Overlooked shelf sediment reductive sinks of dissolved rhenium and uranium in the modern ocean

Domain: earth_science
Published: 2024-05-10
Editorial outcome: major_revision

Abstract
Rhenium (Re) and uranium (U) are essential proxies in reconstructing past oceanic oxygenation evolution. However, their removal in continental shelf sediments, hotspots of early diagenesis, were previously treated as quantitatively unimportant sinks in the ocean. Here we examine the sedimentary reductive removal of Re and U and their coupling with organic carbon decomposition, utilizing the 224Ra/228Th disequilibrium within the East China Sea shelf. We identified positive correlations between their removal fluxes and the rates of sediment oxygen consumption or organic carbon decomposition. These correlations enable an evaluation of global shelf reductive sinks that are comparable to (for Re) or higher than (~4-fold for U) previously established suboxic/anoxic sinks. These findings suggest potential imbalances in the modern budgets of Re and U, or perhaps a substantial underestimation of their sources. Our study thus highlights shelf sedimentary reductive removal as critical yet overlooked sinks for Re and U in the modern ocean.

Introduction
Reconstructing the oxygenation history is vital for understanding the Earth’s geochemical and biological evolution1. Elemental enrichment and isotopic compositions of rhenium (Re) and uranium (U) in sedimentary records are valuable but still developing proxies of global ocean redox conditions for the geological past (e.g., refs. 2–9). Rhenium and U present as soluble perrhenate (ReO4-) and uranyl carbonate ion (UO2(CO3)34-), respectively, and behave conservatively in oxygenated seawater10,11. They are reduced to insoluble species and removed to sediments under suboxic/anoxic conditions (e.g., refs. 2, 3, 12). Overall, their accumulation in sediments predominantly depends on the reductive removal of pore water Re and U supplied via diffusion and irrigation across the sediment–water interface12–15, and deposition of particles (organic matter and/or disordered CaCO3) that scavenge seawater Re and U16–19. The residence times of seawater Re (~130 kyr) and U (~500 ky) are substantially longer than the average ocean mixing timescale of ~1–2 kyr20,21. Hence, their element concentrations and isotopic compositions are relatively uniform across different ocean basins.

The reliable application of these Re and U proxies requires a thorough understanding of their budgets (e.g., sedimentary removal flux) and associated isotope fractionation in the modern ocean. Recent studies posited global mass balance for both elements and proposed suboxic (defined as sediments with oxygen penetration depth (OPD) < 1 cm) and anoxic (no oxygen penetration and H2S ~ 0 μM) sediments as the dominant sink, although there are significant uncertainties3,5,20. Other sinks include coastal lagoons and blue carbon ecosystems (e.g., salt marshes, and mangrove swamps)17,20. Due to the scarcity of available isotope data22–24, the Re isotope systematics in the modern ocean has been poorly evaluated. The suboxic and anoxic sinks in reaching oceanic U isotope (δ238U) mass balance is based on operationally defined suboxic/anoxic seafloors and area-weighted mass accumulation rates of authigenic U in these areas (e.g., refs. 2, 8, 20). The established sinks were based on the extrapolation of limited data (collected at sites below ~100 to thousands of meters) to the suboxic and anoxic seafloor areas at water depths > 200 m3,20. The shelf (water depths: ≤ 200 m) sediments are suboxic settings due to the high sediment and organic matter loadings. Their role in the oceanic Re and U budgets has been dismissed or treated as authigenically neutral using areal burial rates at least two orders of magnitude lower than the representative sinks in suboxic and anoxic seafloors3,20, although shelf sediments show distinct Re and U enrichment (e.g., refs. 13, 18, 20, 25). However, shelf sediments are disproportionately important in global budgets of trace metals, such as iron and rare earth elements26,27. The shelf sediment regime is a hotspot of organic carbon (OC) supply and remineralization, which limits oxygen penetration and causes anoxic conditions. This will spark the sedimentary reductive removal and authigenic accumulation of Re and U, thereby influencing the sink terms in the oceanic Re and U budgets. Recent studies, based on either the sediment accumulation or sedimentary reductive removal rates, found a shelf sediment sink to be up to 600 pmol Re m-2 d-1 and ~200 nmol U m-2 d-1 (refs. 13, 14, 28, 29), which are higher than the representative suboxic sinks of 61.6 pmol Re m-2 d-1 and 1.9 nmol U m-2 d-1, respectively3,20. In view of the higher fluxes than the proposed representative suboxic/anoxic sinks, quantitative evaluation of the reductive removal of Re and U in shelf sediments provides opportunities to improve our understanding of the modern oceanic cycle of these elements and their application as oceanic redox proxies.

The East China Sea (ECS; Supplementary Fig. 1) shelf receives large suspended sediments and OC loadings, resulting in sedimentation rates of a few cm yr-1 (ref. 30). The shelf dynamics are distinctly stronger in winter than in summer31: the eddy diffusivities in the bottom water are 5-fold higher; the sediments are more intensively reworked/irrigated as evidenced by deeper penetration of DO and excess 234Th activity, as well as 224Ra/228Th disequilibria. Additionally, upwelling develops in June, reaches its strongest condition in July and August, and eventually vanishes in late September32. Intensive winter reworking/irrigation enhances the OC decomposition rates to ~3.5–522 mmol m-2 d-1 in the ECS, whereas OC decomposition is relatively low under maximal OC supply and peak seasonal hypoxia in summer31,33. The contrasting conditions make it an ideal region to evaluate the sedimentary reductive removal of Re and U, their relation with OC decomposition, and the role of shelf sediments in the global oceanic Re and U budgets.

In shelf sediments, irrigation, which is driven by the flushing of burrows and coupled with the diffusion of solutes between burrows and the surrounding sediments34,35, acts to supply solutes to sediment pore water when solute concentrations in bottom water exceed pore water levels. Previous studies highlight the importance of irrigation in augmenting the sedimentary reductive removal of Re and U in shallow water sediments14,36. In the ECS, irrigation has been shown to enhance the benthic DO, NH4+, Fe, and Be fluxes31,37,38. The 224Ra/228Th disequilibrium method has been proven suitable for flux estimation over a timescale of ~10 days in highly dynamic systems influenced by sediment reworking/irrigation31,39–41. It is based on the different geochemical behavior between the dissolvable 224Ra (half-life: 3.66 days) and its highly particle-reactive parent 228Th (half-life: 1.91 years) in the marine environment37,42. Processes like diffusion, bioturbation, irrigation, and reworking render the transfer of dissolved 224Ra between sediment and the overlying water, thereby resulting in a deficit of 224Ra relative to 228Th that can be integrated to derive the benthic flux of 224Ra. This flux can be used to quantify the sedimentary reductive removal rates of Re and U based on the ratio of the concentration gradients between dissolved 224Ra and Re (U). A key advantage of this 224Ra/228Th disequilibrium method over traditional sediment incubation and modeling methods is that it captures all physical processes that affect solute transfer across the sediment–water interface but does not alter the system31,39–41.

In this work, we measure the pore water Re and U concentrations in the shelf sediments from the ECS during the summer and winter seasons. Subsequently, we apply the 224Ra/228Th disequilibrium method to estimate the sedimentary reductive removal fluxes of Re and U. Our analysis reveals significant positive correlations between these fluxes and the sediment oxygen consumption rate. Our calculation suggests sedimentary reductive removal processes in continental shelf environments as critical sinks affecting the modern oceanic budgets of Re and U. These findings imply the sources and sinks of Re and U in the modern ocean may be imbalanced and/or their sources are substantially underestimated.

Results
Porewater Re and U concentrations
Porewater geochemistry of DO, nutrients, and trace metals are shown in order to depict the redox conditions regulating sedimentary reductive removal of Re and U. The dataset of 224Ra/228Th, DO, nutrients, and Fe for the cruises of 2017 and 2018 are available from previous studies31,43. New data includes 224Ra/228Th, nutrients, and Fe for the samples collected in the summer of 2021 and RSMs (Mn, Re, and U) for all cruises. These datasets are shown in Fig. 1 (Re, U, and 224Ra/228Th ratio) and Supplementary Fig. 2 (Fe, Mn, and nutrients) and compiled in Supplementary Tables 1 and 2. Here, we focus on the sedimentary reductive removal fluxes and their role in the modern oceanic budgets of Re and U.

Porewater [Re]diss and [U]diss fall in the range of 2.7–115 pM and 0.3–27.3 nM, respectively. [Re]diss and [U]diss in the upper ~1–2 cm of some sites (11 and 8 out of 17 cores, for Re and U, respectively) are higher than the seawater values (Re: 40.3±2.4 pmol L−1, U: 10.7±1.0 nmol L−1; N=23, 1SD; Supplementary Fig. 3). Beneath these layers, [Re]diss and [U]diss are lower than those in the seawater. They decrease smoothly and reach asymptotic minima comparable to those in other margins (~3–14 pM and ~2–4 nM, respectively)14,18,44. Incomplete removal of Re and U from pore water may be associated with a colloidal phase12 and/or a minimum U concentration required for microbial-mediated removal45. However, not all stations show the typical removal pattern. Broad maxima of [Re]diss and [U]diss extend to ~6–10 cm at St. B3-W, E4-S, F1-S, and Y7-S.

Sedimentary removal of Re and U is strongly associated with reducing conditions in marine sediments4. In the ECS, DO exhausts within the upper 0.12–0.69 cm across all sites, and reducing conditions favoring metal oxides and sulfate reduction dominate the OC decomposition31. During summer, [Re]diss and [U]diss generally decrease slowly, together with slow increases in terminal metabolism product of anaerobic OC decomposition ([NH4+]diss, [Mn]diss, [Fe]diss) and less decline of SO42-/Cl ratios. On the contrary, [Re]diss and [U]diss progressively decrease to asymptotic minima at relatively shallower depths in winter (Fig. 1a, b and Supplementary Fig. 2). The stronger OC decomposition and more reducing conditions, evidenced by lower SO42-/Cl ratios, enhanced [NH4+]diss and [Fe]diss maxima, are probably responsible for the relatively faster Re and U removal in winter. The change of gradients and the minima of [U]diss coincide with the broad maxima of [Fe]diss. Re reduction and removal start from the depth of NO3- and MnO2 reduction and could extend to the bottom layer conducive to Fe oxides and sulfate reduction.

Larger 224Ra deficits (224Ra/228Th ratio < 1) in deeper depth intervals were observed at most sites collected in winter (Fig. 1c), revealing more intense irrigation/reworking of pore water, which was also evidenced by deeper penetration of oxygen and excess 234Th into the sediments31.

Sources of Re and U for sedimentary reductive removal
The elevated porewater Re and U concentrations at the sediment–water interface of some sites exceed those found in the seawater (Fig. 1a, b), indicating additional sources supplying pore water Re and U for reductive removal, apart from irrigation and diffusion of dissolved species from bottom seawater (Supplementary Fig. 5)12–15,18. Under oxic bottom water, enhanced bioturbation may result in the re-oxidation of reduced Re and U14,18. However, the most distinct enrichments of core-top porewater [Re]diss and [U]diss are observed in summer with lower bioturbation compared to winter (Fig. 1). In this regard, re-oxidation of reduced Re and U is unlikely to be the dominant source for the observed pattern at these sites. Instead, the decomposition and transformation from freshly deposited particles (particularly organic matter) at the sediment–water interface might have released relatively large amounts of Re and U (i.e., regenerated flux) compared to the reductive removal14,16,18,19. If the “particulate source” scavenged from is not quantitatively removed by sedimentary reductive removal, it could even result in a dissolved flux back to the water column (Supplementary Fig. 5).

Moreover, the broad maxima of [Re]diss and [U]diss at St. B3-W, E4-S, F1-S, and Y7-S could not be supported merely by their release at the sediment–water interface31. Alternatively, they may be indicative of enhanced Re and U release from the solid phases into pore water at these depth intervals, probably due to intense sediment reworking. Below these depths, Re and U are steadily removed to reach the typical asymptotic minima.

Regardless of the routes (dissolved versus particulate) that supply Re and U to the pore waters, it is the reductive removal that ultimately results in the enrichment of Re and U in the modern shelf sediments. Our data of Re and U concentrations in the corresponding solid phases provides further support for the reductive enrichment process (see Supplementary Text 1 and Supplementary Fig. 4). The 224Ra/228Th disequilibria approach, which incorporates the influence of irrigation in regulating solute fluxes, is used to estimate the reductive removal fluxes of Re and U (Fi, positive values denote removal)37. A detailed description of the calculation is provided in Methods.

Sedimentary reductive removal fluxes of Re and U
The total Re and U fluxes vary from 20±5 to 4600±950 (median: 320±61) pmol m−2 d−1 and from 1.5±0.6 to 830±320 (median: 36±13) nmol m−2 d−1, respectively (Fig. 2 and Table 1). They are ~15 times higher than the diffusive fluxes (7.0–66 pmol Re m−2 d−1 and 0.6–3.7 nmol U m−2 d−1, respectively; Table 1) at the same stations, suggesting remarkable enhancement of sedimentary Re and U removal by irrigation. The amplification factor of sediment area (see the Methods), a dimensionless parameter, is used to characterize the enhancement of solute exchange driven by irrigation relative to that driven by molecular diffusion31. Unexpectedly high removal fluxes of Re and U were observed at St. E1-W located near a refuge harbor and probably driven by accidental anthropogenic disturbance, as indicated by the extreme amplification factor of sediment area. These fluxes would be similar to the other cores (1200±250 pmol Re m−2 d−1 and 220±84 nmol U m−2 d−1, respectively) and fall much closer to the trend if the amplification factor decreases by ~4 times to the average of the other cores in winter. In the following discussion, this station is omitted for a more representative estimation of global shelf sedimentary reductive sinks.

The amplification factor of sediment area in the ECS is higher than that estimated with the sediment chamber incubation method in the North American marginal sediments (NAMs: ~3)14,36. This might be partly caused by the temporal and spatial variability between these studies. Another plausible explanation is that the alteration of the physical conditions during sediment incubation may dismiss small-scale advective processes and thus underestimate the fluxes39,43. The sedimentary reductive removal rates of Re and U in the ECS are comparable to the accumulation rates in shelf sediments with well-oxygenated water columns in the NAMs and the East Siberia Sea (8–600 pmol Re m−2 d−1 and 4.6–106 nmol U m−2 d−1; refs. 13, 28, 36, 46).

The sedimentary reductive removal rates show remarkable seasonal variation (Fig. 2 and Table 1), with lower fluxes measured in summer (average: ~180 pmol Re m−2 d−1 and 26 nmol U m−2 d−1) when OC supply and seasonal hypoxia are at maximum, and the seafloor is relatively stagnant. In contrast, they are ~3–4 times higher (average: ~720 pmol Re m−2 d−1 and 78 nmol U m−2 d−1) in winter under well-oxygenated water column and rough seafloor conditions. It is at odds with the expectation that these fluxes would culminate during summer when the maximum OC supply and the upwelling promote the development of hypoxia47 and, consequently, the sedimentary reductive removal of RSMs2,3. Sedimentary OC decomposition dictates the reducing conditions conducive to the removal of Re and U13,14. Intense winter reworking/irrigation was posited to be responsible for the highly efficient decomposition of sedimentary OC in the study area, resulting in relatively homogeneous TOC contents (0.33±0.05%) and stronger reducing conditions31. Therefore, we argue that the distinctly higher winter removal rates of Re and U are driven by intensive reworking/irrigation. It is evidenced by the positive correlations between the removal fluxes of Re and U and the sediment area amplification factors (R2=0.59 and 0.73 for Re and U, respectively, Fig. 3a, b). Irrigation brings DO, labile OC, Re, and U to sediments. The supply of labile OC overwhelms DO in controlling the sediment redox conditions. Therefore, intensive winter reworking/irrigation would trigger the development of more vigorous anaerobic diagenesis and thus enhance the reductive removal of Re and U (see next section). Overall, the observation suggests that the sedimentary reductive removal fluxes of Re and U in shelf sediments are not regulated by water column oxygenation conditions and are significantly higher than the prior estimates (e.g., 0.24 pmol Re m−2 d−1; refs. 3, 20).

Regenerated flux of Re and U out of the sediment–water interface
The regenerated upward fluxes of Re and U estimated with our porewater data range from -860 to 0 pmol m−2 d−1 (median: -49 pmol m−2 d−1; average: -223 pmol m−2 d−1) and -40 to 0 nmol m−2 d−1 (median: 0 nmol m−2 d−1; average: -6 nmol m−2 d−1), respectively (Table 1). For some sites, the regenerated Re and U fluxes are comparable to or even larger than the sedimentary reductive removal fluxes. Regarding the median values, they are ~15% and 0% of the sedimentary reductive removal fluxes of Re and U, respectively, since many sites have zero upward fluxes. Regarding the average values, the regenerated upward fluxes could be ~36% and ~6% of the reductive removal fluxes of Re and U at our studied sites, respectively. It suggests that the regenerated flux could be an important yet not well understood source for the reductive removal of Re and U within the sediments. However, unlike rare earth elements whose riverine particle-bound phases might be actively involved in the shelf cycling (e.g., refs. 26, 48, 49), the regenerated upward fluxes of Re and U are most likely scavenged from seawater, as discussed above. Therefore, they are considered part of oceanic internal recycling. In this regard, we have not included these fluxes as external sources when discussing the oceanic Re and U mass balance. Even if some of the upward fluxes are from riverine particle-bound phases and/or re-oxidation of previously reduced species, our conclusion on the quantitative importance of shelf sediments sinks to the modern oceanic budgets of Re and U remains unchanged. Future studies should aim to better characterize the nature of Re and U released at the sediment–water interface, for example, by combined analyses of their isotope signatures and exchange flux.

Sedimentary reductive removal of Re and U coupled with organic carbon decomposition
In contrast to the NH4+ fluxes that increase exponentially with DO in the bottom water31, no significant correlations of total Re and U fluxes with DO in bottom water and its penetration depth in sediments (Supplementary Fig. 6) were observed, suggesting a minor role of DO levels in regulating the reductive removal of Re and U. This is in line with a small impact of bottom-water anoxia on OC decomposition and burial50. Sedimentary OC decomposition, the primary mechanism dictating the sedimentary reducing conditions, must be governing the Re and U fluxes in the O2-depleted sediments. Accordingly, stronger OC decomposition would accelerate the production and accumulation of NH4+ in pore water and enhance the DO consumption and reductive removal of Re and U from pore water4,14. In support of the assertion are the positive correlations between the reductive removal fluxes of Re and U and the OC decomposition rates (determined as the net NH4+ production rates; R2=0.75 and 0.62 for Re and U, respectively, p<0.005; Fig. 3c, d), as well as the total sediment oxygen consumption rates (R2=0.61 and 0.72 for Re and U, respectively, p<0.0001; Fig. 3e, f).

It is counterintuitive that sedimentary reductive removal of Re and U correlates simultaneously with sediment oxygen consumption and anaerobic OC decomposition rates. However, it is unsurprising since oxygen consumption and Re/U reduction are synergistic rather than competing during OC decomposition within shallow sediments. Microbial metal respiration coupling with sediment reworking/irrigation and anaerobic conditions has been posited to be responsible for the loss of OC in shallow sediments or at the sediment–water interface in the ECS51,52. In these sediments, the oxygen supply is insufficient to fully decompose the OC. Therefore, the higher sediment oxygen consumption fluxes mean a greater demand for electron acceptors53. Intense sediment reworking/irrigation introduces large amounts of labile OC into sediments, exposing anoxic sediments and buried recalcitrant OC. This process produces and accumulates bacterial biomass, whose remineralization would prime high anaerobic OC decomposition rates and the reduction of metal oxides and sulfate34,36,52,54–56. Under this circumstance, OC decomposition triggers anoxic conditions conducive to the reduction of Re and U and shoals the depth of their reductive removal4,44,57,58. Such phenomena are evidenced by lower SO42-/Cl ratios and elevated [NH4+]diss and [Fe]diss maxima in winter (Fig. 1 and Supplementary Fig. 2). Moreover, intense winter reworking/irrigation would compensate for the Re- and U-deficit and increase their availability in porewater, fueling their sedimentary reductive removal rates. Therefore, we suggest that intensive OC decomposition, which triggers severe reducing conditions, is responsible for the strong Re and U removal in the shelf sediments. Note that the correlations between the reductive removal fluxes of Re and U and the oxygen consumption rates should only apply in shelf sediments, where DO is exhausted within 1 cm and reducing conditions prevail. In the deep ocean, OC remineralization in the water column and sufficient DO supply suppresses the development of reducing conditions within sediments, thus hindering the reductive removal of Re and U.

A similar correlation between sedimentary reductive removal of Re and OC decomposition was observed in the NAMs sediments, overlain by oxygenated bottom waters14,36, suggesting a potentially universal linkage between these two processes in shelf sediments. The positive correlations in both ECS and NAMs are consistent with simultaneous increases of Re accumulation and anaerobic OC decomposition14. Notably, the slopes of Re removal vs. anaerobic OC decomposition rates varied between these two studies (29.4±4.9 in the ECS vs. 10.6±2.6 in the NAMs). A plausible explanation is the influence of stronger irrigation in the ECS, which could introduce labile OC, trigger more vigorous anaerobic diagenesis, and shoal the depth of Re removal, thus enhancing sedimentary reductive removal of Re. In the NAMs, Re removal was coincident with Fe(III) and sulfate reduction14, while Re removal in the ECS began from the depth of NO3- and MnO2 reduction and could extend to the bottom layer conducive to Fe oxides and sulfate reduction (Fig. 1 and Supplementary Fig. 2). As a result, a higher slope is expected on the ECS shelf. Furthermore, the correlations in these two studies are based on limited data that might bias the in-between slopes. We, therefore, conclude that the OC decomposition rate should be a good reference for the reductive removal of Re and U in shelf sediments, although more data on their relationship across various environmental and geomorphic conditions is warranted.

Shelf sediments as a critical sink of global oceanic Re and U
For the evaluation of the sedimentary reductive removal fluxes of Re and U over global shelves, the ECS, the NAMs, and the East Siberian Sea (Fig. 4) can be adopted as representative settings since they are among the widest shelves worldwide and characterized by diagenetic conditions (sedimentation rate, and OC supply/decomposition) close to the representative conditions of global shelves (Supplementary Table 3). The compiled flux of Re from these regions (median: 112 pmol m−2 d−1; 95% confidence interval (CI): [120, 299] pmol m−2 d−1; Supplementary Table 4) is comparable to the sinks in suboxic and anoxic seafloor (62 and 197 pmol m−2 d−1, respectively; Fig. 5a). A simple extrapolation of the compiled flux to the global suboxic sediment (area: 1.68 × 107 km2; ref. 3) yields a suboxic sink approximately 1.6 times the dominant Re source from rivers21 (median [95% CI]: 685 [734, 1830] Kmol yr−1; Table 2). For U, the compiled flux (median: 30 nmol m−2 d−1; 95% CI: [25, 41] nmol m−2 d−1; Supplementary Table 4) is much higher than the representative suboxic flux in sediments below 200-m water depth and comparable to the anoxic sink (Fig. 5b). If extrapolated to the global suboxic sediments, it yields a suboxic sink of 184 (95% CI: [154, 252]) Mmol yr−1, which is an order of magnitude higher than previous estimates (Fig. 6 and Table 2; refs. 5, 20).

It is worth noting that the reductive removal rates of Re and U in suboxic seafloor sediments are expected to drop with water depth due to the less reducing conditions associated with lower OC contents and DO consumption13,14,53. Because of the heterogeneity of the fluxes on the global shelves (Supplementary Table 4), simply extrapolating the compiled fluxes to global suboxic sediments may bias the sinks toward the high values of shallow water regions and result in significant uncertainties. We have identified strong correlations between the reductive removal of Re and U and the sediment oxygen consumption due to their synergistic effect during OC decomposition (see last section). The sediment oxygen consumption has been well constrained over the global ocean, with mean areal rates of 25±13 and 9±5 mmol m−2 d−1 on the global inner (10–50 m) and outer (50–200 m) shelves, respectively53. Thus, a more reasonable evaluation of the global shelf sinks of Re and U with narrower uncertainties can be made by taking advantage of the identified correlations between Re and U fluxes and the sediment oxygen consumption rates. Here, the flux ratios between Re and U uptake and sediment oxygen consumption with zero intercept (3.55±0.50 pmol mmol−1 for Re and 0.47±0.05 nmol mmol−1 for U, respectively; Fig. 3e, f) are used because there would be no Re and U removal when no oxygen consumption occurs, i.e., no oxidant is required. By upscaling the flux ratios to the global shelves, a Re sink of 431±175 Kmol yr−1 can be calculated over global shelf sediments (water depths: 10–200 m). It is comparable to the suboxic sink beyond the shelf and the dominant source from rivers (Fig. 6 and Table 2; ref. 3). Accordingly, the Re sink over the global suboxic seafloor sediments can be conservatively updated to 806±175 Kmol yr−1 (Fig. 6 and Table 2), roughly consistent with the lower bound of the shelf sink estimated using the extrapolation of the compiled median areal flux. A similar calculation yields a U sink of 57±22 Mmol yr−1 over global shelves, which is approximately three times the sum of anoxic and suboxic sinks beyond the shelf and still exceeds the total U input5,20.

Based on these updated constraints on the shelf sediment sinks using the correlations between the reductive removal of Re and U and the sediment oxygen consumption, both the Re and U budgets are imbalanced, i.e., the sink is ~80–100% larger than the source (Table 2). Therefore, the residence time of Re and U, expressed as the ratio of the reservoir to the output flux (τ=M/Fout; ref. 59), would be much lower than previous estimates. Possible additional sources or glacial-interglacial cycles of sinks should be invoked to account for the imbalanced budgets of oceanic Re and U. From the source perspective, the lack of a temporal-spatial integration of the rivers may noticeably bias the dominant source, although prior estimates compiled Re and U concentrations from rivers encompassing 37% and 60% of total water runoff to derive this input20,21. Particle disaggregation/dissolution during estuarine mixing could enhance the overall fluxes of trace metals from rivers60. For example, it could enhance the Re and U fluxes from the Amazon River system by ~4–5 times12,61, and enhance the Re fluxes by ~30% and ~15% in the Mississippi River delta and the Jiulong River estuary (southeast China), respectively62,63. Interestingly, it was identified that Re could be buried at a rate ~4-fold higher than oxic marine sediments via clay adsorption and biological uptake in an estuary-like lagoon17, which could be an additional sink within the shelf area. Sorption onto Fe-oxyhydroxides and colloidal flocculation during estuarine transport and/or encompassing coastal blue carbon ecosystems was supposed to be a “coastal retention” sink of U20. However, recent studies suggest U addition in estuarine and blue carbon systems, although the accurate fluxes are not well-constrained64,65. Therefore, the addition/removal of Re and U during transport across estuaries and coastal blue carbon systems may bias their fluxes to the ocean8,20. As high-precision analytical techniques have progressed, it is important to revisit the contribution of Re and U from these systems.

Moreover, glacial-interglacial cycles of the size of shelf sediment sinks may also contribute to the oceanic budgets considering the long ocean residence times of Re and U. The ~50% decrease of total shelf area upon seal level lowering during glacial periods could result in smaller sinks over the global shelf. Furthermore, during the glacial periods, the decreased organic matter supply and increased oxygenation in the upper ocean, including the continental shelves impinging the modern oxygen minimum zone7, result in declines of oxidant (DO, metal oxides) requirement for OC decomposition within the sediments. Therefore, lower sedimentary reductive sinks of Re and U over global shelves during cold periods are expected.

Including the shelf sediment sink is remarkable for the oceanic δ238U budget. Recent studies suggest a systematically higher oceanic δ238U sink than the bulk continental crust2,8 and ascribe it to the non-steady state of modern oceanic U budget, too high δ238U of the net oceanic sink, and/or a higher riverine δ238U than that of bulk continental crust. Our estimate implies that the modern oceanic U budget might indeed be imbalanced. Additionally, the importance of shelf sediment sinks may alter the modern oceanic δ238U budget. For example, combining the updated shelf sediment sink (this study) with the same δ238U of the suboxic sink (-0.30‰; ref. 8) results in a total oceanic δ238U sink of -0.30‰, which is closer to the riverine influx of -0.34‰ than previous estimates (-0.26‰; refs. 2, 8). Thus, this shelf sediment sink helps to close the oceanic δ238U budget.

Methods
Sample collection
Samples were collected on the East China Sea shelf during summer (Aug 2017 and July–Aug 2021; labeled with “-S”) and winter (Dec 2018; labeled with “-W”) (Supplementary Fig. 1). Sediments were collected using a box corer (20×20×60 cm3) and checked visually undisturbed. Subcores were taken by inserting transparent PVC tubes (inner diameter: 65 mm) into bulk sediments. Subcores for DO measurement were temporarily stored with an open top and bubbled with an N2/CO2 mixture to keep the DO concentrations at the bottom water levels. Porewater samples were extracted from sediment subcores using Rhizon samplers and pre-cleaned 10-mL PE syringes66. Aliquots for trace metal analysis were acidified to pH=1.6 with Optima grade HNO3, while aliquots for nutrients and anions (Cl- and SO42-) were stored at -20°C and 4°C until analysis. Seawater samples were collected using Niskin bottles mounted on a CTD rosette. These samples include the bottom water (~2 m above the seafloor) collected during the winter of 2018, as well as the vertical profiles of the sites sampled in the summer of 2021 and at a distal site (C18: 30.08°N; 124.49°E) in the summer of 2019. They were filtered with Polyethersulfone membrane (pore size: 0.22 μm), subsequently acidified and stored in the same manner as porewater.

Analysis of trace metals
Concentrations of Re were measured by Q-ICP-MS (iCAP RQ; Thermo Fisher Scientific) after preconcentration and purification of 5–10 mL porewater/seawater samples using isotope dilution method with anion resin (1 mL AG1-X8 resin, 200–400 mesh). Sample solutions were spiked with 185Re-enriched solution and equilibrate for 24 h. Samples were then processed through anion resin columns conditioned with 1 M HCl solution. The Re fraction was collected with 10 mL 8 M HNO3 after washing the sample matrix with 10 mL 1 M HCl solution. The Re fraction was evaporated and re-dissolved in 4 mL 2% HNO3 for instrumental analysis. Iron, manganese, and uranium concentrations were diluted 20~100-fold and determined using ICP-MS with He as a collision to minimize polyatomic interferences. A matrix-matched external calibration with NaCl was applied using Rh as the internal standard. Certified reference materials NASS-7 and SLRS-6 (for Fe, Mn, Re, and U) were used for quality control. The influence of the porewater/seawater matrix on the accuracy of Fe, Mn, and U measurements was assessed using a spiked seawater standard. The results are consistent within 10% of the certified or previously reported values (Supplementary Table 5). For Re, replicate aliquots of surface seawater from the ECS were measured to be 38.0±0.4 pmol L−1 (N=8). The external reproducibility of replicate analyses was better than 2% for Re and 5% for Mn, Fe, and U.

Analysis of ancillary parameters
The activities of 224Ra and 228Th in bulk sediments and porewater were determined with the RaDeCC systems67. Briefly, the sediment subcores were sliced into 1-cm thick slabs. The slabs were slurried with MilliQ water in Teflon beakers. After adjusting the pH of the slurry to 8–9, KMnO4 (8.0 g L−1) and MnCl2 (3.0 g MnCl2·4H2O L−1) solutions were added to form a suspension of MnO2. The samples were filtered onto a GFF filter (diameter: 142 mm; pore size: 0.7 μm). The filter was initially counted on a RaDeCC system for ~6 h and re-counted after ~10 and ~25 days, respectively. The activities of 224Ra and 228Th in bulk sediment were calculated from the first and second/third measurements. Porewater 224Ra was pretreated and measured in the same fashion but skipped the addition of MilliQ water.

Ammonium and nitrate were determined using a flow injection analyzer (Tri-223 Auto-Analyzer) and a 4-channel segmented flow auto-analyser (Bran-Lube AA3). DO in the water column was measured using the Winkler titration method. Fine-scale DO profiles (resolution: 0.3 mm) in sediment porewater were determined using a Unisense or custom-made gold amalgam microelectrode68. Anions (SO42- and Cl-) were determined after a 200-fold dilution by ion chromatography (Metrohm AG) using IAPSO seawater for calibration and matrix correction. The analytical uncertainties for an in-house seawater sample were <1%. Porosity was estimated with the weight difference of the sediment after drying to a constant weight at 60°C.

Calculation of the Re and U fluxes based on the 224Ra/228Th disequilibria
Sedimentary reductive removal can create [Re]diss and [U]diss gradients in sediment pore waters, which are traditionally used to quantify the diffusive flux following Fick’s first law69: F = φ × DiS × ∂C/∂z. Here, φ is the porosity, DiS is the effective diffusion coefficient in sediments calculated using the molecular diffusion coefficient in seawater (Di) and corrected for tortuosity DiS = Di/(1 - 2 × ln φ), and ∂C/∂z represents the concentration gradient of dissolved components (224Ra, Re, U; Supplementary Table 6). Irrigation dominates solute exchange between pore water and bottom water in shelf sediments14,31,36–38. Thus, the reductive removal fluxes of Re and U (Fi, positive values denote removal) can be estimated with the 224Ra/228Th disequilibria approach, which incorporates the flux enhanced by irrigation37:

Fi = FRa × (DiS/DRa) × (∂Ci/∂z / ∂CRa/∂z) (1)

where FRa denotes the flux of total 224Ra (dpm m-2 d-1) and is calculated as the product of the decay constant of 224Ra (λRa = 0.189 d−1) and the integration of the deficit of total 224Ra (A224Ra) relative to 228Th (A228Th): FRa = λRa × ∫0z (A228Th - A224Ra) dz67. The deficit (224Ra/228Th ratio < 1) of the solids was confined within the depth of ~10 cm, although they could be scattered due to inhomogeneous sediment particle mixing (Fig. 1c). We here integrate the upper 10 cm to derive the total 224Ra depletion fluxes. Amplification factor (ξ) is then defined as31: ξ = FRa / (φ × DiS × ∂CRa/∂z). The calculated amplification factor represents the mean state of irrigation within the upper few cm where 224Ra/228Th disequilibria occur. The fluxes and concentration gradients of 224Ra for the 2021 cruise, together with the previously reported dataset for the 2017 and 2018 cruises (ref. 31), are presented in Table 1 and Supplementary Table 6. We determined the Re and U concentration gradients by modeling a linear fit from the surface layer to the depth where significant gradient change occurs (Supplementary Fig. 7) to account for the influence of irrigation indicated by the 224Ra/228Th disequilibria. This allows us to estimate the reductive removal fluxes of Re and U. For the sites characterized with complicated removal-release in the upper sediments indicated by distinct zig-zag patterns within the upper few cm (Supplementary Fig. 7), their concentration gradients and the resulting removal fluxes were not calculated due to the large uncertainties. The uncertainties associated with the site-specific reductive removal fluxes of Re and U were propagated from the errors of the 224Ra fluxes and the concentration gradients of dissolved 224Ra, Re, and U.

For the sites with Re and U flux out of the sediment–water interface (Fout), their Fout were also estimated using Eq. (1). The concentration gradient at the sediment–water interface is calculated from the first pore water (at 1 cm) and the seawater concentrations (assumed to be equal to those of the overlying water column). When the first pore water concentrations of Re and U are lower than the water column, Fout is set to zero.

Data availability
The data generated in this study are provided in the Supplementary Information and are also available in the Figshare database under accession code: https://doi.org/10.6084/m9.figshare.24807534.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Hong et al. present the Re and U concentrations in the pore water of shelf sediments from the East China Sea. The benthic fluxes of Re and U were estimated using the 224Ra/228 Th disequilibrium method to account for the influence of sediment transformation/irrigation. Although the data presented are a valuable contribution to a better understanding of the biogeochemistry of Re and U in the marine environment, I have significant problems with the authors' interpretation of the data. Unfortunately, I cannot recommend the publication of the manuscript in its current form.\nBut before I go into the interpretation, I have a question about Supplementary Figures 1 and 2. The sampling in summer 2017 is distinguished from the sampling in summer 2021 by red bold circles. However, Supplementary Figure 2 shows that sampling at stations F4 and F3 was also carried out in summer 2017 (the text on sampling – lines 118 to 126 in main manuscript text – does not help to clarify this issue). In addition, the Supplementary Figure 2 shows the station designated as B7, which does not exist on the map in Supplementary Figure 1. The same applies to the sampling station marked as HN (Supplementary Figure 2 compared to Supplementary Figure 1). The same applies to the sampling stations for uranium.\nLooking at the Re pore water depth profiles, we can see that the Re concentration in the upper 1 to 2 cm sediment depth is elevated in 11 out of 17 cores (one more or less) compared to the Re concentration in seawater (accidentally, the Re concentration profiles of the East China Sea are not shown anywhere – at least at one point, while the bottom water Re concentration are shown only in a few cases), if we assume that the dissolved Re concentration in the East China Sea is in the range of reported values. A similar phenomenon has already been observed by Morford et al. (Ref. 1 and 2). In Ref. 2, Morford et al. “the flux into the sediments” and the “flux out of the sediments” for a particular site is also depicted. I don’t think that the observed feature can be called as benthic uptake (e.g. line 137 in the main text), because in most cases the authors have the flow out of the sediment across the sediment-water interface and the Re-porewater maxima below which pronounced Re-uptake occurs. I think that the authors have only tried to estimate the Re uptake occurring below sediment-water interface, while the obvious Re flux from the sediment observed in most pore water profiles is neglected.\nI think the authors are commenting on the Re peak in subsurface pore water observed in the text at station B7. But the same is true for the majority of the data, except that the Re pore water peak is near the sediments surface. Also, next time I would suggest the authors try to do a denser pore water profile for the top 5 cm of the sediments, as Morford et al. (Ref. 1 and 2).\nIt is also unclear that the red line representing the linear regression sometimes reaches the sediment-water boundary and sometimes goes beyond it, although the Re-soil water concentration is not given for these sites. It is therefore not clear which Re-boundary concentrations have been included in the calculations.\nI would also like to draw the authors' attention to Ref. 3.\nReferences\n1. Morford et al., A model for uranium, rhenium, and molybdenum diagenesis in marine sediments based on results from coastal locations. Geochimica et Cosmochimica Acta 73 (2009) 2938-2960.\n2. Morford et al., Rhenium geochemical cycling: Insights from continental margins. Chemical Geology 324-325 (2012) 73-86.\n3. Danish et al. Non-conservative removal of dissolved rhenium from a coastal lagoon: Clay adsorption versus biological uptake. Chemical Geology 580 (2021) 120378.", "reviewer_2": "A lot of different types of carefully executed measurements were made on these cruises. Some of the method details are already published (and so are referenced in the short methods section). As a whole, I found the paper easy to read and attention grabbing. While this isn’t the first estimate ever of shelf sinks, the paper combines the literature data with new ECS measurements to look at the ‘shallow shelf’ question at a global scale. I agree that ‘A key advantage of this 224Ra/228Th disequilibrium method over traditional sediment incubation and modeling methods is that it captures all physical processes that affect solute transfer across the sediment–water interface but does not alter the system’ and that these radioisotope-based estimates are a unique application here to looking at budgets. Some of the data is reused from earlier studies, but the authors utilize interesting applications of existing methods/techniques in a way that provides a unique contribution to the field. I thought the authors did a good job of ‘considering the options’ starting at line 259 (‘Shelf sediments as an overlooked sink’). Budget parameters were often determined in 2 ways to support the conclusions. Implications and ‘what ifs’ were discussed, which shows that the authors have both considered potential issues with higher sinks and, if real, what higher sinks mean for this field and beyond (i.e. paleo-interpretations, etc.). If the larger sink for U is real and the system is not in balance for U, this is very important for the oceanographic community to know.\nI have a few broad comments/concerns that would be helpful for the authors to clarify (whether in responses or in the manuscript text itself). In my opinion, addressing these questions/areas would help to strengthen the conclusions and assuage doubts.\n1. Potential variability of the datasets used (temporal questions) and how representative the dataset is of global shelf conditions for depths <200 m.\nOne thing that I would want to see (without looking at references) is what is the timescale of the summer and winter sampling in Figure 1 and Table 1? Could this go in the caption of Fig 1? Essentially, were all the samples from summer taken the same day? Or the same week? The reason this is of interest is to see how variable the Ra/Th could be (averaging over 10 days) from week to week or month to month on the shelf. Would we see much much higher or much much lower a few weeks later? In short, are these representative conditions on this shelf throughout most of the summer and winter? How does the ’10 day average’ for these radioisotope ratios change at one location over the entire summer? Are there temporal studies in the literature that can be cited here to indicate this range? Two summer dates are shown, although they aren’t from the same locations (Fs are down at the bottom of the map and E/Y are in the middle). The results do seem comparable on the scale in Fig. 2. Since there is so much variability, as noted in lines 166-167 (when comparing to other studies) and Fig 1/Fig 5, it would be helpful to know how changeable the Ra/Th ratios are.\n2. Potential variability of the datasets used (spatial questions) and how representative the dataset is of global shelf conditions for depths <200 m.\nLines 63-68: ‘The contrasting conditions…the global oceanic Re and U budgets.’ I agree that it’s a good region to look at different patterns because of summer/winter changes, my question would be do most shelves globally look like this one and have similar processes? Do we know or have an idea of how representative they might be? When the estimates are extrapolated to the global model, how does this shelf compare to the typical conditions of the ‘average’ shelf?\nIf I look at Fig 3 and Fig 2, two of the really high flux points are northern winter points. Do we know (I assume yes) about the shelf dynamics here and can we attribute these patterns to summer vs. winter and not north shelf vs. south shelf spatial trends? Is there more upwelling in one location vs. other? Are the sediment types similar? Are the regressions in Fig 3. still strong (>0.5) if the two northern points are taken away? I realize these measurements are very difficult and occupying the same locations twice might not have been possible. There is some overlap around 31 in summer and winter points. A short comment/discussion from the authors on how the patterns/trends are clearly related to season and not spatial variability would be welcome (especially for those that don’t know the physics and sedimentary systems of this area well).\nLine 286 (for U) and early for Re use the median flux extrapolated over the are of global suboxic sediments. How realistic is the resulting estimate? Are all shelves expected to be like the ECS or said in another way, if we know shelves are dynamic/different what are the bounds on these estimates? I agree that the locations in Fig 4 are covering some of the ocean’s broadest shelves, however, this doesn’t mean that these conditions will be everywhere. A lot of the Hudson Bay numbers look very low compared to the ECS. In short, how homogenous can we expect the shelves to be (i.e. if there is a mix of types, how many are going to have the ‘interesting’ summer/winter conditions seen in the ECS)? Is there a way to characterize the shelves globally with different types (broad categories) instead of just applying an average or median of the available data to the entire shelf area globally?\nAlong these same lines, how are the ‘uncertainties’ calculated, or errors extrapolated? I see data from this study (with errors) and data from the literature (no errors) in Supp. Table 4. These were the data used to get the budget estimates starting at Line 259. Medians were reported in this section, but ranges and standard deviations might be helpful too. This could go in Table 2. I’m not sure what the +/- in this table mean (i.e. what do they represent?).\nAdditional Line by Line Comments:\nFrom Line 9, it sounds like the authors (from the literature) didn’t anticipate shelves being a sink for Re previously, and so that with this study there is now an extra sink for Re (i.e. if the shelf sink is comparable to suboxic/anoxic sinks that were found previously then the total sink has now increased…because otherwise things would still be the same). Just clarifying here that this is what the authors intended: ‘Our extrapolation suggests the shelf sinks are comparable to (for Re) or higher than (~4–fold for U) previous estimations of the suboxic/anoxic sinks. These results suggest that the modern budget of Re and U may be imbalanced and/or their sources are substantially underestimated.’\nLine 38: ‘In fact, the established sinks were based on limited data from continental margins of water depths >300 m3, 8.’ Based on a quick read of the text, I am assuming that source 8 (Dunk et al, 2022) uses the location in Table 1 to make the shelf sink estimates. I see at least two locations where water depths are <300 m, which would contradict what is said in lines 38-40 of the manuscript. I believe Saanich Inlet is maximum 230-250m and most depths from the Walvis Bay study (Veeh et al) are 119-225m. It would be helpful if the authors could clarify this point regarding the depth range cutoff used. If Table 1 is the source of the estimates being referred to in source 9, then there seems to be a range in settings and depths (vs. a limited one). I could be mistaken how Table 1 is used, but since this is an important point to the significance of this study (i.e. showing that shallow sediments could be greater than assumed), this part of the text about previous estimates should be very clear.\nLines 153, 156 and later uses of ‘amplification’: Please explain what is meant here by ‘extreme amplification of sediment area’ and define this term – I had to go to source 18 to get information about the anthropogenic influences. Since amplification is also mentioned in a different context in Line 188, the term should be defined to account for a wider audience reading Nature Communications. My question here is also, do we anticipate the amplification factor due to anthropogenic disturbance to be common? If this is not going to happen in most locations on most shelves, then omitting is fine. The authors don’t need to a add a huge amount of detail, just indicating how anomalous this is expected to be would help rule this out for use in any extrapolation.\nFig 1: Overall it’s a bit small to read but I like the setup here and it helps the reader quickly understand the Ra/Th and Re, U data. The grey bar is helpful, as are the caption details.\nFig 2: Good visual. Liked it.\nFig 3: Some overlap of text with the points on (e) but overall this was a readable figure. Well structured.\nFig 4: Good figure. If you needed to cut anything, (a) could be removed. What I was hoping to see is the shelf variability in b/c/d (and that was shown). You can see the shelf depths much more clearly in b/c/d and so those panels would be enough for me with a caption indicating (as is already done) where each is from.\nFigure 5: Overall this figure is a good representation of the data and is important. However, the sideways text is a bit confusing/messy and I would recommend using a bracket of some kind at the top or bottom of the plots to indicate data from this study (left, use bracket to cover the red/blue) and the data from the referenced sideways text studies (red/green/purple).\nFigure 6: Same comment as fig. 5. The figure will look more professional without the squished/sideways text.", "reviewer_3": "The importance of shallow shelf sediments on the mass balance for U and Re requires more data and closer study, as both are used for reconstructing past anoxic conditions. U has been used more than Re for this purpose, but this is primarily due to the paucity of available Re data. The authors further our quantitative understanding of the importance of irrigation through using 224Ra/228tTh disequilibria, which is particularly important in these shallow shelf sediments. Overall, I would like to see this manuscript published. We use U extensively (both U concentration in sediments and U isotopes in carbonates), which requires understanding sinks and sources, while Re has the potential to be an extremely valuable tracer, but only if more data is obtained from diverse marine locations. The unexpected greater removal in winter compared to summer is also a surprising twist that will encourage the community to consider the seasonal changes in these shallower sediments in different ways. I do have a few minor comments that the authors should consider.\n• Lines 45-46: The authors suggest that Re and U concentration and isotopic composition are ‘emerging proxies’, but that statement suggests a recentness that may not be appropriate. U concentrations and isotopic reconstruction of past conditions have been used more extensively than indicated in refs 2-5, and it is for exactly this reason that a better understanding of sinks/sources is required.\n• Lines 59-60: the authors cite a lack of Re isotope data, but in 2020 Dickson et al started this process (The rhenium isotope composition of Atlantic Ocean seawater, GCA 287), along with Dellinger et al. in 2021 (Fractionation of rhenium isotopes in the Mackenzie River basin during oxidative weathering, EPSL, 573).\n• Lines 91-93: The authors suggest that authigenic accumulation rates are not used for Re and U due to the high detrital contribution that could hinder the accuracy of the accumulation rate. But is this the case for Re, which has an astonishingly small detrital concentration? More support for this statement would be appropriate.\n• Figure 1: I found it extremely difficult to see the pore water profiles due to the volume of data presented on this figure. I ask whether this might be two figures with some of the profiles separated for each season so that they can be more easily seen, or whether the authors want to consider averaging some of the data and presenting a standard deviation for that profile.\n• Line 133: The authors cite the possibility of colloidal phases to explain incomplete removal, but other ideas instead center around a minimum U concentration required for biological mediation of its removal which should be considered (Lovely et al., 1991, Nature, 350).\nMinor comments:\n• Line 73: Rather than ‘inspires’ I would suggest ‘causes’.\n• Line 166: The decay constant for 224Ra didn’t appear in my version of equation 1, so it was unclear if there was a formatting issue.\n• Line 342: Should be ‘lowering’ instead of ‘lowing’\n• Table 2: The percentage symbols can be removed from within the table since they are in the column heading."}, "author_response": "We thank the reviewers and editor for the detailed comments on our manuscript. We have thoroughly revised the manuscript and provided additional data on Re and U concentrations in seawater and bulk sediments based on the comments. Please see our point-by-point response below.\nReviewer #1 (Remarks to the Author):\nHong et al. present the Re and U concentrations in the pore water of shelf sediments from the East China Sea. The benthic fluxes of Re and U were estimated using the 224Ra/228 Th disequilibrium method to account for the influence of sediment transformation/irrigation. Although the data presented are a valuable contribution to a better understanding of the biogeochemistry of Re and U in the marine environment, I have significant problems with the authors' interpretation of the data. Unfortunately, I cannot recommend the publication of the manuscript in its current form.\nReply 1: We thank the reviewer for valuing the contribution of our study. We have carefully considered all the comments in our revised manuscript and replied below.\nBut before I go into the interpretation, I have a question about Supplementary Figures 1 and 2. The Sampling in summer 2017 is distinguished from the Sampling in summer 2021 by red bold circles. However, Supplementary Figure 2 shows that Sampling at stations F4 and F3 was also carried out in summer 2017 (the text on Sampling – lines 118 to 126 in main manuscript text – does not help to clarify this issue). In addition, the Supplementary Figure 2 shows the station designated as B7, which does not exist on the map in Supplementary Figure 1. The same applies to the sampling station marked as HN (Supplementary Figure 2 compared to Supplementary Figure 1). The same applies to the sampling stations for uranium.\nReply 2: We apologize for the inconsistency of the station names.The stations sampled in the summer of 2017 were initially named as those presented in the original Supplementary Figure 2), but all were re-named in previous papers (Shi et al., 2019; Wei et al., 2022). We have now corrected them for consistency (Supplementary Fig. S7). The sampling was only performed once at all stations.\nReferences:\nShi et al. Large benthic fluxes of dissolved iron in China coastal seas revealed by 224Ra/228Th disequilibria. Geochimica Cosmochimica Acta 260 (2019), 49-61.\nWei et al. Winter mixing accelerates decomposition of sedimentary organic carbon in seasonally hypoxic coastal seas. Geochimica Cosmochimica Acta 317 (2022), 457-471.\nLooking at the Re pore water depth profiles, we can see that the Re concentration in the upper 1 to 2 cm sediment depth is elevated in 11 out of 17 cores (one more or less) compared to the Re concentration in seawater (accidentally, the Re concentration profiles of the East China Sea are not shown anywhere – at least at one point, while the bottom water Re concentration are shown only in a few cases), if we assume that the dissolved Re concentration in the East China Sea is in the range of reported values. A similar phenomenon has already been observed by Morford et al. (Ref. 1 and 2). In Ref. 2, Morford et al. \"the flux into the sediments\" and the \"flux out of the sediments\" for a particular site is also depicted. I don't think that the observed feature can be called as benthic uptake (e.g. line 137 in the main text), because in most cases the authors have the flow out of the sediment across the sediment-water interface and the Re-porewater maxima below which pronounced Re-uptake occurs. I think that the authors have only tried to estimate the Re uptake occurring below sediment-water interface, while the obvious Re flux from the sediment observed in most pore water profiles is neglected.\nReply 3: We thank the reviewer for this important concept-changing comment. We agree that this is an issue that we have neglected in the original manuscript. We have now added a section entitled “Sources of Re and U for sedimentary reductive removal” to discuss the processes that may be responsible for the elevated Re and U concentration in porewater. We then clarified that we are quantifying the sedimentary reductive removal occurring below the sediment-water interface (instead of “benthic uptake from bottom water”) that ultimately results in the enrichment of Re and U in the modern shelf sediments.\n“The elevated porewater Re and U concentrations at the sediment–water interface of some sites exceed those found in the seawater (Fig. 1a,b), indicating additional sources supplying porewater Re and U for reductive removal, apart from irrigation and diffusion of dissolved species from bottom seawater (Supplementary Fig. S5)12, 13, 14, 15, 18. Under oxic bottom water, enhanced bioturbation may result in the re-oxidation of reduced Re and U14, 18. However, the most distinct enrichments of core-top porewater [Re] and [U] are observed in summer with lower bioturbation compared to winter (Fig. 1). In this regard, re-oxidation of reduced Re and U is unlikely to be the dominant source for the observed pattern at these sites. Instead, the decomposition and transformation from freshly deposited particles (particularly organic matter) at the sediment–water interface might have released relatively large amounts of Re and U compared to the reductive removal14, 16, 18, 19. The “particulate source” scavenged from seawater could even have exceeded the reductive removal flux and resulted in net dissolved flux back to the water column (Supplementary Fig. S5).\nMoreover, the broad maxima of [Re] and [U] at St. B3-W, E4-S, F1-S, and Y7-S could not be supported merely by their release at the sediment–water interface30. Alternatively, they may be indicative of enhanced Re and U release from the solid phases into porewater at these depth intervals, probably due to intense sediment reworking. Below these depths, Re and U are steadily removed to reach the typical asymptotic minima.\nRegardless of the routes (dissolved versus particulate) that supply Re and U to the porewaters, it is the reductive removal that ultimately results in the enrichment of Re and U in the modern shelf sediments. Our data on Re and U concentrations in the corresponding solid phases provides further support for the reductive enrichment process (Supplementary Fig. S4). The 224Ra/228Th disequilibria approach, which incorporates the influence of irrigation in regulating solute fluxes, is used to estimate the reductive removal fluxes of Re and U (F, positive values denote removal)36. A detailed description of the calculation is provided in Methods.”\nSee the above text in Line 166-195.\nWe have also added a section of “Calculation of sedimentary reductive removal fluxes of Re and U from the 224Ra/228Th disequilibria” to describe the details for the flux estimation:\n“Sedimentary reductive removal can create [Re] and [U] gradients in sediment porewaters, which are traditionally used to quantify the diffusive flux following Fick's first law66: F Aj–Di – ˜C . Here, jis the porosity, Di is the effective diffusion coefficient in sediments calculated using the molecular diffusion coefficient in seawater (Di) and corrected for tortuosity Di A Di , and ˜C represents the concentration gradient of dissolved components (224Ra, Re, U; Supplementary Table S6). Irrigation dominates solute exchange between porewater and bottom water in shelf sediments14, 30, 35, 36, 37. Thus, the reductive removal fluxes of Re and U (F, positive values denote removal) can be estimated with the 224Ra/228Th disequilibria approach, which incorporates the flux enhanced by irrigation36:\nÈ ˜Ci Ø\nF A1F È É D S i Ø Ù É ˜z Ù (1)\ni Ra DRa É˜CRa Ù\nÊ Ú\nS Ê ˜zÚ\nwhere F denotes the flux of total 224Ra (dpm m-2 d-1) and is calculated as the product of the decay constant of 224)*\"? =0.189 d-1) and the integration of the deficit of total 224Ra (A ) relative to 228Th (A ): F Ap Ô z A 1A dz64. The deficit (224Ra/228Th ratio <1) of the solids was confined within the depth of ~10 cm, although they could be scattered due to inhomogeneous sediment particle mixing (Fig. 1c). We here integrate the upper 10 cm to derive the total 224Ra depletion fluxes. Amplification F .*+957\"@#1890-4,-.14-,*830: |A Ra . The calculated amplification factor represents the mean state of irrigation within the upper few cm where 224Ra/228Th disequilibria occur. The fluxes and concentration gradients of 224Ra for the 2021 cruise, together with the previously reported dataset for the 2017 and 2018 cruises (ref.30), are presented in Table 1 and Supplementary Table S6. We determined the Re and U concentration gradients by modeling a linear fit from the surface layer to the depth where significant gradient change occurs (Supplementary Fig. S7) to account for the influence of irrigation indicated by the 224Ra/228Th disequilibria. This allows us to estimate the reductive removal fluxes of Re and U. For the sites characterized with complicated removal-release in the upper sediments indicated by distinct zig-zag patterns within the upper few cm (Supplementary Fig. S7), their concentration gradients and the resulting removal fluxes were not calculated due to the large uncertainties. The uncertainties associated with the site-specific reductive removal fluxes of Re and U were propagated from the errors of the 224Ra fluxes and the concentration gradients of dissolved 224Ra, Re, and U.” See the text in Line 443-476.\nWe have now added vertical distribution of Re and U in the water column of some stations and found relatively homogeneous concentrations except for one elevated point at St. E1-W near a refuge harbor (Supplementary Fig. S3).\nI think the authors are commenting on the Re peak in subsurface pore water observed in the text at station B7. But the same is true for the majority of the data, except that the Re pore water peak is near the sediments surface. Also, next time I would suggest the authors try to do a denser pore water profile for the top 5 cm of the sediments, as Morford et al. (Ref. 1 and 2).\nReply 4: We have neglected to discuss the subsurface peaks at other stations and the zig-zag patterns at some sites (St. B3-W, E4-S, F1-S, and Y7-S) in the original manuscript. Please see our detailed reply above.\nWe appreciate the reviewer’s suggestion and will perform high-resolution sampling in future work.\nIt is also unclear that the red Line representing the linear regression sometimes reaches the sediment-water boundary and sometimes goes beyond it, although the Re-soil water concentration is not given for these sites. It is therefore not clear which Re-boundary concentrations have been included in the calculations.\nReply 5: During revision, we have refined how we calculate the sedimentary reductive flux, see Supplementary Fig. S7. The regression (denoted by red lines) was done with the data below the sediment-water interface (filled circles). For some sites characterized with distinct zig-zag patterns, their concentration gradients were not calculated due to the large uncertainties.A detailed description of the linear regression was added in the Method (Line 466-473):\n\"We determined the Re and U concentration gradients by modeling a linear fit from the surface layer to the depth where significant gradient change occurs (Supplementary Fig. S7) to account for the influence of irrigation indicated by the 224Ra/228Th disequilibria. This allows us to estimate the reductive removal fluxes of Re and U. For the sites characterized with complicated removal-release in the upper sediments indicated by distinct zig-zag patterns within the upper few cm (Supplementary Fig. S7), their concentration gradients and the resulting removal fluxes were not calculated due to the large uncertainties.\"\nI would also like to draw the authors' attention to Ref. 3.\nReply 6: We appreciate the reviewer for pointing out this work. It supports our interpretation of Re scavenging from the water column by biological organic matter, which serves as another removal mechanism of Re into sediments. The study also documented an unaccounted Re sink in the coastal lagoon that is ~4 times higher than that in oxic marine sediments. We have added the information in the revised manuscript (Line 54, 63-65 and Line 367-370).\nReferences\n1. Morford et al., A model for uranium, rhenium, and molybdenum diagenesis in marine sediments based on results from coastal locations. Geochimica et Cosmochimica Acta 73 (2009) 2938-2960.\n2. Morford et al., Rhenium geochemical cycling: Insights from continental margins. Chemical Geology 324-325 (2012) 73-86.\n3. Danish et al. Non-conservative removal of dissolved rhenium from a coastal lagoon: Clay adsorption versus biological uptake. Chemical Geology 580 (2021) 120378.\nReviewer #2 (Remarks to the Author):\nA lot of different types of carefully executed measurements were made on these cruises. Some of the method details are already published (and so are referenced in the short methods section). As a whole, I found the paper easy to read and attention grabbing. While this isn't the first estimate ever of shelf sinks, the paper combines the literature data with new ECS measurements to look at the 'shallow shelf' question at a global scale. I agree that 'A key advantage of this 224Ra/228Th disequilibrium method over traditional sediment incubation and modeling methods is that it captures all physical processes that affect solute transfer across the sediment–water interface but does not alter the system' and that these radioisotope-based estimates are a unique application here to looking at budgets. Some of the data is reused from earlier studies, but the authors utilize interesting applications of existing methods/techniques in a way that provides a unique contribution to the field. I thought the authors did a good job of 'considering the options' starting at line 259 ('Shelf sediments as an overlooked sink'). Budget parameters were often determined in 2 ways to support the conclusions. Implications and 'what ifs' were discussed, which shows that the authors have both considered potential issues with higher sinks and, if real, what higher sinks mean for this field and beyond (i.e. paleo-interpretations, etc.). If the larger sink for U is real and the system is not in balance for U, this is very important for the oceanographic community to know.\nI have a few broad comments/concerns that would be helpful for the authors to clarify (whether in responses or in the manuscript text itself). In my opinion, addressing these questions/areas would help to strengthen the conclusions and assuage doubts.\nReply 7: We thank the reviewer for the very positive evaluation and constructive comments that help to strengthen the conclusion of our study. We have carefully considered the comments/questions in the revised manuscript, including clarifying the timescale of the sampling, compiling the sedimentary and diagenetic variables to constrain the representativeness of the fluxes used for the estimation of global shelf sinks, clarifying the uncertainties for a more comprehensive understanding of our estimation. Please find the details below.\n1. Potential variability of the datasets used (temporal questions) and how representative the dataset is of global shelf conditions for depths <200 m.\nOne thing that I would want to see (without looking at references) is what is the timescale of the summer and winter sampling in Figure 1 and Table 1? Could this go in the caption of Fig 1? Essentially, were all the samples from summer taken the same day? Or the same week? The reason this is of interest is to see how variable the Ra/Th could be (averaging over 10 days) from week to week or month to month on the shelf. Would we see much much higher or much much lower a few weeks later? In short, are these representative conditions on this shelf throughout most of the summer and winter? How does the '10 day average' for these radioisotope ratios change at one location over the entire summer? Are there temporal studies in the literature that can be cited here to indicate this range? Two summer dates are shown, although they aren't from the same locations (Fs are down at the bottom of the map and E/Y are in the middle). The results do seem comparable on the scale in Fig. 2. Since there is so much variability, as noted in lines 166-167 (when comparing to other studies) and Fig 1/Fig 5, it would be helpful to know how changeable the Ra/Th ratios are.\nReply 8: We agree that the timescale of Ra/Th is an important issue to consider. We performed sampling campaigns within 5 days (2018 and 2021) or 10 days (2017). Unfortunately, time-series measurement within the timescale of a week/month was not performed in this study. However, no significant inter-annual variation of Ra/Th (Response Figure 1) and 224Ra fluxes (2230±210 vs. 2260±340 dpm m-2 d-1; Cai et al, unpublished data) between the summer of 2019 and 2020 was observed in the study area. In addition, a time-series study of the total 224Ra activities and fluxes in the German Wadden Sea showed good agreement over 12 days, although it is not in the study area of our research (Cai et al., 2020). The results suggest that our estimates based on the 224Ra/228Th disequilibrium approach are representative of an entire season. We have added the timescale in the figure captions (Fig. 1 in Line 494 and Fig. S2).\nResponse Figure 2. Comparison of Ra/Th ratios between the sampling campaigns of 2019 and 2020 at St. C12 (122.74°E, 30.94°N) in the East China Sea shelf (Cai et al, unpublished data).\nReferences:\nCai et al. Measurement of 224Ra:228Th disequilibrium in coastal sediments using a delayed coincidence counter. Marine Chemistry 138-139 (2012), 1-6.\nCai et al. Carbon and nutrient export from intertidal sand systems elucidated by 224Ra/228Th disequilibria. Geochim. Cosmochim. Acta 274 (2020), 302-316.\nHong et al. Benthic fluxes of metals into the Pearl River Estuary based on 224Ra/228Th disequilibrium: From alkaline earth (Ba) to redox sensitive elements (U, Mn, Fe). Geochimica Cosmochimica Acta 237 (2018), 223-239.\n2. Potential variability of the datasets used (spatial questions) and how representative the dataset is of global shelf conditions for depths <200 m.\nLines 63-68: 'The contrasting conditions…the global oceanic Re and U budgets.' I agree that it’s a good region to look at different patterns because of summer/winter changes, my question would be do most shelves globally look like this one and have similar processes? Do we know or have an idea of how representative they might be? When the estimates are extrapolated to the global model, how does this shelf compare to the typical conditions of the 'average' shelf?\nReply 9: We thank the reviewer for raising this issue. It is worth noting that seasonal variations of Re fluxes and the coupling with organic carbon decomposition were also observed in the North American margins (see Line 289-292): “A similar correlation between sedimentary reductive removal of Re and OC decomposition was observed in the NAMs sediments, overlain by oxygenated bottom waters14, 35, suggesting a potentially universal linkage between these two processes in shelf sediments.”\nIn fact, the sedimentary and diagenetic variables (sedimentation rate, OC supply/decomposition; supplementary Table S3) that dominate the benthic Re and U fluxes in the study fall within the range of the global 'average' shelf. Therefore, we suggest that the fluxes estimated in our study are representative of the average global shelf. We have added the following information (Line 309-314):\n\"For the evaluation of the sedimentary reductive removal fluxes of Re and U over global shelves, the ECS, the NAMs, and the East Siberian Sea can be adopted as representative settings since they are among the widest shelves worldwide and characterized by diagenetic conditions (sedimentation rate, and OC supply/decomposition) close to the representative conditions of global shelves (Supplementary Table S3).\"\nIf I look at Fig 3 and Fig 2, two of the really high flux points are northern winter points. Do we know (I assume yes) about the shelf dynamics here and can we attribute these patterns to summer vs. winter and not north shelf vs. south shelf spatial trends? Is there more upwelling in one location vs. other? Are the sediment types similar? Are the regressions in Fig 3. still strong (>0.5) if the two northern points are taken away? I realize these measurements are very difficult and occupying the same locations twice might not have been possible. There is some overlap around 31 in summer and winter points. A short comment/discussion from the authors on how the patterns/trends are clearly related to season and not spatial variability would be welcome (especially for those that don’t know the physics and sedimentary systems of this area well).\nReply 10: We concur with the reviewer that shelf dynamics is important for the understanding of the variation. The sampling sites are located in muddy sediments and have relatively homogeneous TOC (0.33±0.05%; Wei et al., 2022). Upwelling in the study area peaks in July and August (summer season) and vanishes in later September (Chen et al., 2021; Hu and Wang, 2016). However, elevated Re and U fluxes are observed in the winter. In addition, there are no spatial patterns related to upwelling (see Response Figure 2). Therefore, we attribute the high fluxes to seasonal rather than spatial variation.\nWe have added the contrasting dynamics for a better understanding:\nLine 92-97 in the Introduction: \"The shelf dynamics are distinctly stronger in winter than in summer30: the eddy diffusivities in the bottom water are 5-fold higher; the sediments are more intensively reworked/irrigated as evidenced by deeper penetration of DO and excess 234Th activity, as well as 224Ra/228Ra disequilibria. Additionally, upwelling develops in June, reaches its strongest condition in July and August, and eventually vanishes in late September31.\" (Response Figure 3).\nLine 229-235 in the Discussion: \"Sedimentary OC decomposition dictates the reducing conditions conducive to the removal of Re and U13, 14. Intense winter reworking/irrigation was posited to be responsible for the highly efficient decomposition of sedimentary OC in the study area, resulting in relatively homogeneous TOC contents (0.33±0.05%) and stronger reducing conditions30. Therefore, we argue that the distinctly higher winter removal rates of Re and U are driven by intensive reworking/irrigation.\"\nWe have renewed the calculation of the fluxes. Only the stations with smooth removal patterns of porewater Re and U were calculated. Therefore, the northern sites with zig-zag patterns (Re and U at St. B3-W; only U in St. A3-W) have been excluded. For Re, the regressions in Fig. 3 would decrease if St. A3-W was removed. However, the p;*2:-8 *7-/-4-7*22=>%$%'\"8-- Response Figure 4)"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Dear editor, dear authors\nI would like to thank the authors for their efforts to consider the criticism in the last round of reviews.\nHowever, I am sorry, but I have to criticize again and will therefore unfortunately suggest a further revision or forwarding the manuscript to another journal.\nMy main criticism relates to what is now the last section of the article (Shelf sediments as an overlooked sink of global oceanic Re and U). While it is debatable whether shelf sediments are an overlooked global sink (see e.g. article: Redox-sensitive trace metals as paleoredox proxies: A review and analysis of data from modern sediments by Bennet and Canfield, which clearly shows that shelf sediments have high Re enrichment, implying that shelf sediments have long been recognized as an important Re sink), my concerns are not related to whether or not shelf sediments are recognized as an important Re sink. Rather, my concerns relate to the lack of recognition of Re flux from the sediments (although the authors have since admitted that it is important and have added a paragraph addressing this issue).\nWhy did the authors neglect the Re flux from the sediment in their mass balance calculations? Did auhors try to take this into account? If so, what is the result of the sum of the reductive Re uptake and the Re flux from the sediment? If the Re flux from the sediment is taken into account, the main conclusion of this article (as far as I understand it) is still the same? That is, modern Re budgets are then still unbalanced and/or their sources are significantly underestimated?\nI am sorry but I hope that authors understand that this is an important issue and therefore I have no other choice than suggest another review round or redirection of the manuscript into some other journal.", "reviewer_2": "I very much appreciated the professional tone of the authors’ responses and the detail with which they responded. I think the authors responded well to all the reviewers’ comments, not just my own. Figures 5 and 6 now look great and I the improvements here really help convey the message better. The additional figures/tables and updates were well chosen to support their argument (and provide direct data responses to the reviewers). I still have a few lingering questions that could only be solved by additional sampling or modeling efforts (e.g. repeated time series work at the same location, as R1 noted higher resolution sampling in the upper 2 cm, and additional modeling work to evaluate the scenarios in S5), but in my opinion, I think the authors have provided a convincing argument with their data as is. Importantly, their presentation in this manuscript leaves room for future refinements and possibilities (e.g. the paper has an open vs. a defensive one) and I will again note that they discussed caveats and ‘what ifs’ in a way that strengthens their arguments. The authors continue to present the data in a way that advances the field and peaks curiosities, while encouraging others to investigate further.\nOne minor question/edit:\nSupplemental Figure 4 (S4) notes vertical bars for the crustal ratios. I don’t see vertical bars in my pdf. I only see points and then a figure label at the bottom of each plot. Perhaps there was a transfer error when the figure was converted to a pdf?", "reviewer_3": "I am writing in response to the manuscript revisions for “Overlooked shelf sediment reductive sinks of dissolved rhenium and uranium in the modern ocean” by Hong et al. This paper continues to be an important addition to the literature and will spur the community to better evaluate shelf sediments as a sink for Re and U and/or re-evaluate sources to obtain balanced budgets. The authors made compelling and helpful changes in this current revision that have clarified their manuscript and strengthened their argument. I have only a few remaining suggestions:\n• In Figure S3, the authors provide the water column Re and U data. Because the one circled data point is not elevated in both Re and U, the sentence should be modified to\n“The elevated (Re) and lower (U) concentrations (circled) occurred at St. E1-W…”\n• Figure S3. Consistent with the units from Fig S7, I am assuming that the units for the U concentration in Fig S3 are nM and not uM.\n• The pore water U profiles from Summer 2021 are still confusing to me. I am not sure there is a flux into sediments for F2-S and F4-S the way it is drawn with the red lines on Fig S7. Wouldn’t it make more sense to look at the gradient across the sediment-water interface between the bottom water concentration (Fig S3) and the first porewater concentration? If the slope is effectively the same as what you have already determined, then it would be a minor but important footnote.\n• I would adjust the caption to Table S3 to be more specific for what you mean by “Globe” by adding “Comparison of key environmental variables between globally representative shelf conditions and the regions chosen for extrapolation.”\n• In Table S5, the authors include the CRM results. However in Text S1 lines 29-31, the authors also mention measuring MESS-4 as a reference standard. Please include the MESS-4 results, as there is a paucity of Re concentrations published for CRM.\n• Figure 3. The authors mention that the red lines in panels e and f denote a regression with zero intercept but I am not seeing any red lines on the plot…?"}, "author_response": "We thank the editor and reviewers for the constructive comments on our manuscript. We have revised the manuscript to address the issue of the Re and U flux out of the sediments based on the comments. Please see our point-by-point response below.\nReviewer #1 (Remarks to the Author):\nDear editor, dear authors\nI would like to thank the authors for their efforts to consider the criticism in the last round of reviews.\nHowever, I am sorry, but I have to criticize again and will therefore unfortunately suggest a further revision or forwarding the manuscript to another journal.\nMy main criticism relates to what is now the last section of the article (Shelf sediments as an overlooked sink of global oceanic Re and U). While it is debatable whether shelf sediments are an overlooked global sink (see e.g. article: Redox-sensitive trace metals as paleoredox proxies: A review and analysis of data from modern sediments by Bennet and Canfield, which clearly shows that shelf sediments have high Re enrichment, implying that shelf sediments have long been recognized as an important Re sink), my concerns are not related to whether or not shelf sediments are recognized as an important Re sink. Rather, my concerns relate to the lack of recognition of Re flux from the sediments (although the authors have since admitted that it is important and have added a paragraph addressing this issue).\nWhy did the authors neglect the Re flux from the sediment in their mass balance calculations? Did auhors try to take this into account? If so, what is the result of the sum of the reductive Re uptake and the Re flux from the sediment? If the Re flux from the sediment is taken into account, the main conclusion of this article (as far as I understand it) is still the same? That is, modern Re budgets are then still unbalanced and/or their sources are significantly underestimated?\nI am sorry but I hope that authors understand that this is an important issue and therefore I have no other choice than suggest another review round or redirection of the manuscript into some other journal.\nReply 1: We thank the reviewer for agreeing with the general conclusion of our study. Indeed, shelf sediments have long been known to be enriched in Re; therefore, it is not surprising that shelf sediments will play an important role in the modern oceanic budget of Re. The novelty of our study is to quantify the contribution of shelf sediment in the oceanic Re and U budgets, as it has been dismissed or treated as authigenically neutral in previous mass balance studies. See Line 66-69.\nWhy did the authors neglect the Re flux from the sediment in their mass balance calculations? Did auhors try to take this into account? If so, what is the result of the sum of the reductive Re uptake and the Re flux from the sediment? If the Re flux from the sediment is taken into account, the main conclusion of this article (as far as I understand it) is still the same? That is, modern Re budgets are then still unbalanced and/or their sources are significantly underestimated?\nI am sorry but I hope that authors understand that this is an important issue and therefore I have no other choice than suggest another review round or redirection of the manuscript into some other journal.\nReply 2: As stated in the first section of the Discussion, the enrichment of core-top [Re] is most likely regenerated from the decomposition of freshly deposited organic matter that scavenged Re from seawater rather than from the re-oxidation of the previously-reduced Re and/or riverine particle-bound phases. We have now added a section (The regenerated flux of Re and U out of the sediment–water interface) to discuss the fluxes out of the sediments on the Re and U mass balance. We clarified that these fluxes are small relative to the reductive removal fluxes and will not change our conclusion on the quantitative importance of shelf sediment sinks to the modern oceanic budgets of Re and U. We have added the following text to the Discussion. See Line 239-261:\n“The regenerated upward fluxes of Re and U estimated with our porewater data range from -860 to 0 pmol m-2 d-1 (median: -49 pmol m-2 d-1; average: -223 pmol m-2 d-1) and -40 to 0 nmol m-2 d-1 (median: 0 pmol m-2 d-1; average: -6 pmol m-2 d-1), respectively (Table 1). For some sites, the regenerated Re and U fluxes are comparable to or even larger than the sedimentary reductive removal fluxes. Regarding the median values, they are ~15% and 0% of the sedimentary reductive removal fluxes of Re and U, respectively, since many sites have zero upward fluxes. Regarding the average values, the regenerated upward fluxes could be ~36% and ~6% of the reductive removal fluxes of Re and U at our studied sites, respectively. It suggests that the regenerated flux could be an important yet not well understood source for the reductive removal of Re and U within the sediments. However, unlike rare earth elements whose riverine particle-bound phases might be actively involved in the shelf cycling (e.g., refs. 26, 48, 49), the regenerated upward flux of Re and U are most likely scavenged from seawater, as discussed above. Therefore, they are considered part of oceanic internal recycling. In this regard, we have not included these fluxes as external sources when discussing the oceanic Re and U mass balance. Even if some of the upward fluxes are from riverine particle-bound phases and/or re-oxidation of previously reduced species, our conclusion on the quantitative importance of shelf sediment sinks to the modern oceanic budgets of Re and U remains unchanged. Future studies should aim to better characterize the nature of Re and U released at the sediment–water interface, for example, by combined analyses of their isotope signatures and exchange flux. ”\nReviewer #2 (Remarks to the Author):\nI very much appreciated the professional tone of the authors’ responses and the detail with which they responded. I think the authors responded well to all the reviewers’ comments, not just my own. Figures 5 and 6 now look great and I the improvements here really help convey the message better. The additional figures/tables and updates were well chosen to support their argument (and provide direct data responses to the reviewers). I still have a few lingering questions that could only be solved by additional sampling or modeling efforts (e.g. repeated time series work at the same location, as R1 noted higher resolution sampling in the upper 2 cm, and additional modeling work to evaluate the scenarios in S5), but in my opinion, I think the authors have provided a convincing argument with their data as is. Importantly, their presentation in this manuscript leaves room for future refinements and possibilities (e.g. the paper has an open vs. a defensive one) and I will again note that they discussed caveats and ‘what ifs’ in a way that strengthens their arguments. The authors continue to present the data in a way that advances the field and peaks curiosities, while encouraging others to investigate further.\nOne minor question/edit:\nSupplemental Figure 4 (S4) notes vertical bars for the crustal ratios. I don’t see vertical bars in my pdf. I only see points and then a figure label at the bottom of each plot. Perhaps there was a transfer error when the figure was converted to a pdf?\nReply 4: We have removed the vertical bars and the caption in this figure in the revision.\nReviewer #3 (Remarks to the Author):\nI am writing in response to the manuscript revisions for “Overlooked shelf sediment reductive sinks of dissolved rhenium and uranium in the modern ocean” by Hong et al. This paper continues to be an important addition to the literature and will spur the community to better evaluate shelf sediments as a sink for Re and U and/or re-evaluate sources to obtain balanced budgets. The authors made compelling and helpful changes in this current revision that have clarified their manuscript and strengthened their argument. I have only a few remaining suggestions:\n• In Figure S3, the authors provide the water column Re and U data. Because the one circled data point is not elevated in both Re and U, the sentence should be modified to\n“The elevated (Re) and lower (U) concentrations (circled) occurred at St. E1-W…”\n• Figure S3. Consistent with the units from Fig S7, I am assuming that the units for the U concentration in Fig S3 are nM and not uM.\nReply 5: Thanks for pointing out. We have changed the units and the figure caption.\n• The pore water U profiles from Summer 2021 are still confusing to me. I am not sure there is a flux into sediments for F2-S and F4-S the way it is drawn with the red lines on Fig S7. Wouldn’t it make more sense to look at the gradient across the sediment-water interface between the bottom water concentration (Fig S3) and the first porewater concentration? If the slope is effectively the same as what you have already determined, then it would be a minor but important footnote.\nReply 6: The concentration gradients are -1.17 for F2-S and -0.95 for F4-S using the bottom water and the first porewater (at 1 cm), and they are broadly consistent with the estimation in Fig S7 (-1.24±0.14 and -1.73±0.30; Table S6) within a factor of ~2. Thus, there should be a flux into sediments for the two sites.\n• I would adjust the caption to Table S3 to be more specific for what you mean by “Globe” by adding “Comparison of key environmental variables between globally representative shelf conditions and the regions chosen for extrapolation.”\nReply 7: We have corrected the caption of Table S3.\n• In Table S5, the authors include the CRM results. However in Text S1 lines 29-31, the authors also mention measuring MESS-4 as a reference standard. Please include the MESS-4 results, as there is a paucity of Re concentrations published for CRM.\nReply 8: The results of MESS-4 have been added.\n• Figure 3. The authors mention that the red lines in panels e and f denote a regression with zero intercept but I am not seeing any red lines on the plot…?\nReply 9: We have removed the sentence because only the regressions with zero intercept are shown (black lines) in panels e and f."}, {"round": 3, "reviewer_comments": {"reviewer_3": "I appreciate the work that the authors have done to determine the relative importance of the Re and U fluxes into sediments for shelf sediments. The additional calculations to put the flux from sediments into context were important additions. The relative magnitudes of these fluxes compared with riverine sources and anoxic sinks provide evidence for the key issue at hand - whether the sinks and sources are balanced and the subsequent impact on residence times. I recommend publication.\nIf there remains an additional concern regarding the title of the section that begins on line 325 \"Shelf sediments as an overlooked sink of global oceanic Re and U\" due to the fact that there has been previous work that highlighted shelf sediments as a relavent sink, the authors could consider rewording this section as \"Shelf sediments as an important (or critical or dominant) sink of global oceanic Re and U\""}, "author_response": "REVIEWERS' COMMENTS\nReviewer #3 (Remarks to the Author):\nI appreciate the work that the authors have done to determine the relative importance of the Re and U fluxes into sediments for shelf sediments. The additional calculations to put the flux from sediments into context were important additions. The relative magnitudes of these fluxes compared with riverine sources and anoxic sinks provide evidence for the key issue at hand - whether the sinks and sources are balanced and the subsequent impact on residence times. I recommend publication.\nIf there remains an additional concern regarding the title of the section that begins on line 325 \"Shelf sediments as an overlooked sink of global oceanic Re and U\" due to the fact that there has been previous work that highlighted shelf sediments as a relavent sink, the authors could consider rewording this section as \"Shelf sediments as an important (or critical or dominant) sink of global oceanic Re and U\"\nReply: We have rephrased this section as “Shelf sediments as a critical sink of global oceanic Re and U”. See Line 331."}]

Decision letter