Dominance of benthic flux of REEs on continental shelves: implications for oceanic budgets
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Keywords: Boundary exchange, Sediment–water interface, Benthic process, East China Sea Shelf, Rare earth element
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Abstract
Figures
 Figure 1 Sediment porewater REE pattern normalised to the bottom seawater (depth of 0 cm) at each station. Analytical uncertainties (see Supplementary Information) are shown here and in later figures. |  Figure 2 Porewater and bottom seawater (depth of 0 cm) Nd, Mn (filled symbols), Fe and P (open symbols) concentrations. “+” symbols refer to the water sample data at station C1 at 1 m. |  Figure 3 Shelf REE cycling shown by a Ce/Ce*-HREE/LREE plot. REE data for the salinity transect (1998) and Mn-Fe leachate are from Wang and Liu (2008). The data source for water mass end members (EMs) is provided in Figure S-3. |  Figure 4 Diffusive sedimentary Nd flux variation with depth. The coloured symbols indicate bottom water DO and open symbols are those with no DO data available. Axes and colour bar are on log scale. Compiled dataset and references are provided in Table S-4. |
| Figure 1 | Figure 2 | Figure 3 | Figure 4 |
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Introduction
The rare earth elements (REEs), as a series of particle-reactive elements, show non-conservative behaviour during transport from continental source to oceanic sink (Elderfield and Greaves, 1982
Elderfield, H., Greaves, M.J. (1982) The rare earth elements in seawater. Nature 296, 214–219. https://doi.org/10.1038/296214a0
; Rousseau et al., 2015Rousseau, T.C.C., Sonke, J.E., Chmeleff, J., van Beek, P., Souhaut, M., Boaventura, G., Seyler, P., Jeandel, C. (2015) Rapid neodymium release to marine waters from lithogenic sediments in the Amazon estuary. Nature Communications 6, 7592. https://doi.org/10.1038/ncomms8592
). As such, REE patterns are widely used in oceanographic studies, to track boundary exchange and internal cycling (Elderfield and Greaves, 1982Elderfield, H., Greaves, M.J. (1982) The rare earth elements in seawater. Nature 296, 214–219. https://doi.org/10.1038/296214a0
; Jeandel and Oelkers, 2015Jeandel, C., Oelkers, E.H. (2015) The influence of terrigenous particulate material dissolution on ocean chemistry and global element cycles. Chemical Geology 395, 50–66. https://doi.org/10.1016/j.chemgeo.2014.12.001
). Nevertheless, source-to-sink processes for oceanic REEs remain poorly understood. Two hypotheses have been proposed to explain oceanic REE distributions: the top-down (Siddall et al., 2008Siddall, M., Khatiwala, S., van de Flierdt, T., Jones, K., Goldstein, S.L., Hemming, S., Anderson, R.F. (2008) Towards explaining the Nd paradox using reversible scavenging in an ocean general circulation model. Earth and Planetary Science Letters 274, 448–461. https://doi.org/10.1016/j.epsl.2008.07.044
) versus the bottom-up control (Abbott et al., 2015Abbott, A.N., Haley, B.A., McManus, J., Reimers, C.E. (2015) The sedimentary flux of dissolved rare earth elements to the ocean. Geochimica et Cosmochimica Acta 154, 186–200. https://doi.org/10.1016/j.gca.2015.01.010
; Du et al., 2020Du, J., Haley, B.A., Mix, A.C. (2020) Evolution of the Global Overturning Circulation since the Last Glacial Maximum based on marine authigenic neodymium isotopes. Quaternary Science Reviews 241, 106396. https://doi.org/10.1016/j.quascirev.2020.106396
). The former emphasises reversible scavenging, while the latter focuses on the dominance of benthic processes. The resolution of this debate would provide valuable insights on the long-standing “Nd (Neodymium) paradox”: while Nd isotopes appear to behave conservatively during water mass mixing, dissolved Nd concentrations ([Nd]diss) reflect the behaviour of a reactive element (Arsouze et al., 2009Arsouze, T., Dutay, J.C., Lacan, F., Jeandel, C. (2009) Reconstructing the Nd oceanic cycle using a coupled dynamical – biogeochemical model. Biogeosciences 6, 2829–2846. https://doi.org/10.5194/bg-6-2829-2009
; Haley et al., 2017Haley, B.A., Du, J., Abbott, A.N., McManus, J. (2017) The Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the Oceans. Frontiers in Marine Science 4, 426. https://doi.org/10.3389/fmars.2017.00426
). Such inconsistency impedes the application of Nd isotopes as a tracer for paleo-circulation (Du et al., 2020Du, J., Haley, B.A., Mix, A.C. (2020) Evolution of the Global Overturning Circulation since the Last Glacial Maximum based on marine authigenic neodymium isotopes. Quaternary Science Reviews 241, 106396. https://doi.org/10.1016/j.quascirev.2020.106396
; Patton et al., 2021Patton, G.M., Francois, R., Weis, D., Hathorne, E., Gutjahr, M., Frank, M., Gordon, K. (2021) An experimental investigation of the acquisition of Nd by authigenic phases of marine sediments. Geochimica et Cosmochimica Acta 301, 1–29. https://doi.org/10.1016/j.gca.2021.02.010
).The ambiguities in the oceanic REE cycling and budget are partially caused by incomplete understanding of REE sources. The mass balance of oceanic REEs requires sources other than riverine input and atmospheric deposition (Elderfield and Greaves, 1982
Elderfield, H., Greaves, M.J. (1982) The rare earth elements in seawater. Nature 296, 214–219. https://doi.org/10.1038/296214a0
), such as a benthic dissolved flux across the sediment–water interface via porewater (Abbott et al., 2015Abbott, A.N., Haley, B.A., McManus, J., Reimers, C.E. (2015) The sedimentary flux of dissolved rare earth elements to the ocean. Geochimica et Cosmochimica Acta 154, 186–200. https://doi.org/10.1016/j.gca.2015.01.010
; Du et al., 2016Du, J., Haley, B.A., Mix, A.C. (2016) Neodymium isotopes in authigenic phases, bottom waters and detrital sediments in the Gulf of Alaska and their implications for paleo-circulation reconstruction. Geochimica et Cosmochimica Acta 193, 14–35. https://doi.org/10.1016/j.gca.2016.08.005
) and/or submarine groundwater discharge (Johannesson et al., 2011Johannesson, K.H., Chevis, D.A., Burdige, D.J., Cable, J.E., Martin, J.B., Roy, M. (2011) Submarine groundwater discharge is an important net source of light and middle REEs to coastal waters of the Indian River Lagoon, Florida, USA. Geochimica et Cosmochimica Acta 75, 825–843. https://doi.org/10.1016/j.gca.2010.11.005
). In particular, recent modelling efforts suggest that continental margin sediments can be a major source of oceanic REEs (Arsouze et al., 2009Arsouze, T., Dutay, J.C., Lacan, F., Jeandel, C. (2009) Reconstructing the Nd oceanic cycle using a coupled dynamical – biogeochemical model. Biogeosciences 6, 2829–2846. https://doi.org/10.5194/bg-6-2829-2009
; Rempfer et al., 2011Rempfer, J., Stocker, T.F., Joos, F., Dutay, J.-C., Siddall, M. (2011) Modelling Nd-isotopes with a coarse resolution ocean circulation model: Sensitivities to model parameters and source/sink distributions. Geochimica et Cosmochimica Acta 75, 5927–5950. https://doi.org/10.1016/j.gca.2011.07.044
). On continental margins, isolating the contribution of a sedimentary REE flux to seawater is particularly difficult because of the complex interaction between riverine input, oceanic currents, and benthic processes. Dissolved REEs have been measured in many estuarine transects and an additional sedimentary source is often proposed to explain their spatial distribution (Wang and Liu, 2008Wang, Z.-L., Liu, C.-Q. (2008) Geochemistry of rare earth elements in the dissolved, acid-soluble and residual phases in surface waters of the Changjiang Estuary. Journal of Oceanography 64, 407–416. https://doi.org/10.1007/s10872-008-0034-0
; Rousseau et al., 2015Rousseau, T.C.C., Sonke, J.E., Chmeleff, J., van Beek, P., Souhaut, M., Boaventura, G., Seyler, P., Jeandel, C. (2015) Rapid neodymium release to marine waters from lithogenic sediments in the Amazon estuary. Nature Communications 6, 7592. https://doi.org/10.1038/ncomms8592
). However, the corresponding sediment porewater REE data, which provide the direct evidence for a benthic flux, are still scarce.Here, we focus on one of the largest land–ocean interfaces in Asia, the Changjiang (Yangtze) River–East China Sea system. The Changjiang River delivers a huge amount of fresh water (∼890 km3/yr) and sediment (∼450 Mt/yr) to the continental margin (Chen et al., 2001
Chen, Z., Li, J., Shen, H., Zhanghua, W. (2001) Yangtze River of China: historical analysis of discharge variability and sediment flux. Geomorphology 41, 77–91. https://doi.org/10.1016/S0169-555X(01)00106-4
), accounting for 2–3 % of global discharge. The East China Sea is characterised by one of the widest continental shelves (shelf area: ∼5 × 105 km2) and highest sedimentation rate (inner shelf: ∼1–6 cm/yr) worldwide (Liu et al., 2006Liu, J.P., Li, A.C., Xu, K.H., Velozzi, D.M., Yang, Z.S., Milliman, J.D., DeMaster, D.J. (2006) Sedimentary features of the Yangtze River-derived along-shelf clinoform deposit in the East China Sea. Continental Shelf Research 26, 2141–2156. https://doi.org/10.1016/j.csr.2006.07.013
). The high dissolved–particulate riverine fluxes make this region ideal for studying the effect of boundary exchange on REE cycling. This paper presents REE data for shelf sediment porewater profiles, as well as for estuarine water from this study and the literature. The main aim is to investigate REE cycling on the East China Sea shelf, with an emphasis on benthic processes, and to provide new insights on the role of the continental shelf in the global benthic REE flux.top
REE Cycling on the Shelf
Along the salinity transect in the Changjiang estuary, estuarine [REE]diss decreases dramatically at salinity <1–2 psu, driven by scavenging, and gradually increases at mid-high salinity (Fig. S-3), hinting at a potential marine sedimentary source (Wang and Liu, 2008
Wang, Z.-L., Liu, C.-Q. (2008) Geochemistry of rare earth elements in the dissolved, acid-soluble and residual phases in surface waters of the Changjiang Estuary. Journal of Oceanography 64, 407–416. https://doi.org/10.1007/s10872-008-0034-0
). We measured porewater [REE]diss for four multi-core stations at water depths of 6–46 m (Figs. 1, 2; locations in Fig. S-1). [REE]diss of shallow porewater is generally higher than that for bottom water, consistent with observations from other continental margins and with release of porewater REEs to the overlying seawater (Haley et al., 2004Haley, B.A., Klinkhammer, G.P., McManus, J. (2004) Rare earth elements in pore waters of marine sediments. Geochimica et Cosmochimica Acta 68, 1265–1279. https://doi.org/10.1016/j.gca.2003.09.012
; Abbott et al., 2015Abbott, A.N., Haley, B.A., McManus, J., Reimers, C.E. (2015) The sedimentary flux of dissolved rare earth elements to the ocean. Geochimica et Cosmochimica Acta 154, 186–200. https://doi.org/10.1016/j.gca.2015.01.010
).Figure 1 Sediment porewater REE pattern normalised to the bottom seawater (depth of 0 cm) at each station. Analytical uncertainties (see Supplementary Information) are shown here and in later figures.
Figure 2 Porewater and bottom seawater (depth of 0 cm) Nd, Mn (filled symbols), Fe and P (open symbols) concentrations. “+” symbols refer to the water sample data at station C1 at 1 m.
At the shallowest site, C6-1 at 6 m, the REE patterns are relatively invariant (Fig. 1). However, porewater [REE]diss increases with core depth, implying a diagenetic source below the studied depth range and upward diffusion. The similarity between the porewater [Mn]diss and [REE]diss profiles (Fig. 2) at C6-1 suggests a source of both at depths at, or beneath, ∼20 cm, most likely the reductive dissolution of Mn oxides. Porewaters at C10 (depth: 12 m) are characterised by a maximum in [REE]diss at shallow core depth (<7 cm), coincident with a maximum in [Mn]diss (Fig. 2). These observations are again consistent with a source of REE linked to Mn reduction. Indeed, REEs are commonly enriched in Mn oxides and they can be released together in a reductive environment (Blaser et al., 2016
Blaser, P., Lippold, J., Gutjahr, M., Frank, N., Link, J.M., Frank, M. (2016) Extracting foraminiferal seawater Nd isotope signatures from bulk deep sea sediment by chemical leaching. Chemical Geology 439, 189–204. https://doi.org/10.1016/j.chemgeo.2016.06.024
). The change in porewater REE patterns also supports the control of Mn reduction. The correlation (R2 = 0.58; <7 cm at C10; Fig. S-4) between porewater [Mn]diss and Ce anomaly (Ce/Ce*) values (Eq. S-1) is consistent with the well-known association of Ce with Mn oxyhydroxide (Schijf et al., 2015Schijf, J., Christenson, E.A., Byrne, R.H. (2015) YREE scavenging in seawater: A new look at an old model. Marine Chemistry 177, 460–471. https://doi.org/10.1016/j.marchem.2015.06.010
). Besides, the Mn-Fe leachate from the Changjiang sediment with low ratios of heavy REEs to light REEs (HREE/LREE, Eq. S-2; <1 when normalised to the post-Archean Australian Shale or PAAS) (Wang and Liu, 2008Wang, Z.-L., Liu, C.-Q. (2008) Geochemistry of rare earth elements in the dissolved, acid-soluble and residual phases in surface waters of the Changjiang Estuary. Journal of Oceanography 64, 407–416. https://doi.org/10.1007/s10872-008-0034-0
) would release more dissolved LREEs (relative to bottom water) as observed (Fig. 1). In comparison, [Fe]diss is generally low (mostly <1 μM) throughout core C6-1 and at shallow depth in C10, and the highest [Fe]diss of all cores (at 16 cm of C10) corresponds to the lowest [Nd]diss in this core. Hence, either Fe cycling is not the major controlling factor of REEs in either core, or its effect is obscured by other factors.At depths exceeding ∼7 cm in C10, [LREE]diss decreases dramatically, accompanied by an HREE-enriched pattern (Fig. 1). This evolution with depth hints at the operation of a second early diagenetic process. Here, lower [REE]diss and preferential scavenging of LREEs suggest removal to an authigenic phase. High porewater [P]diss at depth (13–24 μM), in contrast to ∼1 μM at <4 cm (Fig. 2), could facilitate the precipitation of minor phosphate (Byrne and Kim, 1993
Byrne, R.H., Kim, K.-H. (1993) Rare earth precipitation and coprecipitation behavior: The limiting role of PO43− on dissolved rare earth concentrations in seawater. Geochimica et Cosmochimica Acta 57, 519–526. https://doi.org/10.1016/0016-7037(93)90364-3
). This is consistent with in-situ formation of authigenic P at great sediment depth in this region (Liu et al., 2020Liu, J., Krom, M.D., Ran, X., Zang, J., Liu, J., Yao, Q., Yu, Z. (2020) Sedimentary phosphorus cycling and budget in the seasonally hypoxic coastal area of Changjiang Estuary. Science of the Total Environment 713, 136389. https://doi.org/10.1016/j.scitotenv.2019.136389
), and with the fact that phosphate precipitation would result in an HREE-enriched pattern in solution (Byrne and Kim, 1993Byrne, R.H., Kim, K.-H. (1993) Rare earth precipitation and coprecipitation behavior: The limiting role of PO43− on dissolved rare earth concentrations in seawater. Geochimica et Cosmochimica Acta 57, 519–526. https://doi.org/10.1016/0016-7037(93)90364-3
).More LREEs release at shallow core depth and preferential removal at great core depth can also be observed at greater water depth (33 m at C13 and 46 m at B14) (Fig. 1). Specifically, peaks in [Mn]diss and [Nd]diss are co-located at shallow core depth (≤6 cm; Fig. 2). [Fe]diss also peaks at ≤6 cm and thus its effect on [Nd]diss is difficult to isolate. For both stations, [Nd]diss becomes much lower at great depths (>7 cm) while [P]diss remains high (>10 μM). Note that the clear association of high porewater REE abundance and release of more LREEs (relative to bottom water) with Mn reduction could be obscured sometimes: REE concentrations and patterns reflect the competition between diverse sources and sinks, and the contribution of each component likely varies among basins (Haley et al., 2004
Haley, B.A., Klinkhammer, G.P., McManus, J. (2004) Rare earth elements in pore waters of marine sediments. Geochimica et Cosmochimica Acta 68, 1265–1279. https://doi.org/10.1016/j.gca.2003.09.012
; Abbott et al., 2015Abbott, A.N., Haley, B.A., McManus, J., Reimers, C.E. (2015) The sedimentary flux of dissolved rare earth elements to the ocean. Geochimica et Cosmochimica Acta 154, 186–200. https://doi.org/10.1016/j.gca.2015.01.010
).To further illustrate REE cycling through the Changjiang Estuary–East China Sea transect, we present the relationship between Ce/Ce* and HREE/LREE (Fig. 3). Estuarine scavenging leads to a decrease in Ce/Ce* and an increase in HREE/LREE (towards seawater end members), while the reductive release of REEs in shallow porewater shows a reverse trend, with lower HREE/LREE and higher Ce/Ce* (towards Mn-Fe leachate). At greater core depth, the REE patterns deviate from those controlled by these two processes and are characterised by a sharp rise in HREE/LREE and only a slight decrease in Ce/Ce*, suggesting the operation of a different process (authigenesis).
Figure 3 Shelf REE cycling shown by a Ce/Ce*-HREE/LREE plot. REE data for the salinity transect (1998) and Mn-Fe leachate are from Wang and Liu (2008)
Wang, Z.-L., Liu, C.-Q. (2008) Geochemistry of rare earth elements in the dissolved, acid-soluble and residual phases in surface waters of the Changjiang Estuary. Journal of Oceanography 64, 407–416. https://doi.org/10.1007/s10872-008-0034-0
. The data source for water mass end members (EMs) is provided in Figure S-3.top
Implications for Nd Budget in the Marginal Sea and Global Oceans
Our data are clearly consistent with the interaction between REE scavenging in the estuary and reductive REE release from shallow sediments. We calculate the diffusive Nd flux from sediments based on porewater [Nd]diss gradient (Eq. S-5). The diffusive Nd flux is lowest (0.9 pmol/cm2/yr) at 6 m and increases to a stable level at 6.0 ± 0.8 pmol/cm2/yr (12–46 m). Figure 4 compares these diffusive fluxes with compiled literature porewater data. Our dataset falls within the global trend (Du et al., 2018
Du, J., Haley, B.A., Mix, A.C., Walczak, M.H., Praetorius, S.K. (2018) Flushing of the deep Pacific Ocean and the deglacial rise of atmospheric CO2 concentrations. Nature Geoscience 11, 749–755. https://doi.org/10.1038/s41561-018-0205-6
, 2020Du, J., Haley, B.A., Mix, A.C. (2020) Evolution of the Global Overturning Circulation since the Last Glacial Maximum based on marine authigenic neodymium isotopes. Quaternary Science Reviews 241, 106396. https://doi.org/10.1016/j.quascirev.2020.106396
), which shows higher fluxes in the deeper ocean (R2 = 0.30). Furthermore, there is no clear control of bottom dissolved oxygen (DO) on diffusive Nd flux (Abbott et al., 2015Abbott, A.N., Haley, B.A., McManus, J., Reimers, C.E. (2015) The sedimentary flux of dissolved rare earth elements to the ocean. Geochimica et Cosmochimica Acta 154, 186–200. https://doi.org/10.1016/j.gca.2015.01.010
), with no correlation between the two (Fig. 4; p = 0.26). The spatial trend of diffusive Nd flux is probably affected by multiple depth-related processes. At shallow water depths the exchange between porewater and overlying seawater is fast (Shi et al., 2019Shi, X., Wei, L., Hong, Q., Liu, L., Wang, Y., Shi, X., Ye, Y., Cai, P. (2019) Large benthic fluxes of dissolved iron in China coastal seas revealed by 224Ra/228Th disequilibria. Geochimica et Cosmochimica Acta 260, 49–61. https://doi.org/10.1016/j.gca.2019.06.026
; Patton et al., 2021Patton, G.M., Francois, R., Weis, D., Hathorne, E., Gutjahr, M., Frank, M., Gordon, K. (2021) An experimental investigation of the acquisition of Nd by authigenic phases of marine sediments. Geochimica et Cosmochimica Acta 301, 1–29. https://doi.org/10.1016/j.gca.2021.02.010
), resulting in a small Nd gradient at sediment–water interface. In comparison, in some deep ocean sediments high reactive authigenic [Nd] might contribute to a high benthic flux (Abbott et al., 2016Abbott, A.N., Haley, B.A., McManus, J. (2016) The impact of sedimentary coatings on the diagenetic Nd flux. Earth and Planetary Science Letters 449, 217–227. https://doi.org/10.1016/j.epsl.2016.06.001
; Haley et al., 2017Haley, B.A., Du, J., Abbott, A.N., McManus, J. (2017) The Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the Oceans. Frontiers in Marine Science 4, 426. https://doi.org/10.3389/fmars.2017.00426
).Figure 4 Diffusive sedimentary Nd flux variation with depth. The coloured symbols indicate bottom water DO and open symbols are those with no DO data available. Axes and colour bar are on log scale. Compiled dataset and references are provided in Table S-4.
To estimate the contribution of benthic processes to the Nd budget in the East China Sea shelf, Nd fluxes of all major sources need to be known (Table S-5), including the Changjiang River, atmospheric deposition, the Taiwan Strait Current, the Kuroshio Current intrusion (Liu et al., 2021
Liu, Z., Gan, J., Hu, J., Wu, H., Cai, Z., Deng, Y. (2021) Progress of Studies on Circulation Dynamics in the East China Sea: The Kuroshio Exchanges With the Shelf Currents. Frontiers in Marine Science 8, 620910. https://doi.org/10.3389/fmars.2021.620910
) and shelf benthic flux. The Changjiang-derived Nd flux (after estuarine scavenging) and the atmospheric input are 2.7 ± 0.4 × 104 mol/yr and 1.7 ± 0.4 × 104 mol/yr, respectively. In comparison, Nd fluxes from the Taiwan Strait Current and the intrusion of the Kuroshio Current are much higher at 21.6 ± 3.8 × 104 mol/yr and 25.2 ± 4.4 × 104 mol/yr, respectively (Table S-5). Given the similar porewater REE behaviours (Fig. 1) and small flux variability (Table S-4) at depth of ≥12 m, we use the average diffusion-based flux estimate (6.0 pmol/cm2/yr) at this depth range for area extrapolation. The diffusive Nd flux in the whole East China Sea shelf is 3.0 ± 0.4 × 104 mol/yr, higher than the riverine input. Furthermore, on the continental shelf, with dynamic hydraulic environments, advection via e.g., bio-irrigation, rather than diffusion, may play the dominant role in benthic flux of trace metals (Shi et al., 2019Shi, X., Wei, L., Hong, Q., Liu, L., Wang, Y., Shi, X., Ye, Y., Cai, P. (2019) Large benthic fluxes of dissolved iron in China coastal seas revealed by 224Ra/228Th disequilibria. Geochimica et Cosmochimica Acta 260, 49–61. https://doi.org/10.1016/j.gca.2019.06.026
), implying a higher benthic flux. The benthic Nd flux accounting for advection processes can be estimated using Equations S-7 and S-8 (Shi et al., 2019Shi, X., Wei, L., Hong, Q., Liu, L., Wang, Y., Shi, X., Ye, Y., Cai, P. (2019) Large benthic fluxes of dissolved iron in China coastal seas revealed by 224Ra/228Th disequilibria. Geochimica et Cosmochimica Acta 260, 49–61. https://doi.org/10.1016/j.gca.2019.06.026
). The area-extrapolated advective Nd flux (30.8 ± 4.0 × 104 mol/yr; Table S-5) is ∼10-fold higher than the diffusion-based estimate and becomes the largest source on the East China Sea shelf (38 % of the total input).Our observations and calculations emphasise the role of benthic processes in the Nd cycling of marginal seas, and can provide valuable insights on the global sedimentary Nd flux. The best estimate so far (Abbott et al., 2015
Abbott, A.N., Haley, B.A., McManus, J., Reimers, C.E. (2015) The sedimentary flux of dissolved rare earth elements to the ocean. Geochimica et Cosmochimica Acta 154, 186–200. https://doi.org/10.1016/j.gca.2015.01.010
; Du et al., 2020Du, J., Haley, B.A., Mix, A.C. (2020) Evolution of the Global Overturning Circulation since the Last Glacial Maximum based on marine authigenic neodymium isotopes. Quaternary Science Reviews 241, 106396. https://doi.org/10.1016/j.quascirev.2020.106396
) suggests a global benthic Nd flux of 115 × 106 mol/yr, assuming the dominance of diffusion process. However, advection may play a key role in the benthic Nd flux from the continental shelf (0–200 m). Hence, we can revise the shelf estimate by replacing it (∼32 pmol/cm2/yr) with our advection-based estimate (∼62 pmol/cm2/yr), considering that the average depth of our studied shelf (72 m) is close to the global average shelf depth (∼60 m) and most global observations (73 %; World Ocean Database 2018, Boyer et al., 2018Boyer, T.P., Garcia, H.E., Locarnini, R.A., Zweng, M.M., Mishonov, A.V., Reagan, J.R., Weathers, K.A., Baranova, O.K., Seidov, D., Smolyar, I.V. (2018) World Ocean Atlas 2018. NOAA National Centers for Environmental Information. Dataset. Accessed November 2021. https://www.ncei.noaa.gov/archive/accession/NCEI-WOA18
) on shelves show a bottom water DO within our studied range (Table S-1). Despite the implied increase in the shelf-derived flux, the continental shelf only accounts for 14 % of the global area-integrated benthic Nd flux. This contrasts with previous thoughts that the sedimentary source is mainly from shallow water depths (e.g., continental shelves); in fact, much more extensive deep oceans may dominate the benthic Nd source (Haley et al., 2017Haley, B.A., Du, J., Abbott, A.N., McManus, J. (2017) The Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the Oceans. Frontiers in Marine Science 4, 426. https://doi.org/10.3389/fmars.2017.00426
; Du et al., 2020Du, J., Haley, B.A., Mix, A.C. (2020) Evolution of the Global Overturning Circulation since the Last Glacial Maximum based on marine authigenic neodymium isotopes. Quaternary Science Reviews 241, 106396. https://doi.org/10.1016/j.quascirev.2020.106396
). We thus suggest that future ocean models should reconsider the spatial pattern of this sedimentary source. Our results highlight the need for precise constraints on the benthic source if REE/Nd isotopes are to be robustly used as process/source tracer in both marginal seas and on global scales.top
Acknowledgements
This work was funded by the National Natural Science Foundation of China (Grant Nos. 42006059, 41991324 and 41730531). J.D. was supported by the ETH Zurich Postdoctoral Fellowship 19-2 FEL-32. K.D. thanks the support by the ETH Zurich Postdoctoral Fellowship 20-1 FEL-24. We thank the crew of the Zheyuke-2, Ni Su, Qi Jia and Zhongya Hu for their assistance with field sampling, Yi Sun for providing DO data, Madalina Jaggi for her assistance with TOC analysis, Jörg Rickli, Tim Jesper Suhrhoff and Archer Corey for their help with lab work.
Editor: Eric Oelkers
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References
Abbott, A.N., Haley, B.A., McManus, J., Reimers, C.E. (2015) The sedimentary flux of dissolved rare earth elements to the ocean. Geochimica et Cosmochimica Acta 154, 186–200. https://doi.org/10.1016/j.gca.2015.01.010
Show in context Two hypotheses have been proposed to explain oceanic REE distributions: the top-down (Siddall et al., 2008) versus the bottom-up control (Abbott et al., 2015; Du et al., 2020).
View in article
The mass balance of oceanic REEs requires sources other than riverine input and atmospheric deposition (Elderfield and Greaves, 1982), such as a benthic dissolved flux across the sediment–water interface via porewater (Abbott et al., 2015; Du et al., 2016) and/or submarine groundwater discharge (Johannesson et al., 2011).
View in article
[REE]diss of shallow porewater is generally higher than that for bottom water, consistent with observations from other continental margins and with release of porewater REEs to the overlying seawater (Haley et al., 2004; Abbott et al., 2015).
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Note that the clear association of high porewater REE abundance and release of more LREEs (relative to bottom water) with Mn reduction could be obscured sometimes: REE concentrations and patterns reflect the competition between diverse sources and sinks, and the contribution of each component likely varies among basins (Haley et al., 2004; Abbott et al., 2015).
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Furthermore, there is no clear control of bottom dissolved oxygen (DO) on diffusive Nd flux (Abbott et al., 2015), with no correlation between the two (Fig. 4; p = 0.26).
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The best estimate so far (Abbott et al., 2015; Du et al., 2020) suggests a global benthic Nd flux of 115 × 106 mol/yr, assuming the dominance of diffusion process.
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Abbott, A.N., Haley, B.A., McManus, J. (2016) The impact of sedimentary coatings on the diagenetic Nd flux. Earth and Planetary Science Letters 449, 217–227. https://doi.org/10.1016/j.epsl.2016.06.001
Show in context In comparison, in some deep ocean sediments high reactive authigenic [Nd] might contribute to a high benthic flux (Abbott et al., 2016; Haley et al., 2017).
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Arsouze, T., Dutay, J.C., Lacan, F., Jeandel, C. (2009) Reconstructing the Nd oceanic cycle using a coupled dynamical – biogeochemical model. Biogeosciences 6, 2829–2846. https://doi.org/10.5194/bg-6-2829-2009
Show in context The resolution of this debate would provide valuable insights on the long-standing “Nd (Neodymium) paradox”: while Nd isotopes appear to behave conservatively during water mass mixing, dissolved Nd concentrations ([Nd]diss) reflect the behaviour of a reactive element (Arsouze et al., 2009; Haley et al., 2017).
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In particular, recent modelling efforts suggest that continental margin sediments can be a major source of oceanic REEs (Arsouze et al., 2009; Rempfer et al., 2011).
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Blaser, P., Lippold, J., Gutjahr, M., Frank, N., Link, J.M., Frank, M. (2016) Extracting foraminiferal seawater Nd isotope signatures from bulk deep sea sediment by chemical leaching. Chemical Geology 439, 189–204. https://doi.org/10.1016/j.chemgeo.2016.06.024
Show in context Indeed, REEs are commonly enriched in Mn oxides and they can be released together in a reductive environment (Blaser et al., 2016).
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Boyer, T.P., Garcia, H.E., Locarnini, R.A., Zweng, M.M., Mishonov, A.V., Reagan, J.R., Weathers, K.A., Baranova, O.K., Seidov, D., Smolyar, I.V. (2018) World Ocean Atlas 2018. NOAA National Centers for Environmental Information. Dataset. Accessed November 2021. https://www.ncei.noaa.gov/archive/accession/NCEI-WOA18.
Show in context Hence, we can revise the shelf estimate by replacing it (∼32 pmol/cm2/yr) with our advection-based estimate (∼62 pmol/cm2/yr), considering that the average depth of our studied shelf (72 m) is close to the global average shelf depth (∼60 m) and most global observations (73 %; World Ocean Database 2018, Boyer et al., 2018) on shelves show a bottom water DO within our studied range (Table S-1).
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Byrne, R.H., Kim, K.-H. (1993) Rare earth precipitation and coprecipitation behavior: The limiting role of PO43− on dissolved rare earth concentrations in seawater. Geochimica et Cosmochimica Acta 57, 519–526. https://doi.org/10.1016/0016-7037(93)90364-3
Show in context High porewater [P]diss at depth (13–24 μM), in contrast to ∼1 μM at <4 cm (Fig. 2), could facilitate the precipitation of minor phosphate (Byrne and Kim, 1993).
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This is consistent with in-situ formation of authigenic P at great sediment depth in this region (Liu et al., 2020), and with the fact that phosphate precipitation would result in an HREE-enriched pattern in solution (Byrne and Kim, 1993).
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Chen, Z., Li, J., Shen, H., Zhanghua, W. (2001) Yangtze River of China: historical analysis of discharge variability and sediment flux. Geomorphology 41, 77–91. https://doi.org/10.1016/S0169-555X(01)00106-4
Show in context The Changjiang River delivers a huge amount of fresh water (∼890 km3/yr) and sediment (∼450 Mt/yr) to the continental margin (Chen et al., 2001), accounting for 2–3 % of global discharge.
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Du, J., Haley, B.A., Mix, A.C. (2016) Neodymium isotopes in authigenic phases, bottom waters and detrital sediments in the Gulf of Alaska and their implications for paleo-circulation reconstruction. Geochimica et Cosmochimica Acta 193, 14–35. https://doi.org/10.1016/j.gca.2016.08.005
Show in context The mass balance of oceanic REEs requires sources other than riverine input and atmospheric deposition (Elderfield and Greaves, 1982), such as a benthic dissolved flux across the sediment–water interface via porewater (Abbott et al., 2015; Du et al., 2016) and/or submarine groundwater discharge (Johannesson et al., 2011).
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Du, J., Haley, B.A., Mix, A.C., Walczak, M.H., Praetorius, S.K. (2018) Flushing of the deep Pacific Ocean and the deglacial rise of atmospheric CO2 concentrations. Nature Geoscience 11, 749–755. https://doi.org/10.1038/s41561-018-0205-6
Show in context Our dataset falls within the global trend (Du et al., 2018, 2020), which shows higher fluxes in the deeper ocean (R2 = 0.30).
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Du, J., Haley, B.A., Mix, A.C. (2020) Evolution of the Global Overturning Circulation since the Last Glacial Maximum based on marine authigenic neodymium isotopes. Quaternary Science Reviews 241, 106396. https://doi.org/10.1016/j.quascirev.2020.106396
Show in context Two hypotheses have been proposed to explain oceanic REE distributions: the top-down (Siddall et al., 2008) versus the bottom-up control (Abbott et al., 2015; Du et al., 2020).
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Such inconsistency impedes the application of Nd isotopes as a tracer for paleo-circulation (Du et al., 2020; Patton et al., 2021).
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Our dataset falls within the global trend (Du et al., 2018, 2020), which shows higher fluxes in the deeper ocean (R2 = 0.30).
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The best estimate so far (Abbott et al., 2015; Du et al., 2020) suggests a global benthic Nd flux of 115 × 106 mol/yr, assuming the dominance of diffusion process.
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This contrasts with previous thoughts that the sedimentary source is mainly from shallow water depths (e.g., continental shelves); in fact, much more extensive deep oceans may dominate the benthic Nd source (Haley et al., 2017; Du et al., 2020).
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Elderfield, H., Greaves, M.J. (1982) The rare earth elements in seawater. Nature 296, 214–219. https://doi.org/10.1038/296214a0
Show in context The rare earth elements (REEs), as a series of particle-reactive elements, show non-conservative behaviour during transport from continental source to oceanic sink (Elderfield and Greaves, 1982; Rousseau et al., 2015).
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As such, REE patterns are widely used in oceanographic studies, to track boundary exchange and internal cycling (Elderfield and Greaves, 1982; Jeandel and Oelkers, 2015).
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The mass balance of oceanic REEs requires sources other than riverine input and atmospheric deposition (Elderfield and Greaves, 1982), such as a benthic dissolved flux across the sediment–water interface via porewater (Abbott et al., 2015; Du et al., 2016) and/or submarine groundwater discharge (Johannesson et al., 2011).
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Haley, B.A., Klinkhammer, G.P., McManus, J. (2004) Rare earth elements in pore waters of marine sediments. Geochimica et Cosmochimica Acta 68, 1265–1279. https://doi.org/10.1016/j.gca.2003.09.012
Show in context [REE]diss of shallow porewater is generally higher than that for bottom water, consistent with observations from other continental margins and with release of porewater REEs to the overlying seawater (Haley et al., 2004; Abbott et al., 2015).
View in article
Note that the clear association of high porewater REE abundance and release of more LREEs (relative to bottom water) with Mn reduction could be obscured sometimes: REE concentrations and patterns reflect the competition between diverse sources and sinks, and the contribution of each component likely varies among basins (Haley et al., 2004; Abbott et al., 2015).
View in article
Haley, B.A., Du, J., Abbott, A.N., McManus, J. (2017) The Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the Oceans. Frontiers in Marine Science 4, 426. https://doi.org/10.3389/fmars.2017.00426
Show in context The resolution of this debate would provide valuable insights on the long-standing “Nd (Neodymium) paradox”: while Nd isotopes appear to behave conservatively during water mass mixing, dissolved Nd concentrations ([Nd]diss) reflect the behaviour of a reactive element (Arsouze et al., 2009; Haley et al., 2017).
View in article
In comparison, in some deep ocean sediments high reactive authigenic [Nd] might contribute to a high benthic flux (Abbott et al., 2016; Haley et al., 2017).
View in article
This contrasts with previous thoughts that the sedimentary source is mainly from shallow water depths (e.g., continental shelves); in fact, much more extensive deep oceans may dominate the benthic Nd source (Haley et al., 2017; Du et al., 2020).
View in article
Jeandel, C., Oelkers, E.H. (2015) The influence of terrigenous particulate material dissolution on ocean chemistry and global element cycles. Chemical Geology 395, 50–66. https://doi.org/10.1016/j.chemgeo.2014.12.001
Show in context As such, REE patterns are widely used in oceanographic studies, to track boundary exchange and internal cycling (Elderfield and Greaves, 1982; Jeandel and Oelkers, 2015).
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Johannesson, K.H., Chevis, D.A., Burdige, D.J., Cable, J.E., Martin, J.B., Roy, M. (2011) Submarine groundwater discharge is an important net source of light and middle REEs to coastal waters of the Indian River Lagoon, Florida, USA. Geochimica et Cosmochimica Acta 75, 825–843. https://doi.org/10.1016/j.gca.2010.11.005
Show in context The mass balance of oceanic REEs requires sources other than riverine input and atmospheric deposition (Elderfield and Greaves, 1982), such as a benthic dissolved flux across the sediment–water interface via porewater (Abbott et al., 2015; Du et al., 2016) and/or submarine groundwater discharge (Johannesson et al., 2011).
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Liu, J.P., Li, A.C., Xu, K.H., Velozzi, D.M., Yang, Z.S., Milliman, J.D., DeMaster, D.J. (2006) Sedimentary features of the Yangtze River-derived along-shelf clinoform deposit in the East China Sea. Continental Shelf Research 26, 2141–2156. https://doi.org/10.1016/j.csr.2006.07.013
Show in context The East China Sea is characterised by one of the widest continental shelves (shelf area: ∼5 × 105 km2) and highest sedimentation rate (inner shelf: ∼1–6 cm/yr) worldwide (Liu et al., 2006).
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Liu, J., Krom, M.D., Ran, X., Zang, J., Liu, J., Yao, Q., Yu, Z. (2020) Sedimentary phosphorus cycling and budget in the seasonally hypoxic coastal area of Changjiang Estuary. Science of the Total Environment 713, 136389. https://doi.org/10.1016/j.scitotenv.2019.136389
Show in context This is consistent with in-situ formation of authigenic P at great sediment depth in this region (Liu et al., 2020), and with the fact that phosphate precipitation would result in an HREE-enriched pattern in solution (Byrne and Kim, 1993).
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Liu, Z., Gan, J., Hu, J., Wu, H., Cai, Z., Deng, Y. (2021) Progress of Studies on Circulation Dynamics in the East China Sea: The Kuroshio Exchanges With the Shelf Currents. Frontiers in Marine Science 8, 620910. https://doi.org/10.3389/fmars.2021.620910
Show in context To estimate the contribution of benthic processes to the Nd budget in the East China Sea shelf, Nd fluxes of all major sources need to be known (Table S-5), including the Changjiang River, atmospheric deposition, the Taiwan Strait Current, the Kuroshio Current intrusion (Liu et al., 2021) and shelf benthic flux.
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Patton, G.M., Francois, R., Weis, D., Hathorne, E., Gutjahr, M., Frank, M., Gordon, K. (2021) An experimental investigation of the acquisition of Nd by authigenic phases of marine sediments. Geochimica et Cosmochimica Acta 301, 1–29. https://doi.org/10.1016/j.gca.2021.02.010
Show in context Such inconsistency impedes the application of Nd isotopes as a tracer for paleo-circulation (Du et al., 2020; Patton et al., 2021).
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At shallow water depths the exchange between porewater and overlying seawater is fast (Shi et al., 2019; Patton et al., 2021), resulting in a small Nd gradient at sediment–water interface.
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Rempfer, J., Stocker, T.F., Joos, F., Dutay, J.-C., Siddall, M. (2011) Modelling Nd-isotopes with a coarse resolution ocean circulation model: Sensitivities to model parameters and source/sink distributions. Geochimica et Cosmochimica Acta 75, 5927–5950. https://doi.org/10.1016/j.gca.2011.07.044
Show in context In particular, recent modelling efforts suggest that continental margin sediments can be a major source of oceanic REEs (Arsouze et al., 2009; Rempfer et al., 2011).
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Rousseau, T.C.C., Sonke, J.E., Chmeleff, J., van Beek, P., Souhaut, M., Boaventura, G., Seyler, P., Jeandel, C. (2015) Rapid neodymium release to marine waters from lithogenic sediments in the Amazon estuary. Nature Communications 6, 7592. https://doi.org/10.1038/ncomms8592
Show in context The rare earth elements (REEs), as a series of particle-reactive elements, show non-conservative behaviour during transport from continental source to oceanic sink (Elderfield and Greaves, 1982; Rousseau et al., 2015).
View in article
Dissolved REEs have been measured in many estuarine transects and an additional sedimentary source is often proposed to explain their spatial distribution (Wang and Liu, 2008; Rousseau et al., 2015).
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Schijf, J., Christenson, E.A., Byrne, R.H. (2015) YREE scavenging in seawater: A new look at an old model. Marine Chemistry 177, 460–471. https://doi.org/10.1016/j.marchem.2015.06.010
Show in context The correlation (R2 = 0.58; <7 cm at C10; Fig. S-4) between porewater [Mn]diss and Ce anomaly (Ce/Ce*) values (Eq. S-1) is consistent with the well-known association of Ce with Mn oxyhydroxide (Schijf et al., 2015).
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Shi, X., Wei, L., Hong, Q., Liu, L., Wang, Y., Shi, X., Ye, Y., Cai, P. (2019) Large benthic fluxes of dissolved iron in China coastal seas revealed by 224Ra/228Th disequilibria. Geochimica et Cosmochimica Acta 260, 49–61. https://doi.org/10.1016/j.gca.2019.06.026
Show in context At shallow water depths the exchange between porewater and overlying seawater is fast (Shi et al., 2019; Patton et al., 2021), resulting in a small Nd gradient at sediment–water interface.
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Furthermore, on the continental shelf, with dynamic hydraulic environments, advection via e.g., bio-irrigation, rather than diffusion, may play the dominant role in benthic flux of trace metals (Shi et al., 2019), implying a higher benthic flux.
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The benthic Nd flux accounting for advection processes can be estimated using Equations S-7 and S-8 (Shi et al., 2019).
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Siddall, M., Khatiwala, S., van de Flierdt, T., Jones, K., Goldstein, S.L., Hemming, S., Anderson, R.F. (2008) Towards explaining the Nd paradox using reversible scavenging in an ocean general circulation model. Earth and Planetary Science Letters 274, 448–461. https://doi.org/10.1016/j.epsl.2008.07.044
Show in context Two hypotheses have been proposed to explain oceanic REE distributions: the top-down (Siddall et al., 2008) versus the bottom-up control (Abbott et al., 2015; Du et al., 2020).
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Wang, Z.-L., Liu, C.-Q. (2008) Geochemistry of rare earth elements in the dissolved, acid-soluble and residual phases in surface waters of the Changjiang Estuary. Journal of Oceanography 64, 407–416. https://doi.org/10.1007/s10872-008-0034-0
Show in context Dissolved REEs have been measured in many estuarine transects and an additional sedimentary source is often proposed to explain their spatial distribution (Wang and Liu, 2008; Rousseau et al., 2015).
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Along the salinity transect in the Changjiang estuary, estuarine [REE]diss decreases dramatically at salinity <1–2 psu, driven by scavenging, and gradually increases at mid-high salinity (Fig. S-3), hinting at a potential marine sedimentary source (Wang and Liu, 2008).
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Besides, the Mn-Fe leachate from the Changjiang sediment with low ratios of heavy REEs to light REEs (HREE/LREE, Eq. S-2; <1 when normalised to the post-Archean Australian Shale or PAAS) (Wang and Liu, 2008) would release more dissolved LREEs (relative to bottom water) as observed (Fig. 1).
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Shelf REE cycling shown by a Ce/Ce*-HREE/LREE plot. REE data for the salinity transect (1998) and Mn-Fe leachate are from Wang and Liu (2008).
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Supplementary Information
The Supplementary Information includes:
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Figures
Figure 1 Sediment porewater REE pattern normalised to the bottom seawater (depth of 0 cm) at each station. Analytical uncertainties (see Supplementary Information) are shown here and in later figures.
Figure 2 Porewater and bottom seawater (depth of 0 cm) Nd, Mn (filled symbols), Fe and P (open symbols) concentrations. “+” symbols refer to the water sample data at station C1 at 1 m.
Figure 3 Shelf REE cycling shown by a Ce/Ce*-HREE/LREE plot. REE data for the salinity transect (1998) and Mn-Fe leachate are from Wang and Liu (2008)
Wang, Z.-L., Liu, C.-Q. (2008) Geochemistry of rare earth elements in the dissolved, acid-soluble and residual phases in surface waters of the Changjiang Estuary. Journal of Oceanography 64, 407–416. https://doi.org/10.1007/s10872-008-0034-0
. The data source for water mass end members (EMs) is provided in Figure S-3.
Figure 4 Diffusive sedimentary Nd flux variation with depth. The coloured symbols indicate bottom water DO and open symbols are those with no DO data available. Axes and colour bar are on log scale. Compiled dataset and references are provided in Table S-4.