Abstract: R. Chu, H. Wu, Q. Fang, W. Huang, D. Liu, R. Zhu, S. Zhang, T. Yang, C. Wang (2023)

Orbital forcing is a fundamental driver of Earth surface systems. The understanding of feedback responses to orbital forcing remains limited, with one of the most distinguishing features being its inherent nonlinearity. In this paper, we present geological evidence demonstrating nonlinearity during the greenhouse climate state, as exemplified by the Middle Triassic lacustrine-delta succession of the Ordos Basin in China. By combining the radio-isotopic and astronomical timescale, it suggests that lacustrine black shale facies were developed during early Ladinian hyperthermal episode, while deltaic facies were developed before and after the hyperthermal episode. The lacustrine redox cycles exhibit dominant precession cycles with a period of 20 kyr, whereas these precession cycles tend to be covered by the eccentricity and obliquity cycles with periods of 100 kyr and 33 kyr respectively in the deltaic detrital-input cycles. The diverse manifestations of orbital cycles in various environments can be explained by the nonlinear feedbacks. It is proposed that feedback mechanisms result from the combination effects of direct and indirect hyperthermal events, which are respectively manifested by the climatic processes (pCO2 level) and sedimentary processes. Sedimentary processes can enhance the influence of eccentricity through smoothing and memory effects, as supported by statistical analysis. The variations in atmospheric pCO2 may also contribute to the feedback of eccentricity and obliquity cycles through climate threshold and intertropical convergence zone, as supported by paleoclimate simulation and land-ocean correlations.

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Nonlinear responses to orbital forcing inferred from an analysis of lacustrine-delta sequences spanning the Middle Triassic (Ladinian) hyperthermal episode in the Ordos Basin, China