Milankovitch Cyclicity and Global Time Constraint of Cretaceous Black Shales and Oceanic Anoxic Event 2 at the Demerara Rise, Western Equatorial Atlantic Ocean
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Karoly, Sean
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Abstract
Testing for Milankovitch cycles in stratigraphy enables observations of astronomical forcing impacts on climate change, sedimentary cyclicity, carbon sequestration, and climatic effects on organisms. Since Milankovitch cycles are global in nature, their recognition in stratigraphy also provides a means for geologic time determination and high-resolution global correlation. Here this testing is applied to the organic black marine shales at the Demerara Rise (western equatorial Atlantic Ocean) deposited during Oceanic Anoxic Event 2 (OAE2) delineating the Cenomanian-Turonian boundary in the early Late Cretaceous period. Stratigraphic correlation of the Demerara Rise to OAE2-age sediment in the Western Interior Seaway and North Atlantic and Tethys oceans relies largely on inconsistent biostratigraphy and chemostratigraphy, in particular a large, positive marine carbon isotope excursion (CIE). High-resolution grayscale data from Ocean Drilling Program Leg 207 drill cores at the Demerara Rise depict millimeter to meter scale sedimentological variations before, during, and after OAE2 that can be correlated to the CIE. Superimposed meter-scale cyclicity in the grayscale stratigraphic series provides evidence for Milankovitch forcing, and leads to new key refinements in the time-series analysis and global correlation of OAE2.
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Milankovitch, Cyclostratigraphy, OAE2, Cyclicity, Demerara Rise, Ocean Anotic Event