• Prosenjit Ghosh

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

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

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

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.

• Journal of Earth System Science

Volume 129, 2020
All articles
Continuous Article Publishing mode

• Editorial Note on Continuous Article Publication

Posted on July 25, 2019