Solar activity and ENSO signals in Early Eocene lacustrine oil shale from Green River Basin

Previous studies have shown that the Eocene oil shale sequences in the Green River Basin contain long-period astronomical age information. The fine-scale chronological characteristics of the oil shale laminae remain largely unexplored. We selected finely laminated oil shales formed in deep-water environments characterized by stable water column stratification as the primary focus of this study, using microscopy and micro-area X-ray fluorescence (μ-XRF) techniques. By integrating high-resolution elemental data with time-series analysis, we identified significant periodic signals associated with solar activity (Hale and Schwabe cycles) and ENSO. The results indicate that the alternations of light and dark laminae in the Green River Formation oil shale correspond to alternating dry and wet climate regimes: the light laminae are dominated by carbonate minerals, reflecting drier and milder conditions, while the dark laminae are enriched in terrigenous clastics and organic matter, indicating periods of increased precipitation and warmer temperatures. The detected periodicities (23.5 years, 13.3 years and 5.8 years) are highly consistent with modern observations, demonstrating that the lower Eocene Green River oil shale effectively records short-term solar activity and climate variability. Furthermore, our findings confirm that a persistent "permanent El Niño" state did not develop under Early Eocene greenhouse conditions, providing a refined chronological framework for high-resolution paleoclimate studies during greenhouse intervals.