Quantifying weathering on variable rocks,an extension of geochemical mass balance: Critical zone and landscape evolution

We present a statistical model of soil and rock weathering in deep profiles to expand the capacity to assess weathering to heterogeneous bedrock types, which are common at the Earth's surface. We developed the Weathering Trends (WT) model by extending the fractional mass change calculation (tau) of the geochemical mass balance model in two important ways. First, WT log transforms the elemental ratio data, to discern the log‐linear patterns that naturally develop from thermodynamic and kinetic laws of chemistry. Second, WT statistically fits log‐transformed element concentration ratio data – log(c_j/c_i), the only depth‐varying term in tau – as a function of depth to determine characteristic depths of transitions in weathering processes, along with confidence intervals. With no prior assumptions, WT estimates average parent material composition, average composition of the upper weathered zone and mean fractional mass change of each element over the entire weathering profile. WT displays the mean shape of weathering profiles of log‐transformed geochemical data bounded by calculated confidence intervals. We share the WT model code as an open‐source R package ( https://github.com/fisherba/WeatheringTrends ). The WT model was designed to interpret two 21 m cores from the Laurels Schist bedrock in the Christina River Basin Critical Zone Observatory in the Pennsylvania Piedmont, where our morphological and elemental data provided inconclusive estimates of bedrock depth. The WT model differentiated between rock variability and weathering to delineate the maximum extent of weathering at 12.3 m (CI 95% [9.2, 21.3]) in Ridge Well 1 and 7.2 m (CI 95% [4.3, 13.0]) in Interfluve Well 2. The water table was 5–8 m below fresh rock at Ridge Well 1, but at the same depth as fresh rock at the lower elevation interfluve. We assess statistical approaches to identify the best immobile element for use in WT and tau calculations. Copyright © 2017 John Wiley & Sons, Ltd.     

A low‐dimensional model of bedrock weathering and lateral flow coevolution in hillslopes: 2. Controls on weathering and permeability profiles,drainage hydraulics,and solute export pathways

The advance of a chemical weathering front into the bedrock of a hillslope is often limited by the rate weathering products that can be carried away, maintaining chemical disequilibrium. If the weathering front is within the saturated zone, groundwater flow downslope may affect the rate of transport and weathering—however, weathering also modifies the rock permeability and the subsurface potential gradient that drives lateral groundwater flow. This feedback may help explain why there tends to be neither “runaway weathering” to great depth nor exposed bedrock covering much of the earth and may provide a mechanism for weathering front advance to keep pace with incision of adjacent streams into bedrock. This is the second of a two‐part paper exploring the coevolution of bedrock weathering and lateral flow in hillslopes using a simple low‐dimensional model based on hydraulic groundwater theory. Here, we show how a simplified kinetic model of 1‐D rock weathering can be extended to consider lateral flow in a 2‐D hillslope. Exact and approximate analytical solutions for the location and thickness of weathering within the hillslope are obtained for a number of cases. A location for the weathering front can be found such that lateral flow is able to export weathering products at the rate required to keep pace with stream incision at steady state. Three pathways of solute export are identified: “diffusing up,” where solutes diffuse up and away from the weathering front into the laterally flowing aquifer; “draining down,” where solutes are advected primarily downward into the unweathered bedrock; and “draining along,” where solutes travel laterally within the weathering zone. For each pathway, a different subsurface topography and overall relief of unweathered bedrock within the hillslope is needed to remove solutes at steady state. The relief each pathway requires depends on the rate of stream incision raised to a different power, such that at a given incision rate, one pathway requires minimal relief and, therefore, likely determines the steady‐state hillslope profile.     

Influence of mineral weathering reactions on the chemical composition of soil water,Springs, and ground water,Catoctin Mountains,Maryland

During 1983 and 1984, wet precipitation was primarily a solution of dilute sulphuric acid, whereas calcium and bicarbonate were the major ions in springs and ground water in two small watersheds with a deciduous forest cover in central Maryland. Dominant ions in soil water were calcium, magnesium, and sulphate. The relative importance of mineral weathering reactions on the chemical composition of these subsurface waters was compared to the contribution from wet precipitation, biological processes, and road deicing salts. Mineral reaction models, developed from geochemical mass-balance relationships, involved reactions of primary and secondary minerals in metabasalt and metarhyolite with hydrogen ion. Geochemical weathering reactions account for the majority of total ion equivalents in soil water (46 per cent), springs (51 per cent), and ground water (68 to 77 per cent). The net contribution of total ion equivalents from biological processes was 20 and 16 per cent for soil water and springs, respectively, but less than 10 per cent for ground water. The contribution of total ion equivalents from deicing salts (10 to 20 per cent) was related to proximity to roads. Strong acids in precipitation contributed 44 per cent of the total amount of hydrogen ions involved in mineral-weathering reactions for ground water in contact with metarhyolite compared to 25 per cent for ground water in contact with metabasalt, a less resistant rock type to weathering.     

Surface water and groundwater interactions in an extensively mined watershed,upper Schuylkill River,Pennsylvania, USA

Streams crossing underground coal mines may lose flow, whereas abandoned mine drainage (AMD) restores flow downstream. During 2005–2012, discharge from the Pine Knot Mine Tunnel, the largest AMD source in the upper Schuylkill River Basin, had near‐neutral pH and elevated concentrations of iron, manganese and sulphate. Discharge from the tunnel responded rapidly to recharge but exhibited a prolonged recession compared with nearby streams, consistent with rapid infiltration of surface water and slow release of groundwater from the mine complex. Dissolved iron was attenuated downstream by oxidation and precipitation, whereas dissolved CO₂ degassed and pH increased. During high flow conditions, the AMD and downstream waters exhibited decreased pH, iron and sulphate with increased acidity that were modelled by mixing net‐alkaline AMD with recharge or run‐off having low ionic strength and low pH. Attenuation of dissolved iron within the river was least effective during high flow conditions because of decreased transport time coupled with inhibitory effects of low pH on oxidation kinetics. A numerical model of groundwater flow was calibrated by using groundwater levels in the Pine Knot Mine and discharge data for the Pine Knot Mine Tunnel and West Branch Schuylkill River during a snowmelt event in January 2012. Although the calibrated model indicated substantial recharge to the mine complex took place away from streams, simulation of rapid changes in mine pool level and tunnel discharge during a high flow event in May 2012 required a source of direct recharge to the Pine Knot Mine. Such recharge produced small changes in mine pool level and rapid changes in tunnel flow rate because of extensive unsaturated storage capacity and high transmissivity within the mine complex. Thus, elimination of stream leakage could have a small effect on the annual discharge from the tunnel, but a large effect on peak discharge and associated water quality downstream. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

Evidence for influence of mineral weathering on stream water sulphate in Vermont and New Hampshire (USA)

Mass balance studies in forested catchments in the northeastern USA show that S losses via streamwater SO₄^2? exceed measured atmospheric S inputs. Possible sources of the excess S loss include underestimated dry deposition, mineralization of organic S in soils, desorption of soil sulphate, oxidation of recently formed sulphides and mineral weathering. Evaluating the relative contribution of these sources and processes to SO₄^2? export is important to our understanding of S cycling as well as to policy makers in their evaluation of the efficacy of S emission controls. In order to evaluate the potential for mineral weathering contributions to SO₄^2? export, we measured concentration and isotopic composition (δ³⁴S and δ¹⁸O) of SO₄^2? in stream water, and concentration and δ³⁴S values of four S fractions in bedrock and soil parent material in catchments of varying geological composition. Geological substrates with low S concentrations were represented by catchments underlain by quartzite and granite, whereas geological substrates with high S concentrations were represented by catchments underlain by sulphidic slate, schist and metavolcanic rocks. Catchments with S‐poor bedrock had stream‐water SO₄^2? concentrations <100 µeq L^?1 and isotopic values consistent with those of atmospheric SO₄^2? that had been cycled through the organic soil pool. Catchments with S‐rich bedrock had stream‐water SO₄^2? concentrations ranging from 56 to 229 µeq L^?1. Isotopic values deviated from those of SO₄^2? in atmospheric deposition, clearly indicating a mineral weathering source in some cases, whereas in others spatial variability of mineral δ³⁴S values precluded the isotopic detection of a weathering contribution. These results, along with evidence suggesting formation of secondary sulphate minerals in bedrock weathering rinds, indicate that mineral weathering may be an important source of S in the surface waters of some forested catchments in the northeastern USA. Copyright © 2004 John Wiley & Sons, Ltd.     

Geochemical studies on the intensity of chemical weathering in Luochuan loess-paleosol sequence, China

Major and trace element analyses of the Luochuan loess-paleosol sequence in China were performed in order to understand the cheniical weathering processes occurring on the I.oess Plateau during the last 600 ka. Results reveal that most elements in the loess remain immobile durlng chemical weathering. The typical stable elements are Al, K, Ti, Rb and REE, while the main mobile elements are Ca, Sr, P, Mg and Na. 120ess and paleosol experience the incipient stage of chemical weathering characterized by acid leaching and carbonate dissolution. Alteration of silicates in the sequence seems to be limited. Features of less chemical weathering of the loess and paleosol could he indicators for the dry-cold clinlate dominated on the Loess Plateau during the Quaternary. Project supported hy thc Nzitional Natural Science Foundation of China     

Unsaturated zone fracture flow contributions to stream flow: evidence for the process in South Africa and its importance

Understanding hydrological processes has always been important to the development and successful application of conceptual hydrological models. It can also contribute to informed water resources management, particularly in the context of understanding the potential impacts of both land use and climate change. Improved conceptual and quantitative understanding of near‐surface hydrological processes emerged through field studies during the 1960s to1980s; however, there remains a degree of ambiguity about the processes that link surface water and groundwater. This is especially the case in South Africa where a great deal of confusion has arisen about the source of the ‘baseflow’ signal in stream flow observations. This paper suggests that fracture flow within the unsaturated zone could have a lateral component and therefore re‐emerge and contribute to stream flow in catchments with relatively steep topography. The implication is that ‘baseflows’ could be made up of groundwater contributions (caused by intersection of the water table with stream channels) as well as an unsaturated zone flow component. Evidence for the existence of the process is presented on the basis of small‐scale observations and interpretations of stream flow observations. The potential importance of the process relates to interpreting different methods of recharge estimation, assessing the impacts of groundwater abstraction on stream flow, as well as the application and interpretation of the results of hydrological models. The conclusions are that the process does exist, but that there is less than conclusive evidence for its importance. There is therefore a need for further studies that can quantify the scale of the process and therefore its importance. Only then will it be possible to develop a consistent understanding of the processes of surface water and groundwater interaction and therefore manage water resources in a truly integrated manner. Copyright © 2009 John Wiley & Sons, Ltd.     

Plagioclase weathering in the groundwater system of a sandy,silicate aquifer

The weathering rate of plagioclase was estimated in the groundwater system of a sandy, silicate aquifer formed after the Wisconsin Glacial Stage. The study area is an isthmus lying between Crystal and Big Muskellunge Lakes in northern Wisconsin, USA. Plagioclase occupies 3% of the quartz and K‐feldspar dominated sediments. Groundwater in the study area is recharged in part by precipitation through the isthmus soils and in part by seepage from Crystal Lake, which is of low ionic strength and chemically in steady state. Water analysis revealed that the chemistry of groundwater recharged from Crystal Lake is regulated by mineral dissolution reactions. The rate constant for plagioclase was estimated using mass balances for sodium concentrations along a groundwater flowline from Crystal Lake. For this calculation, various kinds of hydrological/mineralogical information were used: groundwater flow path from oxygen isotope analysis, groundwater travel times from flow modelling, mineral composition from microprobe analysis and surface area of minerals from BET (Brunauer–Emmett–Teller) analysis. The overall range of the estimation was less than an order of magnitude (3·5 × 10^?16 to 3·4 × 10^?15 mol/m²/s). The result is up to three orders of magnitude slower than the previous field estimates, which applied geometric methods in measuring mineral surface areas. However, this result is somewhat higher than the estimates reported by other BET area‐based studies, which were undertaken on soil profiles having different hydrological conditions. This rate difference is interpreted as a result of higher mineral reactivity owing to younger sediment age. The rate difference is smaller when this result is compared with the estimates from the soils of similar age, indicating that the differences in hydrological condition are not sufficient to explain the weathering rate discrepancy between the laboratory and field studies, which is up to five orders of magnitude. Copyright © 2002 John Wiley & Sons, Ltd.     

Temporal variation of chemical and mechanical weathering in NE Iceland: Evaluation of a steady-state model of erosion

This study critically assesses the temporal sensitivity of the steady-state model of erosion that has been applied to chemical and mechanical weathering studies of volcanic islands and the continents, using only one sample from each catchment. The model assumes a geochemical mass balance between the initially unweathered rock of a drainage basin and the dissolved and solid loads of the river.Chemical composition of 178 samples of suspended and dissolved inorganic river constituents, collected in 1998–2002, were studied from five basaltic river catchments in NE Iceland. The Hydrological Service in Iceland has monitored the discharge and the total suspended inorganic matter concentration (SIM) of the glacial rivers for ~ four decades, making it possible to compare modelled and measured SIM fluxes.Concentration of SIM and grain size increased with discharge. As proportion of clay size particles in the SIM samples increased, concentrations of insoluble elements increased and of soluble decreased. The highest proportion of altered basaltic glass was in the clay size particles.The concentration ratio of insoluble elements in the SIM was used along with data on chemical composition of unweathered rocks (high-Mg basalts, tholeiites, rhyolites) to calculate the pristine composition of the original catchment rocks. The calculated rhyolite proportions compare nicely with area-weighted average proportions, from geological maps of these catchments.The calculated composition of the unweathered bedrock was used in the steady-state model, together with the chemical composition of the suspended and dissolved constituents of the river. Seasonal changes in dissolved constituent concentrations resulted in too low modelled concentrations of SIM_mod at high discharge (and too high SIM_mod at low discharge). Samples collected at annual average river dissolved load yielded SIM_mod concentrations close to the measured ones. According to the model, the studied rivers had specific mechanical denudation rates of 1.3–3.0 kg/m²/yr whereas the average measured rates were 0.8–3.5 kg/m²/yr which are among the highest on Earth.This study validates the use of a steady-state model of erosion to estimate mechanical weathering rates at the scale of a river catchment when the collected riverine dissolved load represents the average chemical composition over a mean hydrological year.     

A low‐dimensional model of bedrock weathering and lateral flow coevolution in hillslopes: 1. Hydraulic theory of reactive transport

This is the first of a two‐part paper exploring the coevolution of bedrock weathering and lateral flow in hillslopes using a simple low‐dimensional model based on hydraulic groundwater theory (also known as Dupuit or Boussinesq theory). Here, we examine the effect of lateral flow on the downward fluxes of water and solutes through perched groundwater at steady state. We derive analytical expressions describing the decline in the downward flux rate with depth. Using these, we obtain analytical expressions for water age in a number of cases. The results show that when the permeability field is homogeneous, the spatial structure of water age depends qualitatively on a single dimensionless number, Hi. This number captures the relative contributions to the lateral hydraulic potential gradient of the relief of the lower‐most impermeable boundary (which may be below the weathering front within permeable or incipiently weathered bedrock) and the water table. A “scaled lateral symmetry” exists when Hi is low: age varies primarily in the vertical dimension, and variations in the horizontal dimension x almost disappear when the vertical dimension z is expressed as a fraction z/H(x) of the laterally flowing system thickness H(x). Taking advantage of this symmetry, we show how the lateral dimension of the advection–diffusion‐reaction equation can be collapsed, yielding a 1‐D vertical equation in which the advective flux downward declines with depth. The equation holds even when the permeability field is not homogeneous, as long as the variations in permeability have the same scaled lateral symmetry structure. This new 1‐D approximation is used in the accompanying paper to extend chemical weathering models derived for 1‐D columns to hillslope domains.     

Probing deep weathering in the Shale Hills Critical Zone Observatory,Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface

Weathering is both an acid‐base and a redox reaction in which rocks are titrated by meteoric carbon dioxide (CO₂) and oxygen (O₂). In general, the depths of these weathering reactions are unknown. To determine such depths, cuttings of Rose Hill shale were investigated from one borehole from the ridge and four boreholes from the valley at the Susquehanna Shale Hills Observatory (SSHO). Pyrite concentrations are insignificant to depths of 23 m under the ridge and 8–9 m under the valley. Likewise, carbonate concentrations are insignificant to 22 and 2 m, respectively. In addition, a 5–6 m‐thick fractured layer directly beneath the land surface shows evidence for loss of illite, chlorite, and feldspar. Under the valley, secondary carbonates may have precipited. The limited number of boreholes and the tight folding make it impossible to prove that depth variations result from weathering instead of chemical heterogeneity within the parent shale. However, carbonate depletion coincides with the winter water table observed at ~20 m (ridge) and ~2 m depth (valley). It would be fortuitous if carbonate‐containing strata are found under ridge and valley only beneath the water table. Furthermore, pyrite and carbonate react quickly and many deep reaction fronts for these minerals are described in the literature. We propose that deep transport of O₂ initiates weathering at SSHO and many other localities because pyrite commonly oxidizes autocatalytically to acidify porewaters and open porosity. According to this hypothesis, the mineral distributions at SSHO are nested reaction fronts that overprint protolith stratigraphy. The fronts are hypothesized to lie subparallel to the land surface because O₂ diffuses to the water table and causes oxidative dissolution of pyrite. Pyrite‐derived sulfuric acid (H₂SO₄) plus CO₂ also dissolve carbonates above the water table. To understand how reaction fronts record long‐term coupling between erosion and weathering will require intensive mapping of the subsurface. Copyright © 2013 John Wiley & Sons, Ltd.     

Simulation of rainfall-runoff and watershed convergence process in the upper reaches of Heihe River Basin, July 2002

The watershed flow concentration scheme in the distributed hydrology-soil- vegetation model (DHSVM) is coupled with the mesoscale atmospheric model MM5 version 3.5, in which the Oregen States University land surface model (OSULSM) was involved. The flood event which happened in July 2002 in the upper reaches of Heihe river basin is simulated and the surface flow convergence process is shown with this coupled model. It has been concluded that times water head reaches each place of the basin are different. Water amount at each point is split-flow proportionally as the drops in elevation between it and neighbor points. Large part of the water amount pass away in greater slope direction and small part pass away in smaller slope one.Adding of the slope convergence makes the atmospheric model redistributes the surface water laterally.     

Variations in chemical compositions of the eolian dust in Chinese Loess Plateau over the past 2.5 Ma and chemical weathering in the Asian inland

Major and trace elements as well as strontium isotopic composition have been analyzed on the acid-insoluble (AI) phase of the loess-paleosol sequence from Luochuan, Shaanxi Province, China. Results show that the chemical composition of AI phase of loess and paleosols is distinctive to the average composition of upper continental crust (UCC), characterized by depletion of mobile elements Na, Ca and Sr. The distribution pattern of elements in AI phase reveals that initial dust, derived from a vast area of Asian inland, has suffered from Na- and Ca-removed chemical weathering compared to UCC. Some geochemical parameters (such as CIA values, Na/K, Rb/Sr and⁸⁷Sr/⁸⁶Sr ratios) display a regular variation and evolution, reflecting that the chemical weathering in the source region of loess deposits has decreased gradually since 2.5 Ma with the general increase of global ice volume. This coincidence reflects that the aridity of Asian inland since the Quaternary is a possible regional response to the global climate change.     

Effects of urbanization on stream water quality in the city of Atlanta,Georgia, USA

A long‐term stream water quality monitoring network was established in the city of Atlanta, Georgia during 2003 to assess baseline water quality conditions and the effects of urbanization on stream water quality. Routine hydrologically based manual stream sampling, including several concurrent manual point and equal width increment sampling, was conducted ～12 times annually at 21 stations, with drainage areas ranging from 3·7 to 232 km². Eleven of the stations are real‐time (RT) stations having continuous measures of stream stage/discharge, pH, dissolved oxygen, specific conductance, water temperature and turbidity, and automatic samplers for stormwater collection. Samples were analyzed for field parameters, and a broad suite of water quality and sediment‐related constituents. Field parameters and concentrations of major ions, metals, nutrient species and coliform bacteria among stations were evaluated and with respect to watershed characteristics and plausible sources from 2003 through September 2007. Most constituent concentrations are much higher than nearby reference streams. Concentrations are statistically different among stations for several constituents, despite high variability both within and among stations. Routine manual sampling, automatic sampling during stormflows and RT water quality monitoring provided sufficient information about urban stream water quality variability to evaluate causes of water quality differences among streams. Fecal coliform bacteria concentrations of most samples exceeded Georgia's water quality standard for any water‐usage class. High chloride concentrations occur at three stations and are hypothesized to be associated with discharges of chlorinated combined sewer overflows, drainage of swimming pool(s) and dissolution and transport during rainstorms of CaCl₂, a deicing salt applied to roads during winter storms. One stream was affected by dissolution and transport of ammonium alum [NH₄Al(SO₄)₂] from an alum‐manufacturing plant; streamwater has low pH (<5), low alkalinity and high metals concentrations. Several trace metals exceed acute and chronic water quality standards and high concentrations are attributed to washoff from impervious surfaces. Published in 2009 by John Wiley & Sons, Ltd.     

Mass‐balance modeling of mineral weathering rates and CO2 consumption in the forested,metabasaltic Hauver Branch watershed,Catoctin Mountain,Maryland, USA

Mineral weathering rates and a forest macronutrient uptake stoichiometry were determined for the forested, metabasaltic Hauver Branch watershed in north‐central Maryland, USA. Previous studies of Hauver Branch have had an insufficient number of analytes to permit determination of rates of all the minerals involved in chemical weathering, including biomass. More equations in the mass‐balance matrix were added using existing mineralogic information. The stoichiometry of a deciduous biomass term was determined using multi‐year weekly to biweekly stream‐water chemistry for a nearby watershed, which drains relatively unreactive quartzite bedrock. At Hauver Branch, calcite hosts ~38 mol% of the calcium ion (Ca²⁺) contained in weathering minerals, but its weathering provides ~90% of the stream water Ca²⁺. This occurs in a landscape with a regolith residence time of more than several Ka (kiloannum). Previous studies indicate that such old regolith does not typically contain dissolving calcite that affects stream Ca²⁺/Na⁺ ratios. The relatively high calcite dissolution rate likely reflects dissolution of calcite in fractures of the deep critical zone. Of the carbon dioxide (CO₂) consumed by mineral weathering, calcite is responsible for approximately 27%, with the silicate weathering consumption rate far exceeding that of the global average. The chemical weathering of mafic terrains in decaying orogens thus may be capable of influencing global geochemical cycles, and therefore, climate, on geological timescales. Based on carbon‐balance calculations, atmospheric‐derived sulfuric acid is responsible for approximately 22% of the mineral weathering occurring in the watershed. Our results suggest that rising air temperatures, driven by global warming and resulting in higher precipitation, will cause the rate of chemical weathering in the Hauver Branch watershed to increase until a threshold temperature is reached. Beyond the threshold temperature, increased recharge would produce a shallower groundwater table and reduced chemical weathering rates. Copyright © 2013 John Wiley & Sons, Ltd.     

Switch in chemical weathering caused by the mass balance variability in a Himalayan glacierized basin: a case of Chhota Shigri Glacier

Long-term data (2003–2015) on meltwater chemistry, mass balance and discharge of a benchmark glacier (Chhota Shigri Glacier, India) were studied to determine any association between these variables. To infer the factors governing the alteration of chemical weathering processes in glacierized basins, multi-annual records of the hydrochemical indices (Ca²⁺+Mg²⁺/Na⁺+K⁺) and the C-ratio were also examined. A succession of negative mass balance years has resulted in a decline in solute concentrations in the runoff, as discharge has increased. The (Ca²⁺+Mg²⁺/Na⁺+K⁺) and C-ratio are highest during periods of negative annual mass balance, when the spatial extent of the channelized drainage system increases. Conversely, these ratios are lowest in positive mass balance years, when the spatial extent of the channelized drainage system decreases, and chemical weathering in the distributed drainage system becomes more dominant. This paper is the first to show the inter-annual linkages between meltwater chemistry, mass balance and discharge for a valley glacier.     

Petrography and geochemistry of upper Triassic sandstones from the Tumengela Formation in the Woruo Mountain area,North Qiangtang Basin,Tibet: Implications for provenance,source area weathering,and tectonic setting

The petrography and major and trace element concentrations of the sandstones from the Tumengela Formation in the Woruo Mountain area, North Qiangtang Basin, are studied to determine their provenance, intensity of weathering and tectonic setting. The detrital compositions of the Tumengela sandstone samples are dominated by quartz (58.0–70.1 %, average 64.7 %) and lithic fragments (21.8–35.9 %, average 27.3 %), but low in feldspar content (4.9–12.9 %, average 8.0 %). The sandstones can be classified as litharenite and feldspathic litharenite according to their detrital compositions, which is consistent with the geochemical data. The detrital modal compositions reflect that these sandstones are probably derived from a recycled orogenic source. The index of chemical variability (ICV) and SiO₂/Al₂O₃ ratio values suggest that the compositional maturity and recycling were moderate. The weathering indices such as the chemical index of alteration (CIA), plagioclase index of alteration (PIA), chemical index of weathering (CIW), and Al₂O₃–(CaO* + Na₂O)–K₂O (A–CN–K) diagram indicate that the intensities of weathering in the source area were moderate. The Al₂O₃/TiO₂, Th/Co, La/Sc, La/Co, Th/Sc, Cr/Th ratio values and the discriminant function of the Tumengela sandstones indicate that the sediments were mainly derived from felsic source rocks, while also mixed with intermediate source rocks. The comparison of rare earth element patterns and its Eu anomalies to the probable source rocks infer that the sandstones were derived from the combination of granite, rhyolite, dacite, and gneisses. The proximal central uplift belt was probably the primary provenance area as evidenced by the petrographical and geochemical features of the Tumengela sandstones. The multidimensional tectonic discrimination diagram based on major elements show a collision setting (80 %) combined with a rift setting (20 %) for the Tumengela sandstones, which is consistent with the general geology of the study areas.     

Heterogeneity in ground water–surface water interactions in the hyporheic zone of a salmonid spawning stream

Spatial and temporal variability in ground water–surface water interactions in the hyporheic zone of a salmonid spawning stream was investigated. Four locations in a 150‐m reach of the stream were studied using hydrometric and hydrochemical tracing techniques. A high degree of hydrological connectivity between the riparian hillslope and the stream channel was indicated at two locations, where hydrochemical changes and hydraulic gradients indicated that the hyporheic zone was dominated by upwelling ground water. The chemistry of ground water reflected relatively long residence times and reducing conditions with high levels of alkalinity and conductivity, low dissolved oxygen (DO) and nitrate. At the other locations, connectivity was less evident and, at most times, the hyporheic zone was dominated by downwelling stream water characterized by high DO, low alkalinity and conductivity. Substantial variability in hyporheic chemistry was evident at fine (<10 m) spatial scales and changed rapidly over the course of hydrological events. The nature of the hydrochemical response varied among locations depending on the strength of local ground water influence. It is suggested that greater emphasis on spatial and temporal heterogeneity in ground water–surface water interactions in the hyporheic zone is necessary for a consideration of hydrochemical effects on many aspects of stream ecology. For example, the survival of salmonid eggs in hyporheic gravels varied considerably among the locations studied and was shown to be associated with variation in interstitial chemistry. River restoration schemes and watershed management strategies based only on the surface expression of catchment characteristics risk excluding consideration of potentially critical subsurface processes. Copyright © 2002 John Wiley & Sons, Ltd.     

Dissolved major ions,Sr and 87Sr/86Sr of coastal lakes from Larsemann hills,East Antarctica: Solute sources and chemical weathering in a polar environment

Dissolved major ions, Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios of 10 coastal lakes from the Larsemann Hills, East Antarctica have been studied to constrain their solute sources, transport and glacial weathering patterns in their catchments. In absence of perennial river/streams, lakes serve as only reliable archive to study land surface processes in these low-temperature regions. The lake water chemistry is mostly Na-Cl type and it does not show any significant depth variations. Sr isotope compositions of these lakes vary from 0.7110 to 0.7211 with an average value of 0.7145, which is higher than modern seawater value. In addition to oceanic sources, major ions and Sr isotopic data show appreciable amount of solute supply from chemical weathering of silicate rocks in lake catchments and dissolution of Ca-Mg rich salts produced during the freezing of seawaters. The role of sulphide oxidation and carbonate weathering are found to be minimal on lake hydro-chemistry in this part of Antarctica. Inverse model calculations using this chemical dataset provide first-order estimates of dissolved cations and Sr; they are mostly derived from oceanic (seawater + snow) sources (cations approximately 76%) and (Sr approximately 92%) with minimal supplies from weathering of silicates (cations approximately 15%); (Sr approximately 2%) and Ca-rich minerals (cations approximately 9%); (Sr approximately 7%). The silicate weathering rate and its corresponding atmospheric CO₂ consumption rate estimates for Scandrett lake catchment (3.6 ± 0.3 tons/km²/year and 0.5 × 10⁵ moles/km²/year), are lower than that of reported values for the average global river basins (5.4 tons/km²/year and 0.9 × 10⁵ tons/km²/year) respectively. The present study provides a comprehensive report of chemical weathering intensity and its role in atmospheric CO₂ consumption in low-temperature pristine environment of Antarctica. These estimates underscore the importance of Antarctica weathering on atmospheric CO₂ budget, particularly during the past warmer periods when the large area was exposed and available for intense chemical weathering.