Chemical weathering and long-term CO2 consumption in the Ayeyarwady and Mekong river basins in the Himalayas

Abstract

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 CO2 consumption by silicate weathering in the Ayeyarwady is estimated to be only 63-145 × 109 mol yr-1, 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 CO2 consumption by silicate weathering. Although we need further studies, chemical weathering and associated CO2 uptake in the Himalayas likely played a lesser role in long-term global cooling in the past than previously appreciated.