Centennial Record of Inputs and Biogeochemical Transformations of Sediment Dissolved Organic Matter In a Small Plateau-Lake, Southwest China

Abstract

Small lakes on the Yunnan-Kweichow Plateau are highly sensitive indicators and recorders of global climate change and human disturbance, playing a non-negligible role on regulating the carbon cycle. Dissolved organic matter (DOM) encompasses reactive organic carbon and can mineralized and buried in lake sediments for hundreds of years. However, the lack of chronological sequences makes it challenging to unravel historical dynamic of sediment DOM and its driving mechanisms. In this study, we calculated an age dating (1850-2019 year) of the sediment core based on radioactive lead (210Pb), and used elemental analysis, optical spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and stable carbon isotopic measurements to evaluate the long-term variations of contents, sources and compositions of sediment DOM. We found that DOM contents obviously increased after the 2000s due to intensive socio-economic activities and climatic warming within the watershed. Stable carbon isotopic results were consistent with spectral parameters, suggesting the additional DOM contents were mainly derived from annual growth of autochthonous organics released by aquatic plants. These freshly autochthonous sources shifted 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, sediment 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 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 sediment DOM, and opens a new insight to understand DOM dynamics from a paleolimnological perspective.

Keywords: Dissolved organic matter, Lake sediment, Spectrometry, FT-ICR MS, Historical transformation