Riverine carbon cycle and chemical weathering in the Pearl River basin
Mr CHEN Shuai PhD Student, Department of Geography, HKU
Rivers are one of the most important dynamic systems for carbon transport from land to the ocean and the atmosphere. With increasing scientific interest in the regional and global carbon cycle, it is significant to better understand the riverine carbon cycle and its controlling factors to refine estimates of carbon budgets at various spatial and temporal scales, which is beneficial to meet the requirements of the Paris Agreement. However, published estimations of global riverine carbon fluxes remain highly uncertain. One of the primary reasons for the large uncertainties is the lack of spatial-temporally resolved measurements and the complicated factors regulating these fluxes. To accurately quantify the riverine carbon budgets in the Pearl River basin, a 2-year-long measurement scheme was proposed in this study. Field sampling will be conducted at 62 sites of both headwater catchments (i.e., the Xijiuxi, Xiaojianghe, Liujiang and Nanshanhe River catchments) and large river basins (i.e., the Xijiang, Beijiang and Dongjiang River basins). A field sampling campaign and subsequent laboratory analysis have already been completed in the summer of 2021. Water chemistry characteristics, dissolved/gaseous carbon dioxide (CO2) and methane (CH4) concentrations and stable carbon isotopes (δ13C) have been analysed for the collected water and gas samples. The large variations of major cations and anions in different rivers demonstrate that carbonate weathering is the primary source of ions in the Liujiang, Xijiuxi and Xijiang rivers, while silicate weathering contribution is significant in other rivers, indicating the role of lithological control. Fluxes of CH4 did not show obvious changes between headwater streams and large rivers, but varied in a wide range in these studied rivers, ranging from 0.036 mmol m-2 d-1 to 120 mmol m-2 d-1. In comparison, CO2 effluxes in the four headwater streams (median values were 202, 89, 50 and 82 mmol m-2 d-1 for the Nanshanhe, Liujiang, Xijiuxi, Xiaojianghe rivers, respectively) were much higher than those in the three large rivers (median values were 59, 19, 44 mmol m-2 d-1 for the Xijiang, Beijiang, and Dongjiang rivers, respectively). This suggested that headwater streams are significant sources of CO2 for the atmosphere. The positive relationships between total suspended materials and particulate organic carbon (POC) indicated that physical erosion is the major controlling factor of the riverine POC concentrations. Positive relationships between CO2 partial pressure (pCO2) and dissolved oxygen (DO) and between pCO2 and stable carbon isotopes of dissolved inorganic carbon (δ13CDIC) showed that aquatic photosynthesis and CO2 exchange at the water-air interface play significant roles in controlling the magnitude of stream water pCO2. These preliminary findings suggest that the riverine carbon dynamics in the Pearl River system are affected by multiple factors for different carbon species. Further studies are warranted to examine the riverine carbon cycling processes and their underlying mechanisms in the Pearl River system and to better evaluate the role of the Pearl River basin in the global carbon cycle.