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Rare Earth Elements, including the lanthanide series and Y (REYs), are important tracers and paleo-proxies of biogeochemical processes, water mass transport and oceanic mixing. At the interface between the continents and oceans, the geochemical behaviour of REYs in estuarine environments is generally described as being non-conservative, with large-scale removal by particle scavenging. This conventional interpretation stems from observations carried out in tropical or sub-tropical estuaries. However, major river systems in the mid- and high latitude regions are subjected to winter conditions when a frozen watershed and an ice cover may affect the continental input of particles and the REY geochemistry. Here, we investigate the geochemical behaviour of REYs in the Estuary and Gulf of St. Lawrence (EGSL) in spring 2003 as well as winter and summer 2020. In contrast to the ice-free seasons, REYs and Fe behave conservatively in the estuary during the winter. In addition, we observed a higher REY affinity toward particles and fractionation in the REE patterns in the hypoxic deep waters of the estuary. The latter observation may reflect enhanced REY sorption to mineral-carrier surfaces exposed upon the remineralization of settling organic particles and to manganese oxides of benthic origin. Computed partition coefficient (Kd) values and strong correlations between Y/Ho, Er/Nd and Ce anomalies with dissolved oxygen concentrations support this hypothesis. The estimated annual dissolved load of REYs from the EGSL to the ocean ranges from 0.4 to 75 tons per year, thus contributing significantly to the global marine budget. The global river loading of dissolved Nd to the ocean was also revised to 4,000 tons per year based on data available for 21 river systems.

Rivers engage in carbon cycle by transporting the dissolved products of weathering of rocks to the oceans, and this process is sensitive to the global climatic changes. The present study was carried out with an objective of estimating the spatial and temporal variation in carbon consumption due to rock weathering in Cauvery, which is a major non-perennial tropical river in the peninsular India. The samples of all the rock types of this river basin were collected and subjected to mineralogical analysis. The water samples from this river were collected three times a year from 2013 to 2017 at 28 locations and were analysed for pH, EC and major ions. The spatiotemporal variations in the chemistry of river water were used to understand the amount of carbon dioxide consumed by rock weathering. The contribution of weathering to dissolved load of the river was higher followed by the contribution of anthropogenic activities and rainfall. The contribution of silicate weathering is dominant during the high river flow, whereas during low flow time periods, the contribution of carbonate weathering is on par with silicate weathering. The carbon consumption due to weathering in the Cauvery river was higher when the flow was significant, and it was lower during summer months. It is also evident that the carbon consumption is high in the upper and middle regions of the basin due to the weathering of gneissic and granodiorite rocks. Thus, the carbon consumption and flux in this basin are dynamic, both spatially and temporally. The east flowing rivers draining through the peninsular India, which is mostly composed of massive rocks, also functions as carbon sink, thus benefitting the environment by reducing the excess CO2 in the atmosphere.

Nickel (Ni) is an important bio-active element in the oceans, whose past marine abundance and isotope signature has the potential to be recorded in sediments. The interpretation of such a record in terms of earth system processes relies critically on understanding modern oceanic budgets. All oceanic Ni isotope data collected to date demonstrate that the oceanic dissolved pool is heavier than the largest known input, the dissolved load of rivers. Steady-state requires a light sink from the oceans that is of similar size to the inputs. The currently known sinks are much larger than the known inputs and are not isotopically light. At face value, this scenario requires an additional isotopically heavy source of Ni to the oceanic dissolved pool. Here, we present a comprehensive study of Ni and its isotopes, with auxiliary data, for multiple phases (dissolved (, Geochimica et Cosmochimica Acta, 293, ISSN:0016-7037, ISSN:1872-9533

A new project-based laboratory (PJBL) was developed for general chemistry students to tackle the steps involved in chemical water quality analysis using an accessible, real-world hydrologic system (the lakes of the Lower Grand Coulee, Washington). This PJBL was developed in a joint general chemistry and environmental geology course by a chemist–geologist teaching team, and it emphasized an interdisciplinary approach for a more complete understanding of the geochemistry of the lake system. Students worked through the project following industry protocols for sample collection in the field, chemical laboratory determination of dissolved solids, alkalinity, and dissolved metals, and interpretation of the results within the unique geological context of the lakes. Foundational concepts in solution chemistry, such as solubility product, pH, and alkalinity, were essential in the discussion of the lake water trends and broader geologic processes. In particular, students could relate chemical weathering of basaltic bedrock to the measured lake water constituents and could observe how the groundwater- and evaporation-controlled fluxes of water in the closed basin produced a downstream increase in alkalinity and dissolved load. Collectively, the steps involved in the project meet the essential criteria (as defined by other authors) for a complete chemistry PJBL. This PJBL can be readily adapted to a wide range of groundwater or surface water systems and can be framed in a more or less interdisciplinary context. Many students, without prompting and on confidential evaluations, rated this as the most valuable learning experience in the year-long academic coursework.

Dynamics of totall dissolved solids (TDS) and suspended sediment concentrations (SSC) were monitored on the river Nišava at the hydrological station Dimitrovgrad. Water samples were collected each day in the period from 01.01.2010–31.12.2010. TDS concentration followed a seasonal pattern imposed by the availability of water, with higher concentrations recorded in low-flow periods and lower concentration in the highflow period. SSC was extremely variable and dominate mainly with specific high-flow events. Mean annual discharge (Q=6.68 m3/s, q=13.9 l/s/km2) over the year was 3.3 times higher than the average for the period of 50 years. In the study period, the annual transport dissolved load (Qd) was 36962.8 t (specific load Qds=76.7 t/km2/yr) and suspended sediment load (Qs) was 98861.9 t (specific load Qss=205.1 t/km2/yr). The transport of dissolved load and suspended sediment load shows temporal variations in different seasons. The analysis revealed that the maximum loads was transferred during the winter (Qd=43.3% and Qs=56.8% of the annual transport) and spring season (Qd=37.9% and Qs=38.9% of the annual transport). A comparative analysis of Q, Qd, and Qs show that the suspended sediment shows greater variability and exponential pattern of transport. For instance, 90% of the time was required to export 65% of the total water and transported 77% of the total dissolved load and only 22% total suspended sediment load. The most extreme precipitation episodes and extreme temperatures events was recorded in February 2010. The floods in February contributed by 44.8% of the total Qs transport over the study period. These findings have important implications for water resource management in the context of sediments mobilization, erosion, channel management, water quality and ecological functions.

To investigate the influence of chemical weathering on CO2 consumption, an analysis was performed of water chemistry by applying water chemistry equilibria methods in the Bishuiyan subterranean basin, SW China. The average value of total ion concentrations (TZ+) was 1,854.97 μEq/L, which was significantly higher than the global average value (TZ+ = 1,250 μEq/L). Ca2+ and HCO3− were the main ionic constituents in the waters. SO42− and NO3− concentrations were relatively higher than other anion concentrations, and Cl− concentrations were consistently the lowest. Dissolved load balance models result showed that carbonate weathering, silicate weathering, and atmospheric input were the primary ionic contributors, wherein the effects of carbonate weathering > silicate weathering > atmospheric input for the whole catchment, with the exception of Taiping, where silicate weathering was prominent over carbonate weathering. In addition, these analyses indicated that the erosion via rock weathering was also affected by atmospherically derived CO2 and allogenic acids. The estimated yield by quantitative calculation for the carbonate weathering rate was 59.7 t/(km2 year), which was 4.40 times higher than that of silicate weathering rate. Further, the carbonate and silicate weathering components of the carbon sink accounted for 71.2% and 28.8%, respectively, of the total basin rock weathering carbon sink.

Tectonically active high relief areas are proposed to have a substantial impact on climate regulation throughout the Cenozoic. Here, we present lithium (Li) isotope data of the dissolved load and suspended particulate material from the basins of the Yalong River and the Jinsha River, which drain the high-relief borders of the eastern Tibetan Plateau and constitute the headwaters of the Yangtze River. Dissolved δ7Li values are relatively low and constant in the Jinsha River mainstream (+6.8‰ to +9.7‰) compared to the higher and downstream increasing values in the Yalong River mainstream (+10.8‰ to +17.5‰). Furthermore, dissolved δ7Li values in the major tributaries of the Jinsha and Yalong River are highly variable and increase downstream from +11.4‰ to +29.4‰ and from +9.0‰ to +23.6‰, respectively. Robust correlations between Li, Na, and Cl concentrations identify that evaporite dissolution in the uppermost headwaters of the Jinsha River basin controls the Li budget and riverine δ7Li values of the mainstream and has a significant impact on dissolved Li in the Yangtze River. In contrast to the Jinsha River mainstream, dissolved Li in the Yalong River mainstream and the Jinsha and Yalong River tributaries are dominantly derived from the dissolution of silicates. δ7Li variations in the Yalong River basin and the Jinsha tributaries reflect little Li uptake by clays in the upper reaches and more Li uptake by clays in the lower reaches of the river basins. SWR/D ratios (where SWR is the chemical silicate weathering rate, and D is the total denudation rate) highlight an intermediate silicate weathering intensity under a kinetically limited weathering regime for the study area. However, positive correlations between dissolved δ7Li value and silicate weathering intensity proxies (such as Si/(Nasil + K) and K/(Nasil + K)) indicate an increasing weathering intensity downstream. A positive correlation between dissolved δ7Li values and the annual precipitation and a negative one between δ7Li values and elevation indicates that larger annual precipitation under a flattening relief results in a stronger silicate weathering intensity, thus, more Li uptake in clays and higher riverine δ7Li values in the lower reaches. Compared to the lower reaches of Yangtze River basin (∼+20‰), lower dissolved δ7Li values in its headwater of this study suggest that the increase in δ7Li of seawater over the Cenozoic is influenced by weathering in floodplains that developed due to erosion of high mountains, rather than weathering of mountain belts itself. Combined with silicate weathering rates of the eastern border of the Tibetan Plateau, the highest riverine δ7Li values in its catchments come along with the highest silicate weathering rates and high CO2 consumption, suggesting that a more elevated topography may have an important role in the drawdown of CO2 during the Cenozoic.

Ultra-trace ( HREE-enriched REE + Y patterns, small natural positive Y and Gd anomalies, and negative Eu and Ce anomalies. These REE + Y features are coherent downstream in the OR apart from amplification of Eu and Ce anomalies during REE removal/dilution. The OR samples capture a downstream decrease in sparingly soluble HFSE (Th, Nb, Ta, Zr, Hf), presumably related to their colloid-particulate removal from the dissolved load, accompanied by crustal Zr/Hf (32.5 ± 5.1) and supercrustal Nb/Ta (25.1 ± 7.7) ratios. Subcrustal Th/U (0.17–0.96) and supercrustal Mo/W (12.0–74.5) ratios in all ORB waters indicate preferential release and aqueous solubility of U > Th and Mo > W, with the latter attributed primarily to preferential W adsorption on soil or upstream aquatic (oxy)(hydr)oxide surfaces.

Chemical weathering is a key player of the long-term (Ma) timescale carbon cycle. Weathering processes under alpine periglacial environment could be important during the Quaternary glacial-interglacial cycles, but such timescale is difficult to fully apprehend for the weathering impact on C cycles. The Chayu river catchment in SE Tibet spans a geographical gradient from a periglacial environment in the north to a monsoonal environment in the south and we have investigated the various chemical weathering along this climatological gradient. The lithology of the catchment is dominated by granitoid rocks. Due to the active tectonic activity and the periglacial environment, mass wasting deposits including landslide deposits, talus and rock glaciers are found in the catchment. In order to figure out the influence of mass wasting process on water chemistry, we sampled seepage water from the mass wasting deposits and nearby stream water for comparison. Cyclic input contributes 5.7 to 8.2% to the dissolved load of the water samples, suggesting the dominance of weathering contribution within this granitoid catchment. The contributions from sulfuric acid and carbonate to chemical weathering are estimated based on the elemental data and the oxidation of sulfide as the source of dissolved sulfate. In the monsoonal South Chayu, the K*/Si, Ca*/Si ratios of the bedrock landslide seepages are higher than the streams by 265% and 117%, respectively. Together with the high contribution of carbonate (~80%) and sulfuric acid (20 to 40%), the results indicates that landslide deposits have strong influences on local water chemistry by processes identical to bedrock landslides of greywacke and carbonate lithology in active mountain belts, suggesting that the enhanced weathering by landslide process can apply to granitoid regions as well. In the North Chayu with periglacial environment, the chemistry of the seepages and streams are nearly identical, and are similar to the landslide seepage water in the south. Such result highlights the role of the periglacial critical zone without the influence of mass wasting deposits that possibly enhances chemical weathering. The high porosity created by repeat freeze-thaw cycles provides more fresh mineral surfaces for water-rock interaction. But, the low temperature and the partial availability of water, restricted to thaw periods, provide strong kinetic limitations that inhibit the silicate weathering. Such features of the alpine periglacial critical zone led to the preferential weathering of the most reactive mineral phases including calcite, sulfide, and the interlayer K+ of sheet silicates (biotite, chlorite), resulting in the characteristic periglacial water chemistry. The influence of periglacial environment on local water chemistry may apply beyond the North Chayu and may play an important role in modulating the chemical weathering within the transition between Quaternary glacial and interglacial period.

Riverine suspended particulate matter (SPM) is essential for the delivery of micronutrients such as iron (Fe) to the oceans. SPM is known to consist of multiple phases with differing reactivity, but their role in the delivery of elements to the oceans is poorly constrained. Here we provide new constraints on the source and composition of reactive phases in SPM from the Mackenzie River, the largest sediment source to the Arctic Ocean. Sequential leaching of SPM shows that river sediments contain labile Fe phases. We estimate the labile Fe flux is substantial (0.21(+0.06,−0.05) Tg/yr) by quantifying Fe concentrations in weak leaches of the SPM. The labile Fe phase hosts a considerable amount of rare earth elements (REE), including neodymium (Nd). We demonstrate that the labile Fe phase and dissolved load have radiogenic Nd isotope ratios that are identical within uncertainty, but up to 8 epsilon units distinct from the silicate phase. We interpret this as evidence for dynamic cycling between Fe-oxide phases in SPM and the river water, demonstrating the high reactivity of the labile Fe phase. Nd isotope and elemental molar ratios suggest that a significant amount of labile Fe- and Nd-bearing phases are derived from Fe-oxides within the sedimentary source rock rather than silicate mineral dissolution. Thus, sedimentary rock erosion and weathering provides an important source of labile Fe, manganese (Mn) and by extension potentially other trace metals. Our results imply that both past and future environmental change in the Arctic, such as permafrost thaw, may trigger changes to the supply of reactive trace metals. These results demonstrate that a re-evaluation of sediment reactivity within rivers is required where uplifted sedimentary rocks are present.

International audience; Chemical composition of river sediments and dissolved load is classically used to infer controls on continental weathering and, therefore, exert an important role on the understanding of the global carbon and biogeochemical cycles. To date, most studied river basins are strongly impacted by dam constructions; however, the effects of dams on sediment chemical compositions are little known. The Three Gorges Dam is one of the largest dams in the world and began impounding water in 2003 in the Changjiang basin. In order to investigate the impact of this dam on downstream sediment chemistry, temporal variation of sediment weathering intensity is reported here based on analyzed and compiled data between 1997 and 2018. Downstream sediments collected before 2003 are characterized by weak weathering intensity, in agreement with the overwhelming flux and fast transfer of sediments derived from the mountainous upper watershed. After the Three Gorges Dam operation, strong midlower riverbed erosion changed the roles of the midlower reaches from important sinks to major sources of sediments delivered to the East China Sea. This resulted in a progressive change of the sediment chemistry because the eroded midlower riverbed sediments were more deeply weathered, as confirmed by 150-year-old sediment cored in the lower mainstream and by mass-balance calculations. This more intensive weathering may be explained by warmer climate and longer water-rock interaction time in the midlower basin. Thus, this study suggests the need to quantify potential bias in weathering intensity and controls caused by damming activity in large river systems.

The causal effects among uplift, climate, and continental weathering cannot be fully addressed using presently available geochemical proxies. However, stable potassium (K) isotopes can potentially overcome the limitations of existing isotopic proxies. Here we report on a systematic investigation of K isotopes in dissolved load and sediments from major rivers and their tributaries in China, which have drainage basins with varied climate, lithology, and topography. Our results show that during silicate weathering, heavy K isotopes are preferentially partitioned into aqueous solutions. Moreover, δ(41)K values of riverine dissolved load vary remarkably and correlate negatively with the chemical weathering intensity of the drainage basin. This correlation allows an estimate of the average K isotope composition of global riverine runoff (δ(41)K = −0.22‰), as well as modeling of the global K cycle based on mass balance calculations. Modeling incorporating K isotope mass balance better constrains estimated K fluxes for modern global K cycling, and the results show that the δ(41)K value of seawater is sensitive to continental weathering intensity changes. Thus, it is possible to use the δ(41)K record of paleo-seawater to infer continental weathering intensity through Earth’s history.

Total and continuing denudation rates were estimated using cosmogenic nuclides and intensive field observations of sediment transport in two small basins (C3 and S3) from the Ashio Mountains, Japan. The C3 basin underlain by Triassic bedded chert, while the S3 basin has evolved in altered Jurassic sandstone-shale. Continuing denudation rates were estimated from the records of coarse sediment transport and stream discharge for 6 years (2004–2009), soil grain-size distribution, and dissolved load in stream water. A debris flow was observed and the volume was measured in C3 (chert). Concentrations of cosmogenic 10Be and 26Al in fluvial sand were measured to determine spatially-averaged total denudation rates for these basins on the basis of steady-state assumption. Continuing denudation rate for C3 (chert) basin excluding the debris-flow mass accounted for ~75% of the total denudation rate as measured from the cosmogenic nuclides. Assuming a difference between the total and continuing denudation rates indicates the impact of repeat debris flows, the debris flows with the observed magnitude (the mass of 24 t) occur with the recurrence intervals of 100–550 years (mean 167 years) for C3 (chert) basin. For S3 (sandstone) basin, the continuous denudation rates were significantly smaller than the total rates, where debris flow had not occurred for at least 17 years. The large gap in the denudation rates with different time scales shows contribution of higher magnitude debris-flow events with longer recurrence intervals.

Research on the ionic chemistry of rivers and weathering types provides the basis for elucidating the dynamics of river chemistry and exploring carbon cycling in river systems. There is a lack of water chemistry study in the river systems in the Tibet Plateau, especially in the streams/rivers flowing from and through glaciers and permafrost. Samples in the rivers flowing through different land covers (lakes, glaciers, permafrost, grasslands, peatlands) were collected in different months (April, June, August and October) in 2016, covering various hydrological regimes. The temporal and spatial dynamic variations of major ions and the underlying causes were explored. The results revealed that in the headwater region Ca2+ and HCO3− were the dominant ions, derived primarily from the dissolution of carbonatites and evaporates. However, the concentrations of ions from different land covers were vastly different.The high concentrations of Na+ and K+ in the lakes sample were mainly affected by evaporation and precipitation. The acid deposition caused by atmospheric pollutants resulted in high concentration of SO42− in glacial and permafrost streams. K+ concentration was high in the grassland region with frequent agricultural activities such as the planting and fertilization of highland barleys that applied nitrogen and potassium fertilizers. Although Total Dissolved Load (TDS) was higher for the lakes and streams/rivers from glaciers and permafrost, and its average (287.28 ± 40 mg/L) over the headwater region was lower than that in the middle and lower reaches of the Yellow River because of low temperature. The current study provided the basis of and reference for the overall water chemistry characteristics and carbon cycling processes of the entire Yellow River.

Hydrochemical assessment of the Cauvery River Basin (CRB), an east flowing Western Ghats (WG) river is carried out to understand the dissolved load sources and controlling mechanisms along with quantification of source-wise input to the dissolved load. Silicate weathering rates (SWR) and associated CO2 consumption rates (CCR) are evaluated on account of silicate basement of CRB comprising of granulites and supracrustal rocks. The source-wise solute load contributions estimated using the chemical mass balance model signify that 68% of total load is from chemical weathering followed by 18.5% and 13.5% from anthropogenic and atmospheric inputs respectively, implying that chemical weathering is the major solute load controlling mechanism for CRB. The intensity of silicate chemical weathering occurring in the CRB is measured by index (Re) and found to be > 3, suggesting an incomplete weathering of drainage rocks (primary minerals) resulting in formation of soils comprising of secondary minerals including oxides, alumino-silicates and clay minerals (smectite, kaolinite and montmorillonite). Detailed understanding of chemical weathering mechanisms is carried out using Ca/Na and Mg/Na elemental ratios of different end-members including primary minerals from rocks and secondary minerals from soils. The Na-normalized mixing diagram reveals that chemical weathering of secondary minerals is dominating and the solute load contribution to the total dissolved load is significantly higher from secondary minerals (35.5%) than primary minerals (23.5%). The SWR and associated CCR are estimated to be 13 t.km-2.y-1 and 3.3 × 105 mole.km-2.y-1 respectively at outlet (Musiri) of the CRB. Results also indicate that SWR of the east flowing WG river, Cauvery are several times (~4) lower than the average SWR of west flowing WG rivers even though the associated CCR are comparable for both river systems.

Despite the absence of tectonic activity, cratonic environments are characterized by strongly variable, and in places significant, rock weathering rates. This is shown here through an exploration of the weathering rates in two inter-tropical river basins from the Atlantic Central Africa: the Ogooué and Mbei River basins, Gabon. We analyzed the elemental and strontium isotope composition of 24 water samples collected throughout these basins. Based on the determination of the major element sources we estimate that the Ogooué and Mbei rivers total dissolved solids (TDS) mainly derive from silicate chemical weathering. The chemical composition of the dissolved load and the area-normalized solute fluxes at the outlet of the Ogooué are similar to those of other West African rivers (e.g., Niger, Nyong, or Congo). However, chemical weathering rates (TZsil+ rate expressed as the release rate of the sum of cations by silicate chemical weathering) span the entire range of chemical weathering intensities hitherto recorded in worldwide cratonic environments. In the Ogooué-Mbei systems, three regions can be distinguished: (i) the Eastern sub-basins draining the Plateaux Batéké underlain by quartz-rich sandstones exhibit the lowest TZsil+ rates, (ii) the Northern sub-basins and the Mbei sub-basins, which drain the southern edge of the tectonically quiescent South Cameroon Plateau, show intermediate TZsil+ rates and (iii) the Southern sub-basins characterized by steeper slopes record the highest TZsil+ rates. In region (ii), higher DOC concentrations are associated with enrichment of elements expected to form insoluble hydrolysates in natural waters (e.g., Fe, Al, Th, REEs) suggesting enhanced transport of these elements in the colloidal phase. In region (iii), we suggest that a combination of mantle-induced dynamic uplift and lithospheric destabilization affecting the rim of the Congo Cuvette induces slow base level lowering thereby enhancing soil erosion, exhumation of fresh primary minerals, and thus weathering rates. The study points out that erosion of lateritic covers in cratonic areas can significantly enhance chemical weathering rates by bringing fresh minerals in contact with meteoric water. The heterogeneity of weathering rates amongst cratonic regions thus need to be considered for reconstructing the global, long-term carbon cycle and its control on Earth climate.

This study of Lithium (Li) isotopes in surface waters and sediments in the catchment of Lake Bangong and the Upper Indus on the western Tibetan Plateau aims to identify processes that control Li isotope variations during weathering under a cold and hyper-arid climate. Additionally, Li isotope ratios in the Yarlung Tsangpo – Brahmaputra River were investigated. The lake and river sediments of Lake Bangong catchment display remarkable low δ7Li values between −4.7‰ and −0.6‰ relative to L-SVEC. Li isotopes in river bed sediments correlate with weathering intensity tracers such as the chemical index of alteration (CIA), K/(Na + K), or Na/Ti, and δ7Li values decrease continuously within the sediment cascade. These observations cannot be explained by mixing of different lithologies but reflects increasing intensity of weathering. The hyper-arid climate on the western plateau results in considerably long sediment residence times, which allows for overcoming the limitation of water availability on chemical weathering reactions. Samples from the Lake Bangong basins display low δ7Li values between +8.1‰ and +11.1‰. The major inflows have dissolved δ7Li values of +6.1‰ and +8.9‰. High Li/Na ratios in the stream waters indicate some contribution of hydrothermal Li. However, low δ7Li values in surface waters result from impeded silicate weathering processes in the thin soils. The samples from Indus headwaters and Yarlung Tsangpo provide evidence for low δ7Li all over the western and southern Tibetan Plateau. Using data of the Bangong Co, the Indus headwaters, and Yarlung Tsangpo, as well as published data from the northeastern plateau, we explore what controls Li isotope variation across the Tibetan plateau. Mass balance calculations suggest that similar proportions of dissolved Li and particulate Li are exported by river water and sediments on the western plateau. In contrast, high δ7Li values around +17‰ of the dissolved load in rivers on the northeastern Tibetan Plateau reflect a particulate Li export flux that is about five times higher than the export flux of dissolved Li. There is no first-order control by silicate weathering rates. The δ7Li differences largely follow the precipitation gradient across the Tibetan Plateau, which results in high net-incorporation of Li into clays on the northeastern but limits soil formation on the western Tibetan Plateau and, therefore limited, processing of lithium in the weathering zone.

Rivers carry the dissolved and solid products of silicate mineral\ud weathering, a process that removes CO2 from the atmosphere and\ud provides a key negative climate feedback over geological timescales.\ud Here we show that in some river systems, a reactive exchange pool\ud on river suspended particulate matter, bonded weakly to mineral\ud surfaces, increases the mobile cation flux by 50%. The chemistry\ud of both river waters and the exchange pool demonstrate exchange\ud equilibrium, confirmed by Sr isotopes. Global silicate weathering\ud fluxes are calculated based on riverine dissolved sodium (Na+) from\ud silicate minerals. The large exchange pool supplies Na+ of non-\ud silicate origin to the dissolved load, especially in catchments with\ud widespread marine sediments, or where rocks have equilibrated with\ud saline basement fluids. We quantify this by comparing the riverine\ud sediment exchange pool and river water chemistry. In some basins,\ud cation exchange could account for the majority of sodium in the\ud river water, significantly reducing estimates of silicate weathering.\ud At a global scale, we demonstrate that silicate weathering fluxes\ud are over-estimated by 12-28%. This over-estimation is greatest in\ud regions of high erosion and high sediment loads where the negative\ud climate feedback has a maximum sensitivity to chemical weathering\ud reactions. In the context of other recent findings that reduce the\ud net CO2 consumption through chemical weathering, the magnitude\ud of the continental silicate weathering fluxes and its implications for\ud solid Earth CO2 degassing fluxes needs to be further investigated.

A sediment budget for the 0.5 km2 watershed of Nahal Yael is attempted on the basis of detailed observations of rainfall, streamflow, sediment transport and deposition, and changes in geomorphic features. For the ten year period considered, the mean annual rainfall was 31.6 mm, 99 percent of the geomorphic work was accomplished during 5 days by 7 discrete events. Sediment in suspension, computed from data obtained through an automatic sampling program, accounted for a mean annual yield of 127 tons. Bedload yield is estimated on the basis of, (1) distance of transport determined from traceable particles, (2) the area and depth of the scour layer for the inner channel bed and for the gravel bars, and (3) a comparison of grain sizes on the bed and the bars with sediment in transport as sampled by liquid samplers and bedload traps. A mean annual yield of 66 tons was found. The dissolved load is about one percent of the total load. The resulting mean annual sediment yield of 388 tons/km2 considerably exceeds the accepted norm for arid environments. It also exceeds by a factor of 3 the estimated sediment yield, corrected for drainage area, for the 3,100 km2 watershed of Wadi Watir, located in a similar environment in eastern Sinai. While the internal sediment delivery ratios of the Nahal Yael drainage system are reasonably consistent, the aggradation rates as measured directly on the alluvial fan over a period of six years are one fifth of what they should be on the basis of the sediment transport computation.140 Although inadequacies of the sampling and measurement program may explain a part of the discrepancy, the main reasons are an insufficient understanding of the transport mechanism during violent desert floods, exchanges between suspended and bedload transport modes, and the importance of transient alluvial storage and its intense localization.

Magnesium and lithium stable isotope ratios (δ26Mg and δ7Li) have shown promise as tools to elucidate biogeochemical processes both at catchment scales and in deciphering global climate processes. Nevertheless, the controls on riverine Mg and Li isotope ratios are often difficult to determine as a myriad of factors can cause fractionation from bulk rock values such as secondary mineral formation and preferential weathering of isotopically distinct mineral phases. Quantifying the relative contribution from carbonate and silicate minerals to the dissolved load of glacierized catchments is particularly crucial for determining the role of chemical weathering in modulating the carbon cycle over glacial-interglacial periods. In this study we report Mg and Li isotope data for water, river sediment, rock, and mineral separates from the Leverett Glacier catchment, West Greenland. We assess whether the silicate mineral contributions to the dissolved load, previously determined using radiogenic Sr, Ca, Nd, and Hf isotopes, are consistent with dissolved Mg and Li isotope data, or whether a carbonate contribution is required as inferred previously for this region. For δ7Li, the average dissolved river water value (+19.2 ± 2.5h, 2SD) was higher than bedrock, river sediment, and mineral δ7Li values, implying a fractionation process. For δ26Mg, the average dissolved river water value (−0.30 ± 0.14h, 2SD) was within error of bedrock and river sediment and within the range of mineral δ26Mg values (−1.63 to +0.06h). The river δ26Mg values are consistent with the mixing of Mg derived from the same mineral phases previously identified from radiogenic isotope measurements as controlling the dissolved load chemistry. Glacier fed rivers previously measured in this region had δ26Mg values ∼0.80h lower than those measured in the Leverett River which could be caused by a larger contribution from garnet (−1.63h) dissolution compared to Leverett. This study highlights that dissolved Mg and Li isotope ratios in the Leverett River are affected by different processes (mixing and fractionation), and that since variations in silicate mineral δ26Mg values exist, preferential weathering of individual silicate minerals should be considered in addition to carbonate when interpreting dissolved δ26Mg values., Frontiers in Earth Sciences, 7, ISSN:1863-4621, ISSN:1863-463X

The present study deals with the discharge characteristics, major ion chemistry as well as particulate and dissolved sediment transport of the Bharathapuzha river flowing across the Western Ghats through the Palghat gap - a prominent break in the Western Ghats that acts as a climate corridor in the region. While the western part of the basin in Kerala exhibits a wetter/humid climate, the eastern part in Tamil Nadu experiences a semi-arid climate. This peculiar climatic condition together with human interventions has a decisive role on the discharge characteristics as well as catchment erosion of the river basin. The dissolved and particulate contents of the river waters were found to be directly related to the geo-environmental settings of the river catchments. An intercomparison of elemental loading in the river waters in different seasons reveals notable decrease in monsoon compared to nonmonsoon seasons, which is attributed mainly to dilution of the solute contents by monsoon precipitation. The bivariate plots of Na/(Na+Ca) vs Total Dissolved Solids (TDS) and Cl/(Cl+HCO3) vs TDS spread generally in the rock dominance sector of Gibb’s model indicating the role of chemical weathering in contributing to the major ion chemistry of the Bharathapuzha river. The intensity of weathering varied significantly among the different sub-basins of the Bharathapuzha river basin. The higher particulate load by dissolved load ratio of the Thuthapuzha and Gayathripuzha tributaries draining the humid areas compared to the Chitturpuzha tributary draining the semi-arid zones of the basin indicates the predominance of physical weathering in the former two sub-basins than the latter.

The dissolved and suspended sediment flux in a small headwater in the Southern Alps, New Zealand over a 16-month period showed that mass transferred as dissolved load (91 t km−2 a−1) is a significant flux relative to the suspended sediment flux (210 t km−2 a−1). Export of dissolved load occurs mostly under baseflow, with 268 t being transported over 10 discrete events, relative to a total load of 1171 t over the experimental observation period; which is equivalent to 23% of the ionic flux occurring during event flow. Storm events contribute 2420 t of suspended sediment, accounting for 90% of the total suspended sediment flux. The chemical weathering flux was 43% of the physical weathering flux, and much higher than previous estimates of 1–9% for the Southern Alps. Such a high relative rate of chemical weathering is unlikely to be indicative of the full Southern Alps extent, but suggests that in some regions of the eastern Southern Alps mass loss via chemical weathering may be a significant portion of overall flux. During storm events, temporal exhaustion of sediment occurred and inferred limited sediment source production, rather than limited connectivity with the hillslope environment. Similar supply-limitation is expressed in the ionic flux, where rainfall events do not usually express hysteretic behaviour, and event flow is dominated by a simple dilution recession.

The behaviour and fate of copper (Cu) and zinc (Zn) at river confluences is poorly understood, although chemical and physical processes during mixing of compositionally different tributaries might condition metal availability and fluxes to the ocean. To identify and quantify the effect of such processes in river mixing zones, particulate Cu and Zn isotope signatures (delta(65) Cu-SPM and delta Zn-66(SPM)) were measured along cross sections and vertical profiles at the largest river confluence in the world, the "Encontro das Aguas" mixing zone of the Amazon River, where the organic-rich Negro River meets the sediment-rich Solimoes River. The Negro River suspended sediments, with highly variable Cu and Zn concentrations as well as delta Cu-65(SPM) and delta(66) Zn-SPM, are mostly influenced by organic matter and by the lateritic cover of the watershed. The Solimoes River suspended sediments, more homogeneous in Cu and Zn concentration and isotope composition across the river section, reflect the signature of weathered silicate-rich sediments derived from the Andes. The Solimoes River supplies the majority of the suspended Cu and Zn to the Amazon River, and despite important flux losses across the confluence (-35% for Cu and -27% for Zn), delta(65) Cu-SPM and delta(66) Zn-SPM show a conservative behaviour during the mixing. In the dissolved load, Cu concentrations and delta(65) Cu-diss, mostly supplied by the Solimoes River, behave conservatively whereas Zn, derived mainly from the Negro River, suffers an important loss ( -58%) that can be attributed to Zn adsorption onto the suspended sediments from the Solimoes River. This transfer does not induce a significant delta(66) Zn-SPM shift in the Amazon River suspended sediments. Therefore, Cu and Zn isotope ratios in the suspended sediments behave conservatively through this confluence, which mixes two very chemically-contrasted rivers. Our findings thus suggest that the riverine isotopic information on the sources of particulate Cu and Zn is preserved during tributary mixing.

With the aim of elucidating the spatial and seasonal behaviors of rare earth elements (REEs), we investigated the dissolved REE concentrations of surface water collected during four seasons from middle, lower reaches and estuary of the Minjiang River, southeastern China. The results display that the REE abundances in Minjiang River, ranging from 3.3–785.9 ng/L, were higher than those of many of the major global rivers. The total REE concentrations (ΣREE) were seasonally variable, averaging in 5 937.30, 863.79, 825.65 and 1 065.75 ng/L during second highest flow (SHF), normal flow (NF), low flow (LF) and high flow (HF) season, respectively. The R(L/M) and R(H/M) ratios reveal the spatial and temporal variations of REE patterns, and particularly vary apparently in the maximum turbidity zone and estuary. REE patterns of dissolved loads are characterized by progressing weaker LREEs-enrichment and stronger HREEsenrichment downstream from middle reaches to estuary during all four seasons. Comparing with NF and LF seasons, in which REE patterns are relatively flat, samples of SHF season have more LREE-enriched and HREE-depleted patterns that close to parent rocks, while samples of HF season are more LREEs-depleted and HREE-enriched. REE fractionations from the middle to lower reaches are stronger in the SHF and HF seasons than those in NF and LF seasons. Generally, spatial and seasonal variations in REE abundance and pattern are presumably due to several factors, such as chemical weathering, mixture with rainfall and groundwater, estuarine mixing, runoff, biological production and mountain river characters, such as strong hydrodynamic forces and steep slopes. The highest Gd/Gd * always occurs at north ports during all four seasons, where most of the large hospitals are located. This suggests Gd anomalies are depended on the density of modern medical facilities. Y/Ho ratios fluctuate and positively correlate to salinity in estuary, probably because of the geochemical behavior differences between Y and Ho.

Rivers play a vital role in geochemical processes of the critical zone. As the Third Pole, the geochemical processes of the Qinghai-Tibetan Plateau are vital to global element cycles. The Yellow River source region is located in the northeastern Qinghai-Tibetan Plateau, yet there has been little research concerning water chemistry in this region. In this study, river water and rain samples were collected to analyze the stable isotopes and major ion chemistry in the Yellow River source region. The δ18O and δD values ranged from −12.9‰ to −4.5‰ and from −95.7‰ to −36.7‰, respectively. The stable isotopic values of tributaries were lower than those in the main stream, and the construction of the dam could result in the enrichment of δ18O and δD. The stable isotopic values showed that precipitation was the main source of river water. The total dissolved solids (TDS) in river water ranged from 214.9 to 563.2 mg/L, and calcium and bicarbonate were the dominant ions. The average contributions of atmospheric input, evaporites, silicates, and carbonates to the dissolved load of river water were 1.8%–12.5%, 12.4%–15.9%, 18.1%–18.4%, and 56.7%–64.3%. The silicate and carbonate weathering rates were 2.3–2.5 t/km2/yr and 17.4–19.6 t/km2/yr, respectively. The chemical denudation rate in the study area was about 23.0–26.8 t/km2/yr, comparable to the world average value. The CO2 consumption rates by silicate and carbonate weathering were 91–94 × 103 moL/km2/yr and 185–208 × 103 moL/km2/yr, respectively. The TDS flux in the study area was 6.1 × 109 kg/yr, which accounted for 38–47% of that of the Yellow River into the sea. This study showed that although as an alpine arid area, the Yellow River source region had a comparable chemical denudation rate to the world average value, and even higher CO2 consumption rates by silicate and carbonate weathering than some of the large rivers in the world.

Arsenic (As) is a dangerous and carcinogenic element and drinking water is its main pathway of human exposure. Gold mines are widely recognized as important sources of As pollution. This work proposes the assessment of As distribution along watersheds surrounding "Morro do Ouro" gold mine (Paracatu, southeastern Brazil). A balance approach between filtered As fluxes (As 0.45 μm) and suspended particulate material (AsSPM) in different river segments was applied. Ultrafiltration procedure was used to categorize As into the following classes: particulate 0.1 μm, colloidal 0.1 μm to 10 kDa, dissolved 10 kDa to 1 kDa, and truly dissolved 1 kDa. By applying this approach, arsenic contributions from mining facilities were quantified in order to identify critical fluvial segments and support decision makers in actions of remediation. The mass balance indicated the occurrence of a decreasing gradient from upstream to downstream: (i) of the As concentrations higher than the limit established by Brazilian law (10 μg L

The aim of this paper was to analyse the dissolved elements in water which were sampled from the Sebou River in Northern Morocco. The sampling was conducted at the Mechra Bel station for two different hydrological cycles (2013/2013 and 2013/2014). The study was to determine the type of water facies present in the area. The piper diagram shows that the dominant water facies in the area are that of Nacl water facies, which contribute to 56% of the total amount of major elements in the water. The study also revealed that during high water levels, there is dilution of water and during low water levels, the concentration of elements increases. The presence of halite and gypsum bearing rocks in the study area gives an indication that the supply of Na+ and Cl− is related to the dissolution of evaporates. The results also revealed that the specific export of the dissolved load amounts to 127.6 t/km2/year. Finally, the quantification of the dissolved load, through the determination of dissolved material flows shows that the specific export of material amounts to 127.6 t/km2/year.

Sediment transport from mountainous to lowland areas is considered one of the most important geomorphological processes. In the present study, variations in transported sediment loads and dissolved...

Zinc (Zn) is an essential micronutrient released into soils during rock weathering, and Zn isotope composition has been emerging as a powerful tracer involved in the Earth’s surface processes. To explore the soil development in dry tropical regions and evaluate the influence on the global Zn cycles, stable Zn isotope data were presented for the two pristine low-humic gleysol profiles from the Mun River Basin (MRB), northeast Thailand. The samples exhibit the δ66Zn (relative to JMC 3-0749L standard) from −0.69‰ to 0.32‰. Therein, the Fe nodule-containing soils exhibit a light shift of δ66Zn relative to parent red siltstones (Δ66Znnodule-containing soil–p of S1 = −0.60‰) and the other soils (Δ66ZnII-other zones = −0.66‰ – −0.50‰), indicating that the preferential retention of light Zn isotopes in residual soils is attributed to the adsorption of heavy Zn isotopes onto the surface of secondary Fe oxides. Due to the water insufficiency in the MRB, the redox conditions to control the oxide precipitation are driven by the seasonal alteration of groundwater levels. With the recession of groundwater, Fe oxides precipitate and adsorb heavy Zn isotopes. Besides, heavy Zn isotopes are also incorporated in clay fraction. On the contrary, the saturated water is likely to disrupt the interactions of Zn with Fe phases and clay minerals. In addition, low content of organic matter and Mn oxides and the absence of sulfides limitedly contribute to the light Zn pool in soils. Conclusively, the isotopic signature of Zn indicates that Fe oxide-rich soils are likely to control the heavier Zn isotope composition of dissolved load in rivers relative to continental rocks in dry tropical regions.