Decrypting the sources of dissolved inorganic carbon in river water: Isotopic study from river Ganga, India

Freshwater flux transport large amount of carbon (dissolved and particulate, organic and inorganic) from the continent into the ocean, contributing significantly to the global carbon cycle. The present sources and sinks of natural as well as anthropogenically produced C compounds in the global carbon cycle remain enigmatic. Among the carbon sources in the river ecosystem, the dissolved inorganic carbon (DIC) constitutes a major component of carbon influx from land to ocean. These fluxes are significantly influenced by the terrestrial and estuary processes. The isotopic composition of DIC can be used to understand the sources and cycling of carbon in rivers and estuaries. In this study, δ¹³C values of DIC in river water of Ganga have been used to understand the sources of dissolved inorganic carbon into the river. The river Ganga (2500 km) is the largest river of the Indian subcontinent which originates from the Gangotri glacier and drains into the Bay of Bengal through its vast delta in the Sunderban. The Ganga river basin (GRB) covers an area of 10⁶ km² draining the carbon sources of the entire basin into the mainstream of river Ganga. The river transports nearly 0.2% of the global freshwater flux, 1% of global DIC flux and 5% of the global sediment flux into the ocean. Despite its significant importance to the global carbon transport, the understanding of the DIC sources in the complex Ganga river system remains enigmatic. Therefore to elucidate the carbon sources in the river Ganga, the δ¹³C DIC of river water were measured from source (Gomukh) to sink (Bay of Bengal) of the river Ganga for pre and late-monsoon period. The seasonal variation in the δ¹³C DIC shows enriched isotopic values in pre-monsoon compared to late-monsoon samples. The upper, middle and lower stretch of the river shows distinct enrichment factors for pre and late-monsoon samples. The variation in the δ¹³C DIC of river water might be indicating the DIC signature of the source water. The pre-monsoon samples show enrichment in the δ¹³C DIC values as we move downstream of the river, whereas the late-monsoon samples show a slight depletion trend. The difference between the pre and late- monsoon samples might be indicating the high input of soil CO₂ during the late-monsoon season which is characterized by lower δ¹³C values.