• J R Trivedi

      Articles written in Journal of Earth System Science

    • Major ion chemistry of the Ganga source waters: Weathering in the high altitude Himalaya

      M M Sarin S Krishnaswami J R Trivedi K K Sharma

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      A systematic study of the major ion chemistry of the Ganga source waters—the Bhagirathi, Alaknanda and their tributaries—has been carried out to assess the chemical weathering processes in the high altitude Himalaya. Among major ions, Ca, Mg, HCO3 and SO4 are the most abundant in these river waters. These results suggest that weathering of carbonate rocks by carbonic and sulphuric acids dominates in these drainage basins. On an average, silicate weathering can contribute up to ∼ 30% of the total cations.

      The concentration of total dissolved salts in the Bhagirathi and the Alaknanda is 104 and 115mg/l, respectively. The chemical denudation rate in the drainage basins of the Bhagirathi and the Alaknanda is, respectively, 110 and 137 tons/km2/yr, significantly higher than that derived for the entire Ganga basin, indicating intense chemical erosion of the Himalaya.

    • 187Re-187Os in Lesser Himalayan sediments: Measurement techniques and preliminary results

      J R Trivedi Sunil K Singh S Krishnaswami

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      The applications of the187Re-187Os isotope pair as a petrogenetic and geologic tracer are increasing in recent years due to several advances in the chemical extraction and purification of Re and Os, occurring at ppb levels in environmental samples, and in the precise determination of the Os isotope composition. We have established in our laboratory; based on available methods, chemical procedures and Negative Thermal Ionisation Mass Spectrometric techniques for the measurement of Re-Os concentrations in environmental samples and the Os isotope composition in them. Using these techniques, we are able to determine187Os/186Os ratios with a precision of ∼ 1% (±2σμ; twice the standard error of the mean) in several tens of picogram of Os. Preliminary analysis of black shales from the Lower Tal section of the Maldeota phosphorite mine yields a mean187Re-187Os model age of 597 ± 30 Ma. The187Os/186Os and Os concentration in black shales of the Lesser Himalaya range from 8 to 96 and 0.02 to 13 ng g-1 respectively. The mean187Os/186Os in these samples is ∼ 25, significantly higher than the crustal value of ∼ 10.5, suggesting that these black shales could be an important source of radiogenic Os to the rivers draining the Himalaya and to the steady increase in187Os/186Os of the oceans through the Cenozoic.

    • Trace element and isotopic studies of Permo-Carboniferous carbonate nodules from Talchir sediments of peninsular India: Environmental and provenance implications

      Prosenjit Ghosh S K Bhattacharya A M Dayal J R Trivedi M Ebihara M M Sarin A Chakrabarti

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      Syngenetic carbonate nodules constitute an interesting feature of the glaciogene sediments of various Talchir basins in peninsular India. Petrographic, cathodoluminescence and sedimentary results suggest that many of these nodules contain primary carbonate precipitates whose geochemical signatures can be used for determining environment of deposition and provenance of the sediments and drainage source. Several nodules were collected from Gondwana basins of east-central India and analyzed for stable carbon and oxygen isotope ratios, REE and trace element composition, and Sr isotope ratio. The mean 𝛿18O and 𝛿13C values of the calcites in the nodules are — 19.5‰ and -9.7‰ (w.r.t. PDB) respectively suggesting a freshwater environment (probably lacustrine) for formation of these objects. Trace element ratios (Eu/Eu and La/Yb) of the nodule samples show that the source of the sediments in the Damodar valley basin was the granites, gneisses and intrusives in the Chotanagpur region. The sediments in the Mahanadi valley were derived from granulites, charnockites and granites of the eastern ghat region. The Sr concentration of the carbonate phase of the nodules is low, ranging from 10-60 ng/g . The 87Sr/86Sr ratios of the samples from the west Bokaro basin and Ramgarh basin vary from 0.735 to 0.748 (mean: 0.739) and from 0.726 to 0.733 (mean: 0.730) respectively. These values are consistent with our proposition that water of these basins drained through the granitic rocks of the Chotanagpur region. In contrast, the 87Sr/86Sr ratios of the samples from the Talchir basin (Type area) of Mahanadi valley vary from 0.718 to 0.723 (mean: 0.719). These 87Sr/86Sr ratios are close to those of the granulites in the adjoining eastern ghat belt suggesting that area as the drainage source.

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