Amplification of Obliquity Forcing during Oceanic Anoxic Event 2

The middle Cretaceous Oceanic Anoxic Event 2 (OAE 2; ~94 Ma) is characterized by widespread rhythmic marine sedimentation, commonly inferred to represent Milankovitch orbital forcing. In this study, we develop a new time-frequency implementation of the “Average Spectral Misfit” method (or ASM), which allows us to quantitatively test for the presence of orbital forcing in OAE 2 deposits spanning high-latitude to equatorial sites. The method does not require supplementary time control (e.g., radio isotopic data, biozonation schemes, etc.), provides a means to objectively (and independently) calibrate the orbital chronometers at widely separated sites, and is specifically designed to evaluate orbital signals that are distorted by unsteady sedimentation rate histories. Our analyses indicate that the null hypothesis (no orbital signal) can be rejected with a high degree of confidence at all investigated OAE 2 sites. Temporal calibration of the lithologic rhythms using the method yields new independent, high-resolution astrochronologies at each location. These astrochronologies provide a means to precisely assess the timing of the OAE 2 carbon isotope excursion, and estimate geochemical burial fluxes (e.g., organic carbon burial rate) at each site. Finally, time-frequency analysis of the orbitally-tuned records reveals a progressive amplification of obliquity forcing during OAE 2 in the mid-latitudes, and delayed obliquity amplification in the equatorial region. We attribute the observed temporal lag to progressive cooling of the climate system associated with carbon dioxide sequestration, resulting in gradual propagation of the obliquity signal to lower latitudes.