An Orbital Beat in the Equatorial Atlantic (~18-27 Ma): Reliable Chronometer or Wishful Thinking?

Orbital-climate theory provides a vital framework for the fields of paleoclimatology and geochronology, having spawned advances in our understanding of climate system components, feedbacks, and thresholds, while also leading to a major revision of the geologic time scale. The numerous successes of Pleistocene cyclostratigraphy have motivated the search for orbital influence in strata spanning the Phanerozoic, culminating in the generation of both "anchored" (<50 Ma) and "floating" astrochronologies that can be used to evaluate environmental, biologic and biogeochemical change at very high resolution. Against this backdrop, a common challenge in the development of astrochronologies is the absence of sufficient independent time constraints (e.g., radioisotopic data) to directly calibrate spatial rhythms to temporal periods, and thus quantitatively test for orbital influence. As a consequence, many investigations attempt to test the orbital hypothesis using the "spectral frequency ratio" approach (e.g., the 5:2:1 ratio of short eccentricity, obliquity and precession), and/or by evaluating signal characteristics, such as amplitude modulations of the presumed precession cycle, prior to or following orbital-tuning. None of these approaches - as applied in common practice - explicitly tests the null hypothesis of no orbital influence, leading some to question the veracity of deep-time astrochronology. Here, we revisit proxy data from the equatorial Atlantic Ceara Rise (~18 to 27 Ma; Paelike et al., 2006), a site that has been instrumental in the development of astrochronologies for the Miocene and Oligocene time scale, and has also provided constraints on the theoretical astronomical solutions. Our cyclostratigraphic evaluation employs a method for astrochronologic testing applied to "un-tuned" proxy data, termed Average Spectral Misfit (Meyers and Sageman, 2007). This inverse method explicitly evaluates time scale uncertainty, and provides a formal statistical test of the null hypothesis (no orbital influence), without the requirement of rigorous independent time control. Following several improvements and extensions of the original method, its application to Ceara Rise sediments objectively identifies a strong orbital signature that is consistent with that proposed by Paelike et al. (2006), but also indicates substantial distortion of the orbital periods as preserved in portions of the un-tuned stratigraphic record. Construction of new chronologies that are not anchored to theoretical orbital insolation solutions, using time-frequency approaches, provides an opportunity to independently test the linkages between climate and orbit that have been previously proposed. Meyers, S.R., and Sageman, B.B. (2007), Quantification of Deep-Time Orbital Forcing by Average Spectral Misfit: American Journal of Science, v. 307, p. 773-792. Paelike, H., Frazier, J., and Zachos, J.C. (2006), Extended orbitally forced palaeoclimatic records from the equatorial Atlantic Ceara Rise: Quaternary Science Reviews, v. 25, p. 3138-3149.