The sources and biogeochemical cycling of carbon in the Wudalianchi UNESCO Geopark volcanic system in Northeast China.

Abstract

The biogeochemical cycling and response mechanisms of carbon within the Wudalianchi UNESCO Global Geopark were characterized by the isotopic compositions of dissolved inorganic carbon (δ13CDIC) and dissolved organic carbon in ground and surface (lake) waters and their relating carbon isotopic composition of soil (δ13CSOC) and sediment organic carbon (δ13Corg). In addition to mantle-derived CO2, the oxidation of organic matter was prevalent in shallow groundwater during the summer. Their associated degassing of CO2 produced higher pCO2 values than in autumn or winter and elevated δ13CDIC values. In summer, DIC in the epilimnion showed a wide range of δ13CDIC from - 8.4 to 2.6‰. Waters in open-lake areas with relatively positive δ13CDIC values and the low levels of pCO2 were primarily influenced by CO2 degassing. Photosynthesis elevated the δ13CDIC values and led to minimal pCO2 levels in closed lake areas. Isotopically, δ13Corg was found to be positively related to δ13CSOC. In addition, lake bed sediments generally had lower concentrations and larger δ13C values of organic carbon than the surrounding soils. These results suggest that 12CO2 derived from the degradation of sediment was preferentially utilized by phytoplankton in the epilimnion during photosynthesis. The remaining 13C-rich organic matter was retained in the sediment. Since 2000, δ13Corg increased in lake 3 over time, reflecting the input of sewage and land use changes associated with a resort used for tourism. The values of δ13Corg in lake 5, distant from the resort, did not change substantially, indicating minimal human impacts.