Obliquity Forcing and the Amplification of High-Latitude Climate Processes during Oceanic Anoxic Event 2

Meyers, Stephen R.¹; Sageman, Bradley B.²; Arthur, Michael A.^3
1 Department of Geological Sciences, University of North Carolina at Chapel Hill, Chapel Hill, NC.
² Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL.
³ Department of Geosciences, Pennsylvania State University, University Park, PA.

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. Numerous studies have utilized these rhythmic OAE 2 deposits to construct high-resolution astrochronologies, but disparity among the resultant timescales has hindered an accurate global synthesis of the event. One of the reasons for this disparity is the lack of a consistent and objective methodology for calibration of observed spatial rhythms to temporal periods. In this study, we develop a time-frequency statistical methodology (“Evolutive Average Spectral Misfit”, or E-ASM), which allows us to quantitatively test for the presence of orbital forcing in OAE 2 deposits spanning high-latitude to near-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. Importantly, the technique 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. Finally, time-frequency analysis of the orbitally-tuned records reveals a progressive amplification of obliquity forcing during OAE 2. We attribute the observed obliquity amplification to cooling of the climate system associated with carbon dioxide sequestration, resulting in a globally enhanced sensitivity to high-latitude climate/oceanographic processes. Our results are consistent with an increasing influence of high-latitude climate/oceanographic processes on thermohaline circulation during OAE 2, and/or the growth of high-latitude ice sheets.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009