Export fluxes of dissolved inorganic carbon to the northern Indian Ocean from the Indian monsoonal rivers

Abstract

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.

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“…Such a significantly higher DIC export rate, as compared to POC export rate, has been also observed in various other mangrove dominated estuarine systems. Yearly riverine DIC export rates normalized by catchment area were in the same order of magnitude as those reported for several (sub-)tropical estuarine systems of similar size along the coast of Sumatra, Indonesia (Wit et al, 2018), the coast of India (Krishna et al, 2019) as well as one system in Cameroon (Brunet et al, 2009). These DIC export rates were even comparable to rates from larger (sub-)tropical estuaries (Cai et al, 2008) including the Pearl river estuary, China (Guo et al, 2008) and Yangtze river estuary, China (Wu et al, 2007).…”

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“…PBC is the concentration at the lowest sampling site (i.e., outlet), and Q is the annual discharge taken from the existing literature [29,30].…”

“…Comprehensive export flux estimates of PBC based solely on the suspended matter samples are sparse (figure 1(a)). Flux of PBC in this study was estimated for the Sabarmati, Mahi, and Ambika rivers using the PBC concentrations at downstream sites with discharge data from Krishna et al [29]. This led to export fluxes of 0.589, 4.400, and 0.723 Gg yr −1 of PBC from the Sabarmati, Mahi, and Ambika rivers, respectively.…”

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“…Only a few DIC yield data similar to this study have been reported, for example in the Brahmaputra River, India-Bangladesh (14.4 t C km 2 yr −1 ); Xijiang, China (15.38 t C km 2 yr −1 ); and Pearl, China (13.2 t C km 2 yr −1 ) [99], as well as in other small monsoon rivers in India, such as the Netravati (17.8 t C km 2 yr −1 ) and Baitarani (20.7 t C km 2 yr −1 [138]. Higher DIC yields have been reported in the tropical mountainous watersheds of western India, such as the Ulhas River (48.5 t C km 2 yr −1 [139] or the Kikori River, New Guinea (74.6 t C km 2 yr −1 ), which has the highest value globally [34].…”