Origin and effect factors of sedimentary organic carbon in a karst groundwater-fed reservoir, South China

Reservoirs are commonly recharged by groundwater that is rich in bicarbonate ions in karst regions of South China, and the recharge of this groundwater to the reservoir can affect the biogeochemical processes of carbon sedimentation at the reservoir bottom. In this study, Dalongdong Reservoir, which is mainly recharged by two subterranean streams, was investigated based on a 42-cm-thick sedimentary core and the ²¹⁰Pb/¹³⁷Cs dating technique and isotope analyses to understand the sedimentary history and identify the carbon sources. The ²¹⁰Pb/¹³⁷Cs age model showed that the sediments were accumulated over the last 60 years. The annual increase precipitation and temperature showed no obvious change compared with trends of δ¹³C in total organic carbon (δ¹³Cₒᵣg), δ¹⁵N values in total nitrogen, and the carbon and nitrogen ratio (C/N). This shows that climate was not the main control of the variation in sediment factors. Based on δ¹³Cₒᵣg, δ¹⁵N, C/N, and isotopic mixing modeling, sources of organic carbon in the sediments were derived from plankton (60.84%), soil (22.93%), waste water (14.56%), and terrestrial plants (1.67%). From 1958 to 1978, reservoir establishment and leakage affected the contribution of the four sources. The contribution of the plankton source increased from 1978 to 2015, resulting from change of water level and continued input of external nitrogen. However, because of the revegetation supplied by an economic aid project the contribution of soil showed a considerable decreasing trend from 1978 to 2002. After 2002, For “Grain for Green” project, the contribution from soil further decreased. After reservoir construction, the contribution of waste water stabilized. The contribution of terrestrial plants started increased rapidly after 2002. Karst groundwater, which contains more dissolved inorganic carbon containing lower δ¹³CDIC than the water sources of other lakes or reservoirs, makes the δ¹³Cₒᵣg value of sediment more negative by phytoplankton photosynthesis in the reservoir.