrodynamic forces infl uence the sediment sorting and accumulation of SOM in the coastal area (Hu et al., 2012). Sediment dispersal in our study was infl uenced by the circulation of the Yellow Sea Coastal Current (YSCC), the Subei Coastal Current (SbCC), the CDW, the Zhejiang Fujian Coastal Current (ZFCC), the TWC and the KC. The grain-size composition could characterize the source and transport of sediment to understand the fate of SOM (Sun et al., 2002), with the size ＜4 μm for clay, 4-63 μm for silt and ＞63 μm for sand. The percentages of clay, silt, and sand fraction varied within the range of 6.02%-43.7%, 14.8%-71.3%, and 0-76.7%, in average of 20.4%, 43.4%, and 36.2%, respectively. As displayed in Fig.3, silt and clay predominated in the coast and coarser sediment dominated in the outer shelf. Categorized by the ternary diagram (Shepard, 1954), sediments in this study were mainly composed of clayey silt and silty sand, with only a few sites composed of sandy silt and sand (Fig.4). The predominant of fi ne-grained sediment (clay+silt) formed the depositional mud areas in the inner shelf due to the delivery of SbCC from the old Huanghe (Yellow) River estuary to the northeast area and of ZFCC from the Changjiang River southward to the southern inner shelf.

3.3 Bulk OM property

Fig.3 Distribution of clay, silt and sand fractions in surface sediment of the ECS

Fig.4 Shepard triangular classifi cation of surface sediment in the ECS

Contents of TOC, TN and C/N ratios ranged within 0.14%-0.99%, 0.03%-0.15%, and 5.13-10.5 in surface sediment of the ECS, with the mean value of 0.52%, 0.08%, and 7.92, respectively. Values of δ13C in present study ranged from -22.71‰ to -20.26‰ (averaged of -21.43‰), which was close to that in the ECS Shelf (-22.08‰- -19.99‰, Zhou et al., 2018), the coastal area of the ECS (-22.7‰- -20.6‰, Zhang et al., 2017) and the Sishili Bay (-22.7‰- -21.6‰, Liu et al., 2012), but higher than that in the Changjiang River estuary (-29.8‰- -23.7‰, Liu et al., 2004) and the Zhujiang (Pearl) River estuary (-25.19‰- -23.33‰, Zhang et al., 2009). The δ15N values in the present study varied between 2.77‰ and 4.84‰ (averaged 3.84‰), which was lower than that in the ECS Shelf (3.67‰-6.28‰, Zhou et al., 2018), the Zhujiang River estuary (3.73‰-6.57‰, Zhang et al., 2009) and the Sishili Bay (5.4‰-6.5‰, Liu et al., 2012). The molar C/N ratio in this study fl uctuated from 5.13 to 10.5, which was comparable to that in other riverdominated depositional regions around the world including the ECS Shelf (4.69-9.12, Zhou et al., 2018), the Bohai Bay (3.3-7.7, Hu et al., 2009), the Zhujiang River estuary (8.1-17.2, Zhang et al., 2009) and the Sishili Bay (7.9-10.1, Liu et al., 2012).

As Fig.5 displayed, content of TOC (0.14%-0.99%) increased southward and corresponded to the trend of fi ne-grained constituents, with the correlation coeffi cient of 0.92 to clay ( P ＜0.01), suggesting the potential infl uence of hydrodynamic forces on the accumulation of SOM and the sediment sorting in the coastal ECS. The TN concentration (0.03%-0.15%) was proportionally to the TOC ( R2=0.943 2; P ＜0.01), with the higher values occurred in the mud patches. Previous investigation also pointed out that the close relationship of SOM to the fi ne-grained sediments indicated the possible delivery from the inner shelf to the southern Okinawa Trough (Kao et al., 2003). In addition, it was reported that the TOC contents depleted in the maximum turbidity zone of the Changjiang River estuary where the burial of OC in sediment fraction of ＜32 μm was relatively low (Shi et al., 2016).

4 DISCUSSION

4.1 Source identifi cation

Fig.6 Scatter plot of TOC and TN in the surface sediment of ECS

The molar ratio of C/N was widely employed as a general proxy for OM sources, with the criteria 5-7 for the MOM (Redfi eld et al., 1963), ＞15 for the TOM (Meyers, 1997), 20-500 in the high plant tissues, 4-7 in the plankton organisms and 4.1-4.2 in the bacteria (Pereira et al., 1996; Hedges et al., 1997; Müller and Mathesius, 1999). The TOM from the Changjiang River was mainly from the soil with ratio of 10-15, and while the phytoplankton was the major source of MOM with ratio of 5-8 (Zhang et al., 2007). The C/N ratio in our study (5.13-10.5) indicated the predominant role of marine origin for SOM and the minor eff ect of debris of terrestrial plant on SOM in the ECS compared with the Redfi eld ratio 5.7 (wt/wt) of marine phytoplankton. The regression line between TOC and TN with the small negative intercept (Fig.6) suggested that most of the nitrogen measured, probably in the organic form, was related to the SOM and the infl uence of the sorption of inorganic nitrogen on organic matter was negligible (Goñi et al., 2003). The higher value of the C/N ratios occurred in the Changjiang River estuary and inner shelf along ZFCC, with the decrease seaward suggested the enrichment of MOM and the relative depletion of TOM. In addition, the low C/N ratios are owing to the contribution of microorganism-derived and soilderived OM, which are widely observed in the coastal settings with signifi cant anthropogenic inputs and intensifi ed resuspension of fi ne-grained sediments (Hu et al., 2012). Therefore, hydrological forces have great eff ect on the elemental signature of SOM besides the act on the accumulation of SOM in this region.

As the reliable proxy to distinguish the terrestrial and marine sources of SOM, the relative contribution of TOM and MOM in the marine sediment was traditionally estimated by the δ13C value, with -33‰- -22‰ (-27‰ in average), -16‰- -9‰ (-13‰ in average), -22‰- -18‰, -21‰- -19‰, and -27‰- -25‰ ranged for the terrigenous C3 plant, the terrigenous C4 plant (Pancost and Boot, 2004), the MOM (Cifuentes and Eldridge, 1998), the phytoplankton of the ECS and Changjiang River (Wu et al., 2003, 2007; Zhang et al., 2007), respectively. In this study, the bulk sediment δ13C (pre-aged with Δ14C =-305‰± 102‰ for inner shelf of the ECS; Bao et al., 2016) is around -22.71‰- -20.26‰ (-21.43‰ in average), which is more depleted than, or approximate to the values of MOM. Values of δ13C in the present study were within the ones for the TOM from the Changjiang River and the phytoplankton from the ECS, indicating the mixture of terrestrial and marine sources for SOM. Further, the value is similar to that in the ECS continental shelf (-22.7‰- -20.1‰) (Xing et al., 2011), but higher than that for the suspended OM (-25.4‰- -19.7‰) in the Changjiang River estuary (Tan et al., 1991). As reported, the modern TOM discharged by the Changjiang River into the coastal ECS is mainly derived from the C3 land vascular higher plants (Hu et al., 2012). The depletion of δ13C in the estuary and the inner shelf along the Zhejiang-Fujian coast suggested the signifi cant terrestrial input. Similar to the distribution pattern reported by the previous study (Hu et al., 2012), δ13C generally displayed a seaward increase, with the value as high as that in the phytoplankton of the ECS in the middle-outer shelf, refl ecting the seaward increasing fraction of the MOM relative to the TOM. Therefore, the marine autogenic source was regarded as the predominant source of SOM in this study except the infl uence of the terrigenous input in the nearshore areas.

The change of δ15N during OM decomposition process limited the use as the single evidence to discriminate the provenance compared to the relatively stable of δ13C (Thornton and McManus, 1994; Ramaswamy et al., 2008; Gireeshkumar et al., 2013). Sedimentary δ15N has been thus used as the additional evidence to further constrain the SOM sources, with 3‰-12‰ for MOM and 8.6‰ for marine phytoplankton (Lamb et al., 2006), while -10‰-10‰ for TOM with an average of 2‰ (Dean et al., 1986). The δ15N of OM from nitrogen fi xing land plant was around zero, whereas a positive value (0.4‰) for terrestrial C3 plant was found in plant only using mineral N from soil NO3ˉ or NH4+(Ramaswamy et al., 2008). The δ15N in present study (2.77‰-4.84‰, in average of 3.84‰) was within the range of the value in marine phytoplankton and TOM and comparable to the average value of marine nitrate (Sigman et al., 2000), suggesting the mixed contribution of terrestrial and marine sources of the SOM. The depletion of δ15N in the Hangzhou Bay and southern inner shelf refl ected the dominant terrestrial input from the Changjiang River and the transport of the SOM by the coastal current from the estuary to the inner shelf along the ZFCC. The enriched δ15N in the Changjiang River estuary was ascribed to the incomplete utilization of nitrate in the river due to the light limitation in the turbid water. As reported, the δ15N was generally enriched in the oxic sediments at the sediment-water interface during sinking and residence in the early diagenesis (Altabet et al., 1999; Sachs et al., 1999).

Fig.7 Dual plots and potential sources of OM in the ECS based on C/N-δ 13 C and δ 13 C-δ 15 N compositions

4.2 Quantitative evaluation of OM sources

To evaluate the contribution of the TOM and the MOM in the ECS, C/N-δ13C and δ13C-δ15N in the surface sediment of the ECS, the particulate matter and the phytoplankton in the Changjiang River estuary as the terrestrial and marine end-members, respectively, were listed in Fig.7. In dual plots, C/N, δ13C, and δ15N in the terrestrial sources were defi ned as 10-14, -27‰- -25‰, 3.5‰-6.5‰ and in the marine source as 5-8, -21‰- -19‰, 4.5‰-7.5‰ (Wu et al., 2007; Zhang et al., 2007). The scattered distribution of measured values between the terrestrial and marine sources confi rmed the mixing of TOM and MOM for SOM of the ECS, with the values closer to marine sources, especially three stations completely falling into the ranges of marine source. The δ13C values with little TOM characteristic suggest the obvious addition of MOM even that the majority sediments are from the Changjiang River in the inner shelf.

Considering δ13C as the source tracer, a two endmember mixing model was used to examine the relative importance of TOM and MOM to the SOM in the ECS based on the following equation (Westerhausen et al., 1993; Eadie et al., 1994):

where x and (1- x) represent the fraction for the terrestrial and marine end-member, respectively. Cmeasured, Cterrestrial, and Cmarinerepresent δ13C value in sample, TOM (-26‰) and MOM (-20‰) (Duan et al., 2017). Results showed that SOM of the ECS was a mixture of terrestrial and marine inputs with marine source as the dominance contributor. The contribution of TOM accounted for 4.3%-45.2% (23.9% on average) of the total OM and decreased seaward, with higher proportions found in the mud areas, especially in the Changjiang River estuary and the southern inner shelf as a consequence of the rapid burial of fl uvial carbon (Fig.8). The percentage of MOM ranged from 54.8% to 95.7% (76.1% on average), which was comparable to the estimation in the previous study (65.4%-100%; Zhou et al., 2018). The relative importance of MOM in the middle-outer shelf confi rmed the predominant contribution of MOM. The dynamics of the overlying water can aff ect the OM provenance in the surface sediment. As Fig.5 demonstrated, the pathway of open seawater to the northern part of ECS coincided with the obvious tongue-shaped with relatively low percentage of TOM and the intrusion of KC to the east of Changjiang River estuary detailed the infl uence of Kuroshio water on the distribution of OM (Wang and Oey, 2016). The OM from the marine phytoplankton could be partly decomposed in the surface sediment, but it will increase owing to the higher accumulation rate than the decomposition rate in high sedimentation environment (Andrieux and Aminot, 1997; Rabouille et al., 2008).

4.3 Preliminary estimation of burial fl uxes of OC

The burial fl ux (BF, mg/(cm2·a)) of the organic components in the surface sediments of the ECS was calculated by the following formula (Fang et al., 2015; Wang et al., 2017):

BF=10× CTOC× ρ× ω×(1- φ),

Fig.8 Distribution of the contribution of TOM and MOM in the ECS

Fig.9 Distribution of sedimentation rate and the burial fl ux of TOC, TOM, and MOM in the ECS

where CTOCis the measured content of sedimentary TOC(%), ρ, ω, and φ represent the dry density of the sediment samples (g/cm3), the sedimentation rate (cm/a), and the sediment porosity (dimensionless), respectively. The value of the porosity ( φ) was denoted as 0.75, the median within the range of 0.7-0.8 measured in the sediment mixing layers around the world (Ali et al., 2014). Value of 1.2 g/cm3was used for ρ (Liu et al., 2007). The spatial distribution of ω (Fig.9) was obtained by the Kriging interpolation method based on the accurately dated values collected in previous studies (DeMaster et al., 1985; Alexander et al., 1991; Chung and Chang, 1995; Huh and Su, 1999; Xia et al., 1999, 2004; Wu et al., 2001; Oguri et al., 2003; Guo et al., 2006; Lim et al., 2007; Wei et al., 2007; Yang and Chen, 2007; Duan et al., 2013; Fan et al., 2014).

The burial fl ux of OC in the ECS varied from 0.34 mg/(cm2·a) to 7.56 mg/(cm2·a) and averaged 2.80 mg/(cm2·a), with obvious spatial heterogeneity (Fig.9). The higher burial fl uxes were found in the estuary and the inner shelf and it decreased seaward with the increasing distance from the shore due to the signifi cant input of the land-based sources, as the distribution of the sedimentation rates clearly evidenced (Fig.9). Similar to the variation of the TOC content, high burial fl ux was observed in the fi negrained mud areas as well, manifesting the eff ect of shelf mud depositional process on the fate of SOM. The terrestrial sediment mainly deposited in the Changjiang River estuary, and the Zhejiang-Fujian coast and the mud region. Considering the quantitative contributions of MOM (76.1% on average) and TOM (23.9% on average) to the SOM, the burial fl uxes of terrestrial and marine OC estimated in our study ranged 0.27-5.78 mg/(cm2·a) (2.01 mg/(cm2·a) on average) and 0.05-3.13 mg/(cm2·a) (0.79 mg/(cm2·a) on average), respectively. The sharp decrease of the burial fl ux of OC from the inner to the outer shelf indicated that the majority of the sedimentary OC were deposited in the inner shelf, with only a minor fraction moving eastwards.

5 CONCLUSION

The mixed origins, hydrodynamic condition, and depositional processes of the fi ne-grained sediment aff ect the accumulation and the transport of the SOM from the inner shelf to the open sea. The relationship between TOC and TN suggested their co-variation during the investigation period and the affi nity of nitrogen measured to SOM. The enrichment of δ13C and depletion of C/N and δ15N seaward illustrated the increase of MOM and the relative decrease of TOM. Moreover, the presence of microorganism-derived OM and anthropogenic inputs could also be responsible for the low C/N ratios and enriched δ13C in the coastal region. Terrestrial plant debris played a minor role in the SOM based on the close of average C/N ratio to the Redfi eld ratio of marine phytoplankton. The two end-member mixing model evaluated quantitatively the contribution for the SOM with 54.8%-95.7% (averaged 76.1%) of the MOM and 4.3%-45.2% (averaged 23.9%) of the TOM. The enrichment of the burial fl ux of OC in the fi ne-grained mud areas manifested the eff ect of shelf mud depositional process on the fate of SOM and the depletion seaward further indicated the signifi cant input of the land-based sources. The sharp decrease of the burial fl uxes of terrestrial and marine OC toward sea confi rmed the majority of organic carbon were settled in the inner shelf and only a minor portion were further transported eastwards.

6 DATA AVAILABILITY STATEMENT

The data that support the fi ndings of this study are available from the corresponding author upon reasonable request.

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