Petrogenic Carbon Oxidation and Its Impact on the Carbon Balance in the Ganga River Basin

Rupak Samadder, Tarun Kumar Dalai, Kruttika Mohapatra, Pankaj Kumar, Damodararao Karri, Sanjeev Kumar

Based on sampling and investigation spanning several years, we conduct a comprehensive assessment of petrogenic organic carbon (OCpetro) oxidation and evaluate the net carbon budget in the Ganga–Hooghly River (GHR) basin, extending from the Himalayan mountainous catchments to the floodplains in India. Our multi-proxy approach combines data on rhenium (Re) concentrations, radiocarbon (pMC), stable carbon isotopes (δ13Corg), major and trace element concentrations to determine Re contributions to the Ganga–Hooghly rivers from different sources. We demonstrate that OCpetro is the dominant source across the entire GHR system, accounting for up to 97% of dissolved Re. Such an outcome justifies our approach of determining the CO2 release from OCpetro oxidation by using dissolved Re as a tracer. In the GHR basin, monsoon periods are characterized by the highest CO2 fluxes from OCpetro oxidation, with emissions that match or exceed CO2 drawdown via silicate weathering. In contrast, this pattern reverses during low-discharge periods. The power-law relationship observed between CO2 release from OCpetro oxidation and sediment yield underscores primary role of physical erosion in controlling OCpetro oxidation. We further propose that in mountainous catchments, although carbon release yields are higher, a fraction of OCpetro experiences minimal to negligible oxidation. The emission of CO2 from OCpetro oxidation is largely balanced by its consumption via silicate weathering. However, both these fluxes are surpassed by substantial OCbio burial driven by enhanced sediment transport. Consequently, the GHR catchment functions overall as a net sink of atmospheric CO2.