Lake Poso as a ‘rain gauge’ in the tropical western Pacific: insights into ENSO variability from laminated sediments

Enhanced seasonal to interannual climate variability associated with changes in El Niño–Southern Oscillation (ENSO) can intensify wet and dry seasons extremes in the tropical western Pacific. Characterizing long-term and high-resolution changes in ENSO is crucial for our understanding of the tropical Pacific hydroclimate and its associated global-scale teleconnections. In this study, we anlayze sedimentary records from Lake Poso, a large (up to 400 m deep) tectonic lake on Sulawesi Island at the western edge of the Pacific warm pool. Stratification of Lake Poso’s water column below ~90 m depth is documented by anoxia, elevated concentrations of redox-sensitive metals and nutrients at depth in the water column, and the presence of finely laminated sediments. The reduction of rainfall and regional humidity during the dry season is increased during El Niño-liked events. Water column temperature monitoring data collected between the end of 2022 and early 2024 exhibit surface temperature cooling and deeper vertical water column mixing down to depths below the ~90 m oxycline during the dry season. This effect was particularly exacerbated during the 2023 El Niño event, suggesting a direct relationship between climate variability and water column structure.

To assess how such climate-induced mixing variability is recorded in the sediments, we examine three short sediment cores retrieved along a depth transect spanning oxic, transitional, and anoxic depositional environments in Lake Poso. Sedimentary structures change systematically with water depth, with massive sediments above the oxycline, to massive sediments with intermittent lamination in the transition zone, to continuously laminated sediments below the oxycline. The spatial and temporal variability in sedimentary structures suggests past changes in mixing depth and lake level linked to variations in dry-season intensity. Furthermore, longer sediment piston cores from three locations were recovered from sites above the oxycline at paleoshoreline locations identified by seismic surveys. These records show repeated transition from sandy, structureless nearshore deposits to laminated fine-grained sediments, indicating multiple substantial decimeter-scale changes in lake level on longer time scales. Changes in sediment structure and composition in Lake Poso are therefore prime indicators of past changes in West Pacific Warm Pool hydroclimate.