Abstract. Rivers are an important source of dissolved inorganic carbon (DIC) to the
adjacent coastal waters. In order to examine the spatial variability in the
distribution and major sources of DIC in the Indian monsoonal rivers and to
quantify their export flux to the northern Indian Ocean, 27 major and medium-sized rivers were sampled during the discharge period. Significant spatial
variability in concentrations of DIC (3.4–73.6 mg L−1) was observed,
and it is attributed to spatial variations in the precipitation pattern, the size
of rivers, pollution and lithology of the catchments. The stable isotopic
composition of DIC (δ13CDIC) also showed strong spatial
variability (−13.0 ‰ to −1.4 ‰) in the Indian monsoonal
rivers with relatively depleted δ13CDIC values in rivers of
the northwest of India (-11.1±2.3 ‰) and
enriched values in the southeast of India (-3.5±2.3 ‰). Results of the linear least-squares regression
models of Keeling and Miller–Tan's plots indicated that the chemical weathering of
carbonate and silicate minerals by soil CO2 is the major source of DIC
in the Indian monsoonal rivers. Spatial variability in the deviation of
δ13CDIC from the approximated δ13C of the source may
probably be due to dominant autotrophic production in rivers of the southeastern
region, whereas heterotrophic decomposition of organic matter largely
influences the other Indian monsoonal rivers. It is estimated that the
Indian monsoonal rivers annually export ∼10.3 Tg of DIC to the
northern Indian Ocean, of which the major fraction (75 %) enters into the
Bay of Bengal, and the remaining fraction reaches to the Arabian Sea. This is
consistent with the freshwater flux, which is 3 times higher for the Bay of
Bengal (∼378 km3 yr−1) than for the Arabian Sea (122 km3 yr−1). Despite discharge from the Indian monsoonal rivers
accounting for only 1.3 % of the global freshwater discharge, they
disproportionately export 2.5 % of the total DIC exported by the world's major
rivers. Despite rivers from the region in the southwest (SW) of India exporting DIC that is an order of
magnitude lower (0.3 Tg yr−1) than the rivers from other regions of
India, the highest yield of DIC was found in the rivers of the SW region of India. It is attributed to intense precipitation (∼3000 mm), favorable natural vegetation of tropical moist deciduous and tropical wet evergreen and semi-evergreen forests, tropical wet climate, high soil organic carbon, and the
dominance of red loamy soils in catchments of the rivers of the SW region.
Hydrologic Transport of Dissolved Inorganic Carbon and Its Control on Chemical Weathering
