• M M Sarin

Articles written in Journal of Earth System Science

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

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.

• Natural radionuclides in the Arabian Sea and Bay of Bengal: Distribution and evaluation of particle scavenging processes

Vertical and temporal variations in the activities of234Th,210Po and210Pb have been measured, in both dissolved and paniculate phases, at several stations in the eastern Arabian Sea and north-central Bay of Bengal. A comparative study allows us to make inferences about the particle associated scavenging processes in these two seas having distinct biogeochemical properties.

A common feature of the234Th profiles, in the Arabian Sea and Bay of Bengal, is that the dissolved as well as total (dissolved + particulate) activity of234Th is deficient in the surface 200 m with respect to its parent,238U. This gross deficiency is attributed to the preferential removal of234Th by adsorption onto settling particles which account for its net loss from the surface waters. The scavenging rates of dissolved234Th are comparable in these two basins. The temporal variations in the234Th-238U disequilibrium are significantly pronounced both in the Arabian Sea and Bay of Bengal indicating that the scavenging rates are more influenced by the increased abundance of particles rather than their chemical make-up. In the mixed layer (0–50 m), the scavenging residence time of234Th ranges from 30 to 100 days.

The surface and deep waters of both the seas show an enhanced deficiency of dissolved210Po relative to210Pb and that of210Pb relative to226Ra. The deficiencies of both210Po and210Pb in the dissolved phases are not balanced by their abundance in the particulate form indicating a net loss of both these nuclides from the water column. The scavenging rates of210Po and210Pb are significantly enhanced in the Bay of Bengal compared to those in the Arabian Sea. The mean dissolved210Po/210Pb and210Pb/226Ra activity ratios in deep waters of the Bay of Bengal are ∼ 0.7 and 0.1, respectively, representing some of the most pronounced disequilibria observed to date in the deep sea. The Bay of Bengal and the Arabian Sea appear to be the regions of most intense particle moderated scavenging processes in the world oceans. This is evidenced by the gross disequilibria exhibited by the three isotope pairs used in this study.

• Recent sedimentary records from the Arabian Sea

An attempt is made to understand the redox conditions that prevailed in the north eastern continental margins of the Arabian Sea and in the nearby deep water regions during the past few centuries using short undisturbed sediment cores. The geochronology is accomplished using210Pb excess method and the proxy indicators chosen for productivity and associated redox changes are CaCO3, organic matter (OM), Mn and U along with major elements Fe and Al. Such changes in principle are related to high productivity in the overlying waters which in turn depend on monsoonal intensity that causes upwelling responsible for increase in productivity. Alongwith the published data on gravity cores from the same region, our measurements suggest the following:

At ∼ 300 m water depth, south of 21°N, the sediment-water interface at depths of ∼ 300 m had been anoxic during the time span represented by the presently studied cores for approximately ∼ 700y as evidenced by low Mn/Al (&lt; 0.7 × 10−2) and high U/Al (&gt; 10−4) weight ratios. In some adjacent deeper regions, however, the environment turned oxic around ∼ 200 y BP. Whereas both Mn and Ra were lost to the overlying waters in the anoxic regions (depth ∼340m), the Mn that diffused from deeper sections appears to have mineralized at the sediment-water-interface. Studies of this type on long undisturbed cores from the margins of the Arabian Sea and the Bay of Bengal, involving several proxies and geochronology by more than one method are needed to understand short term environmental (and monsoonal intensity) changes of the recent past with high resolution.

• 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.

• Chemical characteristics of aerosols in MABL of Bay of Bengal and Arabian Sea during spring inter-monsoon: A comparative study

The chemical composition of aerosols in the Marine Atmospheric Boundary Layer (MABL) of Bay of Bengal (BoB) and Arabian Sea (AS) has been studied during the spring and inter-monsoon (March–May 2006) based on the analysis of water soluble constituents (Na+, NH$^{+}_{4}$, K+, Mg2+, Ca2+, $Cl^{−}$, NO$^{−}_{3}$ and SO$^{2−}_{4}$), crustal elements (Al, Fe, and Ca) and carbonaceous species (EC, OC). The total suspended particulates (TSP) ranged from 5.2 to 46.6 𝜇 g m−3 and 8.2 to 46.9 𝜇 g m−3 during the sampling transects in the BoB and AS respectively. The water-soluble species, on average, accounted for 44% and 33% of TSP over BoB and AS respectively, with dominant contribution of SO$^{2−}_{4}$ over both the oceanic regions. However, distinct differences with respect to elevated abundances of NH$^{+}_{4}$ in the MABL of BoB and that of Na+ and Ca2+ in AS are clearly evident. The non-sea-salt component of SO$^{2−}_{4}$ ranging from 82 to 98% over BoB and 35 to 98% over AS; together with $nss-Ca^{2+}/nss-SO^{2−}_{4}$ equivalent ratios 0.12 to 0.5 and 0.2 to 1.16, respectively, provide evidence for the predominance of anthropogenic constituents and chemical transformation processes occurring within MABL. The concentrations of OC and EC average around 1.9 and 0.4 𝜇 g m−3 in BoB and exhibit a decreasing trend from north to south; however, abundance of these carbonaceous species are not significantly pronounced over AS. The abundance of Al, used as a proxy for mineral aerosols, varied from 0.2 to 1.9 𝜇 g m−3 over BoB and AS, with a distinctly different spatial pattern – decreasing north to south in BoB in contrast to an increasing pattern in the Arabian Sea.

• # Journal of Earth System Science

Current Issue
Volume 128 | Issue 8
December 2019

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019