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 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.

Carbon dynamics in the Mekong Delta

“…In parallel, there was a general increase (Fig. 4), as also shown in other major rivers such as the Mississippi (Cai et al, 2008) and the Oubangui Mekong River, and this seems to be also the case in the Upper Mekong River (Manaka et al, 2015). TA in freshwater was also correlated to Na + but with a slope of 0.…”

Reviews and syntheses: Anthropogenic perturbations to carbon fluxes in Asian river systems – concepts, emerging trends, and research challenges

“…Compared to extensive studies conducted in polluted rivers and estuaries in Europe and North America (Frankignoulle et al, 1998;Borges et al, 2006;Hartmann et al, 2007;Borges and Abril, 2011;Griffith and Raymond, 2011;Amann et al, 2012;Joesoef et al, 2015), few efforts have been made to J.-H. Park et al: Anthropogenic perturbations to carbon fluxes in Asian river systems measure pCO 2 in polluted Asian rivers, except for some large rivers and estuaries in East Asia (Zhai et al, 2005;Chou et al, 2013;Ran et al, 2015b;Yoon et al, 2017). These studies, together with a small number of studies that used water chemistry data to estimate pCO 2 levels in major Asian rivers such as the Mekong (Li et al, 2013b), the Yangtze (Ran et al, 2017b), the Ganges-Brahmaputra (Manaka et al, 2015b), and Indian estuaries (Gupta et al, 2009;Sarma et al, 2012), underscored the importance of anthropogenic OM and nutrients for riverine CO 2 dynamics, particularly along lower river reaches and estuaries draining highly populated areas. When published data on pCO 2 were compared between headwater streams and tributaries feeding into the middle and lower reaches of major Asian rivers, tributary pCO 2 levels (mean: 2116 µatm) tended be higher than those for headwaters of Asian rivers (mean: 1170 µatm; Table 3; Fig.…”

“…As summarized in Table 3, many studies of aquatic CO 2 dynamics in India have been conducted in estuaries and coastal areas (Mukhopadhyay et al, 2002;Biswas et al, 2004;Gupta et al, 2009;Sarma et al, 2012;Samanta et al, 2015), except for the secondary data on pCO 2 calculated using C system equations (Pierrot et al, 2006) and water quality data collected in various headwaters (Sarin et al, 1989;Bickle et al, 2003;Chakrapani and Veizer, 2005) and lower reaches (Manaka et al, 2015b) of the Ganges-Brahmaputra. The values of pCO 2 estimated for some headwaters, lower reaches, and tributaries of the Ganges basin (mean: 893; range: 65-2620 µatm) were relatively low compared to other Asian rivers (Table 3; Fig.…”

Carbon dynamics and CO<sub>2</sub> and CH<sub>4</sub> outgassing in the Mekong delta

“…S1 in = 2 : 1). However, Li et al (2014) have shown, based on an extensive water chemistry data set, that carbonate rock weathering largely dominates silicate weathering in the Lower Mekong River, and this seems to also be the case in the Upper Mekong River (Manaka et al, 2015). TA in freshwater was also correlated to Na + but with a slope of 0.5, lower than expected from the weathering of albite (NaAlSi 3 O 8 ; HCO − 3 : Na + = 1 : 1), and to K + but with a slope of 14, higher than expected from the weathering of microcline (K-feldspar, KAlSi 3 O 8 , HCO − 3 : K + = 1 : 1).…”