Diagenesis Decreasing the Mo Isotopic Composition in Estuarine Systems: Implications for Constraining Its Riverine Input to Ocean

Understanding the geochemical behavior of the Mo isotopes in estuarine systems is essential for determining the isotopic composition of riverine inputs to the ocean and for assessing the historical oxidation state of Earth’s ancient oceans. However, the extent and mechanisms of Mo isotope fractionation during estuarine processes are not yet fully understood. This study systematically investigated the seasonal and spatial variations in aqueous and particulate δ98Mo values within the Pearl River Estuary (PRE). Our research found that aqueous δ98Moaque. values in both summer and winter deviated significantly from the theoretical mixing line for the PRE. Based on the geochemical characteristics of the water column, particulate matter, and pore water in the PRE, we first propose that diagenesis release from sediments is the predominant factor resulting in lower than anticipated aqueous δ98Moaque. values. Given the prevalence of suboxic and anoxic sediments in estuarine and coastal areas, such diagenetic release may substantially decrease the global riverine influx of aqueous Mo isotopes to the ocean. Additionally, particulate δ98MoSPM values exhibit an increasing trend (from 0.02 to 1.62‰) with increasing salinity in both seasons, suggesting that the terrestrial input particulate δ98MoSPM value would be heavier than the mean value for UCC. We hypothesize that the adsorption and desorption processes involving Fe (hydro) oxides predominantly influence this trend. This study advances our understanding of the mechanisms of aqueous and particulate Mo isotopic fractionation in estuarine systems and would be helpful in constraining Mo isotopic compositions of rivers and oceans.

Beyond continental rock weathering, the geochemical behavior in estuarine systems is another critical factor affecting their delivery to the open oceans.Estuaries, as dynamic interfaces between terrestrial and marine environments, significantly influence the elemental flux and isotopic composition of materials from continents to the ocean through various physical, chemical, and biochemical processes (Andersson et al., 2001;Bridgestock et al., 2021;Escoube et al., 2009;Goring-Harford et al., 2020;Laukert et al., 2017;Petit et al., 2015;Pogge von Strandmann et al., 2008;Rahaman et al., 2014;Rouxel et al., 2008;Z. Sun et al., 2019;Q. Wang et al., 2021;Weiss et al., 2015;A. Y. Zhang et al., 2015;L. Zhang et al., 2015;Z. Zhang et al., 2020).Theoretically, aqueous Mo isotopic compositions are considered conservative during transport from continents to oceans.However, recent studies of major estuaries show that most display non-conservative behavior in their aqueous Mo isotopic compositions, with δ 98 Mo values that are lower than those predicted by conservative mixing models between freshwater and seawater (Archer & Vance, 2008;Pearce et al., 2010;Rahaman et al., 2014;Scheiderich et al., 2010;Q. Wang et al., 2021).This indicates that estuarine processes reduce the riverine input of aqueous Mo isotopes to the ocean (Pearce et al., 2010;Rahaman et al., 2014;Scheiderich et al., 2010;Q. Wang et al., 2021).Concerning the non-conservative behavior of aqueous Mo isotopic compositions in estuarine systems, earlier research highlights the significance of desorption and adsorption processes involving particulate matter (Pearce et al., 2010;Rahaman et al., 2014;Q. Wang et al., 2021).For instance, Pearce et al. (2010) proposed that desorption from particulate matter could reduce aqueous δ 98 Mo values in the Iceland's Borgarfjörður estuary due to the lighter δ 98 Mo values in the particulate matter.Rahaman et al. (2014) posited that the adsorption of Mo by Fe-Mn oxides in particulate matter contributes to the reduced aqueous δ 98 Mo values observed in the Narmada estuary, India, a phenomenon concurrent with an increase in particulate Mo concentrations and δ 98 Mo values alongside rising salinity.The study by Q. Wang et al. (2021) demonstrates that interactions of Mo, specifically, desorption and adsorption between particulate and aqueous phases, tend to decrease the aqueous δ 98 Mo values in the Changjiang River Estuary, China.Considering the minimal proportion (from 0.08% to 8.51%) of particulate Mo relative to the total Mo content-encompassing both dissolved forms and suspended particulates-in the riverine and estuarine water samples (Archer & Vance, 2008;Rahaman et al., 2014;Revels et al., 2021;Z. B. Wang et al., 2015;Q. Wang et al., 2021), it is improbable that desorption or adsorption processes involving particulate matter are the primary mechanisms reducing the aqueous δ 98 Mo values in estuarine environments.As suboxic and anoxic sediments are widely distributed in estuarine and coastal waters globally, especially in areas with oxygen-depleted water columns (Breitburg et al., 2018;Diaz & Rosenberg, 2008), sediment reduction and release could markedly alter estuarine aqueous δ 98 Mo values.However, this process has not been thoroughly assessed in estuarine environments.
Apart from the non-conservative behavior of aqueous Mo isotopes, there is a need for a deeper understanding of the factors influencing seasonal and spatial variations in particulate Mo isotopic compositions within estuarine systems.In reconstructing ancient oceanic redox conditions using Mo isotope records from marine sedimentary, early paleoredox studies traditionally assumed that the terrestrial-component δ 98 Mo value, derived chiefly from estuarine particulate, aligns with the average value for basalts and granites (0.0-0.30‰), and this value was applied to assess the terrestrial contribution to bulk δ 98 Mo values (Anbar et al., 2007;Thoby et al., 2019;Voegeline et al., 2010).Indeed, the limited data currently available indicate that estuarine particulate δ 98 Mo values are consistent with the average values of basalts and granites (0.0-0.30‰), exhibiting minimal variability with salinity increments (Rahaman et al., 2014;Q. Wang et al., 2021).This observed minimal variability in estuarine particulate δ 98 Mo values may stem from the limited temporospatial distribution of samples investigated in prior research (Rahaman et al., 2014; Q. Wang et al., 2021).Samples were limited to summer periods in mid-to low-salinity regions of estuarine systems, where brief water residence times, influenced using vigorous hydrodynamic processes, could limit the interaction between particulate and aqueous phases (Rahaman et al., 2014;Q. Wang et al., 2021).Estuarine systems exhibit distinct seasonal and spatial variations in hydrologic conditions, such as water residence time, potentially leading to variability in particulate Mo isotopic compositions.Systematic investigations are required to confirm this hypothesis.If so, understanding of the mechanisms behind the seasonal and spatial variability of particulate Mo isotopic compositions in estuarine systems is essential for accurately quantifying terrestrial particulate input to marine sediments.However, until now, seasonal and spatial variations in particulate δ 98 Mo values in estuarine systems have not been systematically studied.
This study measured aqueous and particulate δ 98 Mo values in summer (August) and winter (December), 2014, from the upper (Humen) to lower reaches of the Pearl River Estuary (PRE; Figure 1), along with δ 98 Mo values of bottom-surface sediments and topmost porewater, to uncover the primary mechanisms governing the non-conservative aqueous Mo isotope composition and seasonal and spatial variations in particulate Mo isotope compositions within the PRE.The study also examined fluctuations in aqueous δ 98 Mo values in the Guangzhou section of the upper PRE (Figure 1), together with physical and chemical parameters during summer (July 2016) and winter (February 2017) to enhance our comprehension of the nonconservative characteristics of the aqueous Mo isotope composition.To evaluate the effect of human activity on aqueous δ 98 Mo values in the PRE, the study examined δ 98 Mo levels from various anthropogenic sources, such as industrial, agricultural, and sanitary sewage in upper estuaries.This research would improve our understanding of Mo isotope fractionation during estuarine processes and be useful in constraining Mo isotope compositions during transport from rivers to oceans.
The PRE comprises three sub-estuaries, the Lingdingyang, Modaomen, and Huangmaobai, with the Lingdingyang being the largest and traditionally referred to as the PRE (P.Cai et al., 2015;Dai et al., 2006;B. He et al., 2014;Zhai et al., 2005).This study focused on the Lingdingyang sub-estuary and its upstream Humen and Guangzhou channels.The PRE is delineated into three reaches according to their relative distances from the Humen Outlet, namely the upper, middle, and lower reaches, which are 100 km above the outlet ( 100 km) to the outlet (0 km), 0-40 km below the outlet, and >40 km below the outlet, respectively.The upper reaches are further divided into the Guangzhou and Humen parts (Figure 1).Surface water residence time in the estuary spans 2 days in the wet season and 5 days in the dry season (Hong et al., 2018;J. Sun et al., 2014).In recent decades, the PRE has suffered dramatic oxygen depletion in its near-bottom water.Previous studies have documented hypoxic or suboxic conditions throughout the basin (Dai et al., 2006;B. He et al., 2010B. He et al., , 2014;;X. Li et al., 2018;Su et al., 2017;Y. Zhao et al., 2020), facilitating geochemical exchange reactions between sediment and water (Cao et al., 2023;Hong et al., 2018).The hypoxia generally arises from a combination of eutrophication-induced high biological productivity and restricted water exchange (Dai et al., 2006;B. He et al., 2010).Similar to numerous estuarine ecosystems worldwide, the PRE has been dramatically affected by human activities such as agriculture, industry, and urbanization in the Pearl River Delta over the past several decades (Ye et al., 2018).

Samples
Seasonal research cruises were conducted in August and December 2014 to investigate the salinity gradient in the PRE, extending from the Humen part to the coastal region near the Wanshan islands (Figure 1).Twelve sampling stations were established along transects at depths of 4-28 m, following the salinity gradients from the Humen Outlet to the open ocean (Figure 1).At each sampling site, surface water and suspended-particulate samples were collected 0.5 m below the air-water interface, bottom water samples were taken 0.5 m above the water-sediment interface, and surface sediment samples were retrieved.Water samples intended for the analysis of major and trace elements as well as Mo isotopes were collected using a flow-through water-sampling bottle, filtered through 0.45 μm Millipore nylon membrane filters, acidified immediately to pH < 2 with distilled nitric acid, and then stored in PP bottles.Samples designated for anion analysis were sealed directly in PP bottles without acid treatment (Z.B. Wang et al., 2015;Z. Wang et al., 2019;Z.-W. Wang et al., 2019).Suspended particulate matter was harvested from 10 to 50 L surface water samples by filtration through 0.45 μm Millipore nylon membrane filters.The collected particulate samples were stored at 4°C until analysis.Surface sediments were collected at each site using a grab sampler to a depth of 0-10 cm, representing 5-10 years of sedimentation accumulation.Temperature and salinity measurements were obtained using a Valeport miniCTD instrument (Valeport Ltd., Devon, UK) (Ye et al., 2017(Ye et al., , 2018)).
Non-conservative aqueous Mo isotope compositions in the PRE were studied during two additional cruises in the Guangzhou low-salinity area (Figure 1) in July 2016 and February 2017.Surface water sample collection and pretreatment from Guangzhou to Humen in the upper reaches were conducted in a manner akin to procedures used in the PRE.Parameters such as temperature, pH, DO, and salinity were measured using a pH/conductivity meter (Thermo Orion 4-star Plus) as reported by Z. B. Wang et al. (2015), Z. Wang et al. (2018), Z. Wang et al. (2019), Z.-W.Wang et al. (2019).
Samples from the three tributaries and four coastal locations on the northern coast of the SCS were obtained to delineate the PRE freshwater and seawater endmembers (Figure 1).Freshwater sampling was undertaken in the lower reaches of the three tributaries of the Zhujiang River: Gaoyao station in the Xijiang River basin, Qingyuan station in the Beijiang River basin, and Boluo station in the Dongjiang River basin.These stations are positioned at upstream from the PRE to ensure they were unaffected by tidal influences (Figure 1), which are known to characterize the riverine end-member properties of the PRE (S.-R.Zhang et al., 2007).Considering the highly seasonal nature of water discharge from these tributaries, four samples-one samples per season-were collected over 1 year at each station to determine the average of Mo concentrations and isotopic compositions for each tributary.Four seawater samples were obtained from the northern SCS (Figure 1), with pretreatments similar to those of samples collected in the PRE and subsequently refrigerated until Mo isotope analysis.Moreover, to assess the effect of anthropogenic contributions on aqueous δ 98 Mo values in the PRE, samples from diverse sources such as industrial, agricultural, and domestic sewage from the upper reaches were analyzed (Figure 1).In addition, five topmost surface pore water samples in the Pearl River Estuary sediments were obtained for Mo isotope analysis (Hong et al., 2018).

Methods
Physical and chemical characteristics of water samples, including temperature, DO, salinity, total suspended solids (TSS), and chlorophyll a, NO 3 , NO 2 , and NH 4 + contents; and suspended particulate matter, including particulate organic carbon (POC), particulate nitrogen (PN), δ 13 C POC , and δ 15 N PN recorded in the August and December 2014 cruises have been published in Ye et al. (2017Ye et al. ( , 2018) ) and are shown in Tables 1 and 2, respectively.Characteristics of water samples from the July 2016 and February 2017 cruises, including temperature, pH, DO, and salinity, are shown in Table 3. Major-and trace-element and Mo isotope compositions of water samples, suspended particulate matter, and surface sediment samples, anion contents of water samples, and phase distributions of Mo in particulate samples were determined as described below.These results are presented in Tables 1 through 5, as well as Tables S1 and S2 in Supporting Information S1.All chemical treatments and measurements were performed at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Science, Guangzhou, China.

Sequential Extraction From Particulate Phases
To understand the factors behind the temporal and spatial variations in the isotopic composition of particulate molybdenum (Mo), we studied the distribution of Mo in suspended particulates.This involved a three-step sequential extraction from eight samples, targeting the Mo in exchangeable form within crystalline iron-manganese (Fe-Mn) hydroxides, and associated with organic matter (Marks et al., 2015;Siebert et al., 2015;Z. Wang et al., 2018Z. Wang et al., , 2020;;Wiederhold et al., 2007).
Initially, exchangeable Mo was leached with 0.5 M hydrochloric acid (HCl) at room temperature.Subsequently, Mo embedded in crystalline Fe-Mn hydroxides was extracted using a 1 M hydroxylamine hydrochloride (NH 2 OH-HCl) solution in 1 M HCl.The final extraction targeted Mo bound to the oxidizable organic fraction employing a treatment of the residue with approximately 5 mL of 30% hydrogen peroxide (H 2 O 2 ) in 0.02 M nitric acid (HNO 3 ).Following each extraction, we collected the supernatant via centrifugation for analysis.

Major Ions and Elements
The analysis of anions in Pearl River Estuary (PRE) water samples was conducted using ion chromatography (Dionex ICS-900), achieving a precision of ±5% or better (RSD).The methodology was detailed by Z. B. Wang et al. (2015).Cation concentrations in water samples, along with the major elements in particulate matter postdigestion with HF-HCl-HNO 3 , were quantified using ICP-AES (Thermo Element II) as per the protocol established by X. Li et al. (2002).The major-element composition of surface sediments was assessed via X-ray fluorescence spectrometry (XRF; Rigaku ZSX100e), adhering to the methods outlined by X. Li et al. (2002).Analytical precision for sediment samples was within ±3%, and for particulate matter and water samples, within ±5%.Geochemistry, Geophysics, Geosystems

Trace Elements
Trace element quantification in water, supernatant, and solid samples from the PRE was performed using inductively coupled plasma-mass spectrometry (ICP-MS; Thermo Icap Qc).Water samples collected from the Guangzhou segment were directly analyzed using ICP-MS (Thermo Scientific Element) without dilution.Conversely, samples from the upper (Humen) to the lower reaches required dilutions up to 100-fold, contingent upon their salinity levels.Solid samples, including surface sediment and particulate matter, underwent digestion with HF-HCl-HNO 3 following the procedure described by X. Li et al. (2002).The precision for trace element analysis was maintained at better than ±5% (RSD).

Mo Isotopes
Mo isotope compositions of water, topmost surface porewater, suspended particulate, surface sediment, and wastewater samples were determined using the double-spike method of J. Li et al. (2014).Filtered and acidified water samples were spiked with a 100 Mo-97 Mo double-spike solution and allowed to equilibrate before Mo separation.For suspended particulates and surface sediments, 45-100 mg of each sample was precisely weighed into a 15 mL PFA beaker and mixed with the double-spike solution before digestion overnight at 120°C with about 6 mL of a 2:1 HF (22 mol L 1 ) + HNO 3 (14 mol L 1 ) mixture.After drying at 120°C, the residue was dissolved in 1 mL conc.HCl and again evaporated to dryness.The residue was finally dissolved in 2 mL of a mixture of HF (0.1 mol L 1 ) + HCl (1 mol L 1 ) for chromatographic separation.

Aqueous Mo Concentrations and δ 98 Mo aque. Values
Aqueous Mo concentrations and δ 98 Mo aque.values measured in the PRE, together with relevant geochemical and physical parameters such as salinity, temperature, and DO, and total suspended solid (TSS), chlorophyll a, major-   2c).Across both seasons, Mo concentrations decreased from the Guangzhou section and increased from the Humen section to the lower reaches (Figure 2a).In contrast, δ 98 Mo aque.values exhibit an increasing trend from the Guangzhou section to the lower reaches in both  4.
seasons, with a marked increase at the beginning of the Humen part (Figure 2c).Winter aqueous Mo concentrations and δ 98 Mo aque.values are distinctly higher than those in summer due to the dilution effect.
Establishing a theoretical conservative mixing line for the freshwater and seawater endmembers is critical for evaluating the conservativeness of PRE Mo concentrations and δ 98 Mo values.As the PRE receives freshwater discharge from the three major tributaries of the Pearl River, the Mo content and δ 98 Mo value of the freshwater endmember can be estimated from the weight averages of these tributaries, based on their relative water discharges into the PRE.Table 4 presents the Mo concentrations and δ 98 Mo values for each tributary across four seasons, their discharge to the PRE, and the resultant calculated freshwater endmembers.The seawater endmember corresponds to the average Mo concentration and δ 98 Mo value from four seawater samples from the SCS (Table 4), aligning with global oceanic values (Archer & Vance, 2008;Siebert et al., 2003).Based on these endmembers, the theoretical conservative mixing lines are shown in the aqueous Mo aque.-salinity and δ 98 Mo aque.-(1/Mo) plots (Figures 3a and 3b).In addition, aqueous δ 98 Mo aque.-salinity plot in the PRE is also shown in Figure S1 of the Supporting Information S1.
Measured data reveal trends in aqueous Mo concentration and δ 98 Mo aque.values that diverge from the theoretical mixing line throughout the PRE basin.Aqueous Mo concentrations in the upper and middle reaches markedly exceed the mixing line in both seasons, while in the lower reaches, concentrations are marginally higher than the mixing line (Figure 3a).In contrast, the aqueous δ 98 Mo aque.values in both seasons significantly deviate downward from the theoretical mixing line across the PRE basin (see Figure 3b).Notably, this deviation is most pronounced in the middle and upper reaches, with the extent of this deviation from the theoretical mixing line being much greater than that reported in prior studies (Pearce et al., 2010;Rahaman et al., 2014;Scheiderich et al., 2010;Q. Wang et al., 2021).It is worth noting that this deviation from the theoretical mixing line is more pronounced during winter than in summer in the PRE.Conversely, in the winter, δ 98 Mo SPM values in the particulates exhibited a minor increase from 0.49 to 0.53‰ in the upper reaches (-40-0 km; salinity 3.8-10.8‰),and a marked increase, from 0.48 to 1.29‰, from middle to lower reaches (0-80 km; salinity 14.9-30.5‰).Consequently, particulate δ 98 Mo SPM values are more depleted in the summer than in the winter.

Discussion
This section will discuss two distinct aspects: the mechanisms governing the non-conservation of aqueous δ 98 Mo aque.values and the spatial variation of particulate δ 98 Mo SPM values in PRE will be discussed respectively.

Non-Conservative Aqueous δ 98 Mo aque. Values
Aqueous δ 98 Mo aque.values in most of the PRE basin exhibit non-conservative behavior below the theoretical mixing line (Figures 3a and 3b).This trend could be attributed to factors such as desorption (release) from suspended particulates, anthropogenic input, and diagenetic release from sediments (Audry et al., 2007;Dalai et al., 2005;Gurumurthy et al., 2017;Masson et al., 2011;Mohajerin et al., 2016;Pearce et al., 2010;Rahaman et al., 2010Rahaman et al., , 2014;;Scheiderich et al., 2010;Waeles et al., 2013).This discussion explores the potential mechanisms behind non-conservative behavior of aqueous Mo isotope composition in the PRE.We highlight that the diagenetic release from surface sediments is the most likely mechanism dominantly controlling lower aqueous δ 98 Mo aque.values in the PRE.

Desorption From Particulate Matter
Previous studies have demonstrated that Mo release from particulate matter can reduce aqueous δ 98 Mo aque.values beneath the theoretical mixing line in the Borarfjoreur estuary (Pearce et al., 2010), attributed to the particulate matter's light Mo isotopic signature (Rahaman et al., 2014;Z. B. Wang et al., 2015).Given the elevated concentration of particulate matter in the upper PRE reaches, we focused on this region (Humen part; samples HM-01 to ZJ-01) in winter and summer 2014 to evaluate the effect of Mo release from suspended particulate matter on aqueous Mo concentration and its δ 98 Mo aque.values.In the Humen part, a consistent decline in particulate Mo/Al ratios was observed across both seasons (Figure 4c), suggesting that a release of particulate Mo into the dissolved load and a consequent reduction in aqueous δ 98 Mo aque.values.However, further quantitative analysis fails to support this hypothesis.Utilizing TSS data and Mo concentrations in the Humen part for both summer and winter (Tables 1 and 2), our estimates suggest that suspended particulate matter could contribute 0.05-0.46nmol L 1 and  4.
0.06-1.10nmol L 1 to the aqueous Mo load in winter and summer, respectively, presuming that complete particulate Mo release to the solution.However, the estimated upper limit of Mo release from particulate matter is substantially lower-by two orders of magnitude-than the "excess" aqueous Mo concentration, which amounts to 4.42-16.5 nmol L 1 in winter and 4.28-6.53nmol L 1 in summer.These "excess" aqueous Mo concentrations were determined by deducting the Mo concentrations expected from the theoretical conservative mixing line from the actual concentration at the corresponding salinity.This indicates that the Mo released from particulate matter is insufficient to account for the excess found in the dissolved phase.Concurrently, we assessed the impact of desorption from particulate matter on the isotopic composition of Mo in the water using a three-end-member mixing model (Figure S2 in Supporting Information S1).The end-members in this model are river water,  Taylor and McLennan (1985).Conversely, the pink dashed line, accompanied by its respective values, indicates the revised compositional data for the upper continental crust as provided by Rudnick and Gao (2014).
seawater, and particulate matter.We selected particulate matter from Humen 01 as an end-member, utilizing parameters such as Mo concentration, its isotopic signature, and the concentration of particulates in summer samples.The model's results suggested that after complete liberation of Mo from particulates, the isotopic composition of estuarine water would closely follow the theoretical mixing line.However, these projections diverged from the empirical data obtained from actual samples.We hypothesize that desorption from particulate matter exerts a minimal influence on the Mo isotopic compositions in the aqueous phase within the PRE basin.
The principal cause seems to be the substantially lower proportion of particulate Mo relative to the total Mo content, encompassing both dissolved and particulate forms in the PRE system.Particulate Mo constitutes a mere 0.00%-0.08% of the total Mo content.This aligns with research on various riverine and estuarine systems, which report particulate Mo contributions of 0.00%-8.12% of total Mo content (Archer & Vance, 2008;Dalai et al., 2005;Gurumurthy et al., 2017;Z. B. Wang et al., 2015).

Anthropogenic Contributions
Previous studies have shown that anthropogenic input such as sewage may affect aqueous Mo concentrations in aquatic systems of rivers and estuaries (Audry et al., 2007;Dalai et al., 2005;Ekka, Liang, Huang, Huang, & Lee, 2023;Ekka, Liang, Huang, & Lee, 2023;Gurumurthy et al., 2017;Rahaman et al., 2010Rahaman et al., , 2014)).Rahaman et al. (2014) suggested that significant discharges of Mo from the steel industry into the Tapi estuary may change its aqueous Mo isotopic composition.Numerous megacities situated along the upper reaches of the PRE could contribute anthropogenic Mo, potentially changing the natural input and its aqueous isotopic composition.To assess this effect, anthropogenic-source δ 98 Mo anth.values from industrial, agricultural, and sanitary sewage were determined.Results reveal that anthropogenic sources possess relatively elevated δ 98 Mo anth.values ranging from 0.74 to 1.33‰, averaging 0.98‰ (Table 4).This is consistent with the results of previous investigations into mine-waste drainage (Skierszkan et al., 2016(Skierszkan et al., , 2017(Skierszkan et al., , 2019)).If anthropogenic input were predominantly controlling the non-conservative PRE Mo isotopic composition, its δ 98 Mo aque.values would be substantially lower than those of the middle and upper reaches, positioning them in the bottom-left zone in the (1/Mo)-δ 98 Mo aque.plot (Figure 3b).However, the average value of these sources is higher than most of the upper and middle water, occupying a central position in Figure 3b, suggesting they are unlikely to be the dominant factor for reduction aqueous δ 98 Mo aque.values in the PRE.

Diagenetic Release From Sediment
Reductive release of Mo during sediment diagenesis is a possible contributor to aqueous Mo in the estuary (Dalai et al., 2005;Gurumurthy et al., 2017).Mo demonstrates a robust affinity for Fe-Mn oxides under oxidizing conditions (Z.Wang et al., 2018Wang et al., , 2020)); however, under reductive conditions at the sediment-water interface, unstable Fe-Mn oxides may release Mn, Fe, and Mo to the water column.Our surveys indicate that the dissolved oxygen levels in the bottom water in the upper (Humen part) and middle reaches of the PRE generally fall below 6.0 mg/L and markedly lower, often beneath than 3.0 mg/L, in the Guangzhou part (Table 1).Earlier studies have documented a pronounced depletion of oxygen in the bottom water of these basins over recent decades (Dai et al., 2006;B. He et al., 2010;X. Li et al., 2018;Su et al., 2017;Y. Zhao et al., 2020).Such diminished oxygen concentrations in bottom water may promote the formation of the suboxic or anoxic conditions in their surface sediments.Observation of pore water also suggests that despite oxygenation in the bottom column water, the sedimentary column readily becomes suboxic or anoxic condition, due to the restricted oxygen infiltration from bottom water in the PRE (Hong et al., 2018).According to prior studies and our data, it appears that the PRE surface sediments may be transitioning into a suboxic or anoxic state.Under these conditions, reduction of Fe-Mn oxides and NO 3 in surface sediment may occur, liberating Fe, Mn, and NO 2 into the water column (Froelich et al., 1979).This assertion is corroborated by the observed ranges of Mn and NO 2 concentrations in the PRE.As indicated in Tables 1 and 3, the concentrations are in the ranges of 1.75-237 μg L 1 and 0.00-9.65 mg L 1 , with means of 64.8 μg L 1 and 0.79 mg L 1 , respectively, greatly above than the theoretical mixing line demarcating the freshwater (Pearl River) and seawater (South China Sea) endmembers (Figures 2f and 2h) throughout the PRE basin.The mean of dissolved Fe concentrations was recorded at 9.02 μg L 1 in the water column of the Guangzhou part of the upper estuary (Table 3), with slightly higher than the freshwater endmember (6.57μg L 1 ) reported by R. Zhang et al. (2019).The observed enrichment in Mn and NO 2 , along with modest increase in Fe indicates that the development of redox conditions conductive to the Fe-Mn oxide reduction stage in PRE surface sediments (Froelich et al., 1979).
In this stage, Mn(IV) and Fe (III) are reduced to Mn(II) and Fe (II) within surface sediments and subsequently released into the water column, along with Mo previously adsorbed on the Fe-Mn oxide (Emerson & Huested, 1991;Malcolm, 1985;Morford et al., 2005;Zheng et al., 2000), leading to a concurrent rise in the concentrations of Mn, Fe, and Mo in the water column (M.Beck et al., 2008;A. J. Beck et al., 2010;Z. He et al., 2021;O'Connor et al., 2015;Scholz et al., 2017;Shaw et al., 1990).This is evidenced by a positive correlation between the Mn/S and Mo/S ratios (with Mn and Mo concentrations normalized by salinity to mitigate dilution effects) (Figure 6a).Regrettably, the relationship between Fe/S and Mo/S ratios cannot be depicted owing to the absence of aqueous Fe concentration data from the upper (Humen part) to lower reaches of the PRE in 2014.The release of Mo from the underlying sediment through reductive mobilization of Fe-Mn oxide accounts for the elevated estuarine aqueous Mo concentrations and may also influence their δ 98 Mo aque.values.As the sedimentary Fe-Mn oxide phase is characterized by light δ 98 Mo signature (ranging from 0.62 to 0.05‰, Goswami et al., 2022;Wang et al., 2018), their reduction is likely to yield lighter δ 98 Mo aque.values in the aqueous phase.Consequently, with the rise in aqueous Mn and Mo concentrations, reflected by Mn/S and Mn/S ratios respectively, the freedilution aqueous δ 98 Mo aque.values exhibit a decline (represented by ∆δ 98 Mo Mea-Mix values calculated by subtracting δ 98 Mo Mix values based on the theoretical mixing line from measured δ 98 Mo Mea values at a given salinity), with a negative correlation between ∆δ 98 Mo Mea-Mix values and Mo/S and Mn/S ratios (Figures 6b and 6c).
Reductive mobilization of Fe-Mn oxide is thus likely the cause for δ 98 Mo aque.values being below the theoretical mixing line in the PRE.This process aligns with the mechanism leading to the lower seawater δ 98 Mo values in the Bay of Bengal (Goswami et al., 2022).Goswami et al. (2022) suggested that reductive release of the Fe-Mn oxyhydroxide phases in shelf and slope sediment may supply isotopically lighter Mo into the norther coastal waters of the Bay of Bengal via diffusion and advection.
Mn and Fe concentrations in sediment cores and pore water from the PRE further support our hypotheses.Cao et al. (2023) observed that most Mn/Al and Fe/Al ratios in PRE sediment cores fall below the global crustal averages reported by Rudnick and Gao (2014) and Taylor and McLennan (1985), with the exception of Fe/Al ratios at stations P06 and A09.These ratios exhibit a distinct decrease from the lower to the upper sediment layers, especially at upstream stations P03, P06, and A03.This suggests that Fe-Mn oxides are likely subject to reduction reactions.
The pore water chemistry in PRE sediments offers more direct evidence.Cao et al. (2023) noted elevated Mn and Fe levels in pore water, indicative of a suboxic zone in the upper 8-10 cm, characterized by the reduction of nitrate, manganese, and iron.Similarly, Hong et al. ( 2018) also reported higher pore water concentrations of Mn and Fe compared to the overlying bottom water, suggesting a release of these elements from the pore to the bottom water.Utilizing the 224 Ra/ 228 Th disequilibrium method, they reported significant benthic fluxes of dissolved Mn and Fe ranging up to 97 and 27 mmol m 2 d 1 , respectively.These results align with the hypothesis of a continuous release of Mn and Fe from the PRE sediment pore water to bottom water, driven by early diagenetic reduction of Fe and Mn oxides.Mo concentrations in topmost pore water during summer 2015 ranged from 59.0 to 133.9 nmol/L, which is markedly higher than those in the corresponding bottom waters from summer 2014, as reported in our study (refer to Table 4).This pronounced concentration gradient is indicative of Mo release from pore water to bottom water in the PRE.Employing the 224 Ra/ 228 Th disequilibrium method, as delineated by Hong et al. ( 2018), we have calculated the Mo flux from pore water.Our calculations suggest that benthic Mo fluxes (Fi) within the PRE exhibit variability ranging from 0.21 to 56.4 nmol m 2 d 1 (Table S2 in Supporting Information S1), implying that a majority of the stations are experiencing Mo release from pore water.The most substantial fluxes were observed in the upper and middle estuaries, which correspond to zones of pronounced Mo enrichment, especially in areas of low salinity within the PRE (Table S2 in Supporting Information S1).These observations underscore the significance of benthic release as a major contributor to the Mo levels in the PRE's water column.An exception is found at station A03, where the Mo release flux appears negative, likely due to the substantial error in the F Ra /F M ratio (F Ra and F M represent the 224 Ra flux induced by porewater exchange and molecular diffusion, respectively.Hong et al., 2018).By applying the benthic release Mo fluxes derived from the 224 Ra/ 228 Th disequilibrium method, we can roughly estimate the net effect of benthic input on dissolved Mo concentration (ΔMo) in the water column.The net impact is determined using the following equation: Here, ∆Mo denotes the change in Mo concentration, Fi is the Mo flux, H is the water column height, and τ the estuarine water residence time.The water residence time for the wet season in the Pearl River Estuary (PRE) is approximately 2.2 ± 0.2 days (Hong et al., 2018;J. Sun et al., 2014).By applying this estimate, we ascertain that benthic inputs alter the water column Mo concentration from +0.62 to +17.7 nmol L 1 (with "+" indicating a net increase) except for the A03 station.This calculated range corresponds the "excess" aqueous Mo concentrations in corresponding water samples from the summer, which show an "excess" ranging from 0 to 5.16 nmol L 1 .
Surface sediment diagenesis likely facilitates the release of Mn, Fe, and Mo into the water column, initially through the reduction of Mn (IV) and Fe (III) to their divalent states within the sediment, followed by their release along with adsorbed Mo into the pore water and subsequently into the water column.Dissolved Mn, Fe, and Mo may be released into pore water via diffusion or physical perturbation, particularly in winter, when wind-induced mixing and tidal pumping intensify (X.Li et al., 2018;D. Zhang et al., 2013;L. Zhang et al., 2013).This is corroborated by earlier experiments on the oxidation of resuspended anoxic sediments (Kowalski et al., 2013).The experiment demonstrated that sediment resuspension precipitates a rapid release of substantial Mo quantities into the aqueous phase.
This release Mo from sediments would result in decreasing δ 98 Mo aque.values in the PRE's water column.This conclusion is also supported by the δ 98 Mo PW values in the topmost pore water from the PRE sediment (Table S2 in Supporting Information S1).The δ 98 Mo PW values in topmost pore water from the PRE sediment span a range of 0.63-2.12‰,which is relatively lighter than those from previous studies (Z.He et al., 2021;McManus et al., 2002).Notably, these values are obviously lighter than those of the bottom waters measured in the same region in 2014, suggesting a preferential release of lighter Mo isotopes from pore water into the bottom waters.It is noteworthy that the δ 98 Mo PW value levels in pore water do not reflect those in surface sediments, instead exhibiting a progressive increase from freshwater to marine settings.This increasing trend in the δ 98 Mo PW values likely signify continuous interactions between pore water and the overlying water column, resulting in the elevated δ 98 Mo PW values observed in topmost pore water from freshwater to marine settings.
Significant seasonal variations in the δ 98 Mo aque.values within the PRE basin have been observed.Notably, deviations from the theoretical mixing line are more marked during the winter season compared to the summer.This phenomenon is likely attributable to the extended water residence times in winter (∼5.0 ± 0.5 days, J. Sun et al., 2014), which may lead to an augmented release Mo from the sediments-a conclusion supported by further analysis (Shimmield & Price, 1986).An initial assessment of benthic contributions to the aqueous Mo concentration (ΔMo) in the water column during winter 2014 reinforced this hypothesis.The computational method, encompassing both the flux rate and the calculation of benthic contributions, is consistent with the methodology used for the 2014 summer analysis.Variations arise in the concentration gradient, water depth, and residence time, as detailed in Table S2 of the Supporting Information S1.As reported by J. Sun et al. (2014), the dry season water residence time in the PRE is approximately 5.0 ± 0.5 days.The analysis indicates that the influence of pore water input on the water column is generally more substantial during the winter than in the summer in Table S2 of the Supporting Information S1.
In addition to the reductive release of Fe-Mn oxyhydroxides into the bottom water column, the release of the authigenic Mo-S precipitation compounds such as thiomolybdates formed in the reducing sediments may also modify the Mo isotope composition in the bottom water column.Authigenic Mo is typically removed from the water column and precipitates with hydrogen sulfide in sediments characterized by porewater hydrogen sulfide concentrations exceeding 0.1 μM (Zheng et al., 2000).However, the likelihood of the release from authigenic Mo-S compounds influencing the reduced aqueous δ 98 Mo aque values in PRE is minimal because (a) the PRE surface sediments lack the hydrogen sulfide (Hong et al., 2018), inhibiting the formation of compounds like thiomolybdates, (b) furthermore, even under anoxic/euxinic sedimentary conditions that could foster authigenic Mo-S precipitation formation, their impact on aqueous δ 98 Mo aque values would be minimal due to insignificant fractionation during such processes (Arnold et al., 2004;Matthews et al., 2017;Neubert et al., 2008).

Variability in Particulate δ 98 Mo SPM
The δ 98 Mo SPM values of suspended particulates exhibit notable spatial variability in the PRE (Figure 4g), with a consistent increase from the upper to the lower reaches of the estuary in both seasons.Prior research indicates that the geochemical composition of suspended particulate matter in estuarine systems may be influenced by the resuspension of surface sediment, particulate matter desorption/adsorption, redox reactions, and pH fluctuations (Audry et al., 2006;Bauer et al., 2017;Guinoiseau et al., 2018;Gurumurthy et al., 2017;Little et al., 2018;Mohajerin et al., 2016;Rahaman et al., 2010Rahaman et al., , 2014;;Renjan et al., 2017;Samanta & Dalai, 2016;J. Zhang & Liu, 2002;Zwolsman & van Eck, 1999).In this section, we analyze the factors influencing the spatial variability of particulate δ 98 Mo SPM values from the upper to lower reaches of the PRE.We emphasize that desorption/ adsorption processes on particulate matter dominantly control the δ 98 Mo SPM variability values from the upper to lower PRE.

Resuspension of Surface Sediment
Previous studies have observed alteration in chemical compositions of particulate matter resulting from tidal hydrodynamic-induced sediment resuspension (Dalai et al., 2005;Gurumurthy et al., 2017;Rahaman et al., 2010), which may also affect particulate δ 98 Mo SPM values.If sediment resuspension considerably affects particulate δ 98 Mo SPM values, the observed trends from the upper to lower PRE would align with those of the corresponding surface sediments.However, the δ 98 Mo SPM values in suspended particulate increased from the upper to lower reaches in both seasons, diverging from the trend observed in surface sediments (Figure 4g).Additionally, the δ 98 Mo SPM values in particulates are generally heavier in winter and lighter in summer compared to those in corresponding surface sediments (Figure 4g).Consequently, the resuspension of surface sediments appears to have a negligible effect on particulate matter δ 98 Mo SPM values in the PRE.

Adsorption/Desorption in Particulate Matter
As river-borne particulate matter enters an estuary, its chemical composition can undergo significant modification through exchange processes-desorption and adsorption-with the dissolved load, influenced by environmental shifts such as salinity, pH, and redox conditions.Particulate matter can release elements into the surrounding solution via ion-exchange processes or adsorb elements through flocculation (Bauer et al., 2017;Gurumurthy et al., 2017;Mohajerin et al., 2016;Rahaman et al., 2014;Renjan et al., 2017), both of which can potentially affect the Mo concentration and isotopic composition in particulate.To study the variations in particulate composition caused by desorption/adsorption within an estuarine system, it is essential to normalize the particulate composition against lithogenic elements to account for the effects of territorial material such as clay minerals.Aluminum (Al), predominantly associated with lithogenic matter, was selected as the normalizing element (Bauer et al., 2017;Gurumurthy et al., 2017;Mohajerin et al., 2016;Rahaman et al., 2014;Renjan et al., 2017).

Variations in Particulate δ 98 Mo SPM Values in Summer
During summer, particulate Mo/Al ratios in PER decrease from 0.18 to 0.08, from the upper to lower reaches, following the salinity gradient (Figure 4c).This suggests that the desorption of Mo from particulates may occur, likely driven by enhanced ion-exchange capacity with higher pH levels.Mo adsorbed on particulate matter in riverine conditions is likely to be released into the estuarine dissolved phase due to alkaline pH and elevated ionic strength.Similar trends in particulate Mo/Al ratios have been reported in the Nethravati, Mandovi, and Zuari estuaries (Bauer et al., 2017;Gurumurthy et al., 2017;Renjan et al., 2017), correlating with salinity gradient.Particulate Fe/Al ratios also decrease in a manner akin to Mo/Al ratios and show a positive correlation with them (Figure 4e), indicating that the release Mo from Fe-Mn oxides in suspended particulate matter into solution in summer.Sequential-extraction experiments corroborate this, revealing that the Fe-Mn (hydro) oxide phase predominantly hosts Mo in suspended particulate matter, comprising 64.3% and 60.0% of bulk-sample Mo in samples SZJ-02 and SZJ-04, respectively (Table 5).Exchangeable Mo accounts for 0.53%-2.36% of bulk-sample Mo, while 0.67%-0.99% of Mo is associated with organic matter (Table 5).Consequently, Mo desorption from Fe-Mn oxides in suspended particulate matter may alter the Mo/Al ratios, impacting their Mo isotopic composition.Fe-Mn oxides are typically characterized by lighter Mo isotopic signatures (Z.B. Wang et al., 2015;Z. Wang et al., 2018), and as Mo is increasingly released from Fe-Mn oxides into solution, the δ 98 Mo SPM values in particulate progressively increase.As a result, particulate δ 98 Mo SPM values exhibit a negative correlation with Fe/Al ratios (Figure 4e).

Variations in Particulate δ 98 Mo SPM Values in Winter
During winter, a gradual decline in Mo/Al ratios from 0.15 to 0.09 was observed in the upper reaches (-40-0 km; salinity 3.80-10.8‰),while an increase from 0.12 to 0.34 was noted from the middle to lower reaches (0-80 km; salinity 14.9%-30.5%)(Figure 4c).The prevailing trend suggests that particulate matter desorbs Mo in the upper reaches and adsorbs it in the middle-lower reaches.
Mo desorption in the upper reaches could take place in winter, similar to summer, attributed to the augmented ion-exchange capacity from rising pH levels.Fe-Mn oxides within suspended particulate matter potentially liberate Mo into an aqueous phase in the upper reaches, mirroring the pattern observed in Fe/Al ratios comparable to Mo/Al ratios (Figure 4e).This is supported by the sequential extraction experiment, revealing that the Fe-Mn (hydro) oxide phase predominantly hosts Mo in suspended particulate matter during winter, accounting for 50.2%-66.5% of the total Mo concentration (Table 5), aside from sample WZJ-07 at 35.5%.As in summer (Section 4.2.2.1), the release of lighter δ 98 Mo value results in a heavier residual δ 98 Mo value heavier signature during winter.
In the middle and lower reaches, the Mo/Al ratios indicate the potential adsorption of Mo from solution onto particulate matter (Figure 4c).Concurrent increases in Fe/Al and Mo/Al ratios (Figure 4e) indicate that Mo enrichment in suspended particulate matter is likely governed by the flocculation of dissolved iron, as colloidal Fe and oxyhydroxide integrate into fine aluminosilicate particles under conditions of moderate to high salinity, a process influenced by salinity and particle size dynamics (Guo et al., 2015;Ye et al., 2015Ye et al., , 2016)).Iron oxides associated with particulate matter may sequester or adsorb additional elements such as Mo from the dissolved phase, leading to their enrichment in particulate matter in the middle lower reaches.This notion is further supported by sequential extraction experiments, demonstrating an increasing trend in the Fe (Fe Fe-OX /Fe Bulk ) and Mo (Mo Fe-OX /Mo Bulk ) fractions of the Fe-Mn oxide phase within the bulk Fe and Mo concentrations alongside increasing salinity.A positive correlation was evident between these fractions (with the exception of the Mo Fe-OX /Mo Bulk ratio of sample WZJ-07) (Figures 7d-7f).Therefore, the adsorption of Mo on particulate Fe-Mn oxides could govern the upward trend in particulate δ 98 Mo SPM values concurrent with rising salinity during winter.
The corresponding aqueous δ 98 Mo aque.values exhibit heavier signatures that intensify with salinity (Figure-2c), while particulate δ 98 Mo SPM values show a correlation with the relative proportions of Fe and Mo in the Fe-Mn oxide phase compared to their bulk Fe and Mo concentrations (Figure 7e).Based on these characteristics, the upward trend in particulate δ 98 Mo SPM values with salinity from the middle to lower reaches of the PRE may be explained as follows.The progressively heavier aqueous δ 98 Mo aque.values lead to the adsorption of the heavier isotope (relative to particulate matter) on Fe-Mn oxides from the middle to lower reaches, resulting in an increase in particulate δ 98 Mo SPM values with salinity, although the Fe-Mn oxides also adsorb the lighter isotope from solution (Barling & Anbar, 2004;Goldberg et al., 2009).It is posited that Mo isotope fractionation during adsorption on Fe-Mn oxides remains consistent from the middle to lower reaches.A prior study (Rahaman et al., 2014) reported that particulate-matter δ 98 Mo SPM values and Fe/Al ratios escalate with salinity.
Sample WZJ-07 diverges from the regression line depicted in Figures 7d-7f, exhibiting a notably lower Mo Fe-OX / Mo Bulk ratio compared to other samples.Consequently, it is plausible that an additional substance contributes to Mo enrichment in this sample, alongside Fe oxides (Wichard et al., 2009).The elevated particulate organic carbon (POC) content and heavy δ 13 C POC value of WZJ-07 (Table 2) imply that Mo enrichment (with a high Mo/Al ratio) of this sample may involve adsorption by organic matter (e.g., plankton utilization).Sequential extraction experiments support this, indicating a higher proportion of Mo in the organic-matter phase compared to other samples.While organic matter preferentially adsorbs the lighter Mo isotopes (King et al., 2018), the particulate δ 98 Mo SPM value of sample WZJ-07 would increase correspondingly with the heavier δ 98 Mo values water sample.

Estimating the Fractionation Factor and Its Relationship With δ 13 C Values
In summary, the variability of the PRE particulate δ 98 Mo SPM values in both seasons is predominantly controlled by the desorption/adsorption of Fe-Mn oxides with a consistent fractionation factor.Estimating the Mo isotope fractionation factor between the particulate and aqueous phases, followed by comparison with Mo adsorption on Fe-Mn oxide experiments, effectively verified our speculation.Given that the estuary functions as an open system with a sustained influx of aqueous Mo, it is not feasible to estimate the fractionation factor for particulate adsorption and desorption in the PRE using the Rayleigh fractionation model, which is designed for closed systems.However, assuming isotope equilibrium fractionation, this fractionation factor can be estimated using the following equation.where δ 98 Mo aque.and δ 98 Mo SPM represent the δ 98 Mo values in aqueous and particulate phases, respectively.The results show that the fractionation factor between the particulate and aqueous phases ranges from +0.60 to +1.67‰, averaging 1.25‰.This is within the experimentally derived isotope composition of seawater Mo adsorbed to the Fe (oxyhydr) oxide minerals, which is between +0.61 and +2.64‰, with an average of 1.37‰ (Goldberg et al., 2009).It is therefore concluded that the adsorption and desorption of particulate Fe (oxyhydr) oxides dominantly controls the variation in PRE particulate δ 98 Mo SPM values.
In other words, the increasing trend in particulate matter δ 98 Mo SPM values from the upper (Humen part) to the lower reaches of the PRE may be closely related to the corresponding aqueous δ 98 Mo aque.signatures.The fresh water endmembers are characterized by light δ 98 Mo values, whereas seawater endmembers have heavy values, with authigenic light Mo in particulate matter in the upper reaches being derived mainly from river water and that in the lower reaches from seawater with consistent fractionation factor, ultimately resulting in particulate δ 98 Mo SPM values increasing consistently with salinity.This is also supported by the variation of δ 13 C values in suspended-particulate organic carbon (POC) (Figure 5), which is similar to that of particulate δ 98 Mo SPM values, with the two being strongly correlated (Figure 7c).This trend in POC δ 13 C values is likely due to a decreasing terrestrial organic matter contribution (with light δ 13 C values), and a seaward-increasing contribution of marine POC with heavier δ 13 C values (Ye et al., 2018).

Implications of Riverine Mo Input to the Ocean
Rivers are recognized as the predominant source of dissolved Mo for oceans globally, with estuarine system dynamics critically influencing riverine aqueous Mo isotopic compositions in the open ocean.Prior research posits that estuarine processes may slightly modify the dissolved δ 98 Mo aque.values of the riverine input upon their oceanic entry (Archer & Vance, 2008;Pearce et al., 2010;Rahaman et al., 2014;Q. Wang et al., 2021).However, our calculations-subtracting the δ 98 Mo Mix values derived from the theoretical mixing line (depicted in Figure 3b) from the measured δ 98 Mo Mea values at a given Mo concentration-reveal a significant reduction in the riverine input dissolved δ 98 Mo aque.values conveyed to the ocean by PRE.This decrease ranges from 0.04 to 1.51‰, averaging at 0.67‰.Our study highlights the impact of reductive release from suboxic-anoxic sediments on non-conservative behavior of Mo isotopes in estuaries and their ultimate flux to the open ocean.Given the global prevalence of suboxic-anoxic sediments in estuaries and coastal areas (Breitburg et al., 2018;Diaz & Rosenberg, 2008), their reduction releases substantial amounts of Mo to the water column (M.Beck et al., 2008;A. J. Beck et al., 2010;O'Connor et al., 2015), significantly lowering the Mo isotopic composition of riverine input to oceans.For instance, historical climatic shifts that expand or contract anoxia-suboxia in these sediments can modulate the release of Mo to the ocean, profoundly affecting the isotopic composition of riverine inputs.Therefore, reduction and release of Mo from suboxic-anoxic sediments can substantially decrease the δ 98 Mo aque.values of riverine input to oceans.It is imperative to conduct more systematic studies to quantify the Mo from suboxic-anoxic sediments in estuaries and coastal areas, its isotopic signature, and the degree to which it alters riverine Mo input to oceans prior to utilizing sedimentary Mo isotopic composition to investigate historical shifts in global oceanic redox states.
Prior research indicates minimal variability in particulate δ 98 Mo SPM values correlating with salinity increments in estuarine systems, possibly owing to the subdued interchange between particulate and aqueous phases, a result of short water retention times propelled by vigorous hydrodynamic forces during summer (Rahaman et al., 2014;Q. Wang et al., 2021).This investigation revealed notable seasonal and spatial variability in particulate δ 98 Mo SPM values within the PRE, exhibiting an upward trend in conjunction with salinity.This observed salinity-associated trend is likely related to intensified particulate-aqueous interexchange, especially during winter months.Extended residence times, resulting from diminished hydrodynamic forces, may amplify this interaction in winter, leading to an isotopically heavier particulate phase as salinity rises.This pattern could be widespread across estuarine systems worldwide.A compilation of particulate δ 98 Mo SPM data from various estuarine systems, including the PRE, indicates a consistent incremental trend with salinity (no shown) (Rahaman et al., 2014;Q. Wang et al., 2021).This implies that riverine particulate δ 98 Mo SPM values, after transiting through an estuary and contributing to marine sediments may exceed the average value for basalts and granites (0.00 ∼ 0.30‰), reaching as high as 1.62‰.Early paleoredox studies typically postulated that the terrestrial fraction, mainly derived from estuarine particulate influx, corresponded to the basalts and granites mean value (0.00 ∼ 0.30‰), utilizing this assumption in the evaluation of terrestrial input to marine sediments.Future paleoredox studies should exercise caution when employing this value to assess the terrestrial contribution to bulk δ 98 Mo values.

Conclusions
The investigation of seasonal aqueous and particulate δ 98 Mo values along the Pearl River Estuary (PRE) culminates in the following conclusions: (1) In the PRE, aqueous δ 98 Mo values demonstrate non-conservative behavior across both seasons.According to the geochemical characteristics of the water column, particulate matter, and pore water in the PRE, the reductive release from suboxic-anoxic sediments likely accounts for aqueous δ 98 Mo aque.values falling below the theoretical mixing line, whereas desorption from particulate matter and anthropogenic contributions appear to have minimal effect.As suboxic-anoxic sediments are widely distributed in estuaries and coastal areas globally, their reduction and subsequent release of Mo could substantially reduce the δ 98 Mo values of riverine input to oceans.

•
Aqueous δ 98 Mo values in different seasons were substantially lower than theoretical mixing-line values in the Pearl River Estuary (PRE) • Reductive release from sediments is responsible for the aqueous δ 98 Mo values being lower than theoretical levels • Fe (hydro) oxides are the dominant factors controlling the increasing trend in the particulate δ 98 Mo values with salinity Supporting Information: Supporting Information may be found in the online version of this article.

Figure 1 .
Figure 1.Simplified map of the Pearl River Estuary (after Ye et al. (2018)) showing sampling stations for all cruises and the upper, middle, and lower reaches.

Figure 2 .
Figure 2. Trends in aqueous Mo, Mn, NO 2 concentrations, and δ 98 Mo values along the sampling transect in the PRE.Figures on the left are plots against distance from the Guangzhou part to the lower reaches, and on the right against salinity.Shaded bars represent the three reaches of the PRE; the green dashed line represents theoretical mixing lines for aqueous Mn; NO 2 concentrations are based on the freshwater (East, West, and North rivers) and seawater (South China Sea) endmembers in Table4.

Figure 3 .
Figure 3. Plots of (a) aqueous Mo concentrations versus salinity, and (b) (1/Mo) ratios versus aqueous δ 98 Mo values in the PRE.Theoretical mixing lines are based on the freshwater (East, West, and North rivers) and seawater (South China Sea) endmembers in Table4.

Figure 4 .
Figure 4. Trends in Mn/Al, Mo/Al, and Fe/Al ratios and δ 98 Mo values in suspended particulate matter and surface sediment from Humen to the lower reaches of the PRE (left) and salinity gradient (right).The shaded bars represent the three reaches of the PRE.The blue dashed line, along with the accompanying value, represents the average Mn/Al, Mo/Al, and Fe/Al ratios of the upper continental crust as documented byTaylor and McLennan (1985).Conversely, the pink dashed line, accompanied by its respective values, indicates the revised compositional data for the upper continental crust as provided byRudnick and Gao (2014).

Figure 5 .
Figure 5. Trends of particulate organic δ 13 C POC values, and Fe, Mo, and Mn contents of surface sediment with distance from Humen to the lower reaches (left) and salinity (right) in the PRE.

Figure 7 .
Figure 7. Correlation plots of (a) particulate (Mo/Al) versus (Fe/Al); and (b) δ 98 Mo versus (Fe/Al) in summer of 2014.(c) Particulate δ 98 Mo versus δ 13 C POC in both seasons of 2014.(d) Particulate Fe Fe-OX /Fe Bulk and Mo Fe-OX /Mo Bulk versus salinity; (e) Fe Fe-OX and Mo Fe-OX versus bulk particulate δ 98 Mo; and (f) Fe Fe-OX /Fe Bulk versus Mo Fe-OX /Mo Bulk in winter of 2014.The Fe Fe-OX /Fe Bulk and Mo Fe-OX /Mo Bulk ratios represent the Fe and Mo ratios of Fe-Mn (hydro) oxide phases to the bulk Mo, respectively (data for the Mo Fe-OX /Mo Bulk ratios in sample WZJ-07 (black circle) were not included in regression fitting).

Table 1
Note.S: surface water; B: water 0.5 m above bottom."-" indicates that the data were not measured in the field.

Table 2
The Mo Concentrations, δ 98 Mo Values, and Some Relevant Geochemical Parameters in the Surface Suspended Particulate and Bottom Sediment From the Upper Reaches (Humen Part) to Lower Reaches of the Pearl River Estuary

Table 3
The Dissolved Mo Concentrations, δ 98 Mo Values, and Some Relevant Geochemical and Physical Parameters in Summer and Winter in 2016 in the Guangzhou Part in the Upper Reaches of the Pearl River Estuary

Table 4
Cai et al., 2004;), D. Wang et al. (2012ng et al., 2008)ameters in Three Tributaries of the Pearl River Estuary, South  China Sea, and Calculated Freshwater and Seawater EndmemberNote.The bolded values represent the average values.The data of discharge of three tributaries of the Pearl River Estuary from the previous studies (W.-J.Cai et al., 2004;F.Liu et al., 2017;S.Zhang et al., 2008)and the date of seawater Mn and NO 2 concentrations from Z.Wang et al. (2019), Z.-W.Wang et al. (2019), D. Wang et al. (2012), and Ye et al. (2015).

Table 5
The Mo Concentrations, Its Ratios (Relative to Mo Concentration in Bulk Sample) in the Three Sequential Extractions of the Suspended Particulate in the PRE a The concentration is relative to the bulk sample weight.b The ratios are relative to the bulk sample concentrations.