Environmental change recorded by radionuclides and organic geochemical signatures in a sediment core from Lake Daihai, North China

Abstract Grasslands are one of the most important terrestrial ecosystems, acting as carbon sinks and thus mitigating greenhouse gas emissions. However, large uncertainties remain regarding the effects of long-term anthropogenic activities on the carbon cycling in grassland ecosystems, due to a dearth of high-resolution records. Here we present an organic geochemical lacustrine record from Lake Daihai, in the arable-pastoral ecotone of North China, in order to investigate the response of biogeochemical processes to anthropogenic activities. The chronology, based on radionuclides (210Pb and 137Cs), shows that the record spanning the last ~80 years, with a temporal resolution of 0.3–7.6 years per centimeter. The total organic carbon (TOC) and total nitrogen (TN) molar ratio and the stable carbon isotopic composition of the total organic carbon (δ13CTOC) range from 13.9 to 19.1 and from −27.0 to −23.9‰, respectively, suggesting that the sedimentary organic matter is mainly derived from the terrestrial vegetation-soil system. The significant negative correlation between TOC concentrations and their δ13CTOC is consistent with those in soil profiles, suggesting the δ13CTOC enrichment is mainly due to microbial decomposition of the organic carbon in the vegetation-soil system. From the 1980s to the 2000s CE, rapid agricultural expansion and over-grazing in the watershed reduced the organic carbon input from vegetation to soils, and enhanced the soil organic carbon decomposition, hence affecting the restoration of the carbon sink function of the surrounding grasslands. Furthermore, organic carbon burial rates within Lake Daihai sediments increase significantly with anthropogenic activities, from 4.2 to 39.3 g m−2 yr−1 with a sediment focusing correction. The geochemical record as a whole clearly shows that anthropogenic activities have dominated biogeochemical processes of carbon in the arable-pastoral ecotone of North China over the last ~80 years.