Riverine Hydrochemistry and CO2 Consumption in the Tropic Monsoon Region: a Case Study in a Granite-Hosted Basin, Hainan Island, China

To evaluate the carbon sink capacity caused by rock chemical weathering processes and its controlling factors in the tropical monsoon region, the hydrochemistry of the granite-hosted Changhuajiang River (CHJR) basin in Hainan Island, China, were systematically investigated by collecting water samples from the river mouth to headwater in the dry season and the wet season, respectively. The results show that Na+, Ca2+ and HCO3 − are dominant in the chemical runoff of the CHJR. The spatial and temporal variations of major ions imply the influence of the multi-sources. The concentrations of Ca2+, Mg2+, HCO3 − and DSi, mainly sourcing from rock (silicates and carbonates) weathering, are lower in the wet season than those in the dry season. However, the concentrations of K+, Cl−, SO4 2−and NO3 −, mainly sourcing from the atmospheric precipitation and human activities, are significantly controlled by the monsoon rainfall and hydrological stage in the CHJR basin. The contributions of the silicates chemical weathering, human activities, atmospheric input, evaporites dissolution and carbonates chemical weathering to the chemical runoff are 82.62, 9.05, 5.24, 2.05 and 1.05 % in the CHJR, respectively. The chemical weathering rates of silicates, carbonates and evaporites are 16.25, 0.40 and 0.62 t km−2 year−1, respectively. Chemical weathering rates in global granite-hosted basins significantly correlate with temperatures (P < 0.01) and rapidly increase when temperatures are up to 24 °C. There are two kinds of relationship between chemical weathering rates and runoff depths: logarithmic pattern (temperatures <10 °C) and linear pattern (temperatures >10 °C). On a regional scale, we find that the chemical weathering rates in the southern China appear a weak positive correlation with normalized differential vegetation indexes (NDVI), which need to be confirmed by enough data. The flux of atmospheric CO2 consumption from rock weathering in the CHJR basin is 2.71 × 105 mol km−2 year−1 by the discharge water of 2014, with being close to that (2.9 × 105 mol km−2 year−1) in the other tropical basins and far from that (5.06 × 105 mol km−2 year−1) calculated by the averaged discharge water of over years in the CHJR. The silicates weathering intensity lies at the moderate phase, implying that the weathering processes are still developing in global tropical granite basins. Hence, how the atmospheric CO2 consumption from rock weathering will change with increasing the atmospheric CO2 level and dam construction altering water cycle in the tropical monsoon area is expected to be deserved.