Silicate or Carbonate Weathering: Fingerprinting Sources of Dissolved Inorganic Carbon Using δ13C in a Tropical River, Southern India

Abstract:

Rivers are an inherently vital resource for the development of any region and their importance is highlighted by the presence of many ancient human civilizations adjacent to river systems. δ¹³C - Si/HCO₃ systematics has been applied to large south Indian rivers which drain the Deccan basaltic traps in order to quantify their relative contributions from silicate and carbonate weathering. This study investigates δ¹³C - Si/HCO₃ systematics of the Cauvery River basin which flows through silicate lithology in the higher reaches and carbonate lithology with pedogenic and marine carbonates dominating the terrain in the lower reaches of the basin. The samples for the present study were collected at locations within the watershed during Pre-Monsoon and Monsoon Season 2014. The measurements of stable isotope ratios of δ¹³C_DIC and were accomplished through a Thermo Scientific GasBench II interface connected to a MAT 253 IRMS.

We captured a large spatial variation in δ¹³C and Si/HCO₃ values during both seasons; Pre-Monsoon δ¹³C values ranges between -17.57‰ to -4.02‰ and during Monsoon it varies between -9.19‰ to +0.61‰. These results indicate a two end-member mixing component i.e. a silicate and a carbonate end member; governing the weathering interactions of the Cauvery River. Within the drainage basin, we identified silicate and carbonate dominating sources by using contributions of DIC and δ¹³C. Si/HCO₃ values for Pre-Monsoon ranges between 0.028 - 0.67 and for Monsoon it varies between 0.073 - 0.80. Lighter δ¹³C composition was observed at sampling sites at higher altitude in contrast to sampling sites at flood plain which show relatively enriched δ¹³C which indicate mixing of soil derived CO₂ with C₄ plants. Result suggests dominance of carbonate weathering during the Monsoon Period, while silicate weathering is pronounced during Pre- Monsoon period.