Chemical Weathering Of Basalt Research Articles

Potassium isotope fractionation during chemical weathering of basalts

Non-traditional stable isotopes (e.g., Li, Mg, and Si) are increasingly used as tracers for studying Earth's surface processes. The isotopes of potassium (K), a highly soluble and mobile element during weathering, could be a promising new tracer for continental weathering; however, the K isotopic variations in weathering profile has not been directly studied due to previous analytical difficulties. Recent high-precision measurements revealed that K isotopes in global river waters are fractionated from the Bulk Silicate Earth (BSE) value, indicating they are influenced by chemical weathering of the crust. Isotopic fractionation during chemical weathering is one of several processes that could ultimately lead to ∼0.6‰ difference of δ41K between the BSE and modern seawater. In order to determine the direction and controlling factors of K isotopic fractionation during basalt weathering, especially under intense weathering conditions, we measured K isotopic compositions in two sets of bauxite developed on the Columbia River Basalts, together with fresh parental basalt and aeolian deposit samples using a recently developed high-precision method. Results show that K isotopic variations among fresh basalts and aeolian dust are limited, close to the BSE value. Extreme K depletion (>99%) and K isotopic fractionation (δ41K up to 0.5 ‰) are observed in bauxite drill cores due to intense chemical weathering. The top of the weathering profiles shows less depletion in K abundances and the δ41K values are closer to those of the fresh basalts and aeolian dusts, likely due to addition of aeolian dust at the tops of both profiles. The weathered products are generally depleted in heavy K isotopes, which is consistent with heavier K isotopic compositions observed in river water and seawater. The δ41K in bauxites displays a positive correlation with K2O contents as well as δ7Li, indicating the behaviors of K and Li isotopes are comparable during chemical weathering. This study shows that K concentrations and its isotopic compositions are sensitive tracers of chemical weathering and could be good weathering proxies over Earth's history.

Potassium isotope fractionation during chemical weathering of basalts

Potassium isotope fractionation during chemical weathering of basalts

Is basalt weathering a major mechanism for atmospheric CO<sub>2</sub> consumption?

In the science of global change, a main focus of researchers who investigate the global carbon cycle is determining the fate of missing carbon sinks. Because atmospheric CO2 that is consumed by carbonate chemical weathering is thought to return to the atmosphere through the precipitation of carbonate, it is widely accepted that it is silicate weathering, rather than carbonate weathering, that constitutes the major mechanism of atmospheric CO2 consumption. In particular, the chemical weathering of basalt, which is a type of silicate rock, is considered to be an important “carbon sink” due to the CO2 drawdown that occurs during the basalt-carbonic acid reaction. However, the high CO2 consumption rates of basalt chemical weathering may also derive from the following four aspects, as identified in the extant literature. First, study areas with high carbon fluxes (CF) are usually ocean islands, volcanic arcs or situated in tropical regions, where precipitation is high, which results in large runoff depth. This may be one of the primary reasons for the high CF, since CF is equal to the product of the runoff depth and the concentration of bicarbonate. In addition, because of the presence of chemostatic behaviors of the concentration of bicarbonate, CF is mainly determined by runoff depth. In other words, high runoff depth will directly result in high CF. Furthermore, the interface between water and minerals will be enlarged, while the saturation state of water will be decreased, by the increase of runoff depth. Therefore, more minerals will take part in the dissolution, and the dissolving capacity of water will be increased. However, this kind of high CF results from high runoff depth, rather than basaltic properties. Second, the high concentration of dissolved inorganic carbon (DIC) may result from the chemical weathering of trace amounts of carbonate dispersed in silicate rock, rather than silicate minerals. This should be regarded as the contribution of carbonate chemical weathering, rather than silicate chemical weathering. Third, the reactions between exogenous acid and trace amounts of carbonate will contribute to DIC fluxes, during which no atmospheric CO2 is consumed. Fourth, the CO2 that participates in the rock chemical weathering in basalt watersheds may be from a deep-source, rather than the atmosphere or soil. If this is the case, the deep-source CO2 consumed by rock chemical weathering will be released into the atmosphere with the precipitation of carbonate in the oceans. In other words, the riverine DIC may become a carbon source instead of a carbon sink. Therefore, further research is needed to conclusively determine whether basalt chemical weathering constitutes a major consumption mechanism for atmospheric CO2.

The linkage between geological setting and human health in Ethiopia: a preliminary geo-environmental-health model

Tropical environments are known for their prevalence of a number of geo-chemical diseases. This is related to the very nature of their geo-environments. Little research is conducted in geo-sciences in Ethiopia to investigate the linkage between geo-environments and associated health risks. Fluoride and related diseases are the most widely studied from geological perspectives. Ethiopia is characterized by complex but regionally homogeneous geo-environment. Climate is under the influence of monsoon from two oceans and continental sources. Locally orography modifies the redistribution of rainfall in the lee and wind ward directions. This leads to any combination of warm to cold and wet to dry climate. Prominent geologic features are rifting, volcanism and uplifting. Combination of geology, climate, and topography results in a complex pattern of geo-environment which in turn results in belts of availability or scarcity of certain geo-chemical elements. This work shows the presence of linkage between geo-environmental setting and prevalence of geo-chemical diseases such as goiter, elephantiasis, flourisis, and trace element related health problems. South-western Ethiopia, which is under the influence of Indian Ocean monsoon and underlain by volcanic rocks shows the highest incidence of Iodine Deficiency Disorder (IDD). Regions of Ethiopia under the influence of Atlantic Ocean monsoon or areas underlain by sediments and sedimentary rocks show the lowest IDD. Elephantiasis prevalence is linked to laterite soils derived from chemical weathering of basalts, a typical process commonly taking place in humid plateau of south-western Ethiopia between altitudes of 1000 and 2000 masl. Flouride related diseases are common features in area where water supply for domestic water use depend on aquifers in acid volcanic rocks, commonly in the rift setting. Regardless of the prevalence of geo-chemical diseases, Ethiopia is also endowed with natural geo-chemical materials of significant health benefits ( e.g. , Mineral waters and mud volcanoes). The work proposes a preliminary geo-environmental health model for Ethiopia.

Assessing the role of climate on uranium and lithium isotope behaviour in rivers draining a basaltic terrain

This study presents uranium (U) and lithium (Li) isotope and major and trace element data for the dissolved load, suspended particles and bedload from rivers draining dominantly basaltic catchments on the island of Sao Miguel in the Azores Archipelago. Uranium activity ratios are at secular equilibrium in the bedload, but are higher for suspended material, suggesting sorption of uranium from the solution onto particle surfaces. The ( 234U/ 238U) of the riverine dissolved phase varies between 1.02 and 1.86, and lies on a mixing trend between the values expected from chemical weathering of basalt (high [U]; ( 234U/ 238U) at secular equilibrium) and an endmember with low [U] and high ( 234U/ 238U), which probably reflects α-recoil effects associated with physical weathering and/or soil formation. Lithium isotope ratios are consistently lighter in suspended material than the corresponding bedload, suggesting preferential retention of 6Li in the suspended load. In turn, the δ 7Li of the dissolved load is always isotopically heavier, ranging from 5.9 to 36.2‰, although the lightest values are affected by hydrothermal input. The lithium isotopic composition of the dissolved load reflects the balance of primary mineral dissolution to secondary mineral formation. Comparison of the behaviour of U and Li isotopes between Sao Miguel, Iceland, and other basaltic terrains suggests that whilst U isotopes are dominated by weathering regimes dependent on factors such as weathering rates, temperature, runoff and climate, neither weathering intensity nor climate exerts a direct influence on Li isotope behaviour; rather the formation of secondary minerals, which is indirectly controlled by climate, weathering, primary mineralogy and biology, appears to play the dominant role.

Basalt weathering in Central Siberia under permafrost conditions

Chemical weathering of basalts in the Putorana Plateau, Central Siberia, has been studied by combining chemical and mineralogical analysis of solids (rocks, soils, river sediments, and suspended matter) and fluid solution chemistry. Altogether, 70 large and small rivers, 30 soil pore waters and groundwaters and over 30 solids were sampled during July to August 2001. Analysis of multiannual data on discharge and chemical composition of several rivers of the region available from the Russian Hydrological Survey allowed rigorous estimation of mean annual major element concentrations, and dissolved and suspended fluxes associated with basalt weathering. For the rivers Tembenchi and Taimura that drain monolithologic basic volcanic rocks, the mean multiannual flux of total dissolved cations (TDS_c = Ca + Mg + Na + K) corrected for atmospheric input is 5.7 ± 0.5 t/km 2/yr. For the largest river Nizhniya Tunguska—draining essentially basic rocks—the TDS_c is 6.1 ± 1.5 t/km 2/yr. The overall CO 2 consumption flux associated with basalt weathering in the studied region (∼700,000 km 2) achieves 0.08 × 10 12 mol/yr, which represents only 2.6% of the total CO 2 consumption associated with basalt weathering at the Earth’s surface. The fluxes of suspended matter were estimated as 3.1 ± 0.5, 9.0 ± 0.8, and 6.5 ± 2.0 t/km 2/yr for rivers Taimura, Eratchimo, and Nizhniya Tunguska, respectively. Based on chemical analyses of river solutes and suspended matter, the relative dissolved versus particulate annual transport of major components is C inorg ≥ C org > Na + K > Ca > Mg > Si > Fe ≥ Mn ≥ Ti ≥ Al which reflects the usual order of element mobility during weathering. According to chemical and mineralogical soil and sediment analyses, alteration of basalt consists of (1) replacement of the original basaltic glass by Si-Al-Fe rich amorphous material, (2) mechanical desegregation and grinding of parent rocks, leading to accumulation of “primary” hydrothermal trioctahedral smectite, and (3) transformation of these trioctahedral (oxy)smectites and mixed-layer chlorite-smectite, into secondary dioctahedral smectite accompanied by removal of Ca, Mg, and Fe, and enrichment in Al. No vertical chemical differentiation of fluid and solid phases within the soil profile was identified. All sampled soil pore waters and groundwaters were found to be close to equilibrium with respect to chalcedony, gibbsite, halloysite, and allophanes, but strongly supersaturated with respect to goethite, nontronite, and montmorillonite. Over the annual cycle, the contribution of atmospheric precipitation, permafrost melting, underground reservoirs, litter degradation, and rock and soil mineral weathering for the overall TDS_c transport in the largest river of the region (Nizhniya Tunguska) is 9.3 ± 3, 10 ± 5, 10.5 ± 5, 25 ± 20, and 45 ± 30%, respectively. In the summertime, direct contribution of rocks and soil mineral weathering via solid/fluid interaction does not exceed 20%. The main unknown factors of element mobilization from basalt to the river is litter degradation in the upper soil horizon and parameters of element turnover in the vegetation.

The effect of gross primary production, net primary production and net ecosystem exchange on the carbon fixation by chemical weathering of basalt in northeastern Iceland

The overall objective of this study is to define and interpret the annual dissolved inorganic carbon (DIC) flux in selected river catchments in North Eastern Iceland. The flux stems primarily from chemical weathering of basalt. The DIC flux out of the catchments is compared with the spatial distribution of the various vegetation communities and their gross primary production (GPP), net primary production (NPP) and net ecosystem exchange (NEE). There is no correlation between the DIC flux and the GPP, but one between DIC and NPP. The DIC flux is highly dependent on the NEE, which in turn is governed by the area extent of wetlands in these catchments. A variation by a factor 5 of the NEE results in a variation by a factor 2.8 in the river dissolved inorganic flux.

Basalt weathering and trace elements migration in the boreal Arctic zone

Chemical weathering of basalts in the Putorana Plateau, Central Siberia, has been studied by combining chemical and mineralogical analysis of solids (rocks, soils, river sediments and suspended matter) and fluid solution chemistry in order to quantify CO 2 consumption and to assess the major factors controlling basic rock weathering under permafrost-dominated taiga climate. The chemical status of ∼ 40 major and trace elements (TE) in pristine boreal rivers and interstitial solutions of permafrost soils has been investigated using in-field ultrafiltration procedure. This revealed strong relationships between concentration of TE and that of major inorganic components of colloids, i.e., Fe or Al. Decomposing plant litter and permafrost thawing are considered important sources of most major and trace elements in Arctic rivers during summertime.

Basalt weathering laws and the impact of basalt weathering on the global carbon cycle

This study attempts to characterise the chemical weathering of basalts and to quantify the flux of carbon transferred from the atmosphere to the ocean during this major process at the surface of the Earth. To this aim, we have compiled different published chemical compositions of small rivers draining basalts. Basaltic river waters are characterised by relatively high Na-normalized molar ratios (Ca/Na: 0.2–3.9; HCO 3/Na: 1–10; Mg/Na: 0.15–6) in comparison with those usually observed for river draining silicates. The data also show the climatic influence on basalt weathering and associated CO 2 consumption. Runoff and temperature are the main parameters controlling the chemical weathering rate and derived CO 2 consumption during basaltic weathering. From these relationships and digital maps, we are able to define the contribution of basalts to the global silicate flux. Taking account of this result, we estimate that the CO 2 flux consumed by chemical weathering of basalts is about 4.08×10 12 mol/year. The fluxes from the islands of Indonesia and regions of central America represent around 40% of this flux. The global flux of CO 2 consumed by chemical weathering of basalts represents between 30% and 35% of the flux derived from continental silicate determined by Gaillardet et al. [Chem. Geol. 159 (1999) 3]. Finally, it appears that volcanic activity not only acts as a major atmospheric CO 2 source, but also creates strong CO 2 sinks that cannot be neglected to better understand the geochemical and climatic evolution of the Earth.

Chemical weathering of basalts, southwest Iceland: effect of rock crystallinity and secondary minerals on chemical fluxes to the ocean

River, ground, and peat water from a basaltic catchment area, Laxa in Kjos Southwest Iceland, were sampled and analyzed for major element concentrations to define the effect of glassy versus crystalline basalt, the formation of secondary minerals, and runoff on fluxes of dissolved elements to the ocean. The proportions of the dissolved solids in the water samples derived from precipitation, air, and rock weathering were estimated. The amounts of Na, Mg, Ca, K, and S originating from precipitation were calculated from the respective solute/Cl marine ratios and the estimated aqueous Cl originated from precipitation, which was calculated using measured B content of the waters and B/Cl molal marine and rock ratios. The rock contribution from weathering ranged from 22 to 46 percent of the total dissolved solids of the waters. Iron and Al showed low mobility compared to Na. This was primarily due to consumption of these elements by ferrihydroxides, allophane, imogolite, and clay minerals. Also, Si and to a lesser degree Ca show apparent slightly lower mobility than Na. These results are in good agreement with these phases being dominant in the soils of the study area (Wada and others, 1992) and the main secondary minerals of basaltic glass in Iceland (Crovisier and others, 1992). Reaction progress calculations support these findings. According to a model interaction of basaltic glass and meteoric water Al, Fe, and Si are consumed by amorphous Fe and Al hydroxides and imogolite and allophane at low reaction progress. With increasing reaction progress, Ca-Fe-Mg smectite became the primary secondary minerals, limiting the mobility of Ca, Fe, Mg, Si, and Al. These findings suggest that the weathering in the Laxa in the Kjos catchment area has evolved into the beginning of the smectite weathering stage and that it is dominated by a relatively low reaction progress and a high water to rock ratio. In the present study the effect of runoff and rock crystallinity was quantified for dissolved elemental fluxes. Temperature, lithology (other than rock crystallinity), and rock age were similar in all of the catchment areas in this study. Silica, Ca, F, S, Al, K, Mg, and B had a very similar dependence on runoff. On the other hand, Na fluxes were found to be less dependent on runoff, and Fe fluxes were found to be independent of runoff. Basaltic glass dissolves faster than fully crystallized basaltic rocks (Gislason and Eugster, 1987a). Therefore, increased glass content of the primary rocks is thought to increase the leaching and elemental fluxes at constant runoff. However, because some of the solutes, particularly Al and Fe, were highly influenced or even controlled by the formation of secondary minerals, this increasing dissolution with increasing glass content was hidden. On the other hand, basaltic glass was observed to enhance the fluxes of more mobile elements like Na, Si, Ca, F, and S by a factor of 2 to 5 at a constant runoff of 200 cm/yr, constant vegetative cover, and 0 to 100 percent glass content of the rocks. The K and Mg fluxes were found to be independent of rock crystallinity. However, runoff alone cannot explain the fluxes of these elements, and it seems that vegetative cover and seasonal variations in biomass activities influence the K and Mg fluxes.

Chemical weathering of basalt in Southwest Iceland; effects of runoff, age of rocks and vegetative/glacial cover

Chemical weathering of basalt in Southwest Iceland; effects of runoff, age of rocks and vegetative/glacial cover

Present Chemical Weathering of Basalt in Iceland

Before the initiation of the water cycle on the 'early Earth' all the rocks were volcanogenic (e.g. Ronov, 1964). At present, volcanic rocks cover about 8 % of the land surface of the Earth and more than half of that is of basaltic composition (Meybeck, 1987). The present contribution of the weathering of volcanic rocks to the dissolved load of the world's rivers is less than 3%, the most important contribution comes from carbonate rocks (45%) and shale (20%; Meybeck, 1987). The high contribution of shale is primarily due to the weathering of small portions of the shale, i.e. calcareousand pyritic-shales. The chemical weathering of Ca-Mg silicate rocks is important for the atmospheric COz on a multimillion-year time scale and may therefore influence the temperature of the atmosphere (e.g. Berner, 1992). Thus, even to day, the weathering of basalt is an important factor in the net loss of CO2 from the atmosphere, and it's general importance increases with geological age because the ratio of basalt to sedimentary rocks increases with the age of the crust (e.g. Ronov, 1964). Iceland provides an opportunity to study the effects of the age of rocks, glaciation and vegetation on the nature and rate of chemical weathering of basalts. About 10% of the country is covered with glaciers, and one fourth is covered with vegetation. The interior of the country is unpopulated, providing pristine unpolluted catchment areas of uniform temperature and rock composition (basalt) with a variable glacier-and vegetation cover and variable runoff. The objective of this paper is to define and interpret, the source of supply for the dissolved constituents in Icelandic rivers, the relative mobility of the elements during weathering of basalt, the rate of chemical denudation in Iceland and the effect of age of rocks, and glacier/ vegetation cover on the chemical weathering of basalt.