• MEKALA RAM MOHAN

      Articles written in Journal of Earth System Science

    • Composition of the peninsular India rivers average clay (PIRAC): A reference sediment composition for the upper crust from peninsular India

      SHAIK SAI BABU RAVIPATI VENKATA RAMANA VENIGALLA PURNACHANDRA RAO MEKALA RAM MOHAN ARADHI KESHAV KRISHNA SARIPOOT SAWANT NANNAPANENI SATYASREE

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      We present a new dataset on the average composition of the clay fraction of sediments in 13 rivers draining the entire peninsular India, referred to here as Peninsular India Rivers Average Clay (PIRAC). PIRAC showed relatively low Si and high Fe, Mn and Mg compared to the other reference sediments. The total trace elements ($\Sigma$TE) content of PIRAC was lower than that of Post-Archean average Australian Shale (PAAS), but close to that of Average Suspended Sediment of World Rivers (ASSWR). The total rare earth elements ($\Sigma$REE) content of PIRAC was slightly lower than that of PAAS but close to that of World River Average Clay (WRAC). The $\Sigma$TE and $\Sigma$REE were much higher for PIRAC than in other reference sediments. Anomalously high Cu, Zn and Pb in PIRAC suggest that these trace elements do not reliably indicate the crustal composition. PAAS-normalised REE of PIRAC showed LREE-depleted, MREE- and HREE-enriched REE patterns with positive Ce and Eu anomalies, suggesting that PIRAC is more mafic than that of PAAS and the clays weathered from volcanic rocks and felsic component of the metamorphic rocks dominated the crustal composition of peninsular India. The REE pattern of PIRAC resembles to that of European Shale (ES) and Mud of Queensland (MUQ) but different from PAAS, WRAC, upper continental crust (UCC) and East China Post-Archean Shale (ECPAS), which exhibit LREE-enriched and HREE-depleted REE patterns. It implies that the REE composition of the upper crust is not uniform and it should be thoroughly investigated to determine the composition of PIRAC with more analyses on sediments for better understanding of the evolution of the crust.

    • Mineralogy and geochemistry of the sediments in rivers along the east coast of India: Inferences on weathering and provenance

      SHAIK SAI BABU VENIGALLA PURNACHANDRA RAO NANNAPANENI SATYASREE RAVIPATI VENKATA RAMANA MEKALA RAM MOHAN SARIPOOT SAWANT

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      The clay fraction of sediments in the lower reaches of 15 rivers along the east coast of India showed high kaolinite followed by illite and smectite for the rivers dominantly draining the Archaean–Precambrian Terrain (APT), high smectite followed by illite and kaolinite for those draining the Deccan Trap Volcanic Terrain (DVT) and, high illite followed by kaolinite, smectite or chlorite for those draining through Mixed-Lithology Terrain (MLT). The CIA (Chemical Index of Alteration) and PIA (Plagioclase Index of Alteration) values, depletion of Ca, K, Na and Sr and enrichment of Al, Fe and Ti, high Rb/Sr and Th/U ratios of sediments relative to the Upper Continental Crust indicate moderate to intense chemical weathering on source rocks. The average composition of clays exhibits slight enrichment of Fe, Mg, Sc, V, Co, Cr and Ni and depletion of Nb, Zr, Hf, Y and Ta relative to the Post-Archaean average Australian Shale. The Cu, Zn and Pb contents were in the range ‘significantly polluted to moderately polluted’ in APT- and DVT-sediments. The plots of TiO$_{2}$ vs. Fe$_{2}$O$_{3}$+MgO, Th/Sc vs. Sc and La–Th–Sc showed sediment composition intermediate between granite and basalt, while the plots of TiO$_{2}$ vs. Zr, Th vs. Sc and V–Ni–Th indicate intermediate provenance between mafic and felsic sources. The mineralogy of the sediments indicates mixed sources, but their chemical composition is affected by weathering and the provenance is intermediate between mafic and felsic sources.

      $\bf{Highlights}$

      The clay fraction of the sediments in 15 rivers along the east coast of India showed

      $\bullet$ Mixed-mineral assemblages in accordance with dominant geology and chemical weathering.

      $\bullet$ High CIA and PIA, and low Ca, K, Na and Sr values indicating moderate to intense chemical weathering on source rocks.

      $\bullet$ Enriched transition metals and depleted high-field strength elements supporting dominance of mafic component.

      $\bullet$ Ratio–ratio plots of trace metals indicating intermediate provenance between mafic and felsic sources.

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