Spatial and seasonal variation of surface water pCO2 in the Ganges, Brahmaputra, and Meghna rivers in Bangladesh: implications for its impact on the local and global carbon cycle

Abundant carbon is released from river water directly to the atmosphere as CO₂. To understand the role of rivers in the global carbon cycle, the roles of chemical weathering and CO₂ release from major rivers must be studied. We investigated three major Himalayan rivers in Bangladesh: the Ganges, Brahmaputra, and Meghna rivers. Although pCO₂ is known to be low in the upper reaches of these rivers owing to active chemical weathering, we observed pCO₂ values higher than the atmospheric pCO₂ along their lower reaches, where deep soils have developed and where high air temperatures promote active soil respiration. By a simple mixing calculation, we found that seasonal variations of the river water carbonate system are controlled by subsurface water flows. In the rainy season, most of the lowlands are inundated and the contribution of subsurface flow to the river system increases. As a result, the CO₂ flux to the atmosphere becomes higher. In future work, more detailed spatial and seasonal investigations are required to clarify the role of rivers in the global carbon cycle and how that role will change under global warming.