Astronomical forcing of Middle Permian terrestrial climate recorded in a large paleolake in northwestern China
Abstract Geological records of Milankovitch cycles provide a temporal framework for reconstructing Earth climate evolution and mapping out the history of the ancient Solar System. Understanding the fundamental celestial mechanics driving astronomical rhythms during the Permian Period from stratigraphic record is challenging because of the inherent limitations of astronomical solutions, the chaotic nature of Solar System motion, and the paucity of cyclostratigraphic studies of geological records. The Junggar and adjacent basins of northwestern China constituted a large tectonic lake in the Middle Permian, which deposited one of the thickest and richest petroleum source rock intervals in the world. Constructing an accurate timescale for the Permian Junggar Basin is critical to understand the processes that controlled organic carbon burial as well as paleoclimatic and paleoenvironmental background conditions. In this study, detailed spectral analyses of natural gamma ray (NGR) data were performed on the lacustrine muddy-dominated Lucaogou Formation from five exploitation wells. The results reveal significant meter-scale sedimentary cycles of 37.8 m, 10.7–10 m, 3.85–3.2 m, and 2.0–1.58 m, reflecting oscillations in lithologies (variations in changing clay content within the mudstone, sandstone and dolomite). These cycle wavelength ratios match well with those of the Milankovitch cycles predicted for Middle Permian Period. The depositional duration of the Lucaogou Formation was estimated at ~3 Myr. Correlation of the ~1.2 Myr obliquity modulation cycles among the NGR logs, sedimentation rates, and lake levels of the Junggar Basin, as well as with the Middle Permian global sea level, suggests that long-term astronomical “grand cycles” tightly forced climatic and sedimentary processes throughout the Lucaogou Formation. According to two independent approaches, we reconstruct Middle Permian astronomical parameters (Earth precession constant: k and fundamental frequencies terms: gi; i = 1, 2, 3, 4, 5) and calculate astronomical periods (eccentricity, obliquity and precession). These results document astronomical forcing of Paleozoic lake systems, constrain Earth-Moon orbital evolution, and refine the fundamental astronomical frequencies of the Solar System during the Middle Permian.