Progress in Earth andPlanetary Science

Che et al.
Progress in Earth and Planetary Science(2022) 9:22
https://doi.org/10.1186/s40645-022-00479-2

Open Access

RESEARCH ARTICLE

Refining the contribution of riverine
particulate release to the global marine NdbudgetHong Che1,2, Jing Zhang1,2,3* , Qian Liu1, Huijun He1 and Zhi‑Qi Zhao4

Abstract
The release of neodymium (Nd) from particles along continental margins may contribute to losses in the global of Ndbudget. The Changjiang River, which carries a heavy load of total suspended matter, empties into the East China Sea,
and a strong particulate–seawater interaction process occurs along the salinity gradient. In the low-salinity region(S < 2.0), strong removal of dissolved rare earth elements (dREEs) occurs, but the Nd isotope values are uniform. At
mid- and high-salinity (S = 2.0–28.0 and S > 28.0) areas, the dREE concentrations increase slightly. An Nd isotope massbalance indicates that the release of particulate matter is a source of dREEs in the Changjiang estuary. The release rateof particulate Nd ­(NdSPM) to the dissolved Nd pool in Changjiang estuary is higher than other estuaries, such as Ama‑
zon estuary. Composite all river data available from the previous studies indicate that 5800–9200 Mg per year of Nd
is released to global marine waters from riverine particles. This estimated quantity is on the same order of magnitude
as the calculated global Nd release flux based on the case study in the Amazon estuary. Our study indicates that tobetter constrain the global Nd budget, it is required to consider the release rate of N
­ dSPM in different rivers due to thesignificant difference among various rivers, but with very limited available data as of now.
Keywords: Particulate release, Nd budget, Rare earth elements, Changjiang estuary
1 Introduction
Studies of the behavior of rare earth elements (REEs)
and neodymium (Nd) isotopes as well as the sources andsinks of these elements are important for understanding
the process of Nd cycle in the oceans. Although rivers are
the main source of Nd in the ocean, the transport pathway of REEs requires further research because estuaries
modify the elements that rivers input into oceans (Frank
2002; Goldstein and Hemming 2003; van de Flierdt et al.
2004; Lacan and Jeandel 2005; Tepe and Bau 2015; Merschel et al. 2017a, b). In the early stage, the magnitudes
of both the flux of Nd from riverine and aeolian leads toa calculated Nd residence time in the ocean on the order*Correspondence: jzhang@sci.u-toyama.ac.jp
1
Frontiers Science Center for Deep Ocean Multispheres and Earth
System, and Key Laboratory of Marine Chemistry Theory and Technology,
Ministry of Education, Ocean University of China, Qingdao 266100, China
Full list of author information is available at the end of the articleof ~ 5000 yr (Bertram and Elderfield 1993). However, theNd isotope provides strong evidence that the residence
time of Nd is shorter than the global turnover time of theocean (1000–1500 yr; Broecker and Peng 1982). This discrepancy in the Nd mass balance (which some researchers have termed the “Nd paradox”) indicates there is a
missing source of Nd in the ocean (Bertram and Elderfield 1993; Jeandel et al. 1995; Goldstein and Hemming
2003; van de Flierdt et al. 2004).
The search for the origin source of the missing Nd has
attracted the attention of many researchers who haveproposed a number of explanations. In 2007, Johannesson and Burdige tried to balance Nd budget in theocean emphasizing the importance of the role of submarine groundwater discharge (SGD). Kim and Kim(2014) demonstrated that the REE fluxes through SGD
were two to three orders of magnitude higher than diffusion from bottom sediments and atmospheric dust

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Che et al. Progress in Earth and Planetary Science(2022) 9:22

fallout in the subsequent study. In addition, pore fluid
was also considered to be one of the sources of neodymium in the ocean (Abbott et al. 2015). Later, asubsequent landmark study by Rousseau et al. (2015)
showed that the particulate fraction makes up the missing term in the Nd budget of global marine seawater.
Although Nd release from particulates has been shown
to be a significant source of dissolved marine Nd, theNd release rate from suspended particulate matter(SPM) varies among different river estuaries. For example, the fraction of Nd released from SPM by partial
dissolution ­(NdSPM, given in [%]) in the Amazon estuary is 0.94 ± 0.25% (Rousseau et al. 2015), but no Nd
release from particulate phases has been found in the
Lena River (Laukert et al. 2017). Significant differencesamong various rivers may be related to the pH and theconcentrations of SPM and dissolved organic carbon(DOC) of the water (Pearce et al. 2013; Merschel et al.
2017b; Laukert et al. 2017; Paffrath et al. 2021). Experimental results have shown that the rate of Nd releasefrom particulates is positively correlated with the surface area of the reacting sediments and ranges from0.04 to 0.38% (Pearce et al. 2013). Thus, studying the­NdSPM for different estuaries is important for improving our understanding of the marine Nd budget.
Estuaries are regions with active particulate–seawaterinteractions. Almost all trace metals exhibit nonconservative behavior due to the destabilization of mixed
colloids during mixing with sea water in estuaries (Eckert and Sholkovitz 1976; Hoyle et al. 1984; Rousseau
et al. 2015). It has been suggested that in most rivers,
Nd is efficiently removed from seawater in low-salinity
areas (e.g., > 70% in the Gironde estuary, 88% in the
Great Whale estuary, 90% in the Amazon estuary, and
79% in the Changjiang estuary; see Rousseau et al. 2015
Table 1; Wang and Liu 2008) and the Nd concentration
rebounds in mid- and high-salinity areas. These phenomena can be explained by the coagulation processof riverine colloid matter in low-salinity areas and theTable 1 Average Nd concentration in the groundwater ofdifferent areas
Area

Averageconcentration(pmol ­kg−1)

References

Bangdu Bay Jeju Island

2899

Kim and Kim (2011)

Hwasun Bay Jeju Island

2448

Kim and Kim (2011)

Pearl River estuary

2573

Yuan et al. (2013)

Changjiang estuary

3606 ± 690

Data estimated in thisstudy

Jiangyin (well water)

1054 ± 485

This study

Page 2 of 10

release of Nd from suspended materials in mid- andhigh-salinity areas (Li et al. 2005; Wang and Liu 2008;
Rousseau et al. 2015).
The Changjiang River is the third largest river in theworld, with a water discharge of 940 ­km3 ­yr−1 and highconcentrations of total suspended matter (Wong et al.
1998; Chen et al. 2002; Zhang 2002; Li et al. 2007; Meybeck and Ragu 2012), that strongly influences local biogeochemical cycling. The Changjiang River is the largest
river discharging into the East China Sea (ECS), contributing approximately 60% of water in the eastern ECS andaffecting the Tsushima Strait during the flooding season(Lee et al. 2014; Che and Zhang 2018). A previous study
by Wang and Liu (2008) showed that in the 0–6.0 salinity
range, > 70% of dREEs were removed from solution due
to salt-induced coagulation of river colloids. The dREE
concentrations increased again in the 8.0–19.0 salinity
range, which was attributed to release from particulatematter. A vigorous release process in the estuarine mixing area could cause the Changjiang River to significantly
impact on the biogeochemistry of adjacent seas, such as
the ECS, the Sea of Japan, and even as far as the northwest Pacific Ocean. In this study, we quantify the fluxof dissolved ­NdSPM in the Changjiang estuary instead of
addressing the detailed driving mechanisms of particulates. The findings of this study enable us to explain thesignificance of particle–seawater interaction and moreaccurately assess the contribution of marginal seas to themarine Nd balance.

2 Materials and methods
2.1 Study area and sampling

Surface freshwater and seawater samples from the
Changjiang (Datong) and estuary were collected during the high discharge season in August 2015 (included
datasets from seven published Nd isotopic as reported by
Che and Zhang 2018) (Fig. 1). Upon collection, water was
filtered through a 0.40-μm Whatman membrane (Nuclepore Polycarbonate, 111707-Whatman) and acidified toa pH of ~ 2 with high-purity HCl (30%, Merck Suprapur).
A volume of 15–20 L of each sample was placed in anacid-cleaned bottle for Nd isotope composition analysis,
and 500 mL of each sample was used to determine the
REE concentration. The filter membranes were cryopreserved to determine the SPM. The REE concentrations
and Nd isotope compositions determined for the filteredwater samples were considered to be dissolved-phasemeasurements ­(Nd<0.45 μm). An unfiltered water sample
was directly acidified to a pH < 2 with hydrochloric aciddirectly after sample collection and then filtered througha 0.40-μm Whatman membrane in a cleanroom to determine the acid-soluble REE concentrations (­Nd>0.45 μm).
The above-mentioned processes were also carried out

Che et al. Progress in Earth and Planetary Science(2022) 9:22

Page 3 of 10

Fig. 1 Sampling map (A) and stations in the Changjiang River (black square in Fig. C), Jiangyin well (purple square in Fig. C), the Changjiang estuary(Fig. B; black circles; gray circles refer Nd isotopic published stations as reported by Che and Zhang 2018), the salinity gradient (Fig. B; blue squares);
CDW and TWC indicate Changjiang diluted water and Taiwan warm current, respectively. Green dotted line in the map B shows the study area for
calculation of the Nd budget (approx. 8455 ­km2). Red dotted line (Fig. B) is the main area where ­NdSPM in the Changjiang River mixing zone was
exploredusing rainwater (between stations 13 and 17) and groundwater samples (Jiangyin well water) (120.262°E, 31.938°N),
where all samples were collected in August 2017.
The analytical procedures for the measurement of the
REE concentrations and Nd isotopic compositions in thispaper are from the works of Persson et al. (2011), Hatje
et al. (2014), Che et al. (2018), and Liu et al. (2022). The
data on dissolved REE concentrations were measured
by ICP-MS (Thermo iCAP-Q) with a precision of better
than 4% and a reagent blank of less than 2%, based on the
results of replicated measurements. Nd isotope measurements were made on Nu plasma I and II instruments at
the Key Laboratory of Environmental Geochemistry.
Corrections were made for instrumental mass bias using
a value of 143Nd/144Nd = 0.7219. The Nd isotopic compositions can be expressed by εNd = (143Nd/144Ndmeasured/143
Nd/144NdCHUR − 1) × 10,000, where CHUR represents theChondritic Uniform Reservoir, with a value of 0.512638(Jacobsen and Wasserburg 1980), and 143Nd/144Nd measured represents the measured value.
2.2 Calculation
2.2.1 Mass balance on water and salinity

The hydrography in the study area is largely controlled
by the Changjiang River discharge, exchanges with

the ECS and Yellow Sea (YS), and the open ocean current (including the Taiwan Warm Current (TWC) andKuroshio Water (KW)). Given that the deepwater of
the TWC mainly comprises the subsurface water ofthe Kuroshio Current (Chao 1990; Yang et al. 2011), we
assumed that the water in the study area was directly
governed by the Changjiang River and KW in August.
Riverine inputs, precipitation, groundwater, evaporation, and exchanges between the studied area and otherwaters were accounted for in the water budget. The water
budget was calculated for the continental shelf using theLand–Ocean Interactions in the Coastal Zone (LOICZ)
approach (Swaney et al. 2011) as follows:
VQ + VP + VG + VX + VO + (VE + VR − VX ) = 0(1)SQ VQ + SP VP + SG VG + SO VO + SECS VX
(2)+ SE VE + SR VR − SCJE VX = 0
where V is the water volume (­km3 ­d−1), S is the salinity,
and the subscripts indicate the riverine input (Q), atmospheric deposition (P), groundwater input (G), exchanges
between the study area and open sea (X), other flow volume (O), evaporation (E), and residual flow volume (R).
SCJE and SECS are the salinities in the Changjiang estuary

Che et al. Progress in Earth and Planetary Science(2022) 9:22

Page 4 of 10

area and outer seas, respectively. As the salinity in theriver, rainwater, and other terrestrial waters was generally
zero, the water exchange between the study area and theouter seas was calculated using the following formula:VX = SR VR / SCJE − SECS
(3)
where VX is the water volume exchange between the
study area and the open sea, SR is the mean salinity of
22.4 in the study area, and VR is the residual flow volume.
SCJE and SECS were 0.1 and 30.1, respectively, in August2015.
2.2.2 Quantitative estimates of the Nd budget

A mass balance was performed on the Changjiang estuary
to explore the effects of Nd processes in the river estuary.
The Nd inputs into the Changjiang estuary include river
inputs (FQ), groundwater input (FG), atmospheric inputs(FP), ECS inflow (FR ECS), and YS inflow (FR YS). The Nd outputs include evaporation (FE), the outflow to the ECS (FXECS), and the residual outflow to the YS (FX YS).
The dissolved Nd flux for the different processes FNd was
calculated based on the measurement data for the samples:

FNd = V × c(4)where V represents the volume of the river, atmospheric
input, and water exchange with groundwater and water
output discharge and c is the dissolved Nd concentration.
2.2.3 Nd isotope mass balanceFirst, Formulas 5 and 6 were used to calculate the mixing proportion of different water masses considering thesalinity: 

fECS vol = SM − SCJ / SECS − SCJ ;(5)

fCJ vol = 1 − fECS vol ;(6)

Next, we calculated the theoretically conserved Nd concentration in the ECS using Formulas 7 and 8:[Nd]T-theor = fECS vol × [Nd]ECS + 1 − fECS vol × [Nd]CJ ;(7)[Nd]ECS-theor = fECS vol × [Nd]ECS ;(8)

Finally, a quantitative mass balance was formulated basedon the Nd fractions from different sources (Formulas:9–11):

fNd ECS = [Nd]ECS-theor /[Nd]M ;(9)fNd CJ = [εNd M − εNd SPM − fNd ECS (εNd ECS − εNd SPM )]/ εNd CJ − εNd SPM ;(10)

fNd SPM = 1 − fNd ECS − fNd CJ .

(11)

where fCJ vol and fECS vol denote the calculated salinity fraction for each endmember; ­[Nd]ECS-theor and ­[Nd]T-theor are
the theoretical conservations of the Nd concentration;
fNd ECS, fNd CJ, and fNd SPM are the Nd fractions in the ECS,
the Changjiang River, and the SPM, respectively; and­[Nd]ECS, ­[Nd]CJ, SECS, SCJ, εNd ECS, εNd SPM, and εNd CJ are
the endmembers of the Nd concentration, salinity, and
Nd isotopic composition in the ECS, Changjiang River,and SPM, respectively; and ­[Nd]M, SM, and εNd M are themeasured values of the Nd concentration, salinity, and
Nd isotopic composition measured at the sampling stations, respectively.

3 Results and discussion
3.1 Significance of estuaries as modulators of REEs
in seawater

Numerous works of estuarine research studies suggestthat the coagulation process of riverine colloid matter in low-salinity areas and the release of suspended
materials in mid- and high-salinity areas are the main
factors affecting the REE fractionation in the estuaries (Elderfield et al. 1990; Pearce et al. 2013; Rousseau
et al. 2015). A similar phenomenon was discovered in
the Changjiang River along the salinity gradient, where
all of the REE distributions show strong nonconservative behavior. The concentrations of dREEs (Nd,samarium (Sm), and ytterbium (Yb) are representative
LREEs, MREEs, and HREEs, respectively) decreased
rapidly in the low-salinity area (S < 2.0) and increasedslightly in the mid-salinity area (S = 2.0–28.0), despite
appreciable scattering (Fig. 2). In the low-salinity
area (S < 2.0) of the Changjiang estuary, the [Nd/Yb]Ndecreased rapidly and more than 89 ± 5% of dissolved
LREE (Nd) was removed, such that the Nd concentration decreased from 295.7 pmol ­kg−1 to a minimumof 30.2 pmol ­kg−1 (Fig. 2A, C). Approximately 85% of
the dissolved MREEs and 60% of the dissolved HREEs
were removed in this region (Fig. 2B). The percentage
removal of dissolved REEs shows a large decrease in
the concentrations of a range of REEs, from LREEs to
HREEs. However, in the area of the Changjiang estuary
where the salinity ranges from 2.0 to 28.0, there wasa noticeable increase in the concentration of dissolved
REEs, of 140% for HREEs and 240% for LREEs and
MREEs. The slight increase in [Nd/Yb]N corresponded
to preferential release from LREEs to the dREE pool.
The endmember of the dissolved phase εNd for the
Changjiang freshwater was determined at the Datong station, which is approximately 500 km from the area in theChe et al. Progress in Earth and Planetary Science(2022) 9:22

Page 5 of 10

Fig. 2 Distribution of dissolved Nd (A), Sm and Yb (B) in surface water along the Changjiang estuary salinity area. C and D represent therelationship between [Nd/Yb]N, or Nd isotopic and salinity, respectively. The black-dashed line in the D is the conservative theoretical dissolved Nd
mixing curve

estuary with zero salinity; the sampling results for thechannel of the Changjiang River in August 2017 showedthat εNd =  − 11.9 ± 0.3 and S = 0.0. No difference in the εNdsignatures between the freshwater and the estuary water(S = 0.1, εNd =  − 12.0 ± 0.3) was observed at the mouthof the estuary (Che and Zhang 2018), indicating thatan essentially uniform εNd distribution was maintained(Fig. 2D). For regions where the salinity increased to 28,
the Nd isotopes and dREE served as conservative tracersof the mixing of water masses (Che and Zhang 2018).
3.2 Importance of the effect of particle–seawaterinteractions on the dREE compositionsin the mid‑salinity regions
3.2.1 Neodymium budget for the Changjiang River Estuary

Considering the results of previous studies and theabove-mentioned findings, a mass balance on the waterand salinity was performed to calculate the dissolved Ndbalance in the Changjiang estuary. The details of the calculation are presented below.
3.2.1.1 Changjiang River fluxes By taking the contribution of Nd = 182.4 pmol ­kg−1 at the Changjiang Rivermouth and an annual river discharge of approximately
913.9 ­km3 ­yr−1 (Ministry of Water Resources of the People’s Republic of China, http://​www.​mwr.​gov.​cn/), we
obtained a Changjiang River fluxes of 24.0 Mg ­yr−1 for
Nd. This value should be regarded as mean annual flux
because of the Nd content including the winter data
(Wang and Liu 2008) in November 1998 and summer datain this study. According to the above-mentioned analytical results, the total transport of effectively dissolved Ndfrom the Changjiang River was 2.6 ± 0.2 Mg ­yr−1, for an
Nd removal rate in the river estuary of approximately 89%.
3.2.1.2 Groundwater fluxes Since its discovery,
groundwater has been regarded as an important sourcein the REE balance in seawater (e.g., Johannesson andBurdige 2007). Several investigators have pointed outthat the Nd concentration of the groundwater is approximately 100 times that of the surrounding seawater (Kimand Kim 2011; Yuan et al. 2013). Few Nd data havebeen reported in the groundwater of the Changjiang
estuary; however, groundwater can be inferred to playa significant role in the Changjiang estuary from the
Jiangyin well water data (1054 ± 485 pmol ­kg−1; n = 2)

Che et al. Progress in Earth and Planetary Science(2022) 9:22

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and previous published data (3606 ± 690 pmol ­kg−1)(Table 1). In this study, we used an annual average waterflow of 72 ­km3 ­yr−1 and an Nd concentration of 1054–
3606 pmol ­kg−1 in groundwater to roughly estimate
the total annual input flux of groundwater to the estuary as (11.0–37.4) ± 6.0 Mg ­yr−1 (Gu et al. 2012). Thisestimated Nd content in groundwater is also within theconcentration range summarized by Johannesson andBurdige (2007). Based on the observations, 89% of Nd
was removed during estuary mixing, from which we estimated that the effective groundwater flux of dissolved
Nd to the study area was (1.2–4.1) ± 0.7 Mg ­yr−1. Notably, however, the groundwater fluxes have a large partrecirculated seawater and a small part fresh groundwater(Gu et al. 2012). The actual Nd contribution from fresh
groundwater to the Changjiang estuary should be lower
than the estimate given above. We will explore the large
uncertainty introduced by the diverse range of the Ndcontents in a future work, especially for the contributionof SGD on the ocean dissolved Nd budget.
3.2.1.3 Atmospheric fluxes By taking the average
Nd concentrations in filtered rainwater in the spring(Nd = 199.9 pmol ­kg−1 (unpublished data)) and summer(Nd = 24.3–33.3 pmol ­kg−1) (Zhang and Liu 2004) (avg.
Nd = 86.0 ± 98.5 pmol ­kg−1), an average annual precipitation of 1600 mm (based on the rainfall in Shanghai),and an oceanic area of 8455 ­km2, we obtain an atmospheric wet deposition flux of 0.17 ± 0.19 Mg ­yr−1 Nd.
However, the effects of anthropogenic activities anddust storms on river estuary areas also influence the Ndconcentration in the surface seawater. In addition, there
is a large amount of uncertainty in the atmospheric dry
deposition input estimate, because only data for thespring have been reported, and no seasonal variation
data have been published. The estimated contribution of
atmospheric dry deposition to marine dissolved Nd was
0.05 ± 0.02 Mg ­yr−1 [Nd = 135.9 ± 49.9 pmol ­kg−1; n = 6;
sedimentation rate = 1 cm ­s−1 (Duce et al. 1991; Kadko
et al. 2020)]. Ultimately, the total annual input (including
wet and dry deposition) to the study area was approximately 0.22 ± 0.19 Mg ­yr−1, indicating a nonsignificant
contribution from atmospheric deposition to the Changjiang estuary.
3.2.1.4 Transport fluxes The LOICZ model was
employed to estimate the transport flux of dissolved Ndfrom the Changjiang estuary to the ECS and the YS during
the survey year (Table 2). By taking the mass balance of thewater and salinity model, the average salinity in the study
area (S = 22.4) and the salinity of the ECS and Changjiang estuary endmembers (30.1 and 0.1, respectively) wereused to estimate the water volume exchange between the
Changjiang estuary and ECS (VX ECS) and YS (VX YS) andthe residual flow volumes (VR ECS and VR YS). The outputsof dissolved Nd from the Changjiang estuary to the ECS
were found to be five times higher than those from the YS.
Overall, the results indicate that the total transport ofeffectively dissolved Nd in the Changjiang estuary area
was 4.1–7.0 ± 0.9 Mg ­yr−1, of which 2.6 ± 0.2 Mg ­yr−1
was from rivers, 1.2–4.1 ± 0.7 Mg ­yr−1 was fromgroundwater, and 0.22 ± 0.19 Mg ­yr−1 was fromthe atmosphere (including wet and dry deposition)(Tables 1, 2). Here, we have neglected the effect of
atmospheric dry deposition on marine dissolved Nd
because the dry deposition flux was less than 2% of
the total dissolved Nd flux (0.05 ± 0.02 Mg ­yr−1, n = 5;
deposition rate = 1 cm ­s−1), although dust is one type

Table 2 Estimates of dissolved Nd fluxes for rivers, atmosphere, and groundwater and water exchange between the ECS and YS
Processes

Area

Annual average
Water discharge ­(km3 ­yr−1) Nd (pmol k­ g−1)

Flux (Mg ­yr−1)

913.9a

182.4

24.0

Atmospheric input (FP)0.12b86.0/135.9

0.3

Groundwater input (FG)72c

1054–3606

River input (FQ)

Changjiang River(11.0–40.3) ± 5.0

Water exchange (FX)eECS (FX ECS)

517.2

57.6d

4.3

YS (FX YS)

97.9

64.5

0.9

Water output (FR)e

ECS (FR ECS)

729.6

57.6d

6.1

YS (FR YS)

138.0

64.5

1.3

a

Ministry of Water Resources of the People’s Republic of China (2015)b

Li et al. (2011)c

Gu et al. (2012). The exchange rates between the study area and the YS (FX YS) and the ECS (FX ECS) are based on data reported by Liu et al. (2003)

d

The concentration is 57.6 pmol k­ g−1 in the south YS and 64.5 pmol ­kg−1 in the ECS (Che and Zhang 2018)e

The loading of the residual outflows and exchange outflows to the YS (26%, 10%, 5%, Ave. = 14%) and ECS (74%) was estimated from the corresponding water
discharges (Liu et al. 2003)

Che et al. Progress in Earth and Planetary Science(2022) 9:22

of particulate matter. However, the flux of dissolved Ndin the estuary (2.8–5.7 Mg) was clearly lower than the
total flux input to the ECS and YS (9.8 ± 0.9 Mg ­yr−1),
which implies that the additional sources of dissolved
Nd should exist in the observation area (Fig. 3).
3.2.2 Fraction of Nd released from suspended particulatematterIn addition, an Nd isotope mass balance was performed
to quantify the additional flux of dissolved Nd in the
Changjiang estuary. The three main sources of dissolved
Nd were the Changjiang River (εNd CJ), the mixing shelfwater of the ECS (εNd MSW), and the particulate matter (εNd
SPM), with corresponding isotope values of − 11.9, − 11.3
(Che and Zhang 2018), and − 13 (Yang et al. 2007),
respectively; here, as few data have been reported for
εNd of particulate matter, we used εNd of the sedimentinstead. The endmembers of the Nd concentrations for
the Changjiang River and the mixing shelf water were­[Nd]CJ = 295.7 pmol ­kg−1 and ­[Nd]MSW = 28.9 pmol ­kg−1,

Page 7 of 10

respectively. The specific calculation method is detailed in“Nd isotope mass balance” section.
We estimated that approximately 1.3 ± 0.3% of­NdSPM was released to the dissolved Nd pool in theestuary based on the above-mentioned calculations(28.4 pmol ­kg−1 of Nd released from particles to the dissolved pool at zero salinity, and a particulate N­ d>0.45 μm
concentration of 2247 pmol ­kg−1). Our result is higher
than those reported for other estuaries (Pearce et al.
2013; Rousseau et al. 2015; Laukert et al. 2017), which isprobably because particulate–seawater interactions arestronger in the Changjiang River (Wang and Liu 2008).
In addition, a relatively high SPM (500–1000 mg ­L−1)
could enhance the release rate in the Changjiang River,
because the proportion of fine particulates in the river
sediment flux impacts the release rate of Nd from theparticulate phase (Pearce et al. 2013). By contrast, no
REEs were released from the particulate phases in the
Lena River because of the low amounts of SPM (33 mg­L−1) (Laukert et al. 2017).

Fig. 3 Flux of effectively dissolved Nd in the Changjiang estuary (unit: t ­yr−1). Numbers represent Nd concentration fluxes. FQ: river inputs; FG:groundwater discharge; FP: atmospheric wet deposition; FX YS: exchange flow between the Yellow Sea and the Changjiang estuary; FX ECS: exchange
flow between the East China Sea and the Changjiang estuary; FR YS: residual outflow to the Yellow Sea; FR ECS: residual outflow to the East China Sea.
Red-stripe area represents the salinity gradientChe et al. Progress in Earth and Planetary Science(2022) 9:22

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The Changjiang River ­NdSPM flux to the ECS was estimated based on the detailed Nd observations in thisstudy. Using an Nd concentration of 35.6 mg ­kg−1 for the
Changjiang sediment and a total river suspended flux of
4.8 × ­108 Mg ­yr−1 (Meybeck and Ragu 2012) yielded an­NdSPM flux of 17,000 Mg ­yr−1. We calculated that about
1.3% of the 17,000 t y­ r−1 ­NdSPM flux (220 Mg ­yr−1) was
released from the SPM to the dissolved Nd pool in the
Changjiang estuary. However, the flux of dissolved Ndfrom the Changjiang River to the ECS is approximately
4.8 ± 0.7 Mg ­yr−1, because approximately 80 ± 3% ofriver suspended and 89 ± 5% of the dissolved Nd flux was
removed in the low-salinity area of the estuary. These
results indicate that Nd release from lithogenic sediments is an important additional contributor to REEs in
the dissolved budget of the ocean.
If the dissolved Nd concentrations and Nd removal
were constant throughout the year, these results would
roughly account for the imbalance in the Nd budget
imbalance in the Changjiang estuary. It is difficult to consider the effects of Nd concentrations and the Nd isotope
compositions determined in this study. The maximum
contribution of the Nd concentration and Nd isotope
composition to the ­NdSPM release rate were approximately ± 0.1% and ± 0.7%, assuming variations in the Ndconcentration and Nd isotope composition to be ± 10%
and ± 0.3, respectively. Seasonal variations in the releaseof Nd from particles in the Changjiang estuary will beconsidered in a future study.
3.3 Quantitative estimates of the Nd flux in Chinese
marginal seas and global ocean

Finally, the Nd budget of China’s marginal seas wasroughly assessed based on our observations of ­NdSPM
release. The total river sediment flux for nine Chinese
rivers is approximately 1.8 × ­109 Mg ­yr−1 (Meybeck
and Ragu 2012). Thus, we estimate that approximately
770 Mg ­yr−1 of Nd was released from Chinese river
sediments to coastal marine waters. (The mean river­Ndsediment concentration is 33 ± 12 mg ­kg−1 (Kim and Kim

2014) and the N­ dSPM release of the rivers is 1.3 ± 0.3%.)
According to previous studies, approximately 6–38% of
Nd is transported to adjacent ocean basins such as the
Sea of Japan and the North Pacific Ocean via the ocean
circulation (Che and Zhang 2018). We calculated thatapproximately 46–290 Mg of Nd is transported annually through the northern South sea (Table 3), the ECS,
and the YS to adjacent seas, which is in agreement with aprevious report (295 ± 240 Mg ­yr−1) (Lacan and Jeandel2005).
We categorized the world’s rivers into threegroups according to the SPM: SPM < 40 mg L
­−1,
−1
−1
40 < SPM < 500 mg ­L , and SPM > 500 mg ­L (Meybeck
and Ragu 2012), and recalculated the global marine Ndbudget by incorporating the data obtained for the Amazon estuary (0.94 ± 0.25%), the Lena River (none), andthe Changjiang estuary (1.3 ± 0.3%). The final estimatedresults showed that approximately 5800–9200 Mg ­yr−1
of Nd is released in marginal seas, which is closer to theglobal missing Nd flux of 5500–11,000 Mg per year thana previous estimate (Rousseau et al. 2015). Considering the global riverine sediment flux to the oceans is1.85 ± 0.16 × ­1016 g ­yr−1, the rivers with an SPM concentration greater than 500 mg ­L−1 account for 83% of thetotal sediment flux and the rivers with an SPM concentration between 40 and 500 mg L
­ −1 account for 16.8%.
The range of the total flux is calculated using the rangeof the error in the release rate. And the final estimatedresults show that approximately 7500 ± 1700 t y­ r−1 of Nd
is released in marginal seas (Table 3). In this study, a synthesis of river data for the northwest Pacific Ocean, theSouth and North Atlantic Oceans, and the Arctic Ocean
demonstrated that particulate release is significant forbalancing the dissolved marine Nd budget. To confirm thecontribution of the particulate release from the margins
to the ocean, it is necessary to obtain more N­ dSPM releasedata from various estuaries with different geochemical
properties, such as the composition and particulate surface area. In addition, submarine groundwater discharge

Table 3 NdSPM release rate in different estuaries and dissolved ­NdSPM flux to the ocean
Contact area

SPM discharge Mg ­yr−1

NdSPM release rate %Nd sedimentconcentration mg k
­ g−1

Water masses
mixingChinese rivers

1.8 × ­109 a

1.3 ± 0.3

33 ± ­12c

6–38%dWorld river  1.85 ± 0.16 × ­1016 aaMeybeck and Ragu (2012)b

Rousseau et al. (2015)c

Kim and Kim (2014)d

Che and Zhang (2018)1.53 × ­1016
16

0.31 × ­10

1.3 ± 0.3b0.94 ± 0.25

33 ± ­12c

Nd flux Mg
46–2905800–9200

Che et al. Progress in Earth and Planetary Science(2022) 9:22

and sediment pore water may be a potential source of dissolved Nd, but these contributions require further study.

4 Conclusions
Considerable dREE removal and uniform Nd isotope
signatures were observed in low-salinity regions of the
Changjiang estuary. In the mid-salinity region of the estuary, an Nd mass balance yielded an estimated Nd release
rate higher than the available published data. Our results
indicate that the flux of Nd release from riverine particulates to the seawater pool (5800–9200 Mg per year) close
to the global missing Nd flux of 5500–11,000 Mg per year
is also on the same order of magnitude as that reported by
Rousseau et al (2015). This study provides direct evidence
for exploring the Nd cycle process in the estuaries, and theresult suggests that the release of dissolved Nd from riverine particles should be considered a “missing source” constraint on the Nd mass balance in ocean waters. Moreover,particulate–seawater interactions appear to be an important geochemical control and should thus be considered
in coastal environments when assessing the cycle of traceelements.
Abbreviations
SPM: Suspended particulate matter; DOC: Dissolved organic carbon; ECS: East
China Sea; REE: Rare earth element; CDW: Changjiang diluted water; TWC​: Tai‑wan warm current; KW: Kuroshio water; LOICZ: Land–ocean interactions in the
coastal zone; LREE: Light rare earth element; MREE: Middle rare earth element;
HREE: Heavy rare earth element.
Acknowledgements
We thank the captain and crew of the Zhedaiyuyun and researchers for taking
seawater samples during cruises and performing onboard sample process‑ing. We would especially like to thank Professor Dr. Tristan C. C. Rousseau forcontributing to the Nd mass balance model calculation. We would also like tothank Wenkai Guan, Jie Liang, and Hailong Zhang for assistance in the labora‑
tory and with sampling. We are grateful to the anonymous reviewers for theirhelpful comments and suggestions. This paper is dedicated to the memoryof Huijun He, an outstanding scientist who sadly passed away on September
3, 2021.
Author contributions
JZ proposed the study subject, conceived and designed the study, and was
the project administrator. HC carried out the experimental study and wrote
the manuscript. QL and HH conceived the study and participated in discus‑
sion on the manuscript. ZZQ analyzed and interpreted the data. All authors
read and approved the final manuscript.
Funding
This study was partially supported by the National Natural Science Foun‑dation of China (Grant Nos. NSFC-41890801, 4191154007, 41906038, and41530965); the Key Laboratory of Marine Ecology and Environmental Scienceand Engineering, SOA (Grant No. MESE-2019-04); the Pilot National Laboratory
for Marine Science and Technology (Qingdao) (Grant No. JCZX202019); JSPS
KAKENHI Grant Nos. JP20H04319 and 15H05821.
Availability of data and materials
The datasets generated and analysed during the current study are available
in the [Data Publisher for Earth & Environmental Science] repository, (https://​doi.​org/​10.​1594/​PANGA​EA.​939705; https://​doi.​org/​10.​1594/​PANGA​EA.​939706;
https://​doi.​org/​10.​1594/​PANGA​EA.​939708).

Page 9 of 10

Declarations
Ethical approval
Our research does not involve human participants, or tissue or animals. The
datasets generated and analysed during the current study are available in the[Data Publisher for Earth & Environmental Science] repository, [https://​doi.​org/​
10.​1594/​PANGA​EA.​93970​5,93970​6,939708].
Competing interests
The authors declare that they have no competing interests.
Author details
1
Frontiers Science Center for Deep Ocean Multispheres and Earth System,
and Key Laboratory of Marine Chemistry Theory and Technology, Ministryof Education, Ocean University of China, Qingdao 266100, China. 2 Deep‑sea
Multidisciplinary Research Center, Pilot National Laboratory for Marine Sci‑
ence and Technology, Qingdao 266237, China. 3 Faculty of Science, Universityof Toyama, Toyama 9308555, Japan. 4 School of Earth Science and Resources,
Chang’an University, Xi’an 710054, China.
Received: 13 October 2021 Accepted: 3 April 2022

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