Human impact on the historical change of CO2 degassing flux in River Changjiang

CO2 or carbon flux is the major cause in today's global climate change. Rivers, connecting the terrestrial and oceanic ecosystems, plays a unique role in the transportation of weathering products and pollutants from land to ocean. Global fluvial export of carbon is now a well-documented component in the global carbon cycle and is estimated ca. ~1015 g C.yr-1 [1-4]. Although it is relatively small compared with the fluxes at other interfaces (atmosphere-biosphere, atmosphere-ocean), the fluvial carbon flux contributes an important percentage to the regional carbon budget. In addition, carbon exports via rivers are not limited to fluvial discharge. Recently, it has also been found that river systems actively degasses CO2 into the atmosphere [2,5,6]. As observed in many studies, concentration of CO2 dissolved in rivers, lakes and coastal areas, is higher than its equilibrium concentration relative to CO2 in the atmosphere (i.e. 350 ppm at present), as a result of biogeochemical process imposed by the thermodynamic equilibrium between the riverine and atmospheric CO2 [2,6-15]. Consequently, the excess of CO2 can escape to the atmosphere due to physical water- air equilibration.On the other hand, the increasing anthropogenic activities in river drainage basin, are significantly changing the continuum of land-sea interaction inter-linked by rivers [16,17]. For instance, river damming and eutrophication are widely regarded as the most remarkable and extensive changing events [18,19]. Environment changes (e.g. catchment landscape, water quality) can be recorded in riverine carbon inventory, which is closely related to the terrestrial ecosystem [3]. As a result, over-saturation of dissolved CO2 in rivers may be misestimated if the estimation is made based on a specific temporal and spatial scale. Hence, we need to look into some historical data from Changjiang for a new understanding in the river geochemistry.Changjiang is the largest river in Euro-Asian Continent, draining