No abstract

Recent studies have remarked on the importance of direct CO 2 release from river water into the atmosphere on the global carbon cycle over a short timescale. In this study, we investigated carbonate systems, including spatial and seasonal variations of pCO 2 , in three major Himalayan rivers in Bangladesh: the Ganges, Brahmaputra, and Meghna Rivers, and their potential importance. Although pCO 2 is known to be low in the upper reaches of these rivers, owing to active chemical weathering, we observed pCO 2 values higher than the atmospheric pCO 2 level 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 in these river water carbonate systems are controlled by subsurface water flows. In the rainy season, most of the lowlands are inundated, and the contribution of subsurface flow to river water carbonate systems increases, resulting in higher pCO 2 values. In future research, more detailed spatial and seasonal investigations are required to clarify the role of terrestrial ecosystems, including rivers and the CO 2 flux to the atmosphere, in the global carbon cycle and to examine how that role will change under global warming.

The role of terrestrial river systems in the global carbon cycle on a long timescale has been a subject of interest, especially in the context of past climate changes such as the global cooling in the Cenozoic. The discharges of water and carbon into the ocean from the Himalayan watersheds are among the highest in the world. However, there are few reliable geochemical data from the Ayeyarwady River. This study focused on reevaluating chemical weathering in the Himalayan watersheds based on samples taken from the Ayeyarwady, Mekong, and Chao Phraya Rivers and on chemical analysis of the composition of dissolved substances in these rivers. Comparisons of water quality showed that, unlike in previous studies, the total alkalinity budgets of the Ayeyarwady are dominated by carbonate rather than silicate weathering. Long-term CO 2 consumption by silicate weathering in the Ayeyarwady is estimated to be only 63-145 × 10 9 mol yr À1 , which is only 10% of the previous estimate. Our results also suggest that the total Himalayan watersheds account for only about 10% of the total global CO 2 consumption by silicate weathering. Although we need further studies, chemical weathering and associated CO 2 uptake in the Himalayas likely played a lesser role in long-term global cooling in the past than previously appreciated.