Decoupling of dissolved and particulate Li isotopes during estuarine processes
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Keywords: Changjiang Estuary, Li isotopes, dissolved load, suspended particulate matter, conservative mixing
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Abstract
Figures
 Figure 1 (a) Variations of dissolved Li concentration as a function of salinity, and (b) variations of dissolved δ7Li as a function of the inverse of the molar Li concentration (1/Li). The x-axis in (b) is logarithmic. The dashed lines and shaded areas represent calculated conservative mixing between seawater (dark blue star) and Changjiang River water (white star). The mass balance model is provided in the Supplementary Information. The shaded areas are estimated using the Monte Carlo method, given that analytical uncertainties in Li concentrations and δ7Li are 10 % and 0.6 ‰, respectively. The analytical uncertainty for each δ7Li value is smaller than the symbol size. |  Figure 2 (a) Variations of Li/Si ratios as a function of Al/Si ratios for riverine (orange, including the most inland site C1 in red) and estuarine SPM (blue), and (b) δ7Li versus Li/Al ratios for the same samples. The linear correlations between Li/Si and Al/Si are displayed with 95 % confidence intervals (shaded areas). The igneous rocks and shale endmembers in (b) are modified after Dellinger et al. (2014), and the shaded area reflects their mixing. Changjiang River SPM data are from Wang et al. (2015). |
| Figure 1 | Figure 2 |
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Introduction
As critical interfaces between continents and oceans, estuaries typically feature gradients of ionic strength and suspended particle concentration, providing an ideal environment for studying land–sea and water–sediment interactions. Over the last decades, numerous studies have demonstrated that riverine elemental fluxes and isotope compositions (e.g., Si, Sr, and Nd) can be modified by physical, chemical, and/or biological processes during their transport through estuaries (Lacan and Jeandel, 2005
Lacan, F., Jeandel, C. (2005) Neodymium isotopes as a new tool for quantifying exchange fluxes at the continent–ocean interface. Earth and Planetary Science Letters 232, 245–257.
; Jones et al., 2012Jones, M.T., Pearce, C.R., Jeandel, C., Gislason, S.R., Eiriksdottir, E.S., Mavromatis, V., Oelkers, E.H. (2012) Riverine particulate material dissolution as a significant flux of strontium to the oceans. Earth and Planetary Science Letters 355, 51–59.
, 2014Jones, M.T., Gislason, S.R., Burton, K.W., Pearce, C.R., Mavromatis, V., Pogge von Strandmann, P.A.E., Oelkers, E.H. (2014) Quantifying the impact of riverine particulate dissolution in seawater on ocean chemistry. Earth and Planetary Science Letters 395, 91–100.
; Zhang et al., 2020Zhang, Z.L., Cao, Z.M., Grasse, P., Dai, M.H., Gao, L., Kuhnert, H., Gledhill, M., Chiessi, C.M., Doering, K., Frank, M. (2020) Dissolved silicon isotope dynamics in large river estuaries. Geochimica et Cosmochimica Acta 273, 367–382.
). For instance, the dissolution of basaltic particles and Ca–Na exchange on clay minerals in saline water were considered to play non-negligible roles in global climate stabilisation (Gislason et al., 2006Gislason, S.R., Oelkers, E.H., Snorrason, A. (2006) Role of river-suspended material in the global carbon cycle. Geology 34, 49–52.
; Tipper et al., 2021Tipper, E.T., Stevenson, E.I., Alcock, V., Knight, A.C.G., Baronas, J.J., Hilton, R.G., Bickle, M.J., Larkin, C.S., Feng, L., Relph, K.E., Hughes, G. (2021) Global silicate weathering flux overestimated because of sediment-water cation exchange. Proceedings of the National Academy of Sciences 118, e2016430118.
). Investigating elemental and isotopic alteration of particles and water in estuaries is thus essential for a better understanding of oceanic elemental cycles, and of the carbon cycle in particular.River Li isotopes (δ7Li) are thought to be a powerful proxy of continental weathering (Huh et al., 1998
Huh, Y., Chan, L.H., Zhang, L., Edmond, J.M. (1998) Lithium and its isotopes in major world rivers: Implications for weathering and the oceanic budget. Geochimica et Cosmochimica Acta 62, 2039–2051.
; Wang et al., 2015Wang, Q.L., Chetelat, B., Zhao, Z.Q., Ding, H., Li, S.L., Wang, B.L., Li, J., Liu, X.L. (2015) Behavior of lithium isotopes in the Changjiang River system: Sources effects and response to weathering and erosion. Geochimica et Cosmochimica Acta 151, 117–132.
; Dellinger et al., 2017Dellinger, M., Bouchez, J., Gaillardet, J., Faure, L., Moureau, J. (2017) Tracing weathering regimes using the lithium isotope composition of detrital sediments. Geology 45, 411–414.
). During weathering processes, the light 6Li isotope is preferentially incorporated into the solids, causing the dissolved phase to be isotopically heavy. To date, Li isotopes registered in bulk carbonate or detrital sedimentary archives have been widely applied to assess changes of continental weathering regimes during mass extinctions and long or short term global warming/cooling events (Misra and Froelich, 2012Misra, S., Froelich, P.N. (2012) Lithium isotope history of Cenozoic seawater: changes in silicate weathering and reverse weathering. Science 335, 818–823.
; Pogge von Strandmann et al., 2013Pogge von Strandmann, P.A.E., Jenkyns, H.C., Woodfine, R.G. (2013) Lithium isotope evidence for enhanced weathering during Oceanic Anoxic Event 2. Nature Geoscience 6, 668–672.
; Bastian et al., 2017Bastian, L., Revel, M., Bayon, G., Dufour, A., Vigier, N. (2017) Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa. Scientific Reports 7, 44231.
; Yang et al., 2021Yang, C.F., Vigier, N., Yang, S.Y., Revel, M., Bi, L. (2021) Clay Li and Nd isotopes response to hydroclimate changes in the Changjiang (Yangtze) basin over the past 14,000 years. Earth and Planetary Science Letters 561, 116793.
). All these studies implicitly or explicitly assume that riverine Li fluxes to the ocean and their isotopic signals behave conservatively in estuaries. However, thus far, this assumption has not been verified over large scales. Indeed, few case studies have highlighted either the conservative or non-conservative behaviour of Li isotopes in small estuaries (Pogge von Strandmann et al., 2008Pogge von Strandmann, P.A.E., James, R.H., van Calsteren, P., Gíslason, S.R., Burton, K.W. (2008) Lithium, magnesium and uranium isotope behaviour in the estuarine environment of basaltic islands. Earth and Planetary Science Letters 274, 462–471.
; Murphy et al., 2014Murphy, M.J., Pogge von Strandmann, P.A.E., Porcelli, D., Ingri, J. (2014) Li isotope behaviour in the low salinity zone during estuarine mixing. Procedia Earth and Planetary Science 10, 204–207.
). For instance, Pogge von Strandmann et al. (2008)Pogge von Strandmann, P.A.E., James, R.H., van Calsteren, P., Gíslason, S.R., Burton, K.W. (2008) Lithium, magnesium and uranium isotope behaviour in the estuarine environment of basaltic islands. Earth and Planetary Science Letters 274, 462–471.
observed an increase of particulate δ7Li values (from ∼0 ‰ to ∼5 ‰) along a transect in the Borgarfjörður estuary (Iceland) related to ongoing weathering of suspended particles.Major rivers in Asia, such as the Changjiang (Yangtze) River (Fig. S-1a), drain large continental basins and deliver huge amounts of detrital particles, dissolved elements, and nutrients to the marginal seas and oceans. Detailed investigations of elemental geochemical behaviours in these estuaries provide constraints on the application of Li isotopes as a robust weathering proxy. We present Li isotopic data for the dissolved load, suspended particulate matter (SPM), and their exchangeable phase (see Supplementary Information) in the Changjiang River estuary (Fig. S-1). Our primary goal is to investigate the dissolved and particulate Li isotopic compositions during the mixing processes, and to evaluate the propagation and alteration of terrestrial Li isotopic signals in a large, turbid, and highly dynamic river estuary.
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Evidence for Conservative Mixing of Dissolved Li and δ7Li
As shown in Figure 1a, the dissolved Li concentrations are linearly correlated with salinity, suggesting that the Li-poor Changjiang River water and Li-rich seawater are the only two major contributors of dissolved Li in the mixing zone. This salinity–Li relationship is consistent with observations made in the St. Lawrence estuary and the Gulf of Papua (Stoffyn-Egli, 1982
Stoffyn-Egli, P. (1982) Conservative behaviour of dissolved lithium in estuarine waters. Estuarine, Coastal and Shelf Science 14, 577–587.
; Brunskill et al., 2003Brunskill, G.J., Zagorskis, I., Pfitzner, J. (2003) Geochemical mass balance for lithium, boron, and strontium in the Gulf of Papua, Papua New Guinea (Project TROPICS). Geochimica et Cosmochimica Acta 67, 3365–3383.
). However, the use of Li concentrations alone does not permit identification of all the processes potentially occurring in the mixing zone. Indeed, as indicated by experimental investigations and by previous studies of Icelandic estuaries, isotopic exchanges may modify riverine dissolved isotopic compositions without significantly affecting elemental concentrations (Jones et al., 2012Jones, M.T., Pearce, C.R., Jeandel, C., Gislason, S.R., Eiriksdottir, E.S., Mavromatis, V., Oelkers, E.H. (2012) Riverine particulate material dissolution as a significant flux of strontium to the oceans. Earth and Planetary Science Letters 355, 51–59.
, 2014Jones, M.T., Gislason, S.R., Burton, K.W., Pearce, C.R., Mavromatis, V., Pogge von Strandmann, P.A.E., Oelkers, E.H. (2014) Quantifying the impact of riverine particulate dissolution in seawater on ocean chemistry. Earth and Planetary Science Letters 395, 91–100.
). When we report the dissolved δ7Li as a function of the inverse of the Li concentration (Fig. 1b), all the samples are distributed along the theoretical mixing line between the Changjiang River water and seawater. These results a priori support the conservative behaviour of the dissolved Li and its isotopes along the studied transect.Figure 1 (a) Variations of dissolved Li concentration as a function of salinity, and (b) variations of dissolved δ7Li as a function of the inverse of the molar Li concentration (1/Li). The x-axis in (b) is logarithmic. The dashed lines and shaded areas represent calculated conservative mixing between seawater (dark blue star) and Changjiang River water (white star). The mass balance model is provided in the Supplementary Information. The shaded areas are estimated using the Monte Carlo method, given that analytical uncertainties in Li concentrations and δ7Li are 10 % and 0.6 ‰, respectively. The analytical uncertainty for each δ7Li value is smaller than the symbol size.
The relatively high seawater Li concentration may mask some of the Li release during SPM dissolution (Morin et al., 2015
Morin, G.P., Vigier, N., Verney-Carron, A. (2015) Enhanced dissolution of basaltic glass in brackish waters: Impact on biogeochemical cycles. Earth and Planetary Science Letters 417, 1–8.
) or Li uptake by SPM (Pogge von Strandmann et al., 2008Pogge von Strandmann, P.A.E., James, R.H., van Calsteren, P., Gíslason, S.R., Burton, K.W. (2008) Lithium, magnesium and uranium isotope behaviour in the estuarine environment of basaltic islands. Earth and Planetary Science Letters 274, 462–471.
). The dissolution rates of quartz, albite, and clays, which are major constituent minerals of Changjiang SPM, are relatively slow, because they have undergone intermediate to strong chemical weathering in the catchment. For instance, less than 0.1 % of kaolinite and illite would be expected to dissolve in seawater annually (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.
). As a first approximation, we can assume that 0.1 % of the SPM Li could be released by dissolution in the Changjiang Estuary, although the average residence time of the Changjiang diluted water plume is only 5.4 d (Gu et al., 2012Gu, H.Q., Moore, W.S., Zhang, L., Du, J.Z., Zhang, J. (2012) Using radium isotopes to estimate the residence time and the contribution of submarine groundwater discharge (SGD) in the Changjiang effluent plume, East China Sea. Continental Shelf Research 35, 95–107.
). A mass balance model suggests that, in that case, the dissolved δ7Li would decrease by ∼0.2 ‰ (see Supplementary Information), which is within analytical uncertainties. For the possible influences of Li uptake, modelling results suggest that the dissolved δ7Li values would be significantly altered in the maximum turbidity zone where suspended sediment concentration can reach ∼2 g/L (for detailed calculations, see Supplementary Information and Fig. S-2). This is apparently inconsistent with the conservative behaviour of dissolved Li and δ7Li observed in this study (Fig. 1). Thus, we suggest that the influence of sediment–water interactions on dissolved Li is insignificant in the Changjiang Estuary. Additionally, submarine groundwater discharge (SGD, 0.2–1.0 × 109 m3 d−1) was estimated to be 6–30 % of the river discharge during the flooding season in the Changjiang Estuary (Gu et al., 2012Gu, H.Q., Moore, W.S., Zhang, L., Du, J.Z., Zhang, J. (2012) Using radium isotopes to estimate the residence time and the contribution of submarine groundwater discharge (SGD) in the Changjiang effluent plume, East China Sea. Continental Shelf Research 35, 95–107.
). Although no Li data were reported for this discharge, our results imply that the SGD plays a small role in the isotope compositions of the Li flux to the East China Sea. A similar conclusion has been drawn by Bagard et al. (2015Bagard, M.L., West, A.J., Newman, K., Basu, A.R. (2015) Lithium isotope fractionation in the Ganges–Brahmaputra floodplain and implications for groundwater impact on seawater isotopic composition. Earth and Planetary Science Letters 432, 404–414.
) for assessing the modern Li isotopic budget of the ocean, based on investigation of the Li flux and δ7Li of groundwater in the Ganges-Brahmaputra downstream basin.The conservative mixing of dissolved Li and Li isotopes observed in the Changjiang Estuary exhibits a nonlinear δ7Li variation as a function of salinity. Indeed, the dissolved δ7Li values increase significantly in the head of the estuary, during the initial mixing between river water and seawater (Fig. 1b). This is because seawater is enriched in Li and isotopically heavy compared to river water. We calculate that the addition of 1 % seawater to the Changjiang River water causes the δ7Li value to increase by ∼3 ‰ (Supplementary Information). When salinity exceeds 7 ‰, the dissolved δ7Li value remains more or less constant (between ∼30 ‰ and 31.6 ‰). Due to differences in catchment lithologies, climate regimes, and other environmental parameters, the dissolved Li concentrations and δ7Li values of river waters worldwide yield significant spatial and seasonal variations. Nevertheless, as discussed above, physical mixing of different water masses cannot modify the conservative behaviours of dissolved Li and Li isotopes in estuaries. Therefore, the fast response and significant variation of riverine δ7Li to a small volume of seawater addition observed in this study is expected to be universal. Indeed, in most large rivers, dissolved Li concentrations are two orders of magnitude lower than that of seawater, and δ7Li values are 5–10 ‰ less (Huh et al., 1998
Huh, Y., Chan, L.H., Zhang, L., Edmond, J.M. (1998) Lithium and its isotopes in major world rivers: Implications for weathering and the oceanic budget. Geochimica et Cosmochimica Acta 62, 2039–2051.
). Caution must therefore be paid to this effect when sampling rivers at their mouths for quantifying their contribution of Li and δ7Li flux to the ocean.top
Particulate Li and δ7Li Behaviour in the Estuary
At the XLJ gauging station, SPM Li concentrations, δ7Li values, and suspended sediment concentrations (SSC) exhibit seasonal and depth variations (see Fig. S-4). In contrast, depth profiles performed along the estuarine transect exhibit negligible or small variations, and a noticeable increase can be observed as one moves offshore (Fig. S-5). Generally, depending on the grain size, density, and shape of detrital minerals, hydrodynamic sorting in riverbeds and floodplains may cause mineralogical and geochemical fractionation during SPM transport (Guo et al., 2018
Guo, Y.L., Yang, S.Y., Su, N., Li, C., Yin, P., Wang, Z.B. (2018) Revisiting the effects of hydrodynamic sorting and sedimentary recycling on chemical weathering indices. Geochimica et Cosmochimica Acta 227, 48–63.
). The sediment Al/Si ratio allows us to constrain the effects of hydrological sorting on isotope proxies (Dellinger et al., 2014Dellinger, M., Gaillardet, J., Bouchez, J., Calmels, D., Galy, V., Hilton, R.G., Louvat, P., France-Lanord, C. (2014) Lithium isotopes in large rivers reveal the cannibalistic nature of modern continental weathering and erosion. Earth and Planetary Science Letters 401, 359–372.
). As shown in Figure 2a, Li/Si ratios positively correlate with Al/Si ratios for both riverine SPM (orange symbols) and estuarine SPM (blue symbols), suggesting a dominant control on Li concentrations by Al-rich materials (e.g., clays) present in the SPM (Vigier et al., 2008Vigier, N., Decarreau, A., Millot, R., Carignan, J., Petit, S., France-Lanord, C. (2008) Quantifying Li isotope fractionation during smectite formation and implications for the Li cycle. Geochimica et Cosmochimica Acta 72, 780–792.
; Dellinger et al., 2014Dellinger, M., Gaillardet, J., Bouchez, J., Calmels, D., Galy, V., Hilton, R.G., Louvat, P., France-Lanord, C. (2014) Lithium isotopes in large rivers reveal the cannibalistic nature of modern continental weathering and erosion. Earth and Planetary Science Letters 401, 359–372.
). Nevertheless, the river and estuary data follow significantly different slopes (Fig. 2a). This cannot be attributed to adsorption in the estuary as exchangeable Li accounts for less than 1 % of the SPM Li and thus has a negligible influence on particulate Li (see Supplementary Information). As indicated by previous studies, this difference in slope can be caused either by ongoing weathering (Lupker et al., 2012Lupker, M., France-Lanord, C., Galy, V., Lave, J., Gaillardet, J., Gajurel, A.P., Guilmette, C., Rahman, M., Singh, S.K., Sinha, R. (2012) Predominant floodplain over mountain weathering of Himalayan sediments (Ganga basin). Geochimica et Cosmochimica Acta 84, 410–432.
) or by a change of sediment source (Yang et al., 2019Yang, C.F., Yang, S.Y., Song, J.Z., Vigier, N. (2019) Progressive Evolution of the Changjiang (Yangtze River) Sediment Weathering Intensity Since the Three Gorges Dam Operation. Journal of Geophysical Research: Earth Surface 124, 2402–2416.
). In the former case, the steeper slope defined by the estuarine SPM data may indicate clay formation in the estuarine environment. However, the formation of clay minerals along the sampling transect is not supported by XRD results (Table S-3). On the other hand, δ7Li values of SPM samples show no shift toward seawater (Fig. S-3a), in contrast to the altered 87Sr/86Sr ratios observed in Icelandic estuaries, for instance (Jones et al., 2014Jones, M.T., Gislason, S.R., Burton, K.W., Pearce, C.R., Mavromatis, V., Pogge von Strandmann, P.A.E., Oelkers, E.H. (2014) Quantifying the impact of riverine particulate dissolution in seawater on ocean chemistry. Earth and Planetary Science Letters 395, 91–100.
). This discrepancy may be explained by the low chemical reactivity of Changjiang-derived SPM, which mainly come from highly weathered and recycled continental sediments and shales. All these observations suggest that SPM Li is not controlled by a chemical process (e.g., water-sediment interactions) in the Changjiang Estuary, and a physical process could be the dominant factor, as discussed below.Figure 2 (a) Variations of Li/Si ratios as a function of Al/Si ratios for riverine (orange, including the most inland site C1 in red) and estuarine SPM (blue), and (b) δ7Li versus Li/Al ratios for the same samples. The linear correlations between Li/Si and Al/Si are displayed with 95 % confidence intervals (shaded areas). The igneous rocks and shale endmembers in (b) are modified after Dellinger et al. (2014)
Dellinger, M., Gaillardet, J., Bouchez, J., Calmels, D., Galy, V., Hilton, R.G., Louvat, P., France-Lanord, C. (2014) Lithium isotopes in large rivers reveal the cannibalistic nature of modern continental weathering and erosion. Earth and Planetary Science Letters 401, 359–372.
, and the shaded area reflects their mixing. Changjiang River SPM data are from Wang et al. (2015)Wang, Q.L., Chetelat, B., Zhao, Z.Q., Ding, H., Li, S.L., Wang, B.L., Li, J., Liu, X.L. (2015) Behavior of lithium isotopes in the Changjiang River system: Sources effects and response to weathering and erosion. Geochimica et Cosmochimica Acta 151, 117–132.
.More information can be inferred from the δ7Li vs. Li/Al diagram (Dellinger et al., 2017
Dellinger, M., Bouchez, J., Gaillardet, J., Faure, L., Moureau, J. (2017) Tracing weathering regimes using the lithium isotope composition of detrital sediments. Geology 45, 411–414.
). As shown in Figure 2b, all the river SPM samples are isotopically fractionated compared to their likely parent lithologies because they contain weathering products formed within the basin (Yang et al., 2021Yang, C.F., Vigier, N., Yang, S.Y., Revel, M., Bi, L. (2021) Clay Li and Nd isotopes response to hydroclimate changes in the Changjiang (Yangtze) basin over the past 14,000 years. Earth and Planetary Science Letters 561, 116793.
). SPM collected in the lower reaches logically represents an average composition of fine particulates from the whole basin. Accordingly, δ7Li and Li/Al variations of SPM collected at the XLJ station are best explained by a simple binary mixing between sediments collected in the upper Changjiang basin and those from the middle basin (Fig. 2b). In contrast, δ7Li and Li/Al ratios of SPM collected along the estuarine transect show a distinct trend. From the C1 site to offshore sites, these values progressively increase towards the binary mixing trend defined between un-weathered shale and igneous rocks (Dellinger et al., 2014Dellinger, M., Gaillardet, J., Bouchez, J., Calmels, D., Galy, V., Hilton, R.G., Louvat, P., France-Lanord, C. (2014) Lithium isotopes in large rivers reveal the cannibalistic nature of modern continental weathering and erosion. Earth and Planetary Science Letters 401, 359–372.
). Quartz, feldspar, illite, and kaolinite are four major minerals (i.e. each accounting for >10 %) of terrigenous sediments from the Changjiang River (Yang et al., 2002Yang, S.Y., Jung, H.S., Choi, M.S., Li, C.X. (2002) The rare earth element compositions of the Changjiang (Yangtze) and Huanghe (Yellow) river sediments. Earth and Planetary Science Letters 201, 407–419.
). According to the XRD results, quartz contents in the estuarine SPM increase from ∼26 % to ∼41 % in the offshore direction, while illite and kaolinite contents both decrease by ∼10 % (Table S-3). Thus, the trend observed for estuarine SPM in Figure 2b can be explained by a physical loss of clay minerals and/or a gain of Si-rich primary minerals.Flocculation and resuspension of fine sediments are two fundamental processes occurring in river estuaries. When encountering alkaline seawater, river-borne clay minerals, especially kaolinite, can be easily aggregated and deposited rapidly onto the seafloor. Milliman et al. (1985)
Milliman, J.D., Shen, H.T., Yang, Z.S., Meade, R.H. (1985) Transport and desposition of river sediment in the Changjiang Estuary and adjacent continental shelf. Continental Shelf Research 4, 37–45.
once estimated that nearly 40 % of the sediment load can be trapped in the Changjiang Estuary during flood season. Seafloor sediments can be resuspended by strong tidal and wind-driven currents, as supported by the several orders of magnitude higher SPM concentration observed at sites C6–C8 than at landward or seaward sites (see Fig. S-5c). Therefore, the offshore transport of SPM in the Changjiang Estuary may result in the preferential flocculation and deposition of clay minerals during the flooding season, while primary minerals or other fine-grained particles tend to be resuspended and carried seaward by currents. Consequently, these physical processes result in a visible increase of SPM δ7Li values (by ∼1.2 ‰) from the most inland site (C1) to the most offshore sites.top
Implications for Li Isotopes as Tracers of Continental Weathering
Previous studies suggest that both the dissolved and particulate δ7Li values related to clay formation are powerful tracers of weathering processes. Our results demonstrate that their behaviours are visibly decoupled in the estuary. During estuarine flocculation and resuspension of river-borne fine particles, the SPM δ7Li values progressively approach the upper continental crust value. This process is distinct from the general control of hydrodynamic sorting on elemental geochemical compositions during source-to-sink river sediment transport (Lupker et al., 2012
Lupker, M., France-Lanord, C., Galy, V., Lave, J., Gaillardet, J., Gajurel, A.P., Guilmette, C., Rahman, M., Singh, S.K., Sinha, R. (2012) Predominant floodplain over mountain weathering of Himalayan sediments (Ganga basin). Geochimica et Cosmochimica Acta 84, 410–432.
; Dellinger et al., 2014Dellinger, M., Gaillardet, J., Bouchez, J., Calmels, D., Galy, V., Hilton, R.G., Louvat, P., France-Lanord, C. (2014) Lithium isotopes in large rivers reveal the cannibalistic nature of modern continental weathering and erosion. Earth and Planetary Science Letters 401, 359–372.
). Although the effects of ongoing weathering are negligible in Changjiang-like estuaries, an increase of particulate δ7Li values would be expected in rivers draining basaltic terrains (Pogge von Strandmann et al., 2008Pogge von Strandmann, P.A.E., James, R.H., van Calsteren, P., Gíslason, S.R., Burton, K.W. (2008) Lithium, magnesium and uranium isotope behaviour in the estuarine environment of basaltic islands. Earth and Planetary Science Letters 274, 462–471.
). Consequently, the utility of detrital δ7Li values for tracing past continental weathering in coastal and marginal seas seems to be more complicated than for the isotopic values of the dissolved phase (Yang et al., 2021Yang, C.F., Vigier, N., Yang, S.Y., Revel, M., Bi, L. (2021) Clay Li and Nd isotopes response to hydroclimate changes in the Changjiang (Yangtze) basin over the past 14,000 years. Earth and Planetary Science Letters 561, 116793.
). Caution should therefore be exercised when using δ7Li values, and other similar sediment geochemical proxies, measured in bulk sediments from marginal seas to reconstruct past continental weathering.Our data along a salinity transect provide the first and clear evidence of the conservative behaviour of dissolved Li and Li isotopes during estuarine mixing in a large, particle-rich river. Differing from the case study on Li isotopes in a small Icelandic estuary (Borgarfjörður) draining dominantly basaltic terrains (Pogge von Strandmann et al., 2008
Pogge von Strandmann, P.A.E., James, R.H., van Calsteren, P., Gíslason, S.R., Burton, K.W. (2008) Lithium, magnesium and uranium isotope behaviour in the estuarine environment of basaltic islands. Earth and Planetary Science Letters 274, 462–471.
), the large Changjiang basin contains various rock types, including intensely weathered shales and Ca-Mg-depleted sediments. This strongly supports the notion that information on continental weathering carried by the dissolved loads (i.e. δ7Li) of large rivers can be propagated to the ocean without significant modification. Scavenging processes thus have negligible influence on aqueous Li behaviour, at least in Changjiang-like basins and estuaries, which verifies the assumption of conservative behaviour when using Li isotopes in marine authigenic archives to reconstruct past continental weathering variations and related carbon cycles (Misra and Froelich, 2012Misra, S., Froelich, P.N. (2012) Lithium isotope history of Cenozoic seawater: changes in silicate weathering and reverse weathering. Science 335, 818–823.
). Additionally, it is noteworthy that δ7Li values of estuarine waters significantly increase with seawater addition at low salinities. This highlights the need for precise salinity assessments when sampling estuarine waters for quantifying global Li and δ7Li budgets and continental fluxes to the ocean.top
Acknowledgements
This work was funded by the National Natural Science Foundation of China (Grant No. 41991324, 41730531), the ANR INTOCC project (ANR-15-CE31-0013), and the China Postdoctoral Science Foundation (Grant No. 2021M692416). Chengfan Yang was supported by the China Scholarship Council for two years of PhD study at the Laboratory of Oceanography of Villefranche-sur-Mer (LOV, France). The KECES cruise was supported by the State Key Laboratory of Marine Geology, Tongji University. We thank Editor Eric H. Oelkers, Morgan T. Jones, and an anonymous reviewer for their constructive comments that greatly improved the quality of this paper. We thank the crew of the Zheyuke-2 for their assistance with field sampling, Jiantao Cao for his assistance with pH measurements, Hao Wu for his assistance with suspended sediment concentration measurements, Yanli Li for her assistance with XRD analyses, and Juan Xu for her assistance with elemental concentration measurements. Many thanks to the CHOC (Chemistry-Ocean-Climate) team at the LOV for their help with sample pre-treatment.
Editor: Eric H. Oelkers
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References
Bagard, M.L., West, A.J., Newman, K., Basu, A.R. (2015) Lithium isotope fractionation in the Ganges–Brahmaputra floodplain and implications for groundwater impact on seawater isotopic composition. Earth and Planetary Science Letters 432, 404–414.
Show in context A similar conclusion has been drawn by Bagard et al. (2015) for assessing the modern Li isotopic budget of the ocean, based on investigation of the Li flux and δ7Li of groundwater in the Ganges-Brahmaputra downstream basin.
View in article
Bastian, L., Revel, M., Bayon, G., Dufour, A., Vigier, N. (2017) Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa. Scientific Reports 7, 44231.
Show in context To date, Li isotopes registered in bulk carbonate or detrital sedimentary archives have been widely applied to assess changes of continental weathering regimes during mass extinctions and long or short term global warming/cooling events (Misra and Froelich, 2012; Pogge von Strandmann et al., 2013; Bastian et al., 2017; Yang et al., 2021).
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Brunskill, G.J., Zagorskis, I., Pfitzner, J. (2003) Geochemical mass balance for lithium, boron, and strontium in the Gulf of Papua, Papua New Guinea (Project TROPICS). Geochimica et Cosmochimica Acta 67, 3365–3383.
Show in context This salinity–Li relationship is consistent with observations made in the St. Lawrence estuary and the Gulf of Papua (Stoffyn-Egli, 1982; Brunskill et al., 2003).
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Dellinger, M., Gaillardet, J., Bouchez, J., Calmels, D., Galy, V., Hilton, R.G., Louvat, P., France-Lanord, C. (2014) Lithium isotopes in large rivers reveal the cannibalistic nature of modern continental weathering and erosion. Earth and Planetary Science Letters 401, 359–372.
Show in context The sediment Al/Si ratio allows us to constrain the effects of hydrological sorting on isotope proxies (Dellinger et al., 2014).
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The igneous rocks and shale endmembers in (b) are modified after Dellinger et al. (2014), and the shaded area reflects their mixing.
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From the C1 site to offshore sites, these values progressively increase towards the binary mixing trend defined between un-weathered shale and igneous rocks (Dellinger et al., 2014).
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As shown in Figure 2a, Li/Si ratios positively correlate with Al/Si ratios for both riverine SPM (orange symbols) and estuarine SPM (blue symbols), suggesting a dominant control on Li concentrations by Al-rich materials (e.g., clays) present in the SPM (Vigier et al., 2008; Dellinger et al., 2014).
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This process is distinct from the general control of hydrodynamic sorting on elemental geochemical compositions during source-to-sink river sediment transport (Lupker et al., 2012; Dellinger et al., 2014).
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Dellinger, M., Bouchez, J., Gaillardet, J., Faure, L., Moureau, J. (2017) Tracing weathering regimes using the lithium isotope composition of detrital sediments. Geology 45, 411–414.
Show in context More information can be inferred from the δ7Li vs. Li/Al diagram (Dellinger et al., 2017).
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River Li isotopes (δ7Li) are thought to be a powerful proxy of continental weathering (Huh et al., 1998; Wang et al., 2015; Dellinger et al., 2017).
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Gislason, S.R., Oelkers, E.H., Snorrason, A. (2006) Role of river-suspended material in the global carbon cycle. Geology 34, 49–52.
Show in context For instance, the dissolution of basaltic particles and Ca–Na exchange on clay minerals in saline water were considered to play non-negligible roles in global climate stabilisation (Gislason et al., 2006; Tipper et al., 2021).
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Gu, H.Q., Moore, W.S., Zhang, L., Du, J.Z., Zhang, J. (2012) Using radium isotopes to estimate the residence time and the contribution of submarine groundwater discharge (SGD) in the Changjiang effluent plume, East China Sea. Continental Shelf Research 35, 95–107.
Show in context As a first approximation, we can assume that 0.1 % of the SPM Li could be released by dissolution in the Changjiang Estuary, although the average residence time of the Changjiang diluted water plume is only 5.4 d (Gu et al., 2012).
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Additionally, submarine groundwater discharge (SGD, 0.2–1.0 × 109 m3 d−1) was estimated to be 6–30 % of the river discharge during the flooding season in the Changjiang Estuary (Gu et al., 2012).
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Guo, Y.L., Yang, S.Y., Su, N., Li, C., Yin, P., Wang, Z.B. (2018) Revisiting the effects of hydrodynamic sorting and sedimentary recycling on chemical weathering indices. Geochimica et Cosmochimica Acta 227, 48–63.
Show in context Generally, depending on the grain size, density, and shape of detrital minerals, hydrodynamic sorting in riverbeds and floodplains may cause mineralogical and geochemical fractionation during SPM transport (Guo et al., 2018).
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Huh, Y., Chan, L.H., Zhang, L., Edmond, J.M. (1998) Lithium and its isotopes in major world rivers: Implications for weathering and the oceanic budget. Geochimica et Cosmochimica Acta 62, 2039–2051.
Show in context Indeed, in most large rivers, dissolved Li concentrations are two orders of magnitude lower than that of seawater, and δ7Li values are 5–10 ‰ less (Huh et al., 1998).
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River Li isotopes (δ7Li) are thought to be a powerful proxy of continental weathering (Huh et al., 1998; Wang et al., 2015; Dellinger et al., 2017).
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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.
Show in context For instance, less than 0.1 % of kaolinite and illite would be expected to dissolve in seawater annually (Jeandel and Oelkers, 2015).
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Jones, M.T., Pearce, C.R., Jeandel, C., Gislason, S.R., Eiriksdottir, E.S., Mavromatis, V., Oelkers, E.H. (2012) Riverine particulate material dissolution as a significant flux of strontium to the oceans. Earth and Planetary Science Letters 355, 51–59.
Show in context Indeed, as indicated by experimental investigations and by previous studies of Icelandic estuaries, isotopic exchanges may modify riverine dissolved isotopic compositions without significantly affecting elemental concentrations (Jones et al., 2012, 2014).
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Over the last decades, numerous studies have demonstrated that riverine elemental fluxes and isotope compositions (e.g., Si, Sr, and Nd) can be modified by physical, chemical, and/or biological processes during their transport through estuaries (Lacan and Jeandel, 2005; Jones et al., 2012, 2014; Zhang et al., 2020).
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Jones, M.T., Gislason, S.R., Burton, K.W., Pearce, C.R., Mavromatis, V., Pogge von Strandmann, P.A.E., Oelkers, E.H. (2014) Quantifying the impact of riverine particulate dissolution in seawater on ocean chemistry. Earth and Planetary Science Letters 395, 91–100.
Show in context Indeed, as indicated by experimental investigations and by previous studies of Icelandic estuaries, isotopic exchanges may modify riverine dissolved isotopic compositions without significantly affecting elemental concentrations (Jones et al., 2012, 2014).
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On the other hand, δ7Li values of SPM samples show no shift toward seawater (Fig. S-3a), in contrast to the altered 87Sr/86Sr ratios observed in Icelandic estuaries, for instance (Jones et al., 2014).
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Over the last decades, numerous studies have demonstrated that riverine elemental fluxes and isotope compositions (e.g., Si, Sr, and Nd) can be modified by physical, chemical, and/or biological processes during their transport through estuaries (Lacan and Jeandel, 2005; Jones et al., 2012, 2014; Zhang et al., 2020).
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Lacan, F., Jeandel, C. (2005) Neodymium isotopes as a new tool for quantifying exchange fluxes at the continent–ocean interface. Earth and Planetary Science Letters 232, 245–257.
Show in context Over the last decades, numerous studies have demonstrated that riverine elemental fluxes and isotope compositions (e.g., Si, Sr, and Nd) can be modified by physical, chemical, and/or biological processes during their transport through estuaries (Lacan and Jeandel, 2005; Jones et al., 2012, 2014; Zhang et al., 2020).
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Lupker, M., France-Lanord, C., Galy, V., Lave, J., Gaillardet, J., Gajurel, A.P., Guilmette, C., Rahman, M., Singh, S.K., Sinha, R. (2012) Predominant floodplain over mountain weathering of Himalayan sediments (Ganga basin). Geochimica et Cosmochimica Acta 84, 410–432.
Show in context As indicated by previous studies, this difference in slope can be caused either by ongoing weathering (Lupker et al., 2012) or by a change of sediment source (Yang et al., 2019).
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This process is distinct from the general control of hydrodynamic sorting on elemental geochemical compositions during source-to-sink river sediment transport (Lupker et al., 2012; Dellinger et al., 2014).
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Milliman, J.D., Shen, H.T., Yang, Z.S., Meade, R.H. (1985) Transport and desposition of river sediment in the Changjiang Estuary and adjacent continental shelf. Continental Shelf Research 4, 37–45.
Show in context Milliman et al. (1985) once estimated that nearly 40 % of the sediment load can be trapped in the Changjiang Estuary during flood season.
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Misra, S., Froelich, P.N. (2012) Lithium isotope history of Cenozoic seawater: changes in silicate weathering and reverse weathering. Science 335, 818–823.
Show in context Scavenging processes thus have negligible influence on aqueous Li behaviour, at least in Changjiang-like basins and estuaries, which verifies the assumption of conservative behaviour when using Li isotopes in marine authigenic archives to reconstruct past continental weathering variations and related carbon cycles (Misra and Froelich, 2012).
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To date, Li isotopes registered in bulk carbonate or detrital sedimentary archives have been widely applied to assess changes of continental weathering regimes during mass extinctions and long or short term global warming/cooling events (Misra and Froelich, 2012; Pogge von Strandmann et al., 2013; Bastian et al., 2017; Yang et al., 2021).
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Morin, G.P., Vigier, N., Verney-Carron, A. (2015) Enhanced dissolution of basaltic glass in brackish waters: Impact on biogeochemical cycles. Earth and Planetary Science Letters 417, 1–8.
Show in context The relatively high seawater Li concentration may mask some of the Li release during SPM dissolution (Morin et al., 2015) or Li uptake by SPM (Pogge von Strandmann et al., 2008).
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Murphy, M.J., Pogge von Strandmann, P.A.E., Porcelli, D., Ingri, J. (2014) Li isotope behaviour in the low salinity zone during estuarine mixing. Procedia Earth and Planetary Science 10, 204–207.
Show in context Indeed, few case studies have highlighted either the conservative or non-conservative behaviour of Li isotopes in small estuaries (Pogge von Strandmann et al., 2008; Murphy et al., 2014).
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Pogge von Strandmann, P.A.E., James, R.H., van Calsteren, P., Gíslason, S.R., Burton, K.W. (2008) Lithium, magnesium and uranium isotope behaviour in the estuarine environment of basaltic islands. Earth and Planetary Science Letters 274, 462–471.
Show in context For instance, Pogge von Strandmann et al. (2008) observed an increase of particulate δ7Li values (from ∼0 ‰ to ∼5 ‰) along a transect in the Borgarfjörður estuary (Iceland) related to ongoing weathering of suspended particles.
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Indeed, few case studies have highlighted either the conservative or non-conservative behaviour of Li isotopes in small estuaries (Pogge von Strandmann et al., 2008; Murphy et al., 2014).
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Although the effects of ongoing weathering are negligible in Changjiang-like estuaries, an increase of particulate δ7Li values would be expected in rivers draining basaltic terrains (Pogge von Strandmann et al., 2008).
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Differing from the case study on Li isotopes in a small Icelandic estuary (Borgarfjörður) draining dominantly basaltic terrains (Pogge von Strandmann et al., 2008), the large Changjiang basin contains various rock types, including intensely weathered shales and Ca-Mg-depleted sediments.
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The relatively high seawater Li concentration may mask some of the Li release during SPM dissolution (Morin et al., 2015) or Li uptake by SPM (Pogge von Strandmann et al., 2008).
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Pogge von Strandmann, P.A.E., Jenkyns, H.C., Woodfine, R.G. (2013) Lithium isotope evidence for enhanced weathering during Oceanic Anoxic Event 2. Nature Geoscience 6, 668–672.
Show in context To date, Li isotopes registered in bulk carbonate or detrital sedimentary archives have been widely applied to assess changes of continental weathering regimes during mass extinctions and long or short term global warming/cooling events (Misra and Froelich, 2012; Pogge von Strandmann et al., 2013; Bastian et al., 2017; Yang et al., 2021).
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Stoffyn-Egli, P. (1982) Conservative behaviour of dissolved lithium in estuarine waters. Estuarine, Coastal and Shelf Science 14, 577–587.
Show in context This salinity–Li relationship is consistent with observations made in the St. Lawrence estuary and the Gulf of Papua (Stoffyn-Egli, 1982; Brunskill et al., 2003).
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Tipper, E.T., Stevenson, E.I., Alcock, V., Knight, A.C.G., Baronas, J.J., Hilton, R.G., Bickle, M.J., Larkin, C.S., Feng, L., Relph, K.E., Hughes, G. (2021) Global silicate weathering flux overestimated because of sediment-water cation exchange. Proceedings of the National Academy of Sciences 118, e2016430118.
Show in context For instance, the dissolution of basaltic particles and Ca–Na exchange on clay minerals in saline water were considered to play non-negligible roles in global climate stabilisation (Gislason et al., 2006; Tipper et al., 2021).
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Vigier, N., Decarreau, A., Millot, R., Carignan, J., Petit, S., France-Lanord, C. (2008) Quantifying Li isotope fractionation during smectite formation and implications for the Li cycle. Geochimica et Cosmochimica Acta 72, 780–792.
Show in context As shown in Figure 2a, Li/Si ratios positively correlate with Al/Si ratios for both riverine SPM (orange symbols) and estuarine SPM (blue symbols), suggesting a dominant control on Li concentrations by Al-rich materials (e.g., clays) present in the SPM (Vigier et al., 2008; Dellinger et al., 2014).
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Wang, Q.L., Chetelat, B., Zhao, Z.Q., Ding, H., Li, S.L., Wang, B.L., Li, J., Liu, X.L. (2015) Behavior of lithium isotopes in the Changjiang River system: Sources effects and response to weathering and erosion. Geochimica et Cosmochimica Acta 151, 117–132.
Show in context Changjiang River SPM data are from Wang et al. (2015).
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River Li isotopes (δ7Li) are thought to be a powerful proxy of continental weathering (Huh et al., 1998; Wang et al., 2015; Dellinger et al., 2017).
View in article
Yang, C.F., Yang, S.Y., Song, J.Z., Vigier, N. (2019) Progressive Evolution of the Changjiang (Yangtze River) Sediment Weathering Intensity Since the Three Gorges Dam Operation. Journal of Geophysical Research: Earth Surface 124, 2402–2416.
Show in context As indicated by previous studies, this difference in slope can be caused either by ongoing weathering (Lupker et al., 2012) or by a change of sediment source (Yang et al., 2019).
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Yang, C.F., Vigier, N., Yang, S.Y., Revel, M., Bi, L. (2021) Clay Li and Nd isotopes response to hydroclimate changes in the Changjiang (Yangtze) basin over the past 14,000 years. Earth and Planetary Science Letters 561, 116793.
Show in context As shown in Figure 2b, all the river SPM samples are isotopically fractionated compared to their likely parent lithologies because they contain weathering products formed within the basin (Yang et al., 2021).
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Consequently, the utility of detrital δ7Li values for tracing past continental weathering in coastal and marginal seas seems to be more complicated than for the isotopic values of the dissolved phase (Yang et al., 2021).
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To date, Li isotopes registered in bulk carbonate or detrital sedimentary archives have been widely applied to assess changes of continental weathering regimes during mass extinctions and long or short term global warming/cooling events (Misra and Froelich, 2012; Pogge von Strandmann et al., 2013; Bastian et al., 2017; Yang et al., 2021).
View in article
Yang, S.Y., Jung, H.S., Choi, M.S., Li, C.X. (2002) The rare earth element compositions of the Changjiang (Yangtze) and Huanghe (Yellow) river sediments. Earth and Planetary Science Letters 201, 407–419.
Show in context Quartz, feldspar, illite, and kaolinite are four major minerals (i.e. each accounting for >10 %) of terrigenous sediments from the Changjiang River (Yang et al., 2002).
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Zhang, Z.L., Cao, Z.M., Grasse, P., Dai, M.H., Gao, L., Kuhnert, H., Gledhill, M., Chiessi, C.M., Doering, K., Frank, M. (2020) Dissolved silicon isotope dynamics in large river estuaries. Geochimica et Cosmochimica Acta 273, 367–382.
Show in context Over the last decades, numerous studies have demonstrated that riverine elemental fluxes and isotope compositions (e.g., Si, Sr, and Nd) can be modified by physical, chemical, and/or biological processes during their transport through estuaries (Lacan and Jeandel, 2005; Jones et al., 2012, 2014; Zhang et al., 2020).
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Supplementary Information
The Supplementary Information includes:
- Materials and Methods
- Estimation of Dissolved Li and δ7Li Values Assuming Conservative Mixing
- The Potential Influences of SPM Li Release and Uptake on Dissolved Li Isotopes
- Isotopic Exchange Between Water and Particles
- Tables S-1 to S-4
- Figures S-1 to S-5
- Supplementary Information References
Download the Supplementary Information (PDF).
Figures
Figure 1 (a) Variations of dissolved Li concentration as a function of salinity, and (b) variations of dissolved δ7Li as a function of the inverse of the molar Li concentration (1/Li). The x-axis in (b) is logarithmic. The dashed lines and shaded areas represent calculated conservative mixing between seawater (dark blue star) and Changjiang River water (white star). The mass balance model is provided in the Supplementary Information. The shaded areas are estimated using the Monte Carlo method, given that analytical uncertainties in Li concentrations and δ7Li are 10 % and 0.6 ‰, respectively. The analytical uncertainty for each δ7Li value is smaller than the symbol size.
Figure 2 (a) Variations of Li/Si ratios as a function of Al/Si ratios for riverine (orange, including the most inland site C1 in red) and estuarine SPM (blue), and (b) δ7Li versus Li/Al ratios for the same samples. The linear correlations between Li/Si and Al/Si are displayed with 95 % confidence intervals (shaded areas). The igneous rocks and shale endmembers in (b) are modified after Dellinger et al. (2014
Dellinger, M., Gaillardet, J., Bouchez, J., Calmels, D., Galy, V., Hilton, R.G., Louvat, P., France-Lanord, C. (2014) Lithium isotopes in large rivers reveal the cannibalistic nature of modern continental weathering and erosion. Earth and Planetary Science Letters 401, 359–372.
), and the shaded area reflects their mixing. Changjiang River SPM data are from Wang et al. (2015Wang, Q.L., Chetelat, B., Zhao, Z.Q., Ding, H., Li, S.L., Wang, B.L., Li, J., Liu, X.L. (2015) Behavior of lithium isotopes in the Changjiang River system: Sources effects and response to weathering and erosion. Geochimica et Cosmochimica Acta 151, 117–132.
).