Evaporite Weathering Research Articles

Statistical categorization geochemical modeling of groundwater in Ain Azel plain (Algeria)

Water analysis data of 54 groundwater samples from 18 uniformly distributed wells were collected during three campaigns (June, September and December 2004). Q-mode hierarchical cluster analysis (HCA) was employed for partitioning the water samples into hydrochemical facies. Interpretation of analytical data showed that the abundance of major ions was identified as follows: Ca ⩾ Mg > Na > K and HCO 3 ⩾ Cl > SO 4. Three major water facies are suggested by the HCA analysis. The samples from the area were classified as recharge area waters (Ca–Mg–HCO 3 water), transition area waters (Mg–Ca–HCO 3–Cl water), and discharge area waters (Mg–Ca–Cl–HCO 3 water). Inverse geochemical modeling suggests that relatively few phases are required to derive the water chemistry in the area. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into two categories: (1) evaporite weathering reactions and (2) precipitation of carbonate minerals.

Seasonal variations of physical and chemical erosion: A three-year survey of the Rhone River (France)

Numerous studies of weathering fluxes have been carried out on major world rivers during the last decade, to estimate CO 2 consumption rates, landscape evolution and global erosion rates. For obvious logistical reasons, most of these studies were based on large scale investigations carried out on short timescales. By comparison, much less effort has been devoted to long term monitoring, as a means to verify the temporal variability of the average characteristics, their trends, and the representativeness of short-term investigations. Here we report the results of a three-year survey (November 2000 to December 2003) of the major and trace element composition of dissolved and suspended matter in the lower Rhone River (France), the largest river of the Mediterranean area. Subsurface water samples were collected in Arles, about 48 km upstream of the estuary, twice a month routinely, and at higher frequency during flood events. During each flood event, the suspended particulate matter (SPM) show the usual trend of clockwise hysteresis with higher SPM concentrations on the rising limb of the flood than at the same discharge on the falling limb. We show that the annual average SPM flux of the Rhone River to the Mediterranean Sea (7.3 ± 0.6 × 10 6 tons yr −1) was largely controlled by the flood events (83% of the solid discharge occurred in less than 12% of the time), and that the precision on the total output flux depends strongly on the precise monitoring of SPM variations during the floods. The chemical composition of water and SPM are characterized by the predominance of Ca 2+ due to the abundance of carbonate rocks in the Rhone watershed. Chemical budgets have been calculated to derive the contributions of atmospheric deposition, carbonate, silicate and evaporite weathering, and anthropogenic inputs. The chemical weathering rate of carbonates is estimated to be 89 ± 5 t km −2 yr −1 compared to 14.4 ± 3 t km −2 yr −1 from silicates. By contrast, the physical erosion rate of silicates is about 51 t km −2 yr −1 against 19 t km −2 yr −1 for carbonates. The steady-state model of Gaillardet et al. (1995) has been applied to the chemical composition of dissolved and solid products. The results show that the Rhone River currently exports much less material than produced at steady-state by weathering in its watershed. The sediment flux inferred from the steady-state calculation (21–56 × 10 6 t yr −1) is on the same order as that estimated in literature for the 19th and the beginning of the 20th centuries. This imbalance may suggest that the Rhone is under a transient erosion regime following climate change (i.e. significant decrease of the flooding frequency since the beginning of the 19th century). On the other hand, the imbalance may also be due to the trapping of alluvion by the numerous dams on the river and its tributaries. Our data corroborate with previous studies that suggest a strong coupling between chemical and physical erosion fluxes, during the hydrological seasonal cycle of the Rhone River. The correlation between physical and chemical transport rates is, however, clearly different from that reported for global annual averages in large world rivers.

Temporal variability of selected dissolved elements in the lower Orinoco River, Venezuela

AbstractMonthly analyses of pH, conductance, alkalinity, total suspended solids, dissolved major cations (Na, K, Ca and Mg), and selected dissolved trace elements (Fe, Al, Mn, Cu, Zn and Cr) were conducted on waters of the lower Orinoco River from February 2004 to May 2006. The data show strong seasonal variability. Major cations had maximum concentrations at low water, and were correlated with pH, conductance and alkalinity because of the tributaries coming from the Andean zones, where the weathering of evaporites and carbonate shales controls water chemistry. K concentrations did not show any relation with evaporite weathering, probably because large amounts of K come from the Guayana Shield rivers as a consequence of feldspar weathering. The concentrations of dissolved Fe, Mn and Cu were highest during the high water stage. Concentrations of the elements K, Cr, Zn and Cu are correlated with each other but not with dissolved Fe and Al, which probably are complexed with humic and fulvic substances. Mn concentrations did not show relationships with other variables. Inter‐annual variability of dissolved elements is explained by temporal changes in precipitation. Copyright © 2008 John Wiley & Sons, Ltd.

Major ion chemistry of the Yarlung Tsangpo–Brahmaputra river: Chemical weathering, erosion, and CO 2 consumption in the southern Tibetan plateau and eastern syntaxis of the Himalaya

The Yarlung Tsangpo–Brahmaputra river drains a large portion of the Himalaya and southern Tibetan plateau, including the eastern Himalayan syntaxis, one of the most tectonically active regions on the globe. We measured the solute chemistry of 161 streams and major tributaries of the Tsangpo–Brahmaputra to examine the effect of tectonic, climatic, and geologic factors on chemical weathering rates. Specifically, we quantify chemical weathering fluxes and CO 2 consumption by silicate weathering in southern Tibet and the eastern syntaxis of the Himalaya, examine the major chemical weathering reactions in the tributaries of the Tsangpo–Brahmaputra, and determine the total weathering flux from carbonate and silicate weathering processes in this region. We show that high precipitation, rapid tectonic uplift, steep channel slopes, and high stream power generate high rates of chemical weathering in the eastern syntaxis. The total dissolved solids (TDS) flux from the this area is greater than 520 tons km −2 yr −1 and the silicate cation flux more than 34 tons km −2 yr −1. In total, chemical weathering in this area consumes 15.2 × 10 5 mol CO 2 km −2 yr −1, which is twice the Brahmaputra average. These data show that 15–20% of the total CO 2 consumption by silicate weathering in the Brahmaputra catchment is derived from only 4% of the total land area of the basin. Hot springs and evaporite weathering provide significant contributions to dissolved Na + and Cl − fluxes throughout southern Tibet, comprising more than 50% of all Na + in some stream systems. Carbonate weathering generates 80–90% of all dissolved Ca 2+ and Mg 2+ cations in much of the Yarlung Tsangpo catchment.

Constraints on paleowater dissolved loads and on catchment weathering over the past 16 ka from 87Sr/ 86Sr ratios and Ca/Mg/Sr chemistry of freshwater ostracode tests in sediments of Lake Constance, Central Europe

Sr isotope and Ca/Mg/Sr chemical compositions of freshwater ostracode tests separated from a sediment core represent the last 16 ka of sedimentation in Lake Constance, Central Europe. The chemical evolution of the paleowater's dissolved load of Lake Constance was estimated by correcting the ostracode data for Ca/Mg/Sr fractionation due to biogenic calcification. Since the Late Pleistocene deglaciation, the Ca/Sr molar ratios of paleowaters increased systematically from about 100 (a near marine signature) to about 200. Ca/Mg molar ratios varied in the range of 1–25. The 87Sr/ 86Sr ratios indicate Late Pleistocene paleowater compositions of 0.7086–0.7091, significantly more radiogenic than present day waters (0.7085). Sr isotopes and Ca/Mg/Sr chemical data together show that weathering of Mesozoic evaporites consistently dominated the dissolved Sr load (80–90%). Carbonate and silicate weathering were less important (1–10%). Trends of Sr dissolved loads were therefore not related to Mg which was mainly mobilized by carbonate weathering. Biotite weathering was an important source of radiogenic Sr in the paleowaters. The short-term release (duration about 600–800 years) of radiogenic Sr during glacier retreat started 15.2 ka ago and was due to enhanced biotite weathering at the glacier base. Long-term release of radiogenic Sr was due to biotite weathering in glacial soils and silicate rocks, and has gradually declined since the Late Pleistocene/Holocene transition.

Water chemistry of the Zhujiang (Pearl River): Natural processes and anthropogenic influences

The temporal and spatial variations of major ions in the Zhujiang (Pearl River) were analyzed using long‐term water chemistry data of major dissolved ions (Ca2+, Mg2+, the sum of Na+ and K+, HCO3−, SO42−, Cl−) and dissolved silica (SiO2) from 75 hydrological stations (1958–2002). The total dissolved solids (TDS) within the Zhujiang basin varies from 34.0 mg/l to 416.1 mg/l generally decreasing from upstream to downstream along the main stem of the Zhujiang. Rock weathering is the dominant controlling factor for the water chemistry of the Zhujiang, and more specifically, on average, 68% (22–92%) of total dissolved load comes from carbonate weathering, 22% (2–68%) from silicate weathering, and 10% (3–24%) from evaporite weathering, respectively. The flux calculations indicate that in total about 41.8 × 106 tonnes/year of TDS are transported out of the Zhujiang (excluding the Delta Region), averaged for the period 1958–2002. Changes in water chemistry can be observed from long‐term trend analysis, notably for SO42− and Cl−, as a result of anthropogenic influences, such as acid deposition, domestic and industrial wastewater discharge, and basin water resource development. An intense reforestation policy coupled with rapid reservoir development in the Zhujiang Basin would trigger more significant anthropogenic impacts on water chemistry in the future.

Chemical weathering in the Upper Huang He (Yellow River) draining the eastern Qinghai-Tibet Plateau

We examined the fluvial geochemistry of the Huang He (Yellow River) in its headwaters to determine natural chemical weathering rates on the northeastern Qinghai-Tibet Plateau, where anthropogenic impact is considered small. Qualitative treatment of the major element composition demonstrates the dominance of carbonate and evaporite dissolution. Most samples are supersaturated with respect to calcite, dolomite, and atmospheric CO 2 with moderate (0.710–0.715) 87Sr/ 86Sr ratios, while six out of 21 total samples have especially high concentrations of Na, Ca, Mg, Cl, and SO 4 from weathering of evaporites. We used inversion model calculations to apportion the total dissolved cations to rain-, evaporite-, carbonate-, and silicate-origin. The samples are either carbonate- or evaporite-dominated, but the relative contributions of the four sources vary widely among samples. Net CO 2 consumption rates by silicate weathering (6–120 × 10 3 mol/km 2/yr) are low and have a relative uncertainty of ∼40%. We extended the inversion model calculation to literature data for rivers draining orogenic zones worldwide. The Ganges-Brahmaputra draining the Himalayan front has higher CO 2 consumption rates (110–570 × 10 3 mol/km 2/yr) and more radiogenic 87Sr/ 86Sr (0.715–1.24) than the Upper Huang He, but the rivers at higher latitudes are similar to or lower than the Upper Huang He in CO 2 uptake by silicate weathering. In these orogenic zones, silicate weathering rates are only weakly coupled with temperature and become independent of runoff above ∼800 mm/yr.

Cretaceous and Cenozoic evolution of seawater composition, atmospheric O2 and CO2: A model perspective

A new box model for the global cycles of sulfur, oxygen, and magnesium has been developed and coupled with a carbon-calcium-strontium model ([Wallmann, 2001][1]). The model accounts for sulfur masses in the ocean, in evaporite rocks, in sedimentary pyrite, in oceanic crust, and in the mantle. Sulfur fluxes considered are burial and weathering of evaporites and sedimentary pyrite, volcanic emission of sulfur gases originating from the mantle and subducted reservoirs, and alteration of oceanic crust and serpentinization of oceanic peridotites. Oxygen fluxes are derived from sulfur turnover, carbon cycling, and iron redox processes. Model outputs are seawater concentrations of SO4, Ca, Mg, Sr, CO2, and HCO3, atmospheric partial pressures of O2 ( p O2) and CO2 ( p CO2), as well as the sulfur, carbon, and strontium isotopic composition of seawater. The secular trends of δ34S, δ13C, and 87Sr/86Sr recorded in marine evaporites and carbonates are used for model optimization and control. According to sensitivity analysis, tectonic/volcanic processes affect oceanic δ34S values considerably, whereas δ13C values are determined mainly by turnover of particulate organic carbon (POC). The different control mechanisms induce a significant deviation between the secular trends of sulfur and carbon isotope data. Dependence of POC burial on atmospheric p O2 via redox-dependent phosphate recycling provides an effective negative feedback on p O2. Moreover, phosphate supply to the ocean and thus POC burial depends on weathering, which in turn is related to the prevailing p CO2 level. Therefore, emission of volcanic CO2 induces O2 production via POC burial while O2 is consumed by the coeval release and oxidation of reduced sulfur compounds from the mantle. This coupling between tectonic/volcanic processes and POC burial provides additional stabilization of p O2. Consequently, predicted p O2 values fall into a narrow range of 0.17 to 0.25 atm during the entire model period. Changes in p O2 are mainly driven by changes in tectonic activity, erosion rates, and size of exposed carbonate areas. The model indicates that carbonate weathering, unlike silicate weathering, establishes a positive feedback on atmospheric p CO2 via enhanced burial and recycling of pelagic carbonates at subduction zones. [1]: #ref-82

Major ion chemistry in the headwaters of the Yamuna river system:: Chemical weathering, its temperature dependence and CO 2 consumption in the Himalaya

The Yamuna river and its tributaries in the Himalaya constitute the Yamuna River System (YRS). The YRS basin has a drainage area and discharge comparable in magnitude to those of the Bhagirathi and the Alaknanda rivers, which merge to form the Ganga at the foothills of the Himalaya. A detailed geochemical study of the YRS was carried out to determine: (i) the relative significance of silicate, carbonate and evaporite weathering in contributing to its major ion composition; (ii) CO 2 consumption via silicate weathering; and (iii) the factors regulating chemical weathering of silicates in the basin. The results show that the YRS waters are mildly alkaline, with a wide range of TDS, ∼32 to ∼620 mg l −1. In these waters, the abundances of Ca, Mg and alkalinity, which account for most of TDS, are derived mainly from carbonates. Many of the tributaries in the lower reaches of the Yamuna basin are supersaturated with calcite. In addition to carbonic acid, sulphuric acid generated by oxidation of pyrites also seems to be supplying protons for chemical weathering. Silicate weathering in YRS basin contributes, on average, ∼25% (molar basis) of total cations on a basin wide scale. Silicate weathering, however, does not seem to be intense in the basin as evident from low Si/(Na*+K) in the waters, ∼1.2 and low values of chemical index of alteration (CIA) in bed sediments, ∼60. CO 2 drawdown resulting from silicate weathering in the YRS basin in the Himalaya during monsoon ranges between (4 to 7) × 10 5 moles km −2 y −1. This is higher than that estimated for the Ganga at Rishikesh for the same season. The CO 2 consumption rates in the Yamuna and the Ganga basins in the Himalaya are higher than the global average value, suggesting enhanced CO 2 drawdown in the southern slopes of the Himalaya. The impact of this enhanced drawdown on the global CO 2 budget may not be pronounced, as the drainage area of the YRS and the Ganga in the Himalaya is small. The CO 2 drawdown by silicates in the YRS basin is marginally higher than the reported values of CO 2 release from oxidation of organic rich sediments, estimated using Re as a proxy. This comparison shows the need to constrain CO 2 sources and sinks better to balance its budget in a regional scale. The results also show that silicate weathering rate in the YRS basin is ∼10 mm ky −1 and on the Ganga basin, it is ∼5 mm ky −1, which are several times lower than the carbonate weathering rates. The significantly higher silicate weathering rate observed in the YRS basin seems to be governed by rapid physical erosion in this region. The apparent activation energy for overall silicate weathering in the YRS basin, derived from Na* and Si concentrations and water temperature, ranges from ∼50 to 80 kJ mol −1. These values are comparable to those reported for granitoid weathering in natural watersheds and feldspar weathering in laboratory experiments. This study brings to light the sources contributing to major ions, enhanced chemical weathering rates in the Yamuna River Basin and interdependence of silicate weathering on physical erosion and temperature.

Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers

The main problem associated with the study of silicate weathering using river dissolved load is that the main control of solute chemistry is lithology and that all rivers are influenced by carbonate and evaporite weathering. In this paper, newly compiled data on the 60 largest rivers of the world are used to calculate the contribution of main lithologies, rain and atmosphere to river dissolved loads. Technically, an inverse method is used to solve a model containing of a series of mass budget equations relating river concentrations to chemical weathering products and atmospheric inputs. New estimates of global silicate weathering fluxes and associated CO2 consumption fluxes are given. The role of basalt weathering on oceanic islands and volcanic arcs is emphasized. For each large river, an attempt is made to calculate chemical weathering rates of silicates per unit area. Only relative chemical weathering rates can be calculated. The relationships between the chemical weathering rates of silicates and the possible controlling parameters are explored. A combined effect of runoff-temperature and physical denudation seems to explain the variability of modern silicate chemical weathering rates. The results of this study highlight the coupling between the physical and the chemical processes of silicate weathering. Only an active physical denudation of continental rocks seems to be able to maintain high chemical weathering rates and significant CO2 consumption rates.

Chemical and Strontium, Oxygen, and Carbon Isotopic Compositions of Carbonates from the Lesser Himalaya: Implications to the Strontium Isotope Composition of the Source Waters of the Ganga, Ghaghara, and the Indus Rivers

Samples of Precambrian carbonate (mostly dolomite) outcrops collected across the Lesser Himalaya have been analysed for their mineralogy, chemical composition, and isotope ratios of Sr, O, and C to assess the extent of their preservation and their role in contributing to the high radiogenic strontium isotope composition of the source waters of the Ganga, Ghaghara, and the Indus. Their Sr concentrations range from 20 to 363 ppm, δ 18O PDB −1.4 to −12.8‰ and Mn 11–2036 ppm. The petrography of the samples, their low Sr concentrations, and wide range of δ 18O values are suggestive of their postdepositional alteration. The 87Sr/ 86Sr of the bulk samples and their carbonate fractions are similar to one another with values ranging from 0.7064 to 0.8935 and are generally more radiogenic than that of contemporaneous seawater. Comparison of the 87Sr/ 86Sr and Sr/Ca ratios among the carbonates and silicates from the Lesser Himalaya and the source waters of the Ganga, Ghaghara, and the Indus shows that the values for the source waters overlap with those of the silicates but are much higher than those in carbonates. An upper limit of carbonate Sr in the various source waters is calculated to be between 6% and 43%, assuming that all the Ca in the rivers is of carbonate origin. The results show that on the average, weathering of the Precambrian carbonates is unlikely to be a major contributor to the highly radiogenic strontium isotope composition of these source waters; however, they can be a dominant supplier of radiogenic Sr to some rivers on a regional scale. The silicate Sr component in some of the source waters of the Ganga (Bhagirathi, Bhilangna, Alaknanda, and Ganga), Ghaghara (Kali and Sarju), and the Indus (Sutlej) was calculated from the Ca/Na, Sr/Na ratios, and strontium isotope compositions of these rivers and the silicate endmember. These calculations suggest that 33–89% of Sr in the Bhagirathi, Bhilangna, Alaknanda, Ganga, and Sarju rivers is of silicate origin, whereas in the Kali and the Sutlej it is much lower, only ∼8%. The remaining Sr to all these waters has to be supplied from other sources such as weathering of carbonates and evaporites. This study underscores the importance of weathering of silicates, carbonates, and evaporites in contributing to the Sr mass balance and 87Sr/ 86Sr of the source waters of the Ganga, Ghaghara, and the Indus. The present day silicate and carbonate Sr contributions to the Sr budget of the rivers vary considerably, but among the major source waters of the Ganga, silicate Sr exerts a more dominant control on their Sr abundance and 87Sr/ 86Sr.

Chemical and physical denudation in the Amazon River Basin

We present major and trace element data on the suspended and dissolved phases of the Amazon River and its main tributaries. The Sr isotopic composition of the dissolved load is also reported. Special attention is paid to the abundances of REE and to their fractionation between the dissolved and suspended phase. The rivers of the Amazon Basin are among the richest in dissolved REE and are similar to the rivers of the Congo system. However a greater range of fractionation between LREE and HREE is reported here. At a global scale the rivers have intermediate patterns between those of the Congo system and those of high pH rivers such as the Indus and Mississippi rivers. Only few elements (Rb, U, Ba, K, Na, Sr and Ca) are mobilized by silicate weathering. These elements are strongly depleted in the suspended phase with respect to upper continental crust. In the dissolved load, these elements are controlled by atmospheric inputs and the weathering of the main lithologies. We propose a model based on mass budget equations, that allow the proportions derived from the different sources to be calculated. As a consequence silicate, carbonate and evaporite weathering rates can be estimated as well as the consumption of CO2 by weathering of each of these lithologies. Physical weathering rates can be estimated by two complementary approaches. On the one hand, the multi-year average of suspended sediments yields can be used to estimate physical denudation. On the other hand, we have developed a steady-state model of erosion that allows us to calculate physical erosion rates on the basis of the dissolved load of rivers. A mean crustal composition is assumed in this model for the rock sources of the drainage basins. Comparison of the rates predicted by the model to the observed rates shows good agreement for the lowland rivers, but a strong discrepancy for the rivers derived from the Andes. Andean rivers (Solimoes, Madeira and Amazon) have observed sediment yields much greater than those predicted according to the steady-state model of chemical and physical weathering. Two interpretations can account for this inconsistency. The first is that these rivers are not in steady state and hence that the soils are being destroyed. The second requires that the local continental crust is different from the average continental crust of Taylor and McLennan, and contains a large proportion of sedimentary rocks. Using the measured sediment yields, and assuming a steady state, we can estimate the amount of sediment recycling for each drainage basin. For the Amazon at Santarem, we find that at least 25% of the mass of the upper continental crust of the Amazon drainage basin is constituted of recycled material.

Fluvial geochemistry of the eastern slope of the northeastern Andes and its foredeep in the drainage of the Orinoco in Colombia and Venezuela

The fluvial geochemistry of the tributaries of the Orinoco draining the eastern branch of the northern Andes in Colombia and Venezuela is determined by lithology and ranges from rivers dominated by aluminosilicate weathering, mainly of shales and mafic rocks, to those bearing the signatures of dissolution of marine limestones and evaporites and of continental playa deposits. These left bank tributaries of the Orinoco provide the overwhelming proportion of the suspended and dissolved loads transported by the main stem although feeding only half the discharge. However, due to the extreme severity of weathering on the Guayana Shield, the right bank tributaries supply about half the silica and 40% of the K carried by the Orinoco. There is a similar partitioning between the Andes and the lowlands in the Amazon drainage. In contrast the Mackenzie, which drains the northern extremity of the continental arc of the Western Americas, is completely dominated by the weathering of limestones and evaporites and transports very little silica. This, again, appears to be a reflection of lithology since the Yukon, in similar latitudes and terrain, has silica concentrations comparable to the Tropical systems. In addition, a conservative deconvolution of the data gives estimates of the net consumption of atmospheric CO2 by aluminosilicate weathering that follows the same pattern. Thus, the available evidence from the Western Americas indicates that exposure and lithology, rather than the climatic variables, temperature, and runoff, dominate the weathering yields in active orogenic belts. These observations contradict the conventional view of weathering processes and their controls and are at complete variance with the assumptions parameterised in most models of the geochemical cycle.

The Indus river system (India-Pakistan): Major-ion chemistry, uranium and strontium isotopes

The Indus River is one of the large river systems draining the Himalaya. We report in this paper the major-ion chemistry, Sr and U isotope systematics of the Indus system, particularly its headwaters. The results show that: (1) on an average about a third of the cations in the waters can be from silicate weathering; however, most of the (Ca + Mg) is likely to be from the weathering of carbonates and evaporites; (2) the 87 Sr 86 Sr of the waters ranges between 0.7085 and 0.7595, the higher values (> 0.72) are typical of the tributaries draining the Precambrian granite/gneissic terrains. The 87 Sr 86 Sr of the Indus main channel, throughout its entire stretch, shows only minor variations, 0.7104–0.7116. The Indus transports ∼8.8 · 10 6 mol Sr to the Arabian Sea annually with a 87 Sr 86 Sr of 0.7111 if our results of Sr concentration and 87 Sr 86 Sr measured at Thatta can be considered typical of the Indus throughout the year; and (3) the U concentration in the Indus and its tributaries is generally high, 0.37–10.3 μg 1 −1. The source for the high uranium can be the weathering of granites, zones of uranium mineralisation and black shales. The Indus results when compared with our earlier data on the Ganga-Brahmaputra show that in all these three river systems carbonate weathering is the dominant source of (Ca + Mg) and HCO 3 and that U concentrations are high. In the case of 87 Sr 86 Sr , the Indus waters are less radiogenic; however, its tributaries draining the Precambrian granites/gneisses have 87 Sr 86 Sr in excess of 0.72, similar to that in the rivers of the Ganga system. The low 87 Sr 86 Sr of the Indus, 0.7111, suggests that its contribution to the Sr isotope evolution of oceans since the Cenozoic is less significant than that of the Ganga-Brahmaputra.

Drainage basin weathering and major element transport of two large Chinese rivers (Huanghe and Changjiang)

Long‐term records show high concentrations of dissolved major elements in the Changjiang (Yangtze River) and the Huanghe (Yellow River). The Huanghe drainage basin is characterized by intense weathering and erosion of carbonates and evaporites, while weathering and erosion of carbonates and silicates over the drainage basin is the main source of major elements in the Changjiang. The rock/soil composition and tectonics of the drainage basin are more important than atmospheric precipitation in affecting major element composition of these two rivers. Compared with other large rivers of the world, the high suspended sediment and major element loads of the Changjiang and Huanghe reveal intense physical and chemical erosion over the Chinese continent.

The Distribution and Significance of Evaporitic Weathering Products on Antarctic Meteorites

Abstract— The distribution of white evaporitic deposits differs among different meteorite compositional groups and weathering categories of Antarctic meteorites. Evaporites occur with unusual frequency on carbonaceous chondrites, and are especially common in carbonaceous chondrites of weathering categories A and B. Among achondrites, weathering categories A and A/B show the most examples of evaporite weathering. Unlike carbonaceous chondrites and achondrites, most evaporite‐bearing ordinary (H and L) chondrites are from rustier meteorites of weathering categories B and, to a lesser degree, B/C and C. LL chondrites are conspicuous by their complete lack of any evaporitic weathering product.Almost two‐thirds of all evaporite‐bearing meteorites belong to weathering categories A, A/B, and B. Where chemical data are available, surficial evaporite deposits are associated with elemental anomalies in meteorite interiors. Meteorites of weathering classes B, A/B, and even A may have experienced significant element redistribution and/or contamination as a result of terrestrial exposure. Evaporite formation during terrestrial weathering cannot be neglected in geochemical, cosmochemical, and mineralogical studies of Antarctic meteorites. A lower‐case “e” should be added to the weathering classification of evaporite‐bearing Antarctic meteorites, to inform meteorite scientists of the presence of evaporite deposits and their associated compositional effects.

Geochemistry of the Amazon: 2. The influence of geology and weathering environment on the dissolved load

In the Amazon Basin, substrate lithology and erosional regime (seen in terms of transport‐limited and weathering‐limited denudation) exert the most fundamental control on the chemistry of surface waters within a catchment. Secondary effects, such as the precipitation of salts within soils and in stream beds, biological uptake and release, and cyclic salt inputs, are more difficult to discern. Samples can be separated into four principal groupings based on relationships between total cation charge (TZ+) and geology. (1) Rivers with 0<TZ+<200 μeq/l drain the most intensely weathered materials (Upper Tertiary sediments, soils of the Negro Basin, and similarly weathered regions). These rivers show high levels of Fe, Al, H+, and coloration and are enriched in Si relative to other major species; they exhibit cation ratios similar to those of substrate rocks. (2) Rivers with 200<TZ+<450;μeq/l drain siliceous terrains. These rivers are also rich in silica relative to other species. Rivers draining weathering‐limited siliceous terrains exhibit the highest TZ+ and their cation load is typically preferentially enriched in Na over K and Ca over Mg when compared to the rocks in their catchments. (3) Rivers with 450<TZ+<3000 μeq/l drain marine sediments or red beds with high cation concentrations (resulting from the presence of carbonates and minor evaporites in the Peruvian Andes and reduced shales and minor carbonates in the Bolivian Andes). These rivers exhibit relatively high levels of Ca, Mg, alkalinity, and SO4(in rivers draining reduced shales and minor evaporites). (4) Rivers with TZ+>3000 μeq/l drain massive evaporites. These rivers are rich in Na and Cl. In the third and fourth categories, rivers tend to have 1:1 (equivalent) ratios of Na:Cl and (Ca+Mg):(alkalinity+SO4), caused primarily by the weathering of carbonates and evaporites. Supplement available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document C83‐002; $2.50. Payment must accompany order.

Major ion chemistry of some large Chinese rivers

There is very little information available in the western literature on the chemical compositions of the major rivers of the People's Republic of China1,2. Since these include the largest in terms of sediment transport3 (Huang He or Yellow), the third largest in terms of flow4 (Chiang Jiang or Yangtze) and major streams draining the Tibetan plateau, this lack of data represents a significant gap in our knowledge of the chemical denudation rates of the continents as a whole and of south, central and eastern Asia in particular. To begin to rectify this situation, advantage has been taken of recent visits by US scientists to the People's Republic of China to collect suitable samples for analysis. It was found that the chemistry of such rivers is dominated by the weathering of carbonates and evaporites, with no pronounced effects of the degradation of aluminosilicates.