Pre-Cenozoic Cyclostratigraphy and Palaeoclimate Responses to Astronomical Forcing

Astronomical insolation forcing is well established as the underlying metronome of Quaternary ice ages and Cenozoic climate changes. Yet its effects on earlier eras (Mesozoic, Palaeozoic and pre-Cambrian) are less understood. In this Review, we explore how cyclostratigraphy can help to distinguish climate modes over the pre-Cenozoic era and aid our understanding of climate responses to astronomical forcing over geological time. The growing uncertainties with geologic age mean that pre-Cenozoic astronomical solutions cannot be used as tuning targets. However, they can be used as metronomes to identify the pacing of distinct climate states. Throughout the pre-Cenozoic, global average temperature differences between climate states were even more extreme (5-32 degrees C) than in the Cenozoic (14-27 degrees C), and these, combined with an evolving biosphere and changing plate tectonics, led to distinct Earth-system responses to astronomical forcing. The late Palaeozoic icehouse, for example, is characterized by a pronounced response to eccentricity, caused by nonlinear cryosphere and carbon-cycle behaviour. By contrast, the Devonian warmhouse and the Late Cretaceous hothouse featured recurrent episodes of marine anoxia that may have been paced by astronomical forcing. Formally defining 405,000-year eccentricity cycles as chronostratigraphic units (astrochronozones) throughout the Phanerozoic eon will enable a more comprehensive understanding of how astronomical forcing has shaped Earth's climate over geologic time. Pre-Cenozoic cyclostratigraphy differs from the Cenozoic owing to its greater uncertainty in astronomical solutions regarding the phase of precession, obliquity and eccentricity. Nevertheless, astronomical solutions do offer precise periodicity estimates for the pre-Cenozoic, enabling the use of astronomical cycles as an astrochronological metronome.Warmer climate states often exhibit an Earth-system response dominated by precession-driven variability in monsoon intensity. Nevertheless, Devonian and Cretaceous anoxia could have been paced by long eccentricity cycles.The stratigraphic record of the early Palaeozoic icehouse often displays dominant precession cycles, whereas the late Palaeozoic icehouse principally features eccentricity cycles. This shift can be attributed to nonlinear carbon-cycle and cryosphere-related mechanisms that have a more prominent role in the later icehouse.The Permian-Triassic extinction is marked by a geologically instantaneous carbon cycle disruption, the timing and pacing of which is constrained by astrochronology, that resembles the Anthropocene delta 13C signature. However, recovery after the Permian-Triassic spanned millennia, emphasizing the impact of positive feedback loops and tipping points.To achieve a more comprehensive understanding of how astronomical insolation forcing has shaped the Earth's climate over geologic time, we advocate the implementation of astrochronozones (cycles formally defined as chronostratigraphic units) to establish a fully astronomically calibrated timescale for the Phanerozoic era. Earth's climate responds to astronomical forcing cycles that occur over tens of thousands to hundreds of thousands of years. This Review explores the distinct Earth-system responses to astronomical forcing over the pre-Cenozoic era and explains how astronomical cycles are used to calibrate geologic time.