Transformation of organic matter input to sediments of a typical alpine lake over the past approximately 240 years and its link to atmospheric nitrogen deposition and climate warming

Understanding past organic matter (OM) input patterns to lake sediments and the factors controlling OM input transformation are fundamental for predicting lake organic carbon burial under changing environmental conditions; however, this topic remains poorly addressed in alpine lakes. Here, we examined the transformation of OM input to sediments of a typical alpine lake (Wodi Co) on the southeastern margin of the Tibetan Plateau over the past approximately 240 years by analyzing the trajectories of biomarker proxies in a dated sediment core. The associated drivers were confirmed by comparing the timing of changes in geochemical results and the regional environment. The results indicated a relatively natural condition of OM input before ca. 1891 CE. In the sediments of this phase, short-chain n-alkanes, C₂₀ HBI, i-C₁₈, pristane, phytane, and middle-chain n-alkanes exhibited the lowest concentrations, Paq remained low, and long-chain n-alkane concentrations were high, indicating a low input from phytoplankton and submerged macrophytes and a high input from terrestrial plants relative to the rest of the record. During ca. 1905–1978 CE, all biomarker proxies described above turned toward an inverse direction relative to before, reflecting a departure from the natural state, notably an expansion in phytoplankton and submerged macrophyte input and a recession in terrestrial plant input. Atmospheric nitrogen deposition was considered the major driver, considering its intensification in this phase and its ability to enhance lake nutrient levels and aquatic productivity. After ca. 1988 CE, trends of middle- and long-chain n-alkane concentrations were further reversed, and Paq decreased to a minimum and stabilized below 0.4, revealing a notably increasing input from terrestrial plants and a coeval regressive input from submerged macrophytes. This might have been caused by climate warming since the 1980s because freeze–thaw of permafrost and glaciers under this scenario can facilitate the transfer of terrestrial OM and clastic materials to the lake, suppressing submerged macrophyte productivity and resultant OM input via weakening water transparency. Additionally, the importance of phytoplankton input in this lake was highlighted by a multi-lake analogy approach, best explained by a massive accumulation of active nitrogen from atmospheric deposition owing to its large catchment area. This study provides novel insights into the transformation of OM input to alpine lake sediments forced by different environmental drivers.