Historical inputs and biogeochemical transformations of dissolved organic matter since 1850 CE in a small plateau-lake, Southwest China

Abstract

Small lakes on the Yunnan-Kweichow Plateau of Southwest China are significant carbon sinks, and highly sensitive recorders of global climate change and human disturbance. However, the lack of robust sediment chronologies makes it challenging to unravel the dynamics of dissolved organic matter (DOM) accumulation in these sedimentary successions, and the driving mechanisms of change over historical timescales. In this paper, we constructed a robust chronology (1850–2019 CE) based on radioactive lead (²¹⁰Pb) for Xihu Lake, and used elemental analysis, optical spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and stable carbon isotopic measurements to evaluate long-term variations in the abundance, source and composition of sedimentary DOM. We found that DOM abundance increased after 2000 CE due to intensive socio-economic activities and climatic warming within the watershed. Stable carbon isotopic results are consistent with spectral parameters, suggesting the additional DOM abundance was mainly derived from annual input from autochthonous aquatic plants. These autochthonous sources shifted the DOM pool into saturated and S-containing aliphatic compound and protein-like fluorescence component, which had higher bio-lability and mineralization potential. After successive mineralization for hundreds of years, sedimentary DOM underwent a transition characterized by reduced bio-lability and increased persistence with time gradients. More oxygenated, aromatic and high-molecular-weight DOM fractions were preserved in deeper and older sediments, which became a key component of lake carbon sinks. Furthermore, massive diagenetic transformations resulted in an increasing tendency in diversity and richness of molecular assemblage. This work provides a comprehensive understanding of historical inputs, degradations and preservations of sedimentary DOM, and offers new insights into our understanding of DOM dynamics from a paleolimnological perspective.