Seasonal variations of dissolved inorganic carbon isotope in the Beijiang River

Abstract

In watershed with occurrence of carbonate minerals, information of carbonate weathering can be easily traced by river chemistry due to its higher weathering rate, whereas signals of silicate weathering tend to be concealed. In this study, temporal and spatial variations of carbon isotope of dissolved inorganic carbon are employed to trace silicate weathering in the Beijiang River, whose watershed is covered by both carbonate and silicate. Water samples were collected from six sites from upstream to downstream for all seasons. Our results show that there is an obvious seasonality in isotope values at sites in the midstream and downstream, displaying marked lighter values in June (-16‰ to -19‰) except for the upstream sites showing less varying carbon isotope values. Taking into consideration of constrains of various processes on dissolved inorganic carbon isotope, we propose that silicate weathering is greatly enhanced in summer in addition to carbonate weathering, which strengthens carbon sink significantly.