Dissolved inorganic carbon export by mountainous tropical rivers of the Western Ghats, India

Abstract

Large rivers that are known for carrying huge fluxes are few in numbers; nevertheless, the contributions of thousands of small mountainous coastal rivers to global biogeochemical cycles is also measured to be equally important. Small catchment sizes, high channel gradients yet high water discharge are prominent characters of the coastal rivers draining the Western Ghats (WG) of India. Three geo-tectonic divisions, namely, Southern Granulite Terrain (SGT) in southern, Western Darwar Craton (WDC) in central and Deccan Traps (DT) in the north constitute the entire Western Ghats. Based on the seasonal sampling of 70 coastal rivers draining into the Arabian Sea, this study provides a comprehensive discharge-weighted estimate of DIC for the Western Ghats (WG) region. The concentration of bicarbonate measured in-situ using gran plot method. Fluxes and yields were calculated using seasonal discharge and area of respective rivers. The discharge-weighted DIC concentration of WG region is 629 μM, which is slightly higher than the average global DIC concentration for the low latitude (< 30°) rivers. Despite, covering 113×103km2 area, this region yearly exports 175×109 mol DIC to the Arabian Sea. The DIC flux contributed by the WG region is among the highest in peninsular rivers and comparable with several large global rivers. DIC export rates suggest that within the region, the chemical weathering rate of DT is ∼4 times and WDC is ∼2.5 times higher than that of SGT region. The DIC flux of the WG region (covering 0.076% of global land area) is approximately 0.55% of the global riverine DIC flux to the ocean. Thus, in terms of global DIC export to seas, this region is the 28th largest contributor. Due to high annual DIC yield (1547×103 mol km−2) WG region ranks as the 9th most active region of the world. DIC yields of rivers draining granite-dominated (SGT and WDC) and basalt-dominate (DT) regions of WG are remarkably higher than the global rivers draining similar lithology from temperate and arctic regions. The WG rivers display a latitudinal gradient in DIC transport. Type of silicate rocks, rainfall-runoff, temperature range, and sediment yields control the variability in DIC fluxes and export rates of Western Ghats region.