Sources of sulfur in Deccan Trap rivers: A reconnaissance isotope study Anirban Das a,⇑ , N.J. Pawar b , Jan Veizer a a Ottawa-Carleton Geoscience Centre, Ottawa, Canada K1S 5B6 b Shivaji University, Kolhapur 416004, Maharashtra, India article info Article history: Available online 9 December 2010 Editorial handling by S. Bottrell abstract Sulfur isotopic composition (d 34 S) of dissolved SO 2 4 was measured in waters of rivers draining the Deccan Trap basalts in order to elucidate the source(s) of elevated SO 2 4 content in some of these rivers, observed in an earlier study. The d 34 S values range from 8.7‰ to 19.5‰, with an average of 14.5 ± 2.8‰, values gen- erally enriched in 34 S compared to data available from other Indian rivers. The Bhima River (a tributary of the Krishna) and most of its tributaries have d 34 S within the 14 ± 2‰ range and high dissolved SO 2 4 con- centrations (256–1556 lM). Measured d 34 S of 7 of the saline/alkaline soils suggest that the high dissolved SO 2 4 in the Bhima river system is likely derived from this source and/or gypsum concretions in the soils. The d 34 S–SO 4 relationship for the Krishna River and its smaller tributaries follows a mixing pattern of two end-members, rainwater with low SO 2 4 and low d 34 S and an unknown, likely a pollution, source with intermediate SO 2 4 content and heavy d 34 S. The most important finding that this study brings out is the minimal role that the weathering of basalt-sulfides play in contributing to the dissolved SO 2 4 in the riv- ers. This rules out the role of H 2 SO 4 -, and supports H 2 CO 3 -mediated weathering as the dominant process in the Deccan settings. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Dissolved SO 2 4 in rivers can originate from a multitude of sources, such as oxidation of igneous and sedimentary sulfides, dis- solution of evaporites, atmospheric deposition or anthropogenic input (Berner and Berner, 1996), and the isotopic composition of S may enable attribution to specific sources. Apart from immediate environmental applications, such knowledge can help to under- stand weathering processes in watersheds. The weathering is med- iated by proton-supplying H 2 CO 3 and/or H 2 SO 4 , but it is only the silicate weathering mediated by the carbonic acid that may impact the atmospheric CO 2 balance on longer time scales (Spence and Telmer, 2005) and thus play a role in climate considerations. The rivers flowing across the Deccan Traps in India have been previously studied for major ion chemistry, C and Sr isotopes, and elemental chemistry of sediments (Dessert et al., 2001; Das et al., 2005a,b, 2006; Das and Krishnaswami, 2006, 2007) and some showed high concentrations of SO 2 4 . The Deccan Trap eruptions were linked to climate change at the Cretaceous–Tertiary Bound- ary (KTB); it has been argued that the contemporaneous 87 Sr/ 86 Sr (Dessert et al., 2001; Das et al., 2006) and 187 Os/ 188 Os (Ravizza and Peucker-Ehrenbrink, 2003) signals in seawater were related to weathering of basalts. If weathering of basalt-sulfides is indeed an important source of Os to the ocean, the S in rivers draining the Deccan Traps should reflect the isotopic composition of this source, that is, of their sulfide minerals. If, however, there are other sources of SO 2 4 (and not of Os) to the rivers, overwhelming the sul- fide source, then the isotopic signals of S from the sulfides can be obscured. Das et al. (2005a), however, observed that the elevated concen- trations of dissolved SO 2 4 in Deccan Trap rivers (DTR) coincided with high dissolved Cl and Na. This was particularly the case for the Bhima River and its tributaries (Fig. 1), a watershed with a semi-arid climate. The high SO 2 4 content was, therefore, attributed primarily to the weathering/dissolution of salt affected soils (Bhargava and Bhattacharjee, 1982; NRSA, 1998), with industrial and anthropogenic activities potentially a contributing factor. Fol- lowing earlier studies that utilized the isotopes of S and O in SO 2 4 for discrimination of ‘natural’ and ‘anthropogenic’ sources in rivers (Robinson and Bottrell, 1997; Gislason and Torssander, 2006) and groundwater systems (Hughes et al., 1999; Moncaster et al., 2000; Bottrell et al., 2008), it intended to apply this isotopic ap- proach to the present study, the more so that the anthropogenic component in ‘global’ rivers today is typically about 50% (Berner and Berner, 1996). While characterization of sources for high SO 2 4 in the Deccan Trap rivers is of interest in and of itself, the present study aims pri- marily at testing the proposition that weathering of basaltic sul- fides is an important source of acidity in these watersheds and, by analogy, the cause of isotopic anomalies in seawater at the K/ T transition. 0883-2927/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2010.12.003 ⇑ Corresponding author. Address: School of Petroleum Technology, Raisan, Gandhinagar 382 007, India. E-mail addresses: anirban.das@spt.pdpu.ac.in, ani260175@gmail.com (A. Das). Applied Geochemistry 26 (2011) 301–307 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem