Cyclostratigraphy and the problem of astrochronologic testing

Abstract

When Milankovitch cycles are preserved in the geologic record they provide a direct link between chronometer and climate change, and thus a remarkable opportunity to constrain the evolution of the surficial Earth System. The identification of such cycles has allowed exploration of the geologic record with unprecedented temporal resolution, and has spurred the development of a rich theoretical framework for climatic change. Accompanying these successes, however, has been a persistent skepticism: how does one reliably test for astronomical forcing/pacing in stratigraphic and paleoclimate data, especially when time is poorly constrained? From this perspective, it would seem that the merits and promise of astrochronology – a Phanerozoic time scale measured in 20,000 to 400,000 year increments – also serves as its Achilles heel, if the confirmation of such geologically short temporal rhythms defies rigorous hypothesis testing. The implications are substantial, since much of our understanding of paleoclimate change throughout the Cenozoic (and beyond) is firmly rooted in astrochronologic interpretation.In this study, a conceptual framework for assessing Earth System response to astronomical-insolation changes, and the propagation of that signal into the geologic record, is used as a guide to understand the nature of the problem of astrochronologic testing. This framework emphasizes three challenges – contamination, stratigraphic distortion, and temporal calibration. A statistical optimization method (TimeOpt; Meyers, 2015) is formulated as a solution to these three challenges, providing an approach for astrochronologic testing that objectively evaluates time scale uncertainty while simultaneously identifying an optimal model for climate and depositional system response to astronomical forcing. New extensions to the technique are presented, allowing explicit reconstruction of distortions to the primary forcing that are known to be omnipresent in the stratigraphic record. To illustrate the utility of this approach, it is applied to five well-studied stratigraphic series throughout the Phanerozoic, supporting their astronomical origin, and yielding constraints on the evolution of the Earth System and the astronomical solutions themselves. Future directions that build on this foundation are discussed, including the utility of process-based null models, approaches for Earth System transfer function reconstruction, and mapping out ancient Solar System behavior and Earth-Moon history using the geologic archive of Milankovitch cycles. The TimeOpt approach recognizes astronomy, geochronology, paleoclimatology and depositional system reconstruction as a unified geoscientific inverse problem.