Articles written in Journal of Earth System Science

    • 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.

    • An experimental set-up for carbon isotopic analysis of atmospheric CO2 and an example of ecosystem response during solar eclipse 2010

      Tania Guha Prosenjit Ghosh

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      We present here, an experimental set-up developed for the first time in India for the determination of mixing ratio and carbon isotopic ratio of air-CO2. The set-up includes traps for collection and extraction of CO2 from air samples using cryogenic procedures, followed by the measurement of CO2 mixing ratio using an MKS Baratron gauge and analysis of isotopic ratios using the dual inlet peripheral of a high sensitivity isotope ratio mass spectrometer (IRMS) MAT 253. The internal reproducibility (precision) for the 𝛿13C measurement is established based on repeat analyses of CO2 ± 0.03‰. The set-up is calibrated with international carbonate and air-CO2 standards. An in-house air-CO2 mixture, ‘OASIS AIRMIX’ is prepared mixing CO2 from a high purity cylinder with O2 and $N_2$ and an aliquot of this mixture is routinely analyzed together with the air samples. The external reproducibility for the measurement of the CO2 mixing ratio and carbon isotopic ratios are ± 7 ($n = 169$) 𝜇 mol·mol−1 and ± 0.05 ($n = 169$)‰ based on the mean of the difference between two aliquots of reference air mixture analyzed during daily operation carried out during November 2009–December 2011. The correction due to the isobaric interference of $N_{2}O$ on air-CO2 samples is determined separately by analyzing mixture of CO2 (of known isotopic composition) and N2O in varying proportions. A +0.2‰ correction in the 𝛿13C value for a N2O concentration of 329 ppb is determined. As an application, we present results from an experiment conducted during solar eclipse of 2010. The isotopic ratio in CO2 and the carbon dioxide mixing ratio in the air samples collected during the event are different from neighbouring samples, suggesting the role of atmospheric inversion in trapping the emitted CO2 from the urban atmosphere during the eclipse.

    • Moisture rainout fraction over the Indian Ocean during austral summer based on ¹⁸O/¹⁶O ratios of surface seawater, rainwater at latitude range of 10°N–60°S

      K Prasanna Prosenjit Ghosh S K Bhattacharya P Rahul Kei Yoshimura N Anilkumar

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      Oxygen isotope ratios (¹⁸O/¹⁶O) of surface seawater and rainwater samples from the Indian Ocean region (10◦N–60◦S) during austral summer collected onboard ORV Sagar Nidhi during 2011–2013 have been measured along with salinity, sea surface temperature and relative humidity. The rainwater is isotopically lighter (by 4.6±2.70/00) compared to the equilibrium condensation of the vapour arising from the seawater at the ambient condition. The isotopic composition of the vapour at high altitude responsible for the rain formation at the sampling location is estimated from a global atmospheric water isotope model (IsoGSM2). The apparent deficit of ~5⁰/₀₀ can be explained by invoking a high degree of rainout (on average, about 70% of the overhead atmospheric moisture) during transport of the source vapour to the sampling location undergoing a Rayleigh fractionation. The required rainout fraction is higher (~80%) in the latitude belt 40◦–60◦S compared to the equatorial belt (~60%). The pattern of variation in the rainout fraction with latitude is consistent with the well-known evaporation/precipitation processes inthe Indian Ocean.

    • Seasonal freshwater flux estimation using mollusc from the tropical Mandovi Zuari estuary, Goa, India


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      Seawater incursion and freshwater discharges into the tropical Mandovi Zuari (MZ) estuary is investigated here using stable oxygen isotope data on monthly water samples from locations spatially separated within the estuary. Surface water samples were analysed for $\delta^{18}$O and salinity relationship,$\delta^{18}$O=0.0843$\times$S−2.1, where freshwater end member is designated as −2.1 per thousand; similar to the composition recorded for the rain water. We have estimated percentage of monthly freshwater fluxes into the estuary using mass balance equation adopting an isotopic value of seawater end member. Our estimates showed freshwater contribution to the estuary water during monsoon time was 63%, while in the post-monsoon season the fraction drops to 34.7%. The contribution of the freshwater registered a minimum value of 6.8% during the pre-monsoon season. Further, we analysed the seasonal growth band secreted by the mollusc from the same estuary to understand the potential of mollusc as a recorder of seasonal water composition. The $\delta^{18}$O of mollusc shell growth layers varied over a range between −4.3 per thousand and −2.1 per thousand. We interpreted this as seasonal signal assuming the growth rate from the culture experiment. The $\delta^{18}$O of estuary water and observed temperature are used to simulate the isotopic composition of seasonal growth bands. The lighter $\delta^{18}$O value of −4.3 per thousand precipitated during the month of July 2010, which coincides with the time of low productivity ($\delta^{13}$C = −3.5 per thousand). While the heavier $\delta^{18}$O (−2.1 per thousand) is recorded in the growth layer generated during November 2010 defining the period of post-monsoon growth.


      $\bullet$ Here we presented our observation on the spatiotemporal variability of salinity and $\delta^{18}$O$_{water}$ across a tropical estuary, Mandovi Zuary located at the western coast of India and fed by rivers originating from the peninsular region which receives rainfall during period of Indian summer monsoon.

      $\bullet$ We designed a two-component mixing model for the estimation of freshwater fluxes at monthly time intervals and demonstrated its efficacy with the tide gauge data.

      $\bullet$ The study highlighted the scope of isotope mass balance approach in estimating freshwater fluxes at seasonal time domain.

      $\bullet$ Further, we demonstrated that the isotopic signature preserved in the growth bands of the bivalve shell can be useful proxy for the reconstruction of freshwater fluxes in an estuary and thus add a new tool for reconstruction of seasonal runoff.

      $\bullet$ The isotope approach used here can also be extended to other estuaries in the peninsular India and also in other tropical continental settings where hydrological process is driven by seasonal reversal of wind direction.

    • Relic surface water (clay-pore water) input triggers arsenic release into the shallow groundwater of Bengal aquifers


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      The geogenic source of arsenic (As) in groundwater is an indisputable fact. Arsenic contamination in the shallow groundwater of West Bengal is an alarming health issue with progressive growth of regional population and demand for water. The As contamination in shallow groundwater is facilitated by microbial activities, which depend on the inflow of dissolved organic carbon (DOC), providing a temporary reducing condition conducive to the release of As from aquifer sediments. Here, we present a documentation over several years of seasonal observations on dissolved total As contents and stable isotope ratios in shallow groundwater, ‘deep pond water’ (collected from the deepest part of the pond just above the sediment–water interface), river water and rainwater from the Chakdaha region, Haringhata block, Nadia district, West Bengal. The current study shows, for the first time, a large similarity in the isotopic composition of seasonal deep pond water with the shallow aquifer-intercalated clay sediment-trapped pore water (data adopted from the literature on a Bangladesh region). We highlight the possible processes wherein seasonal eutrophication during dry time followed by monsoon time flooding and sedimentation allows the burial of organic-rich clayey sediments with trapped DOC rich pore water, which serve as a source of clay-pocket pore water. The isotopic composition of claypore water formed at a historical time scale resembles the deep-bottom water composition of surface water bodies and provides an ideal composition responsible for triggering the seasonal release of As into the shallow groundwater. The processes such as excess withdrawal of groundwater during dry periods and consequent squeezing of aquifer sediment-intercalated clay-lenses can expel its organic-rich contents to trigger anaerobic microbial activities and As release into the adjoining water. Using D-excess and ${\delta}^{18}$O as conservative tracers, we have shown that ${\sim}$13–14% mixing (exclusion) of clay-pore water to the adjoining As-uncontaminated (${\le}$10 ${\mu}$g/L) shallow regional groundwater is sufficient to cause high As (${\ge}$10 ${\mu}$g/L) mobilisation.


      $\bullet$ Shallow groundwater samples (${\le}$60 m bgl) collected at seasonal time intervals (from Nadia) show increasing dissolved total arsenic (As) contents during dry pre-monsoon periods, featuring mixing of aquifer-intercalated clay-lens-trapped pore water with the adjoining groundwater.

      $\bullet$ Excessive groundwater withdrawal during dry periods generates compaction of the intercalated clay-lenses placed within the shallow aquifer and actuates the process of clay-pore water expulsion which causes land subsidence.

      $\bullet$ An estimation of ~13–14% mixing (exclusion) of aquifer-intercalated clay-pore water (high As, organic-rich) to the adjoining As-uncontaminated (≤10 µg/L) regional shallow groundwater is sufficient to trigger high As mobilisation.

      $\bullet$ Aquifer-intercalated clay-pore water is equivalent to ‘relic surface water’, making the presence of palaeo-water of quaternary time scales.

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