Factors controlling basalt weathering in the Deccan traps: a small catchment perspective

Abstract

Chemical weathering fluxes from small basalt catchments, the Ambika and the Girna in the Deccan Traps, have been compared. The two catchments are characterized by distinct climates: humid in the Ambika and semi-arid in the Girna River. We collected water samples at the sub- catchment scale of these basins and analysed concentrations of major ions and dissolved Sr in them to estimate the weathering rates and to assess their controlling factors. The catchment area, slope and relief of the catchments were derived using Digital Elevation Model (DEM) on ArcGIS platform whereas the Soil and Water Assessment Tool (SWAT) was used to derive the sub-catchment scale runoffs. The SWAT simulated annual discharge shows a nearly three times larger discharge in the Ambika River compared to the Girna River. The various sources, viz., basalt, carbonates, sea-salts and saline alkaline soils (SAS) contributing to major ions in dissolved load, were quantified using inverse modelling approach. The total chemical denudation rates (TCDR*) during monsoon, 1033 ± 512 tons km−2 y-1, is ∼ 12 times higher compared to non-monsoon (82 ± 42 tons km−2 y-1) in the Ambika River, and ∼ 3 times higher in the Girna River (monsoon: 185 ± 155 tons km−2 y-1 vs. non-monsoon: 51 ± 25 tons km−2 y-1). Annually, TCDR* is around 6-fold higher in the Ambika River (∼683 tons km−2 y-1) than in the Girna River (∼113 tons km−2 y-1) at their outlets. Basalt (silicate) weathering rates (BasWR) in the Ambika and the Girna rivers are 88 ± 19 and 12 ± 3 tons km−2 y-1, respectively. The chemical weathering rates in the two adjacent catchments is primarily controlled by rainfall/runoff, while relief plays secondary control. CO2 consumption rates (CCR) of the Ambika with higher rainfall are much higher ((13 ± 4) × 105 mol km−2 y-1) compared to most of the inactive basalt cathchments globally.