Paper No. 7
Presentation Time: 10:05 AM
MAJOR IONS AND RADIOGENIC STRONTIUM ISOTOPIC COMPOSITION IN A HUMID TROPICAL RIVER (NETHRAVATI-GURUPUR BASIN), SOUTHWESTERN INDIA
The study aims to improve our understanding of chemical weathering process and elemental transport in the humid tropical river basins. The dissolved and particulate major ion/element, trace element and 87Sr/86Sr isotopic compositions were measured from Nethravati-Gurupur river and its main tributary channels. The river basin drains Archean silicate basement (Dharwar Craton) such as granite gneisses, charnockites and metasediments. The surface area of the river basin is 4540 km2 and the river discharges 388 m3s-1 water and 14x105 tons of sediments. These rivers exhibit low major cation concentrations i.e., Na, Mg, K, Ca. but high concentrations of Si, Al, Fe. The total dissolved concentrations (TDS) are less (46 mg L-1) and dominated by silica. The major and minor elements in the basin are higher during base flow and lesser during peak flow season and the relationship can be explained by Power Law reduction equation. The chemical weathering is intense in the basin which is controlled by higher surface runoff and warm temperature (Gurumurthy et al. 2012). The basin is characterized with lesser dissolved Sr concentration (avg. 150 µmol L-1) and radiogenic 87Sr/86Sr isotopic ratios (avg. 0.72041 at outlet) that can be explained by the weathering of silicate basement rock in the basin. These values are higher than the world river average (avg. 0.7119) and other tropical major rivers. Strontium concentration and 87Sr/86Sr isotopic compositions are strongly correlating with silicate derived cations (corrected for atmospheric and carbonate contribution). The 87Sr/86Sr isotopic composition shows strong seasonal variation in the basin which corresponds to variations in the silicate weathering components in the basin. There is a strong seasonal variation in the isotopic composition i.e., highly radiogenic values during the dry period of sampling whereas less radiogenic values during peak flow season. The calculated Chemical Index of Alteration (CIA) suggests intense weathering in the basin and the variation in intensity of weathering in the basin is negligible.
Ref: Gurumurthy GP, Balakrishna K, Riotte J, Audry S, Braun J-J, Udayashankar HN, Manjunatha BR (2012). Controls on intense silicate weathering in tropical river basins, southwestern India, Chemical Geology, 300-301, 61-69.