Radiocarbon age offset of lake sediments from central eastern China modulated by both hydroclimate and human activity

The radiocarbon (14C) dating of lake sediments is often beset by the problem of the existence of a carbon reservoir and its effect on the dated materials, which in turn influences the reliability of paleoclimatic reconstructions from lake sediments. Changes in the carbon reservoir of lake sediments are usually attributed to changes in old carbon input from the surrounding catchment caused by climatic variations or human activities. However, there is much uncertainty regarding the main driver of changes in the carbon reservoir when both climate changes and human activities coexist. We compared the 14C ages of bulk sediment samples and charcoals with the published age-model for Lake Nvshan, which is based on terrestrial plant macrofossils. The results show that the use of charcoals 14C ages for dating the sediments of Lake Nvshan is unreliable. We then obtained a time series of changes in the carbon age offset (Δt) of the bulk sediments from Lake Nvshan. The results show that prior to a significant increase in human activities, changes in Δt coincided with changes in precipitation and in the pollen concentration/total organic carbon (TOC) contents of the lake sediments; whereas after the onset of increasing human activities, changes in Δt coincided with the size of the human population of surrounding area. Prior to the increase in human activities, a higher Δt likely resulted from decreases in rainfall and phytomass which caused a reduction in contemporaneous carbon inputs. Thereafter, a higher Δt was caused by intensive human land-use and the resulting input of more old carbon. Our findings reveal a shift in the main factors influencing the Δt of bulk sediments, which was caused by the intensification of human activities in the anthropic zone. Our findings contribute to an improved understanding of the factors modulating the Δt in lake sediments.