Ratios In Rivers Research Articles

Ecosystem-level effects of re-oligotrophication and N:P imbalances in rivers and estuaries on a global scale.

Trends and ecological consequences of phosphorus (P) decline and increasing nitrogen (N) to phosphorus (N:P) ratios in rivers and estuaries are reviewed and discussed. Results suggest that re-oligotrophication is a dominant trend in rivers and estuaries of high-income countries in the last two-three decades, while in low-income countries widespread eutrophication occurs. The decline in P is well documented in hundreds of rivers of United States and the European Union, but the biotic response of rivers and estuaries besides phytoplankton decline such as trends in phytoplankton composition, changes in primary production, ecosystem shifts, cascading effects, changes in ecosystem metabolism, etc., have not been sufficiently monitored and investigated, neither the effects of N:P imbalance. N:P imbalance has significant ecological effects that need to be further investigated. There is a growing number of cases in which phytoplankton biomass have been shown to decrease due to re-oligotrophication, but the potential regime shift from phytoplankton to macrophyte dominance described in shallow lakes has been documented only in a few rivers and estuaries yet. The main reasons why regime shifts are rarely described in rivers and estuaries are, from one hand the scarcity of data on macrophyte cover trends, and from the other hand physical factors such as peak flows or high turbidity that could prevent a general spread of submerged macrophytes as observed in shallow lakes. Moreover, re-oligotrophication effects on rivers may be different compared to lakes (e.g., lower dominance of macrophytes) or estuaries (e.g., limitation of primary production by N instead of P) or may be dependent on river/estuary type. We conclude that river and estuary re-oligotrophication effects are complex, diverse and still little known, and in some cases are equivalent to those described in shallow lakes, but the regime shift is more likely to occur in mid to high-order rivers and shallow estuaries.

Drivers of multi-decadal nitrate regime shifts in a large European catchment

Long-term monitoring shows evidence of persistent changes in the magnitude and timing of the seasonal pattern of nitrate concentrations in streams, with possibly grave effects on aquatic ecosystems. Seasonal patterns of stream nutrient concentrations are determined by a complex interplay of inputs, transport, and turnover. Over multi-decadal periods, each of these factors may change due to socio-economic factors such as consumption patterns, governance regimes, or technological control measures. Here we test the hypothesis that observed multi-decadal changes in stream nitrate seasonality could be explained by changes in the relative importance of catchment nutrient sources over time. We analyze 66 years of shifting nitrate seasonality in a large, central-European river (Elbe) during a period of significant socio-political changes (1954–2019), with correspondingly significant changes in the sources of anthropogenic nitrate emissions. We develop a mixed-source succession model to test how the multi-decadal evolution of the composition of nitrate sources (point and diffuse) influences in-stream seasonality. We show that the in-stream nitrate seasonality of the River Elbe changed significantly from a weak seasonal pattern with peak concentrations during summer in the 1950s to a strong seasonal pattern with peak concentrations during winter in the 1990s. We link these shifts to a succession of technical and political developments which influence the contribution of point and diffuse sources over time. Such shifts in seasonal concentration patterns can significantly impact the macronutrient (carbon, nitrogen, phosphorus) ratios in rivers, which in turn highly affect the health of aquatic ecosystems.

Changes in Escherichia coli to enteric protozoa ratios in rivers: Implications for risk-based assessment of drinking water treatment requirements

Minimum treatment requirements are set in response to established or anticipated levels of enteric pathogens in the source water of drinking water treatment plants (DWTPs). For surface water, contamination can be determined directly by monitoring reference pathogens or indirectly by measuring fecal indicators such as Escherichia coli (E. coli). In the latter case, a quantitative interpretation of E. coli for estimating reference pathogen concentrations could be used to define treatment requirements. This study presents the statistical analysis of paired E. coli and reference protozoa (Cryptosporidium, Giardia) data collected monthly for two years in source water from 27 DWTPs supplied by rivers in Canada. E. coli/Cryptosporidium and E. coli/Giardia ratios in source water were modeled as the ratio of two correlated lognormal variables. To evaluate the potential of E. coli for defining protozoa treatment requirements, risk-based critical mean protozoa concentrations in source water were determined with a reverse quantitative microbial risk assessment (QMRA) model. Model assumptions were selected to be consistent with the World Health Organization (WHO) Guidelines for drinking-water quality. The sensitivity of mean E. coli concentration trigger levels to identify these critical concentrations in source water was then evaluated. Results showed no proportionalities between the log of mean E. coli concentrations and the log of mean protozoa concentrations. E. coli/protozoa ratios at DWTPs supplied by small rivers in agricultural and forested areas were typically 1.0 to 2.0-log lower than at DWTPs supplied by large rivers in urban areas. The seasonal variations analysis revealed that these differences were related to low mean E. coli concentrations during winter in small rivers. To achieve the WHO target of 10−6 disability-adjusted life year (DALY) per person per year, a minimum reduction of 4.0-log of Cryptosporidium would be required for 20 DWTPs, and a minimum reduction of 4.0-log of Giardia would be needed for all DWTPs. A mean E. coli trigger level of 50 CFU 100 mL−1 would be a sensitive threshold to identify critical mean concentrations for Cryptosporidium but not for Giardia. Treatment requirements higher than 3.0-log would be needed at DWTPs with mean E. coli concentrations as low as 30 CFU 100 mL−1 for Cryptosporidium and 3 CFU 100 mL−1 for Giardia. Therefore, an E. coli trigger level would have limited value for defining health-based treatment requirements for protozoa at DWTPs supplied by small rivers in rural areas.

A Study on the Distribution Characteristics of NATURALIZED PLANTS: For Gyeongsangbuk-do Area

Purpose: Naturalized plants come in artificially or naturally, producing and spreading large numbers of seeds. Naturalized plants are not culled in the natural ecosystem and coexist in their own way. They have a wide range of tolerance and fast growth rates. Therefore, this study analyzed the differences of naturalized plants in the Gyeongsangbuk-do area by population density, forestland ratio, farmland ratio, land ratio, road ratio, and river ratio. We investigated the correlation with the naturalization index to understand the distribution characteristics of naturalized plants. Method: We conducted one-way ANOVA and multivariate analysis of variance(MANOVA) to determine the differences between the number of naturalized plant species and the items related to the naturalization index. Post-hoc analysis was performed through the Scheffe test method. We performed curvilinear regression analysis to find out the correlation. We used the Spss Statistics 21 statistics program to perform the above statistical analysis. Results: The naturalization index of naturalized plants showed differences in four items: population density, farmland ratio, land ratio, and river ratio, but there were no statistical differences in the two items, such as forestland ratio and road ratio. The farmland ratio did not affect the increase or decrease in the naturalization index. When the land ratio and road ratio increased, the naturalization index was also positively correlated. The river ratio had a positive correlation, but it had a negative correlation again after a certain point. Conclusion: The results of this study are expected to be used as data for selecting priority areas to manage naturalized plants. In addition, the data of this study can be used as basic data to study the naturalization index and environmental change of naturalized plants. However, we propose that comprehensive management for naturalized plants, such as analysis of the inflow path and distribution characteristics of naturalized plants between domestic regions, should be ca rried out continuously at the administrative level of local governments.

Organic sulfur fluxes and geomorphic control of sulfur isotope ratios in rivers

Pyrite oxidation plays a critical role in the relationship between weathering and climate, and its impact on the global carbon cycle has previously been constrained through inversion models utilizing observations of river sulfate (SO42−) and its 34S/32S isotope ratio (δ34SSO4). However, measurements from some rivers have suggested that δ34SSO4 can be substantially impacted by processes such as microbial sulfate reduction and/or sulfur assimilation and cycling, rather than simply reflecting a weighted mixture of lithologic sulfur sources. To study the prevalence and controls on SO42− transformations, we measured dissolved major element concentrations and δ34SSO4 in river water samples from throughout western Iceland. Our analyses focused on samples from a small catchment hosting the Efri Haukadalsá river, a system with relatively uniform and isotopically constrained basaltic bedrock. We also measured sediment δ34S and sulfur speciation using sulfur K-edge X-ray absorption spectroscopy on sediment and vegetation samples from this catchment. Values of dissolved δ34SSO4 in the Efri Haukadalsá ranged from 2.5‰ to 23.7‰ and had a linear relationship with Cl−/SO42− ratios, indicating that SO42− predominantly derived from basalt weathering and meteoric precipitation. The lower δ34SSO4 values were found in fluvial valleys with V-shaped cross sections, while higher values of δ34SSO4 occurred in U-shaped, glacially eroded valleys with thick alluvial fills blanketing the valley floor. Spectroscopic observations identified organic sulfur phases in suspended river sediment, floodplain deposits, and vegetation. Mass balance calculations quantified the organic sulfur flux as less than 10% of SO42− export, and sediment δ34S values were comparable to river δ34SSO4. We interpreted these isotopic and chemical patterns as reflecting differences in the availability of unweathered bedrock across the Efri Haukadalsá catchment, with V-shaped valleys having greater access to fresh sulfide-bearing minerals than alluviated U-shaped valleys; this interpretation is in contrast to one in which the elevated δ34SSO4 values reflect fractionation during sulfur transformations along alluvial reaches. These results validated the application of river inversion models for constraining weathering fluxes and affirmed that pyrite oxidation globally, even in the presence of river sulfur cycling, modulates the abundance of atmospheric carbon dioxide.

Patterns of nutrients distribution along salinity gradient in the estuaries of the rivers Sukhodol and Razdolnaya/Suifen (Peter the Great Bay, Japan Sea)

Sign of production-destruction balance is determined for different parts of two rivers estuaries in Peter the Great Bay (Razdolnaya/Suifen and Sukhodol) on the data of chemical samplings in different seasons of 2010-2013. For this purpose, the nutrients concentration dependence on ratio of river and sea waters mixing is determined. In the case of conservative behavior of nutrients, their concentration should have negative linear dependence on increasing salinity, but its non-linearity shows removal or addition of the nutrients by processes of biological production or destruction: negative deviations indicate production and positive deviations indicate destruction. The concentration of mineral nitrogen is close to zero everywhere in the estuaries in spring, so below the line of conservative behavior that is the evidence of active nitrogen consumption. Patterns of silicon and phosphorus distribution are similar to the distribution of nitrogen. Summer nutrients concentrations are mainly below the conservative values, as well, with exception of the areas at the river bar, so production prevails in the external estuaries of both rivers, but remineralization prevails in the lower parts of the internal estuaries in this season. In autumn, destruction of organic matter becomes more important in both estuaries, so heightened values of nutrients are observed in all their parts, in particular in the «silicon belt» of brackish water (≈ 13-17 eps). The silicon concentration increasing at the river bar (above the concentration in the river water) is the most significant in the Razdolnaya estuary which is generally poor by this nutrient. There is concluded that terrigenous nutrients of the river water are utilized for new production mostly in the external estuaries, whereas the process of organic matter mineralization dominates in the internal estuaries. Besides of the production-destruction processes, changes of the mineral silicon concentration are determined by transformation of its inorganic forms, as dissociation of low-soluble silicates to silicon acid under heightened pH. Principal difference of the production-destruction balance between the internal and external parts of estuary could be an ecological basis for formation of different ecosystems in these zones, with principally different structure and functioning patterns.

The Li isotope response to mountain uplift

Research Article| January 01, 2015 The Li isotope response to mountain uplift Philip A.E. Pogge von Strandmann; Philip A.E. Pogge von Strandmann 1Institute of Earth and Planetary Sciences, University College London and Birkbeck College, University of London, Gower Street, London WC1E 6BT, UK Search for other works by this author on: GSW Google Scholar Gideon M. Henderson Gideon M. Henderson 2Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Philip A.E. Pogge von Strandmann 1Institute of Earth and Planetary Sciences, University College London and Birkbeck College, University of London, Gower Street, London WC1E 6BT, UK Gideon M. Henderson 2Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK Publisher: Geological Society of America Received: 08 Aug 2014 Revision Received: 23 Oct 2014 Accepted: 29 Oct 2014 First Online: 09 Mar 2017 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2014 Geological Society of America Geology (2015) 43 (1): 67–70. https://doi.org/10.1130/G36162.1 Article history Received: 08 Aug 2014 Revision Received: 23 Oct 2014 Accepted: 29 Oct 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Philip A.E. Pogge von Strandmann, Gideon M. Henderson; The Li isotope response to mountain uplift. Geology 2015;; 43 (1): 67–70. doi: https://doi.org/10.1130/G36162.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Silicate weathering is a key process by which CO2 is removed from the atmosphere. It has been proposed that mountain uplift caused an increase in silicate weathering, and led to the long-term Cenozoic cooling trend, although this hypothesis remains controversial. Lithium isotopes are tracers of silicate weathering processes, and may allow this hypothesis to be tested. Recent studies have demonstrated that the Li isotope ratio in seawater increased during the period of Himalayan uplift (starting ca. 45 Ma), but the relationship between uplift and the Li isotope ratio of river waters has not been tested. Here we examine Li isotope ratios in rivers draining catchments with variable uplift rates from South Island, New Zealand. A negative trend between δ7Li and uplift shows that areas of rapid uplift have low δ7Li, whereas flatter floodplain areas have high δ7Li. Combined with U activity ratios, the data suggest that primary silicates are transported to floodplains, where δ7Li and (234U/238U) are driven to high values due to preferential uptake of 6Li by secondary minerals and long fluid-mineral contact times that enrich waters in 234U. In contrast, in mountainous areas, fresh primary mineral surfaces are continuously provided, driving δ7Li and (234U/238U) low. This trend is opposite to that expected if the increase in Cenozoic δ7Li in the oceans is driven directly by mountain uplift. These data suggest that the increase in seawater δ7Li reflects the formation of floodplains and the increased formation of secondary minerals, rather than weathering of mountain belts. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Inorganic Nitrogen Deposition and Its Impacts on N:P-Ratios and Lake Productivity

The pronounced increase in the cycling and deposition of biologically reactive dissolved inorganic nitrogen (DIN) over large areas globally not only cause increased concentrations of DIN in surface waters, but it will also affect nutrient ratios in rivers, lakes and coastal areas. This review addresses the flux and fate of DIN, focusing NO3 in lakes of boreal and alpine catchments. Not only DIN-deposition, but also catchment properties strongly affect the concentrations of NO3 in lakes, as well as NO3:total P (TP) ratios. This ratio displays an extreme variability, and does also serve as an indicator of shift between N and P-limitation of aquatic autotrophs. A high share of forests and bogs in the catchment generally decreases NO3:total P ratios, while alpine and subalpine catchments with sparse vegetation cover may have high NO3:total P ratios, especially in regions with high DIN-deposition. Several empirical and experimental studies indicate a shift from an initial N to P-limitation, but for N-limited lakes, an increased growth of phytoplankton, periphytes and macrophytes may be accredited to elevated inputs of DIN. An intensified P-limitation may also be a consequence of elevated DIN-deposition. This P-limitation may again yield higher C:P-ratios in autotrophs with negative impacts on grazers and higher trophic levels.

Global distribution and long‐term fate of anthropogenic 129I in marine and surface water reservoirs

Since the advent of the nuclear age in the mid‐1940s, the mass of radioactive 129I (t1/2 = 15.7 Myr) circulating in the Earth's hydrosphere has increased nearly fortyfold from its natural background level of 140 kg. Nuclear fuel reprocessing has been by far the major contributor, responsible for releasing 5400 kg of 129I, primarily into the North Atlantic Ocean. Regional and global trends in the distribution of the 129I inventory are elucidated from an examination of more than 600 determinations of 129I in environmental samples from around the world. Because the major point sources are located in Europe and the United States, more than 99% of the present 129I reservoir is distributed in the Northern Hemisphere, where both 129I concentrations and 129I/I ratios in rivers, lakes, and shallow seawater are several orders of magnitude above the preanthropogenic background. Downwelling in the North Atlantic presently provides a major sink for marine 129I; however, marine upwelling along the margins of the Pacific will eventually return part of this anthropogenic input to the ocean surface, where it will find its way back into surface waters and the atmosphere. Iodine‐129 has a long half‐life (15.7 Myr), and consequently, there is also the possibility that climate change will influence the dynamics of iodine transfer in surface reservoirs. We model the effect of a collapse in thermohaline circulation and project a concentration increase of more than 3 orders of magnitude in shallow oceans over the 10,000 years that follow if nuclear reprocessing is to continue at the present rate.

Nitrogen sources in the South China Sea, as discerned from stable nitrogen isotopic ratios in rivers, sinking particles, and sediments

Stable nitrogen isotopic ratios were measured in sinking particles and surface sediments from the South China Sea (SCS) in order to study recent nitrogen sources and degradation. Average δ 15N values of 16 sediment traps deployed at seven locations in the northern, central and southern SCS were uniformly low, ranging between 2.7 and 4.5‰ with a winter minimum in the northern and central SCS. Enhanced nitrogen contents and δ 15N values were noted in samples affected by swimmers, comprising between 5 and 20% of total nitrogen fluxes. Nitrate sources were subsurface waters from the western Pacific, which were isotopically depleted due to the remineralization of nitrogen from nitrogen fixation in surface waters. Nitrogen fixation in the SCS contributed up to 20% to the settling particles. In the southern SCS, resuspended matter close to the shelf added to the sinking particulates. The long-term trap record from the central SCS revealed decreasing δ 15N values during the 1990s, which correspond with findings from the North Pacific Subtropical Gyre and may be attributable to increased nitrogen fixation due to global warming-related stratification. This trend may be restricted to the 1990s but could also persist due to the projection of more frequent occurrence of El Niño conditions. The δ 15N increase from swimmer-free trap averages of 2.7–3.6‰ to values of 5–6‰ in underlying deep-sea sediments was in the same range as in other deep ocean areas. Similar to results from the northern Indian Ocean, this increase could be related to isotopic enrichment during amino acid degradation. The lowest sedimentary δ 15N values characterize the Pinatubo ash layer deposited off Luzon in an event of mass sedimentation in 1991. The fast deposition of organic matter drawn from the surface waters with the ash in the form of vertical density currents evidently preserved the planktonic δ 15N signal.

The isotopic signature of the global riverine molybdenum flux and anoxia in the ancient oceans

Despite important biological and biogeochemical consequences of extensive ocean anoxic events, their identification is controversial. The marine isotope geochemistry of molybdenum can help quantify the past oxygenation state of the ocean if the riverine input of Mo isotopes is known. Analysis of a set of rivers that account for 28% of global river runoff suggests more variable Mo isotopic ratios in rivers that are also isotopically enriched in the heavy isotopes, suggesting near-total anoxia in the Proterozoic ocean and during Mesozoic ocean anoxic events. Despite its important biological and biogeochemical consequences1,2,3, the identification of extensive oceanic anoxia in the geological record is controversial. In particular, global anoxia is difficult to distinguish from spatially restricted anoxia in the deep ocean, or in tectonically isolated basins such as the modern Black Sea. The marine isotope geochemistry of molybdenum (Mo) can help quantify the past oxygenation state of the ocean4,5,6,7,8,9,10,11,12,13,14, because to first approximation under oxic conditions lighter isotopes of Mo are preferentially removed to sediments, whereas in euxinic conditions quantitative removal leads to no fractionation. However, the isotopic composition of the Mo input from rivers, the main contributor of Mo to the oceans, is poorly constrained and had been assumed to be isotopically comparable to the narrow range found in a small dataset of continental rocks4,5,6,7,8,9,10,11,12,13,14. Here we present an isotopic analysis of Mo in a set of rivers that together account for 22% of the global riverine water discharge. We find a broad range of variability in the Mo isotopic composition of these rivers, with almost all samples enriched in the heavy isotopes compared with continental rocks. Our data remove key uncertainties associated with the marine Mo isotope budget14 and strongly suggest near-total anoxia in the mid-Proterozoic ocean8 as well as during Mesozoic ocean anoxic events13.

Calcium and magnesium isotope systematics in rivers draining the Himalaya-Tibetan-Plateau region: Lithological or fractionation control?

In rivers draining the Himalaya-Tibetan-Plateau region, the 26Mg/ 24Mg ratio has a range of 2‰ and the 44Ca/ 42Ca ratio has a range of 0.6‰. The average δ 26 Mg values of tributaries from each of the main lithotectonic units (Tethyan Sedimentary Series (TSS), High Himalayan Crystalline Series (HHCS) and Lesser Himalayan Series (LHS)) are within 2 standard deviation analytical uncertainty (0.14‰). The consistency of average riverine δ 26 Mg values is in contrast to the main rock types (limestone, dolostone and silicate) which range in their average δ 26 Mg values by more than 2‰. Tributaries draining the dolostones of the LHS differ in their δ 44 42 Ca values compared to tributaries from the TSS and HHCS. The chemistry of these river waters is strongly influenced by dolostone (solute Mg/Ca close to unity) and both δ 26 Mg (−1.31‰) and δ 44 42 Ca (0.64‰) values are within analytical uncertainty of the LHS dolostone. These are the most elevated δ 44 42 Ca values in rivers and rock reported so far demonstrating that both riverine and bedrock δ 44 42 Ca values may show greater variability than previously thought. Although rivers draining TSS limestone have the lowest δ 26 Mg and δ 44 42 Ca values at −1.41 and 0.42‰, respectively, both are offset to higher values compared to bedrock TSS limestone. The average δ 26 Mg value of rivers draining mainly silicate rock of the HHCS is −1.25‰, lower by 0.63‰ than the average silicate rock. These differences are consistent with a fractionation of δ 26 Mg values during silicate weathering. Given that the proportion of Mg exported from the Himalaya as solute Mg is small, the difference in 26Mg/ 24Mg ratios between silicate rock and solute Mg reflects the 26Mg/ 24Mg isotopic fractionation factor ( α silicate – dissolved Mg ) between silicate and dissolved Mg during incongruent silicate weathering. The value of α silicate – dissolved Mg of 0.99937 implies that in the TSS, solute Mg is primarily derived from silicate weathering, whereas the source of Ca is overwhelmingly derived from carbonate weathering. The average δ 44 42 Ca value in HHCS rivers is within uncertainty of silicate rock at 0.39‰. The widespread hot springs of the High Himalaya have an average δ 26 Mg value of −0.46‰ and an average δ 44 42 Ca value of 0.5‰, distinct from riverine values for δ 26 Mg but similar to riverine δ 44 42 Ca values. Although rivers draining each major rock type have δ 44 42 Ca and δ 26 Mg values in part inherited from bedrock, there is no correlation with proxies for carbonate or silicate lithology such as Na/Ca ratios, suggesting that Ca and Mg are in part recycled. However, in spite of the vast contrast in vegetation density between the arid Tibetan Plateau and the tropical Lesser Himalaya, the isotopic fractionation factor for Ca and Mg between solute and rocks are not systematically different suggesting that vegetation may only recycle a small amount of Ca and Mg in these catchments. The discrepancy between solute and solid Ca and Mg isotope ratios in these rivers from diverse weathering environments highlight our lack of understanding concerning the origin and subsequent path of Ca and Mg, bound as minerals in rock, and released as cations in rivers. The fractionation of Ca and Mg isotope ratios may prove useful for tracing mechanisms of chemical alteration. Ca isotope ratios of solute riverine Ca show a greater variability than previously acknowledged. The variability of Ca isotope ratios in modern rivers will need to be better quantified and accounted for in future models of global Ca cycling, if past variations in oceanic Ca isotope ratios are to be of use in constraining the past carbon cycle.

The magnesium isotope budget of the modern ocean: Constraints from riverine magnesium isotope ratios

We have measured the magnesium (Mg) isotope ratios in 45 rivers including 16 of the largest rivers in the world, covering a range of geologic, tectonic and climatic drainage basin environments. The range in riverine δ 26Mg is 2.5‰, half the variation in terrestrial rock. Although the Mg isotopic composition of the source rock is important for small rivers, at a global scale lithology is of limited significance for Mg isotope ratios in rivers. Part of the variability at a global scale may be attributed to fractionation in the weathering environment. The rivers analysed constitute 30% of the global Mg riverine flux to the oceans and a flux weighted Mg isotope composition of global runoff has been estimated at − 1.09‰. Even taking into account uncertainty, this is distinct from seawater at − 0.82‰. This difference arises either from the fractionation of Mg isotope ratios in the ocean and/or a Mg budget which is not at steady state. The difference is consistent with fractionation by carbonate precipitation. In the simplest steady state scenario, where the oceanic mass balance is maintained by riverine input and hydrothermal and dolomite output, Mg isotope ratios imply a minimum dolomite Mg flux of 9% of the total output Mg flux. The implications of the distinct isotopic composition of the riverine flux from modern seawater, is that the Mg isotope ratio of seawater has probably varied outside of current analytical uncertainty. Mg isotope ratios may therefore provide a valuable new tracer in palaeo-oceanography.

Carbon isotope ratio of dissolved inorganic carbon (DIC) in rivers draining the Deccan Traps, India: Sources of DIC and their magnitudes

Chemical weathering of silicate and carbonate rocks by carbonic acid is important in the context of atmospheric CO 2 sequestration and its delivery to the oceans via rivers as dissolved inorganic carbon (DIC). In this work, silicate weathering contribution to DIC in the Krishna river system, draining the Deccan basalts in south-western India has been determined using a new approach based on δ 13C–Si / HCO 3 systematics of the waters. δ 13C in the samples ranges from − 8.5‰ to − 20.7‰ and shows a strong linear inverse trend with Si / HCO 3 ( r 2 = 0.80). The Si / HCO 3 ratios in rivers vary from ∼0.55 to ∼0.10, the higher value matches the expected ratio for chemical weathering of Deccan basalts based on their composition. The δ 13C–Si / HCO 3 trend suggests the mixing of two end members, a carbonate derived endmember with low Si / HCO 3 and enriched in 13C and a silicate derived endmember with higher Si / HCO 3 and depleted in 13C. Small rivers of the western ghats are depleted in 13C ( δ 13C: − 18.7 ± 2‰) and have high Si / HCO 3. These properties are interpreted as signatures of basalt (silicate) weathering with CO 2 from C 3 vegetation. Three samples from the Krishna mainstream and two of its larger tributaries, the Bhima and the Ghod, are enriched in 13C ( δ 13C: − 8.5‰ to − 15‰) with higher HCO 3 and lower Si / HCO 3 ratios, indicating that a significant fraction of DIC in these samples is derived from carbonate weathering. Carbonate dispersed in river sediments can be a likely source of DIC.

Mobility of U-series nuclides during basalt weathering: An example from the Deccan Traps (India)

U-series disequilibria have been measured by TIMS and MC-ICP-MS in dissolved phases and suspended sediments of the main basaltic rivers from the Deccan Trap region (India). For dissolved phases ( 234U/ 238U), ( 230Th/ 238U), and ( 226Ra/ 238U) range between 1.11 and 1.28, between 0.03×10 −2 and 1.35×10 −2, and between 0.02 and 0.16 respectively. For suspended particles, ( 234U/ 238U), ( 230Th/ 238U), and ( 226Ra/ 238U) range from 0.95 to 1.1, from 1.23 to 1.59 and from 0.1 to 0.47 respectively. 230Th– 238U and 234U– 238U systems show direct links with indices of weathering. The data also indicate possible redistributions between the solid and aqueous phases and indicate that erosion processes do not presently operate at steady-state because present-day physical erosion rates are higher than expected (100–400 mm/ka versus 40 mm/ka for a steady-state process) for the Narmada and the Tapti basins. Models assuming either discrete or continuous particle leaching yield timescales for chemical erosion of 40–90 ka. In contrast with basins located in Northern latitudes, erosion in the Deccan basins does not appear to have been significantly disturbed during the last glaciation. 234U– 238U disequilibria in the dissolved phase of large rivers world-wide are also explored and two key factors are highlighted in this study: the dissolution of carbonates, releasing dissolved U at secular equilibrium and, the silicate weathering rate estimated at the scale of the watershed. In particular, a positive correlation is found between dissolved ( 234U/ 238U) and basaltic weathering rates for the Deccan that could reflect a key role of physical erosion since, for silicate monolithological basins, chemical erosion rates are primarily controlled by physical erosion rates. For basins with mixed lithology, dissolved organic matter also plays a key role in the preferential release of 234U from silicate minerals. ( 234U/ 238U) ratios in rivers therefore reflect a mixture between U released by carbonate dissolution and 234U preferentially released from silicate minerals located in high physical erosion and peat areas.

87Sr/86Sr and Sr/Ca Investigation of Jurassic mollusks from Scotland: Implications for paleosalinities and the Sr/Ca ratio of seawater

Measurements of 87 Sr/ 86 Sr, Sr/Ca, ! 18 O, and ! 13 C were performed on molluskan fos- sils from the Great Estuarine Group, Scot- land, to deduce paleosalinity patterns and hydrodynamic environments in this classic Jurassic marginal marine succession. Fos- sils are found as winnowed accumulations in shell beds that bear low-diversity assem- blages, consistent with deposition in a la- goonal paleoenvironment. Previous studies have concluded that estuarine mixing of seawater and freshwater took place based on correlated trends in ! 18 O and ! 13 C, most notably in the mussel Praemytilus strathair- densis from the Kildonnan Member. This interpretation is shown to be incorrect based on 87 Sr/ O, ! 13 C, and the Sr/Ca ratio. This is consistent with a lagoonal hydrology controlled more by seasonal changes in evaporation, precipitation, and flooding than by direct inputs of seawater or river- ine water transported over large distances. Lagoons containing the oyster Praeexogyra in the Duntulm Formation have similar in- ferred O and C isotope characteristics, but based on their 87 Sr/ 86 Sr ratios, they have a higher proportion of seawater than the Praemytilus lagoons of the Kildonnan Member. The data thus far suggest that many of the lagoonal environments in the Great Es- tuarine Group were nearly hydrologically closed, at least over the interval represent- ing the molluskan invasion of these habi- tats, rendering O and C isotopes unreliable proxies of paleosalinity. Because strontium isotopes are not directly affected by evap- oration of the lagoons, knowledge of marine and freshwater 87 Sr/ 86 Sr ratios, along with estimates of Sr concentration, are used to calculate the proportion of seawater in the brackish water mixture. Unio is nearly al- ways the most radiogenic taxon compared to other genera in closely adjacent sedimen- tary strata, confirming its freshwater affin- ity. The purported marine-brackish bivalve Tancredia yields a low paleosalinity of 4‰, only slightly more saline than Unio and Praemytilus at 1-3‰. The oyster Praeexo- gyra hebridica yields paleosalinities between 2‰ and 26‰, attesting to the wide salinity tolerance of this species. Thus far, only Neomiodon from the Dun- tulm and Staffin Bay formations show co- variant relationships between 87 Sr/ 86 Sr and Ca/Sr ratio that can be reconciled with a dynamic response to seawater-freshwater mixing. The slopes of these trends converge at 0.7068, which is close to the early Cal- lovian seawater composition of 0.7069. Us- ing values of DSr from modern aragonite se- creting mollusks, the Sr/Ca ratio of middle Jurassic seawater is constrained from the best correlated mixing line to be 5.4 0.6 millimoles/mole. In contrast, Sr/Ca ratios in the Great Estuarine Group lagoons and continental watershed range from 5.4 to 19.0. These are much higher than the av- erage value of 2.3 for modern rivers but typical of Sr/Ca ratios in rivers draining arid climate regions. The semirestricted hy- drodynamic regime inferred from the high seawater-like ! 18 O values and high but uni- form Sr/Ca ratios in many of the Great Es- tuarine Group mollusks is consistent with a seasonally dry, Mediterranean-style climate for the Middle Jurassic of Britain.

The significance of Himalayan rivers for silicate weathering rates: evidence from the Bhote Kosi tributary

The significance of weathering by Himalayan runoff for both the Sr-isotope marine record and the removal of atmospheric CO 2 through silicate dissolution has been examined by systematic sampling of dissolved loads and bedloads from the Bhote Kosi, a tributary of the Ganges that rises in Tibet from Tethyan sediment bedrock and traverses the major Himalayan lithologies of eastern Nepal before debouching onto the Gangetic plains. Throughout the section, the cation geochemistry of water samples is dominated by Ca and Mg ions, suggesting that carbonates are the predominant lithology undergoing dissolution particularly within the Lesser Himalayas. As the river transects the metasedimentary and granitic lithologies of the High Himalayas the Sr-isotope ratio of the bedload rises rapidly, closely reflecting the isotope geochemistry of the bedrock. In contrast the 87Sr/ 86Sr ratio of the dissolved load remains roughly constant (0.719–0.723). Downstream of the Main Central Thrust, where the river transects the carbonate-bearing lithologies of the Lesser Himalayas, the 87Sr/ 86Sr ratio of the dissolved load rises sharply (>0.768). The relative contributions of silicate and carbonate weathering from each of the main Himalayan units has been estimated from major cation, Sr concentration and Sr isotope mass-balance equations. These calculations suggest that the high Sr and high 87Sr/ 86Sr characteristics of riverine analyses arise initially from a component dissolved from the Tibetan Sedimentary Series which is substantially enhanced by input weathering fluxes, particularly as the river traverses the Lesser Himalayas. Whilst mechanical erosion is maximised within the High Himalayan Crystalline Series, as confirmed by 143Nd/ 144Nd ratios from the bedload, at least 63% of the dissolved load is acquired by chemical weathering of bedrock lithologies and/or of transported particulates within the Lesser Himalayas, enhanced by higher ambient temperatures and slower discharge rates. This may involve continued dissolution of the High Himalayan Crystalline Series particulates in addition to Lesser Himalayan lithologies. Although Himalayan rivers collectively have a major influence on the Sr-isotope marine record, the high 87Sr/ 86Sr ratios of their dissolved load results from the mixing of a small component (<10%) of silicate-derived material with an unusually high 87Sr/ 86Sr ratio (0.75–1.0) and a large component (>90%) of carbonate-derived material some of which is characterised by a high 87Sr/ 86Sr ratio (up to 0.8). Elevated 87Sr/ 86Sr ratios in rivers are therefore not necessarily indicative of anomalously high dissolution rates of silicates.

The Origin of Strontium and the Strontium Isotope Budget of the River Nile

The river Nile has been used as an example to study the evolution of the Sr content and the Sr isotope ratios in rivers in dependence on rock types of the catchment areas, the mixing of the waters from tributaries, the levelling out in a reservoir and evaporation losses. The Blue Nile, Sobat and Atbara rivers drain the Ethiopian Highlands, which consist partially of oceanic crustal rocks. Hence, the Sr content of these rivers is comparatively high and the Sr isotope ratio exhibits an old oceanic crustal signature. The tributaries of the White Nile, in contrast, drain mainly supracrustal metamorphic rocks with higher Sr isotope ratios. The seasonal change of the Sr contents in the Blue Nile and Atbari waters can be modelled using a factor deduced from published – experimental data for Sr contents in river waters at high and low stage conditions. It is demonstrated that the Aswan High Dam reservoir levels out seasonal fluctuations of the Sr contents and Sr isotope ratios. The rate of evaporation loss along the river was determined to be 0.038% per km. Contrary to the situation in highly industrialized urban areas, there is no measurable effect on the Sr isotope ratio from waste waters of Cairo.