Time scale evaluation and the quantification of obliquity forcing

Abstract

The geologic time scale serves as an essential instrument for reconstructing Earth history. Astrochronology, linking regular sedimentary alternations to theoretical quasi-periodic astronomical rhythms, often provides the highest resolution age models for strata that underlie the time scale. Although various methods for testing astronomically-tuned time scales exist, they often present challenges, such as the problem of circularity. Here, we introduce an approach to extract a reliable obliquity envelope from astronomically tuned data, avoiding the effects of frequency modulations that can artificially introduce astronomical beats. This approach includes (1) the application of a broad obliquity filter followed by (2) a Hilbert transform and (3) a low-pass filter of the amplitude envelope to (4) test the significance of correlation between amplitude envelope and astronomical solution. These data amplitudes provide a robust means to evaluate the climate response to obliquity forcing and, more specifically, to test the significance of correlation with the theoretical astronomical solution, in a manner similar to the phase-randomized surrogate approach previously introduced for the evaluation of precession tuning. Synthetic astronomical/ice-sheet models and several Quaternary climate proxy records – where obliquity can be a dominant component of astronomically driven climate variability – are used to demonstrate the feasibility of the proposed method and yield new insight into climate system evolution.