Sources and mineralization determine the generation and transformation pathway of sedimentary organic matter over the past century.

Sedimentary organic matter (SOM), an organic mixture of complex compositions with diverse structures, plays a crucial role in lake carbon cycling. The quantity, sources, compositions and biogeochemical behaviors of SOM typically vary with lake ecosystem evolution and mineralization process. However, the dynamic response and transformation pathway of SOM during these joint processes remain incompletely understood, particularly in time dimension. Geochronology, total organic carbon (TOC), nutrients and SOM sources in a sediment core from Erhai Lake were determined using radionuclides, geochemical analyses and stable carbon and nitrogen isotopes. Fluorescence spectroscopy and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were used to detect the optical and molecular compositions of SOM. A remarkable increase in TOC, total nitrogen (TN), and total phosphorus (TP) contents was observed after the 2000s, accompanied by a rise in autochthonous sources. These changes were largely attributed to lake eutrophication caused by intensive human activities and climate warming, and increased water residence time. Autochthonous sources shifted SOM compositions toward more protein-like component and aliphatic compound, which are more labile and susceptible to microbial degradation. Long-term mineralization led to a gradual decrease in TOC, TN and TP contents over time. Concurrently, the accumulation of humic-like component and CHO-type highly unsaturated and phenolic (HUPs) compound characterized by higher aromaticity and molecular weight promoted the preservation of refractory SOM in deeper sediments, thereby contributing significantly to lake carbon sinks. Network analysis based on mass difference revealed that oxygen addition, dealkylation and decarboxylation were the dominant reactions under the transformation pattern of molecular reactants to products. The CHO formation was related to deamination and desulfonation within N- or S-containing formulae. This study demonstrates that SOM characteristics are sensitive and responsive to lake evolution and mineralization on geological timescales, and offers some new insights about managements of endogenous pollution in lakes.