A HIGH-RESOLUTION, MULTI-PROXY STUDY OF ORGANIC RICH MUDROCKS FROM THE MID CENOMANIAN EVENT IN NORTH AMERICA

The Mid Cenomanian Event (MCE) is one of several extreme Cretaceous climatic events, which are generally characterized by extended anoxic conditions through the water column and/or at the sediment-water interface. Whilst in-depth studies of the MCE are in their relative infancy, the event is considered a major turning point within the mid-Cenomanian in terms of climatic and oceanographic systems. However, the nature of the MCE is not well understood, with ambiguity over whether it represents a truly global perturbation to the global carbon cycle, or is a prelude to OAE2 (~93.9 Ma).

Total organic carbon (TOC), organic carbon isotope (δ¹³C_org), trace element (TE) geochemistry, biomarker, and palynological data, along with SEM-EDS and microscopy provide a high-resolution, integrated insight into the nature of the MCE within the Western Interior Seaway (WIS). Key palynological bio-events and isotope stratigraphy allow correlation with other successions in the WIS (e.g. Eagle Ford Gr., Texas), whilst compound-specific isotope analyses allow for discrimination between local and global carbon isotope signals through comparisons between Kansas and the Demerera Rise (tropical North Atlantic).

The MCE initiated during the early part of the 2^nd order Greenhorn transgression connecting surface waters from the Tethys with those of the Boreal influenced epicontinental WIS. High TOC levels (≤9.2%) associated with a positive δ¹³C_org excursion identify these study sites as one of few localities where the MCE is characterized by organic-rich mudrock deposition. Consistent with global records, the event can be split into two phases: MCE 1a and MCE 1b, both with distinctive δ¹³C_org peaks. The relationship between TOC and δ¹³C_org differ dramatically across the MCE with an inverse trend present prior to MCE 1a and MCE 1b, and conversely, a concordant trend during the δ¹³C_org maxima.

Redox-sensitive TE enrichment is mostly absent throughout the MCE, indicating long-term anoxia did not develop. However, through integrated geochemical and visual studies of organic matter, and statistical analyses, this study clearly delimits changes in environmental facies through the MCE in the WIS; identifying periods of intensified water column stratification indicated by high Mo, U and pyrite during times of maximum TOC preservation.