Global Biogeochemical Cycles
RESEARCH ARTICLE
10.1029/2018GB005921
Key Points:• A survey of oceanographic
conditions, as well as physical andgeochemical properties ofcontinental margin surface
sediments, is presented• Hydrodynamic processes on shallow
continental shelves emerge as an
14
important control on the C
contents of OC• General characteristics of OC in
global active and passive continental
margin sediments are shown

Supporting Information:• Supporting Information S1• Table S1• Data Set S1Correspondence to:
R. Bao,rui_bao@fas.harvard.edu

Citation:
Bao, R., van der Voort, T. S., Zhao, M.,
Guo, X., Montluçon, D. B., McIntyre, C., &Eglinton, T. I. (2018). Inﬂuence ofhydrodynamic processes on the fate of
sedimentary organic matter oncontinental margins. Global
Biogeochemical Cycles, 32. https://doi.
org/10.1029/2018GB005921
Received 6 MAR 2018
Accepted 4 SEP 2018
Accepted article online 20 SEP 2018

Inﬂuence of Hydrodynamic Processes on theFate of Sedimentary Organic Matteron Continental Margins
Rui Bao1,2 , Tessa S. van der Voort1, Meixun Zhao3,4
Cameron McIntyre1,6,7 , and Timothy I. Eglinton1, Xinyu Guo5, Daniel B. Montluçon1,

1

Geological Institute, ETH Zurich, Zurich, Switzerland, 2Now at Department of Earth and Planetary Sciences, Harvard
University, Cambridge, MA, USA, 3Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory
for Marine Science and Technology, Qingdao, China, 4Key Laboratory of Marine Chemistry Theory and Technology (Ocean
University of China), Ministry of Education, Qingdao, China, 5Center for Marine Environmental Studies, Ehime University,
Matsuyama, Japan, 6Laboratory for Ion Beam Physics, Department of Physics, ETH Zurich, Zurich, Switzerland, 7Scottish
Universities Environmental Research Centre, Glasgow, UK

Abstract Understanding the effects of hydrodynamic forcing on organic matter (OM) composition isimportant for assessment of organic carbon (OC) burial in marginal seas on regional and global scales.
Here we examine the relationships between regional oceanographic conditions (bottom shear stress), andthe physical characteristics (mineral surface area and grain size) and geochemical properties (OC content[OC%] and carbon isotope compositions [13C, 14C]) of a large suite of surface sediments from the Chinese
marginal seas to assess the inﬂuence of hydrodynamic processes on the fate of OM on shallow continental
shelves. Our results suggest that 14C content is primarily controlled by organo-mineral interactions andhydrodynamically driven resuspension processes, highlighted by (i) positive correlations between 14C
content and OC% (and surface area) and (ii) negative correlations between 14C content and grain size (andbottom shear stress). Hydrodynamic processes inﬂuence 14C content due to both OC aging during lateraltransport and accompanying selective degradation of OM associated with sediment (re) mobilization, theseeffects being superimposed on the original 14C characteristics of carbon source. Our observations support the
hypotheses of Blair and Aller (2012, https://doi.org/10.1146/annurev-marine-120709-142717) and Leithold
et al. (2016, https://doi.org/10.1016/j.earscirev.2015.10.011) that hydrodynamically driven sedimenttranslocation results in greater OC 14C depletion in broad, shallow marginal seas common to passive margin
settings than on active margins. On a global scale, this may inﬂuence the extent to which continental
margins act as net carbon sources and sinks. Our ﬁndings thus suggest that hydrodynamic processes areimportant in shaping the nature, dynamics, and magnitude of OC export and burial in passive marginal seas.

1. Introduction©2018. The Authors.
This is an open access article under theterms of the Creative Commons
Attribution-NonCommercial-NoDerivs
License, which permits use and distribution in any medium, provided theoriginal work is properly cited, the use isnon-commercial and no modiﬁcations
or adaptations are made.

BAO ET AL.

Understanding the fate of organic matter (OM) deposited in continental margin sediments is critical for
constraining carbon cycle models, including carbon exchange between terrestrial, oceanic, and atmosphericcarbon reservoirs (Bauer et al., 2013; Bianchi et al., 2018; Bianchi & Allison, 2009; Hedges & Keil, 1995). Largeamounts of marine and terrestrial organic carbon (OC) are produced in and delivered to river-dominatedmarginal seas (Bianchi et al., 2018; Blair & Aller, 2012; Hedges & Keil, 1995). Preservation of OM in continental
margin sediments is mainly attributed to physical protection via association with minerals (Arnarson & Keil,2007; Burdige, 2005, 2007; Mayer, 1994). Marginal sea systems are highly dynamic and heterogeneous, withspatially diverse sediment transport processes as well as OC inputs that inﬂuence the distribution and composition of sedimentary OM (Bao et al., 2016; Bianchi et al., 2018). Signiﬁcant gaps remain in our understanding of relationships between transport processes and OM characteristics; spatially comprehensive
investigations are needed to deconvolve the intertwined inﬂuences of carbon sources and transport
processes on the composition and distribution of OM accumulating in continental shelf sediments (e.g.,van der Voort et al., 2018).
Surface sediments of river-dominated margins typically exhibit signiﬁcant variability in OC radiocarbon(14COC) content (Bao et al., 2016; Bianchi et al., 2016; Canuel & Hardison, 2016; Grifﬁth et al., 2010;
Wakeham & McNichol, 2014). While this variability can be partly attributed to variations in OC sources,

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Figure 1. (a) The Chinese Marginal Sea (CMS) study area. The white arrows indicate prevailing current directions (modiﬁed
from Chen, 2009): the Chinese Coastal Current (CCC), the Yellow Sea Warm Current (YWC), the Kuroshio Current (KC), Yangtze(Changjiang) Diluted Water (CDW), and the Taiwan Warm Current (TWC). (b) Subregions of the CMS (delineated by thewhite dashed line): The Yellow Sea-Bohai Sea (BS-YS) and the East China Sea (ECS) as deﬁned by (Harris et al., 2014; Saito et al.,
1998; Lie & Cho, 2016). Bubble sizes represent the approximate water depth at each sample location (larger size meaning
greater water depth; minimum and maximum bubble sizes corresponding to approximately 20 and 1,000 m, respectively).

there is evidence that hydrodynamic processes also inﬂuence 14COC content of marine sediments (Bao et al.,2016, 2018; Bröder et al., 2018; Cathalot et al., 2013; Inthorn et al., 2006; Keil et al., 2004; Kusch, Eglinton, et al.,2010; Kusch, Kashiyama, et al., 2010; Ohkouchi et al., 2002; Pedrosa-Pàmies et al., 2013; Mollenhauer & Eglinton,
2007; Mollenhauer et al., 2003, 2005, 2006, 2007, 2008; Shah et al., 2008; Tesi et al., 2008, 2010, 2014, 2016;
Table S1). In addition to inducing widespread dispersal of sediments, hydrodynamic processes also affectthe extent of degradation and compositional characteristics of associated OM (Bao et al., 2016, 2018; Bröder
et al., 2018; Pedrosa-Pàmies et al., 2013; Thomsen & Gust, 2000). For instance, the sortable silt component of
sediment that is most prone to resuspension and redistribution exhibits relatively low OC loadings (Baoet al., 2016), likely due to enhanced degradation and preferential removal of labile OC during entrainment
in deposition-resuspension loops (Aller & Blair, 2004, 2006; Bröder et al., 2018; Keil et al., 2004). These
processes are considered to alter both stable carbon (13COC) and 14COC compositions of residual OM (Baoet al., 2016). However, incomplete knowledge of the interplay between hydrodynamic processes, OMmineral associations, selective protection, and attenuation or modiﬁcation of OM during sedimenttranslocation presently limits our understanding of carbon cycling on continental shelves.
In this study, we have undertaken an extensive survey of mineral-speciﬁc surface area (SA), mean grain size,
and the OC content (OC%) and 13COC, 14COC in surface sediments from an large passive marginal sea system:
the Chinese marginal seas (abbreviation: CMS), encompassing the Bohai Sea (BS), the Yellow Sea (YS), and theEast China Sea (ECS; Figure 1). Spatial variations in these sedimentological and geochemical properties are
examined within a hydrodynamic context (bottom shear stress). Broader, global-scale implications are discussed in the context of carbon cycling on passive and active continental margins.

2. Sampling and Methods
2.1. Study Area and Sampling
The CMS, as a passive marginal sea system, constitutes the interface between the Eurasian continent and
Paciﬁc Ocean. The CMS receives vast quantities of terrestrial material primarily from the Yellow River(Huang He) and Yangtze (Changjiang) River that originate in the highlands of the Tibetan Plateau and integrate materials emanating from their extensive drainage basins. The Yellow River alone exports ~150 Mt/

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year sediment to the BS (Wang et al., 2010, 2011). While the coastal area adjacent to the Yellow River delta
acts as a sediment sink in summer, these sediments are subject to subsequent remobilization triggered by
winter storms (Yang et al., 2011) and are exported to the YS through the seasonal currents (e.g., Chinese
Coastal Current [CCC]; Yang & Liu, 2007; Figure 1a). About 30% of the Yellow River sediment is estimatedto reach the central YS (Liu et al., 2004; Yang & Liu, 2007). The Yangtze River discharges ~110 Mt/year sediment into the ECS (Yang et al., 2014, 2015). Although a large fraction of Yangtze-derived sediments is deposited proximal to the river mouth, signiﬁcant amounts of material are dispersed by seasonal shelf currents(Gao & Collins, 2014). In winter, Yangtze-derived, ﬁne-grained sediments are carried southward by an intensiﬁed CCC that transports materials parallel to the coastline. Due to these prevailing southward ﬂowing
coastal currents, an elongated distal subaqueous mud wedge overwhelmingly derived from Yangtze Rivermaterials has developed on the inner shelf of the ECS. In spring, Changjiang Diluted Water strengthens
and turns eastward, while the CCC in the ECS weakens and the Taiwan Warm Current begins to intensify.
In summer, Taiwan-derived sediments are delivered westward, while the Changjiang Diluted Water transports ﬂuvially derived sediment eastward across the ECS (Figure 1a). In fall, the Yellow Sea Warm Current thatoriginates from the Kuroshio Current dominates the circulation from the ECS to the YS.
These seasonal currents play a crucial role in the mobilization, transport, and dispersal of ﬂuvially derived
sediments and associated OM of both terrestrial and marine origin (Liu et al., 2007; Yang & Liu, 2007; Yao
et al., 2014, 2015), with broader-scale hydrodynamic processes likely driven by interactions between both
local and regional current systems (Chen, 2009; Figure 1a). These spatially and temporally complex hydrodynamic conditions can lead to widespread dispersal of suspended sediment and associated OM in the CMS. In
order to develop regional-scale assessments of OC fate inﬂuenced by hydrodynamic processes and keep a
uniform regional delineation for comparative analysis with prior studies, we separate the CMS into the BSYS and ECS, delineated by a line between Shanghai and Jeju Island (Figure 1b), according to prior studies
of Saito et al. (1998), Harris et al. (2014), and Lie and Cho (2016).
Surface sediment (0–2 cm) samples (n = 270) were collected in 2011, 2013 on the R/V Dongfanghong II, and in2014 on the R/V Yanping II. The samples were retrieved by box-corer or grab sampler and stored at 20 °C
prior to analysis. Combined with published results, the database including oceanographic conditions, physical characteristics, geochemical properties, and carbon isotope compositions of surface sediment samples in
the CMS is shown in Figure 2 and Table S2 (n = 358).
2.2. Mineral-Speciﬁc Surface Area Analysis
After freeze-drying, aliquots of a subset of sediment samples (n = 183) were heated at 350 °C for 24 hr in orderto remove OM (Mayer, 1994), outgassed at 350 °C under vacuum for 2 hr to ensure complete removal ofmoisture, and then measured using a 5-point Brunauer-Emmett-Teller (BET) method on a NOVA 4000
(Quantachrome Instruments) in order to determine mineral-speciﬁc SA (Keil et al., 1997; Mayer, 1994; Taoet al., 2015). Based on replicate measurements (n = 4) of one standard sample, we estimate a standard errorof ± ~2–3%.
2.3. Bulk Organic Carbon Isotopic Analysis
Aliquots of freeze-dried bulk sediment samples (n = 270) were analyzed for stable carbon isotope composition of bulk OM (δ13COC) on a Vario MICRO cube elemental analyzer (Elementar Analysensysteme GmbH)coupled to a VisION stable isotope mass spectrometer (Isoprime Ltd, UK) that was calibrated against standard
materials (Laboratory of Ion Bean Physics, ETH Zurich). Prior to analysis, inorganic carbon was removed by
fumigation in the presence of HCl (37%, 72 hr) and drying over NaOH pellets (72 hr) in a desiccator at
60 °C (Bao et al., 2016; Tao et al., 2015). Resulting δ13COC values were measured to a precision of less than
0.1‰, based on standards.
Methods and results from 14C analysis of bulk OC in sediment samples have been reported previously(n = 358; Bao et al., 2016). Sampling dates are shown in Table S2.
2.4. Bottom Shear StressResuspension phenomena are pervasive in shallow marginal seas under conditions when bottom shear stressexceeds critical threshold for initiation of surface sediment motion (McCave, 1986; McCave & Hall, 2006). In
this context, the magnitude of bottom shear stress is an important consideration in sediment mobilization

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Figure 2. Spatial variability in (a) radiocarbon content of bulk OM (Δ COC; Bao et al., 2016), (b) stable carbon isotope composition of bulk OM (δ COC), (c) bulk OC22
2
content (OC%; Bao et al., 2016), (d) mineral-speciﬁc surface area (SA; m /g), (e) organic carbon loading (OC:SA, mg C/m ), (f) bottom shear stress (N/m ), (g) mean
grain size (μm; Bao et al., 2016), and (h) sedimentation rate (Qiao et al., 2017, reference therein).

and dispersal. Spatial variability in shear stress in the CMS is poorly constrained and certainly varies
temporally. Nonetheless, some insights can be gleaned from tidal currents.
Based on established relationships between bottom current velocity (shear stress), sediment grain size, andsediment resuspension (McCave, 1986; Thomsen & Gust, 2000), there is ample potential for sediment mobilization and dispersal over this large continental shelf system (Milliman et al., 1985). To understand thesepotential inﬂuences on observed spatial variations in surface sediment properties, we calculated the bottom
stress (τ!
b ) at our sampling sites using a hydrodynamic model (Wang et al., 2008) as follows:
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
τ xb ¼ ρC d u u2 þ v 2
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
τ yb ¼ ρC d v u2 þ v 2

(1)
(2)

where τ xb and τ yb are the eastward and northward components of bottom shear stress, u and v are theeastward and northward component of bottom currents, ρ is water density, and Cd is the bottom drag coefﬁcient. The bottom current is composed of tidal components and a subtidal component, in which the tidal currents are a superposition of several tidal constituents (M2, S2, K1, and O1) with each tidal constituent having aﬁxed period. In the BS, YS, and ECS (<200-m water depth), the amplitude of the M2 tidal current (principal
lunar semidiurnal and the strongest tidal constituent) is over 0.5 m/s (Guo & Yanagi, 1998), whereas

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Table 114
Signiﬁcant Spearman Correlations Between Δ C and Various Parameters in
the CMS

subtidal currents have a magnitude of 0.2 m/s or less (Lie & Cho, 2016). We
thus argue that the former is the major contributor to bottom shear stress.
A harmonic analysis program (T-tide; Pawlowicz et al., 2002) is then used to
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Parameters
Δ C (‰)
obtain the amplitude and phase of tidal current and bottom shear stresswith the periods encompassing all tidal constituents at each of the grid
Water depth (m)20.29 (p < 0.0005)
SA (m /g)points in our hydrodynamic model. We use a grid size of 1/18 degree in
TOC (%)
0.32 (p < 0.0005)
meridional and zonal directions and 20 layers in the vertical direction
2
OC/SA (C mg/m )(Wang et al., 2008). Among tidal constituents used in the model (Wang13δ C (‰)
et al., 2008), M2 tidal current is the strongest with a magnitude at least
Mean grain size (μm)
0.19 (p < 0.05)
2
twice that of other tidal constituents (Guo & Yanagi, 1998; Lie & Cho,
Bottom shear stress (N/m )0.46 (p < 0.0005)
2016). Consequently, the bottom shear stress associated with M2 tidal perNote: Hyphen indicates nonsigniﬁcant correlation.
iod is at least 4 times stronger than that related to other tidal constituent
periods because the bottom shear stress is proportion to square of bottom
current (equations (1) and (2)). As an approximation, we therefore use theamplitude of bottom shear stress with M2 tidal period to represent the amplitude of bottom shear stress in
the CMS (Figure 2f). Wave-induced bottom shear stress is over one magnitude smaller than the tide-induced
bottom shear stress (Luo et al., 2017) and is therefore not considered.
2.5. Statistical Analysis
In order to evaluate and quantify the signiﬁcance of correlations between various key parameters related tohydrodynamic processes (e.g., water depth and bottom shear stress) and geochemical characteristics (e.g.,
OC% Δ14COC), Spearman correlations were determined (signiﬁcant for p < 0.05) for the subset of CMS samples (n = 194), which was determined for all parameters (Table 1). The latter method was chosen to accountfor nonparametric nature of the data. The statistical analyses were applied using the open-source statisticalsoftware, “R,” version 3.2.3.

3. Results and Discussion
3.1. OC Characteristics of Surface Sediments in the BS-YS and ECS
Spatial variations in Δ14COC for bulk sediments from the CMS were previously described by Bao et al. (2016).
The measured Δ14COC values exhibit a general decrease with decreasing OC% (Figure 3). The lower Δ14COC
values in BS-YS samples may be due to prolonged resuspension andreworking of sediments on the inner shelf (Bao et al., 2016, 2018), as wellas inputs of preaged OC from the Yellow River (Tao et al., 2015; Wanget al., 2012). While some lower-OC% and 14C-depleted samples from the
ECS are from shallow regions, a substantial proportion (≳35%) of samples
with the low Δ14COC values (i.e., Δ14COC less than ~ 400‰) originate
from deeper regions (Figure 3). The latter observation might be explainedby export and supply of preaged OM from small rivers draining the
Taiwan Island (Hilton et al., 2011; Kao et al., 2014), while the lower
Δ14COC values from shallower, inner-shelf regions may reﬂect the supplyof 14C-depleted OM from the Yangtze River and subsequent sedimentremobilization processes (Bao et al., 2016; Li et al., 2012; Wu et al.,2013). Provenance analyses based on clay mineralogy indicate that the
Yangtze River is the predominant source for ﬁne-grained sediments inshallower regions of the ECS (Qiao et al., 2017; Xu et al., 2009), and it istherefore unlikely that preaged OM from Taiwanese rivers is the primary
driver of the depleted 14COC in shallow ECS regions. Furthermore, vander Voort et al. (2018) found that Taiwan acts as a point source with a primarily local impact. In addition, sedimentation rates along the coast of the
14
Figure 3. Relationship between Δ C values and OC% in the CMS surface ECS are high (>5 cm/year; Figure 2h), and given this as well as sedimentsediments. Bubble sizes correspond to approximate water depth (ranging
mixed layer depths on the order of ~10 cm (based on 210Pb analysis; Su &between approximately 20 and 1,000 m for smallest and largest symbols).
Huh, 2002; Liu et al., 2006)—which are signiﬁcantly greater than that of
The two dashed circles in the right panel correspond to marine sediments
the sampling interval (0–2 cm)—upward mixing of older OC due tosurround Taiwan Island.

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bioturbation is unlikely to account for the low Δ14COC values observed.
We also note that many low-OC% samples exhibit a range of Δ14COC
values (Figure 3, dashed square). Therefore, while the Δ14COC of sedimentary OM in the CMS exhibits signiﬁcant positive correlation with OC%(0.32, p < 0.0005; Table 1), our ﬁndings suggest that a complex suite ofprocesses underpins this relationship and hence inﬂuences the fate of
OC in this shallow marginal sea system.
The δ13COC values of surface sediments in the CMS exhibit large spatial
variability, ranging from 25.9 to 19.1‰ (Figure 2b; n = 358). In general, depleted δ13COC values are found in the prodelta regions, along
the coast (inner shelf) of the CMS, Northern YS, and adjacent to Taiwan,
whereas relatively high δ13COC values are evident on the outer-shelfand slope of the ECS extending to the Okinawa Trough (Figure 2b).
These δ13COC values reﬂect contributions from different sources of sedimentary OC (Blair & Aller, 2012), with the observed spatial pattern ofδ13COC values suggesting that point sources (i.e., OC discharge from rivers) may have regional impacts. In the BS-YS, δ13COC values generally fallwithin a narrower range, from ~ 23 to 21‰ (ave. 22.1 ± 0.6‰,
n = 137), lying between values for corresponding marine and ﬂuvial(Yellow River) end-members (Figure 4a). Despite the likely inﬂuence of
aging and/or selective degradation of OM in the BS-YS during sedimentredistribution (Bao et al., 2016, 2018; Tao et al., 2016), these processes
do not clearly manifest themselves in a Δ14COC-δ13COC scatter plot
(Figure 4a). This may be due to the replacement of terrestrial OM by
13
C-enriched (Keil et al., 1997) and younger (higher Δ14COC values) marine OM (Blair & Aller, 2012).

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Figure 4. Bulk Δ COC versus δ COC values of samples from the (a) Bohai
Sea-Yellow Sea (BS-YS) and (b) East China Sea (ECS). Bubble sizes correspond to approximate water depth. The dashed boxes indicate approximate
14
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ranges of Δ COC and δ COC values of marine OM (Blair & Aller, 2012; Wanget al., 2016) and of Yellow and Yangtze River suspended particle organic
matter (SPM; Marwick et al., 2015; Tao et al., 2015; Wang et al., 2012). The datafrom Wu et al. (2013) and Kao et al. (2014) (solid symbols) are plotted withoutwater depth information in the lower panel.

In contrast to the BS-YS system, Δ14COC-δ13COC values of ECS sediments falloutside of a simple mixing between two end-members of marine and terrestrial OM (Figure 4b). In this regime, the Yangtze River is the dominant
sediment source (Qiao et al., 2017), with mean annual sediment discharge
for the other smaller rivers contributing <25% (~1.0 × 105 Mt) of that of
the Yangtze (Qiao et al., 2017). Based on the studies of Wang et al.
(2012), the Δ14COC values of suspended particulate matter (SPM) in the
Yangtze River are relatively high (~ 110‰) compared to those of the
Yellow River. Moreover, ECS surface sediments exhibits a larger variationof 14C contents compared with the BS-YS (Figure 4). Again, while
Taiwanese rivers may supply preaged OC to the ECS, the sediment loadfrom western Taiwan is much smaller (<16%) than that of the Yangtze
River (Qiao et al., 2017). Additionally, the bulk sediment emanating from
Taiwan ﬂuvial systems empties into the Taiwan Strait or southern and eastern trough areas (Kao et al., 2014; Liu et al., 2013; Xu et al., 2009) and istherefore unlikely to contribute signiﬁcantly to the variability in Δ14COC
values of surface sediments across the entire ECS (Figure 4b).

Recently, Bao et al. (2016, 2018) provided evidence for OM aging during the lateral sediment redistribution in
the CMS. Both 14C decay associated with protracted entrainment in repeated resuspension-deposition loops
on the shallow inner shelf (Zhu et al., 2006) and removal of labile (i.e., 14C- and 13C-enriched) OM could resultin lower Δ14COC (δ13COC) values (Bao et al., 2016; Mollenhauer & Eglinton, 2007). Samples with lower δ13C
values in Figure 2b (blue regions in the ECS) are also primarily sediments from the inner shelf, suggesting that
this region is characterized by enhanced terrestrial OC contributions and/or selective degradation of
13
C-enriched OC. Additionally, sediments underlying deeper waters (e.g., >100 m) are relatively enrichedin 14COC and 13COC (Figures 2b and 4b), implying that 14C aging and selective degradation processes are mostpronounced on the inner shelf relative to deeper regions.

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3.2. Inﬂuence of Hydrodynamic Processes on Fate of OC
Grain size inﬂuences both hydrodynamic properties and SA of marine sediments (Mayer, 1994; McCave & Hall, 2006). Effects of hydrodynamic processes on OC% and carbon isotope composition are closely intertwined
due to physical protection through OM association with mineral surfaces(positive Spearman correlation [0.29] of Δ14COC with SA, p < 0.0005;
Table 1), thereby complicating assessment of the inﬂuence of hydrodynamic processes on 14COC-13COC characteristics. Hydrodynamic processes
should manifest themselves through differential (re) mobilization and dispersal of sediment grain sizes (McCave, 1986; Thomsen & Gust, 2000;
Thomsen & McCave, 2000), which, in turn will inﬂuence the Δ14COC valuesof associated OM. For instance, Bao et al. (2016) suggested that older OC in
the sortable silt fraction (20–63 μm) of inner shelf sediments reﬂects thesusceptibility of this fraction to cyclic resuspension, transport, and deposition within the benthic nepheloid layer.
In the CMS, grain size is negatively correlated with SA (Figure S1), with thelatter invoked as a control on OM preservation in continental margin
sediments (Blair & Aller, 2012; Mayer, 1994). The sortable silt fractions(20–63 μm) and coarser sediments (>63 μm) correspond to SA values witha range of 10–20 m2/g and < 10 m2/g, respectively. Although high SA sediments can be expected to have generally higher OC% as compared tolower SA sediments, and sediment characterized by higher OC% also exhibit higher Δ14COC values (Figure 3), we note that there is no signiﬁcant difference in Δ14COC values among the three corresponding SA ranges (>20,
10–20, and <10 m2/g, respectively; Figures 5 and S2). This suggests thatmineral surface protection of fresher OM associated with sortable silt fractions (i.e., SA 10–20 m2/g) may be undermined by hydrodynamic processes. Consequently, although OC% exhibits a general correlation with
Δ14COC values (Figure 3), hydrodynamic processes independently exert
an inﬂuence on surface sediment Δ14COC values in the CMS.
General relationships between abundance of terrestrial and marine OC
with respect to mineral-speciﬁc surface area (OC:SA) have emerged fromstudies of continental shelf sediments (Blair et al., 2003; Blair & Aller, 2012;
14
Figure 5. Relationship between Δ COC values and SA in the (a) Bohai Sea- Burdige, 2005; Coppola et al., 2007; Goñi et al., 2005; Wang et al., 2015;
Yellow Sea (BS-YS) and (b) East China Sea (ECS). The bubble size represents
Wu et al., 2013). Typical OC:SA ratios for river-dominated continental shelf
the OC% (ranging between approximately 0.1 and 1.5% for smallest andsediments range from 0.5 to 1.0 mg C/m2 (Bianchi et al., 2018; Blair &largest symbols).
Aller, 2012; Eglinton & Repeta, 2003; Hedges & Keil, 1995; Mayer,1994). This ratio has been used as an index to assess terrestrial OM loading on sedimentary particles(Burdige, 2005; Keil et al., 1997; Wu et al., 2013) and to place constraints on terrestrial OC burial efﬁciency
in marginal sea sediments (Burdige, 2005; Keil et al., 1997; Nuwer & Keil, 2005). In the CMS, surface sediment
OC:SA ratios exhibit distinctly spatial variability (Figure 2d), with generally lower ratios in the BS-YS than in
the ECS (signiﬁcant difference, t test; Figure S3). Furthermore, while BS-YS sediments exhibit large variability
in OC:SA ratios (<0.05 to >0.55 mg C/m2; ave. 0.32 mg C/m2, n = 117), they are generally higher than thoseof Yellow River SPM (0.18 mg OC/m2, mean grain size: 8–17 μm; Tao et al., 2015). Interestingly, δ13COC valuesof BS-YS sediments are relatively uniform (ave. 22.1 ± 0.6‰, n = 123; mean grain size: 59 μm, n = 119;
Figure 6a), despite signiﬁcant variability in Δ14COC values (Figure 6b). Some samples with lower OC:SA ratios,
particularly those originating from the shallower parts of the BS-YS, exhibit more scattered and generally
lower Δ14COC values (dashed circle, Figure 6b). These differing relationships between OC/SA ratios and
Δ14COC and δ13COC values suggest that hydrodynamic processes inﬂuence OC isotopic composition, especially for sediments from shallower inner-shelf settings.
It has been previously observed that marine sediments accumulating proximal to the Fly (Papua New Guinea)
and Columbia (northwest USA) rivers exhibited lower OC/SA ratios and higher δ13COC values than those of
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Figure 6. Relationships between carbon isotopic characteristics (Δ COC, δ COC) and OC:SA in surface sediments of the(a and b) Bohai Sea-Yellow Sea (BS-YS) and the (c and d) East China Sea (ECS). The bubble size represents the sample
13
water depth; (a) the shaded area highlights the low variability in δ COC values (ave. 22.1 ± 0.6‰); (b) the dashed circle
14
indicates the samples from shallow (<25 m) locations that are characterized by lower OC% and Δ COC values; (c and d) the13
green envelope shows the samples with larger OC:SA ratios and higher δ COC than SPM, and the orange envelope
14shows the samples with larger OC:SA ratios and lower Δ COC than SPM. The box circle indicates approximate corresponding
values for SPM from the Yellow River (a and b) and Yangtze River (c and d).

corresponding riverine SPM (Keil et al., 1997). This change in OC loading has been attributed to a process ofloss of terrestrial OM and replacement with marine OM on mineral surfaces. In the case of the ECS, we notethat some sediments from deeper regions exhibit OC:SA ratios and δ13COC values that are both higher thanriverine SPM (Figure 6c, green area), whereas Δ14COC values are lower than those of Yangtze River SPM(Figure 6d, orange area). This may reﬂect the inﬂuence of other hydrodynamic processes, such aspreferential displacement of certain grain size fractions, on OM composition. Therefore, loss-andreplacement processes are unlikely to be the only cause of the observed changes of δ13COC and Δ14COC
with OC:SA ratios.
While hydrodynamic processes are a driving factor promoting sediment resuspension (McCave, 1986;
McCave et al., 1995) and contributing to the above geochemical characteristics, they are difﬁcult to constrain
from direct ﬁeld observations. We therefore use bottom shear stresses calculated from a hydrodynamic
model that is based on tidal forcing and circulation patterns in the CMS in order to examine the dependenceof surface sediment Δ14COC on hydrodynamic conditions. Our results reveal a signiﬁcant negative
correlation between Δ14COC and bottom shear stress for the entire CMS (Spearman correlation 0.46,
p < 0.0005; Table 1); that is, greater bottom shear stress corresponds to lower Δ14COC values (14C aging;
Figure 2f), presumably as a consequence of processes associated with enhanced sediment mobilization

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Figure 7. Fm × OC% versus OC% relationships for surface sediments from (a) passive margins and (b) active margins (supporting information; the samples from this study are highlighted by black circles). The schematic highlights the contrasting
characteristics of active and passive margins (modiﬁed from Blair and Aller, 2012). In previous studies, straight-line relationships between Fm × OC% (Blair et al., 2003; Cui et al., 2017; Galy et al., 2008; Tao et al., 2015) and OC% were observed.
14
Here Fm × OC% reﬂects both marine and terrestrial biogenic OM (assuming a binary mixture of end-members: C-free
14
ancient carbon and modern carbon with the C content of the modern atmosphere).

and hydrodynamically induced particle sorting. Once the shear stress exceeds the critical erosion threshold,this will lead to resuspension and an abrupt increase in the concentrations of SPM in the water column.
Entrainment in repeated suspension-deposition loops results in enhanced OC remineralization (Aller &Blair, 2006), with selective degradation of younger OC and hence an apparent increase in 14C age(decrease in 14C content) of residual OM. The OM associated with those sediments subjected to greaterbottom shear stress is thus likely to exhibit lower 14C contents (older OC ages). These ﬁndings suggest thathydrodynamic processes are a key factor in dictating 14C content (age) of underlying sediments in the CMS.
3.3. Contrasting OC Fates on Passive and Active Margins
Sediment resuspension and redistribution is widespread on and beyond, continental margins (Hwang et al.,2010). The underlying hydrodynamic processes inﬂuence OC characteristics, likely through a combination
particle sorting effects and OM-mineral surface interactions, of which 14C content is perhaps most diagnostic(Table S1). Such processes may exert greater inﬂuence on passive margins that host expansive shallow continental shelf seas (e.g., BS-YS and ECS), leading to longer sediment residence and transport times (aging) andprotracted entrainment in resuspension-deposition loops (Bao et al., 2018; Blair & Aller, 2012; Bröder et al.,2018). Examination of OC versus SA relationships in shelf and upper slope sediments from passive margins
suggests that these systems are generally characterized by lower OC loadings (i.e., OC:SA <0.5 mg C/m2) thanthose from active margins (OC:SA >1.0 mg C/m2; Figure S4). Thus, on a global scale, OC may experience contrasting fates on active and passive margins, underlining a need to better understand relationships between
physical dynamics and nature and efﬁciency of OC burial (Blair & Aller, 2012). While data coverage remains
sparse, and additional factors (e.g., bottom oxygen concentration) are also likely important (Bianchi et al.,2018), we suggest that the distinctive physical dynamics of active and passive margins strongly inﬂuences
the fate of OC.
A plot of 14C content (expressed as Fraction modern [Fm]; Stuiver & Polach, 1977) × OC% versus OC% reveals
the contrasting characteristics of surface sediments from active and passive margins (Figure 7). Those fromBAO ET AL.

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active margins fall close to a straight line, suggesting that OC content in these sediments strongly inﬂuences
14
C content, whereas passive margin sediments exhibit considerable scatter and fall below the 1:1 line. Thisimplies that other factors, independent of OC%, control 14C content. These may include selective degradationof OC, preferential export of OC from different carbon sources (Zonneveld et al., 2010), or lateral transport
times. Decreasing 14C contents associated with the latter phenomenon is independent on OC%, composition,
and/or carbon source. Due to likely inﬂuence of each of these factors in passive margin systems, lower 14C
contents caused by lateral transport is likely often masked. However, in the case of the CMS, ancillaryevidence points to a clear role for these processes as a cause for the observed scatter in this relationship.
We therefore speculate that sediment resuspension and other hydrodynamically driven processes that
impact on 14C content are a hallmark of passive margin systems. Consequently, the type of plot exempliﬁed
in Figure 7 may prove useful in assessing relationships between continental margin dynamics and the fate of
OC (Galy et al., 2008; Tao et al., 2015).
In addition to varying degrees of OC incineration linked to margin type and length scales for sediment resuspension (Figure 7; Bao et al., 2018; Blair et al., 2003; Blair & Aller, 2012; Bröder et al., 2018), the different geometries of active and passive margins have implications for dynamics and nature of OC delivered to, andburied in the deep ocean, both for the present and the geologic past (Blair et al., 2003, 2004; Drenzek
et al., 2007; Leithold et al., 2016; Muller-Karger et al., 2005). For example, reduced accommodation spaceand sediment resuspension on continental shelves during intervals of lower sea level may help to explain elevated OC% of margin sediments during glacial maxima (Cartapanis et al., 2016; Hilton et al., 2015; Wagner
et al., 2014). In this context, diminished OC remineralization on active margins and more direct OC transfer
to deep ocean would correspond to a short-term OC sink, whereas more extensive processing on OM on
passive margins acts as a OC source. This balance of OC source-to-sink processes on active versus passive
margins may thus play an important role in regulating global carbon cycle.

4. Summary and Conclusions
Hydrodynamic processes are known to inﬂuence carbon cycling on continental margins, however, the underlying mechanisms and their impact on the distribution and characteristics of sedimentary OM remain elusive.
Our in-depth survey of shallow CMS sediments reveals the following:

Acknowledgments
This study was supported by SNF “CAPSLOCK” project 200021_140850 (T. I. E.),by the National Natural Science
Foundation of China (grants
41520104009 and 41521064, M. Z.) andby the “111” project (B13030). We
acknowledge Shuqing Qiao (First
Institute of Oceanography, State
Oceanic Administration, Qingdao,
China) and Qingqian Cui (Massachusetts
Institute of Technology, USA) for assistance in collecting data. Readers canaccess or ﬁnd the data from supporting
information. There are no any conﬂicts
of interest for any author that is apparent from our afﬁliations or funding.

BAO ET AL.

1. In addition to carbon provenance, bulk 14COC contents are impacted by both organo-mineral interactions
and hydrodynamically-driven resuspension processes, highlighted by (a) positive correlations between
Δ14COC, and OC% and SA, and (b) negative correlations between Δ14COC, grain size, and bottom shear
stress (Table 1).
2. Relationships between 14COC contents, OC%, and SA reveal widespread inﬂuence of resuspension processes on the 14C age and OC% in sediments from shallow, broad marginal seas (Figures 3 and 5).
3. Our sediment data set supports prior hypotheses that resuspension processes exert greater inﬂuence on
OC fate on passive margins compared with active margins (Figure 7). This in turn may inﬂuence the extent
to which continental margins act as net carbon sources and sinks globally, both for the present-day andthe past.
4. The differing extents of OM processing on active and passive margins have implications of the characteristics and fate of OC exported to the interior ocean (Figure 7). Remobilization and redistribution of sedimentary OM on the continental margins thus emerge as an important component of the global
oceanic carbon cycle and should be considered in budgets and ﬂuxes of oceanic OC burial and export.

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