δ15N EXCURSION IN THE HEEBNER SHALE: SIGNIFICANCE FOR THE DURATION OF POST-GLACIAL TRANSGRESSIONS IN THE LATE PENNSYLVANIAN MIDCONTINENT SEA

Understanding the evolution of nitrogen cycling is an important factor for the investigation of paleo-environmental and biogenic conditions. Nitrogen is not only often a limiting nutrient in marine systems, but also impacts global climate. The Late Pennsylvanian Midcontinent Sea (LPMS) is a unique environment to study nitrogen cycling due to the fact that it has no modern basin analogues. Recent studies of the LPMS documented a δ¹⁵N excursion in the lower black shale member of several cyclothemic core shales from the Missourian Stage, with a peak excursion of ca. +7‰ that was correlative with post-glacial eustatic transgression (Algeo et al., 2008; Herrmann et al., 2012).

The purpose of this study is to investigate and correlate an analogous nitrogen isotopic excursion in the lower Virgilian Heebner Shale across the LPMS in order to investigate its geographic extent and origins. At six sites in Kansas and Iowa, the Heebner Shale and the underlying transgressive limestone were examined for elemental and isotopic content. Samples were powdered and analyzed using X-Ray Fluorescence for bulk content and Thermo Delta V IRMS coupled to Costech 4010 EA for nitrogen isotopic composition analysis. A nitrogen isotopic excursion ranging from +6.5‰ to +7.5‰ was identified in all sample locations. Initial results show onset of the nitrogen isotopic excursion before the maximum flooding surface, near the transgressive limestone-shale boundary.

The δ¹⁵N excursion appears to progress stratigraphically lower, with respect to the onset of shale deposition, at study sites that are further towards the paleo-north and away from the open-ocean water source. Current models propose that an extreme chemocline in the large, shallow, semi-restricted sea resulted in strong restriction of the bottom waters, favoring watermass denitrification. Strong upwelling of pre-conditioned, denitrified waters from the deepwater Anadarko Basin to the south contributed to this isotopic signal. The non-homogenous onset of the δ¹⁵N excursion suggests there may be a longer timescale associated with the eustatic transgression than current basin models predict and must be studied further.