Observations of modern processes in Nam Co, Tibetan Plateau, China

The lakes of the Qinghai-Tibet Plateau form China’s largest lake cluster, whose sediments are pivotal for reconstructing paleoenvironmental changes. Studying modern lake dynamics is key to improving the reliability of sediment-based paleoenvironmental reconstructions. Recent research indicates that these lakes act as atmospheric carbon sinks with a capacity comparable to grasslands, yet interpretations remain tentative due to insufficient modern process observations.

 

Buoy-based monitoring, especially time-series sediment trap moorings, is an effective tool for tracking contemporary lacustrine sedimentary processes, but such observations are scarce on the plateau, with only Qinghai Lake having successful implementations. We conducted multi-year observations using a self-developed trap moored buoy at Ranwu Lake in southeastern Tibet, identifying a consistent correlation between sediment flux and varve thickness. At central Tibet’s Nam Co, we deployed time-series trap moorings alongside thermometer chains in 2018; since 2019, we have added buoys fitted with multi-parameter water quality sensors, ADCPs and meteorological stations.

 

Key findings include: 1) High-precision temperature profiling detected a thermal inversion layer in Nam Co during the Win1 phase. 2) Sedimentation mainly takes place during autumn and spring water turnover periods. 3) Resuspension during turnover at Nam Co appears to be insignificant. 4) The biogenic carbon pump alone cannot fully explain Nam Co’s carbon sequestration, as lake water absorbs more carbon, with sedimentation and carbon sequestration occurring synchronously. 5) Elevated autumn chlorophyll concentrations in the plateau’s deep oligotrophic lakes may stem from the upwelling of stratified-period deep chlorophyll maxima (DCM) to the surface, supported by modern observations and remote sensing data. 6) Carbon sequestration in these deep, oligotrophic, hard-water lakes is primarily driven by enhanced atmospheric CO₂ absorption during the upwelling of high-pH water in turnover periods.