NITROGEN ISOTOPE DYNAMICS OF THE LATE PENNSYLVANIAN MIDCONTINENT SEA: AN EXAMPLE FROM THE HEEBNER SHALE

The Late Pennsylvanian Midcontinent Sea (LPMS) covered broad areas of central North America during glacio-eustatic highstands, depositing anoxic black shales beneath a halo/pycno/chemocline that extended from the Illinois Basin to at least the Anadarko Basin. These black shales exhibit substantial variation in N isotope compositions that may be linked to eustatically controlled denitrification. This study examines δ¹⁵N values and elemental concentrations of the LPMS across eight sites from the lower Virgilian Heebner Shale and its equivalent. The study sites span a >1000-km transect from the Anadarko Basin across the Midcontinent Shelf to the Illinois Basin. δ¹⁵N values are integral to the understanding of the nitrogen cycle of the LPMS and can give insight into its paleo-environmental and biogenic conditions. Isotopic analyses were carried out using an Elemental Analyzer coupled to an Isotope Ratio Mass Spectrometer. Samples containing high inorganic carbon concentrations were pretreated with sulfurous acid to isolate isotopic signatures of the organic portion. XRF measurements were taken to determine elemental concentrations. Using elemental and isotopic crossplots, distinct sediment provenances are shown, representing multiple terrestrial and marine sources.

Recent studies have documented a δ¹⁵N excursion of ~+7‰ in the lower black shale member of cyclothemic core shale of the Missourian Stage in the LPMS, analogous to the Heebner. This excursion correlates with post-glacial eustatic transgressions and is attributed to advection of denitrified water masses from an upwelling zone in the Eastern Panthalassic Ocean onto the LPMS. Our findings document a similar excursion in Heebner samples across the LPMS of +5 to +9‰ from a background of ~+8‰. Additionally, we document previously unreported δ¹⁵N excursions of +2-3‰ occurring in cores more proximal to the paleoshore in the Illinois Basin. These new δ¹⁵N peaks occur up core from the larger excursion, near the boundary of the middle and upper black shale members. The peaks are present only in cores deposited furthest from the open-ocean connection, suggesting an intensification of denitrification or an additional mechanism for preferentially heavy δ¹⁵N sequestration in the Illinois Basin.