The construction of time models (orbital time scales), and their integration with proxy data to develop a conceptual model for Oceanic Anoxic Event 2

The late Cretaceous Oceanic Anoxic Event 2 (OAE 2; ~94 Ma) represents an extreme perturbation to the global carbon cycle, characterized by widespread deposition of organic carbon-rich marine sediments. Due to the common occurrence of rhythmic marine sedimentation within the OAE 2 interval, this event has emerged as a model case study for the investigation of paleoceanographic events via the integration of high-resolution orbital time scales with paleobiologic and geochemical data. While numerous studies have utilized these rhythmic OAE 2 deposits to construct astrochronologies, disparity among the resultant time scales has hindered an accurate global synthesis of the event.  This problem is not unique to OAE 2, and a principal reason for such disparities is the lack of a consistent and objective methodology for calibration of observed spatial rhythms to temporal periods. In this study, we quantitatively test for the presence of orbital forcing/pacing in OAE 2 deposits spanning high-latitude to near-equatorial sites, using an inverse method that provides a means to objectively and independently calibrate the orbital chronometers at widely separated locations. The technique is specifically designed to evaluate orbital signals that are distorted by unsteady sedimentation rate histories, and importantly, it also provides a formal statistical test to evaluate the null hypothesis (no orbital signal).

 

Our analyses indicate that the null hypothesis can be rejected with a high degree of confidence at four investigated OAE 2 sites, spanning paleolatitudes from 5°N to 60°N.  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.  Analysis of the orbitally-tuned records reveals a progressive amplification of obliquity forcing during OAE 2, which we attribute to cooling of the climate system associated with carbon dioxide sequestration, resulting in a globally enhanced sensitivity to high-latitude climate/oceanographic processes. Candidates for this enhanced high-latitude response include changes in thermohaline circulation and/or the growth of high-latitude ice sheets. Our new results will be evaluated within the context of recently published paleoceanographic proxy data, to develop a conceptual model for OAE 2.