The Role of Astronomical Forcing in Forming Cyclostratigraphic Rhythmites

The influence of astronomical forcing on Earth’s hydroclimate dynamics can be reconstructed from sedimentary rhythmite successions in marine and continental depositional settings. However, the mechanistic chain linking insolation forcing, hydroclimatic responses, environmental variability, and the ultimate formation of sedimentary rhythmites is not well constrained. We propose two proxies of sedimentary rhythmites, lithologic rank and lithologic couplet, which involve hypothesized hydroclimate responses to insolation forcing in synthetic models and six Cenozoic and Mesozoic rhythmite successions. The models simulate threshold-sensitive rhythmite successions by imposing a series of increasing severe of threshold responses to insolation forcing time series dominated by climatic precession. Wavelet analysis indicates significant coherence between all of the simulated lithologic rank and couplet time series for 405-kyr and ~100-kyr orbital eccentricity cycles. This reflects strong and sustained control of climatic precession modulations on rhythmite formation. The Cenozoic and Mesozoic rhythmite successions, on the other hand, reveal variability in coherence and phase relationships of orbital eccentricity cycles between their rank and couplet proxies. Both numerical models and geological data indicate that orbital eccentricity, through modulation of climatic precession-forced insolation, exerts a persistent influence on hydroclimate responses recorded in the sedimentary rhythmites. During orbital eccentricity maxima, high-amplitude insolation forcing and precession-driven seasonality surpass the environmental stability thresholds, leading to insolation-sensitive hydroclimate responses and rhythmic lithofacies. By contrast, during orbital eccentricity minima, low-amplitude insolation forcing maintains conditions below these same thresholds, resulting in static depositions and monotonous lithofacies.