River geochemistry, chemical weathering, and atmospheric CO 2 consumption rates in the Virunga Volcanic Province (East Africa (original) (raw)
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Basalt weathering laws and the impact of basalt weathering on the global carbon cycle
Chemical Geology, 2003
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 Â 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.
Geomorphology, 2015
The chemical weathering rate and atmospheric/soil CO 2 consumption of Paraná flood basalts in the Preto Stream basin, São Paulo State, Brazil, were evaluated using major elements as natural tracers. Surface and rain water samples were collected in 2006, and analyses were performed to assess pH, temperature, dissolved oxygen (DO), electrical conductivity (EC) and total dissolved solids (TDS), including SO 4 2− , NO
Global Biogeochemical …, 2003
The silicate rock weathering followed by the formation of carbonate rocks in the ocean, transfers CO 2 from the atmosphere to the lithosphere. This CO 2 uptake plays a major role in the regulation of atmospheric CO 2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks. This leads us to develop the first world lithological map with a grid resolution of 1°Â 1°. This paper analyzes the spatial distribution of the six main rock types by latitude, continents, and ocean drainage basins and for 49 large river basins. Coupling our digital map with the GEM-CO2 model, we have also calculated the amount of atmospheric/soil CO 2 consumed by rock weathering and alkalinity river transport to the ocean. Among all silicate rocks, shales and basalts appear to have a significant influence on the amount of CO 2 uptake by chemical weathering.
Global Biogeochemical Cycles, 2003
1] The silicate rock weathering followed by the formation of carbonate rocks in the ocean, transfers CO 2 from the atmosphere to the lithosphere. This CO 2 uptake plays a major role in the regulation of atmospheric CO 2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks. This leads us to develop the first world lithological map with a grid resolution of 1°Â 1°. This paper analyzes the spatial distribution of the six main rock types by latitude, continents, and ocean drainage basins and for 49 large river basins. Coupling our digital map with the GEM-CO2 model, we have also calculated the amount of atmospheric/soil CO 2 consumed by rock weathering and alkalinity river transport to the ocean. Among all silicate rocks, shales and basalts appear to have a significant influence on the amount of CO 2 uptake by chemical weathering.
Aquatic Geochemistry, 2008
Analyses of 72 samples from Upper Panjhara basin in the northern part of Deccan Plateau, India, indicate that geochemical incongruity of groundwater is largely a function of mineral composition of the basaltic lithology. Higher proportion of alkaline earth elements to total cations and HCO 3 [Cl + SO 4 reflect weathering of primary silicates as chief source of ions. Inputs of Cl, SO 4 , and NO 3 are related to rainfall and localized anthropogenic factors. Groundwater from recharge area representing Ca + Mg-HCO 3 type progressively evolves to Ca + Na-HCO 3 and Na-Ca-HCO 3 class along flow direction replicates the role of cation exchange and precipitation processes. While the post-monsoon chemistry is controlled by silicate mineral dissolution + cation exchange reactions, pre-monsoon variability is attributable chiefly to precipitation reactions + anthropogenic factors. Positive correlations between Mg vs HCO 3 and Ca + Mg vs HCO 3 supports selective dissolution of olivine and pyroxene as dominant process in post-monsoon followed by dissolution of plagioclase feldspar and secondary carbonates. The pre-monsoon data however, points toward the dissolution of plagioclase and precipitation of CaCO 3 supported by improved correlation coefficients between Na + Ca vs HCO 3 and negative correlation of Ca vs HCO 3 , respectively. It is proposed that the eccentricity in the composition of groundwater from the Panjhara basin is a function of selective dissolution of olivine [ pyroxene followed by plagioclase feldspar.
Chemical Geology, 2016
Chemical weathering rates and atmosphere/soil CO 2 consumption of igneous and metamorphic rocks under tropical climate in southeastern Brazil were evaluated using the chemical composition of surface waters and fresh rocks and soil (horizon C) in the Upper Sorocaba River basin. Surface water samples were collected between June/2009 and June/2010, and analyses were performed to assess pH, electrical conductivity (EC), temperature and total dissolved solids (TDS), including Na + , K + , Ca 2+ , Mg 2+ , Cl − , SO 4 2− , PO 4 3− , NO 3 − and SiO 2. Fresh rocks and C horizon samples were also collected, taking into account their geological context, abundance and spatial density, to analyze major elements and mineralogy. The concentration of TDS and dissolved cations, anions and silica increased during the dry period in relation to the wet period, and the same behavior was observed for pH, EC and temperature. After corrections of anthropogenic contributions (ca. 21 t/km 2 /yr) and atmospheric inputs (ca. 19 t/km 2 /yr), the annual flux due to chemical weathering involving the igneous and metamorphic rocks was ca. 29 t/km 2 /yr. The CO 2 atmospheric/soil consumption in the Upper Sorocaba River basin was ca. 0.2 × 10 6 mol/km 2/ yr, and when extrapolated to the entire Mantiqueira Orogenic Belt, accounted an estimated consumption of 0.07 × 10 12 mol/yr, representing 0.6% of the total CO 2 consumption flux derived from global average silicate weathering. The chemical weathering rates of igneous and metamorphic rocks in the Upper Sorocaba River basin were estimated at 15 m/My, respectively. The main weathering process in this watershed was the monossialitization, with partial hydrolyses of bedrock minerals, except quartz, which was not weathered and remained in the soil profile. The annual specific flux derived from igneous and metamorphic rocks at Upper Sorocaba River basin could be compared with watersheds in tropical climates. However, this value is higher than in other North American, European, Asian and African granitoid watersheds, and lower than in montane watersheds.
Chemical weathering and consumption of atmospheric carbon dioxide in the Alpine region
Global and Planetary Change, 2016
To determine the CO 2 consumption due to chemical weathering in the Alps, water samples from the 32 main Alpine rivers were collected and analysed in two periods, spring 2011 and winter 2011/2012. Most of the river waters are characterized by a bicarbonate earth-alkaline composition with some samples showing a clear enrichment in sulphates and other samples showing a slight enrichment in alkaline metals. The amount of total dissolved solids (TDS) ranges between 96 and 551 mg/L. Considering the major ion composition and the Sr isotopic composition of water samples, coherently with the geological setting of the study area, three major reservoirs of dissolved load have been recognized: carbonates, evaporites and silicates. Based on a chemical mass balance, the flux of dissolved solids, and the flux of carbon dioxide consumed by chemical weathering have been computed for each basin and for the entire study area. Results show that the flux of dissolved solids, ranges from 8 × 10 3 to 411 × 10 3 kg km −2 y −1 , with an average value of 127 × 10 3 kg km −2 y −1 , while the flux of carbon dioxide consumed by chemical weathering in the short-term (b 1 Ma) is 5.03 × 10 5 mol km −2 y −1 1 on average. Since part of the CO 2 is returned to the atmosphere through carbonate precipitation and reverse weathering once river water reaches the ocean, the CO 2 removed from the atmosphere/soil system in the long-term (N 1 Ma) is much smaller than the CO 2 consumed in the short-term and according to our calculations amounts to 2.01 × 10 4 mol km −2 y −1 on average. This value is almost certainly a minimum estimate of the total amount of CO 2 fixed by weathering on the long-term because in our calculations we assumed that all the alkaline metals deriving from rock weathering in the continents are rapidly involved in the process of reverse weathering in the oceans, while there are still large uncertainties on the magnitude and significance of this process. The values of CO 2 flux consumed by weathering are strongly correlated with runoff while other potential controlling factors show only weak correlations or no correlation. Our estimation of the CO 2 consumed by weathering in the Alpine basins is in the same order of magnitude, but higher than the world average and is consistent with previous estimations made in river basins with similar climatic conditio and similar latitudes.