NEGATIVE NITROGEN ISOTOPE EXCURSION AT THE ONSET OF CRETACEOUS OAE2: NEW RECORD FROM THE HAWARDEN CORE, IOWA

The Hawarden (D-7) Core was recovered from Sioux County in Northwest Iowa and contains nearly five hundred feet of Cretaceous strata. The Late Cretaceous Oceanic Anoxic Event OAE2 is well preserved in the core and the mixed carbonate and clastic lithologies allow for the recovery of both carbonate and organic carbon isotope stratigraphy. Both carbon isotope records display an enigmatic drop in values during the initial rise during the onset of the event in the uppermost Graneros Formation below the base of the Greenhorn Formation. This transient ‘negative’ excursion in carbon isotope values during the onset of a major positive excursion is a feature seen in many other positive carbon isotope excursions of the Paleozoic and is well developed in the Hawarden Core.

Previous studies of the Nitrogen isotope record of OAE2 have demonstrated that a pronounced negative excursion in Nitrogen isotopes occurred immediately prior to, and during, the initial rise in carbon isotope values during OAE2. Unfortunately, most isotopic records either do not include carbonate and organic carbon, as well as nitrogen, or they are produced at relatively low resolution, and it remains unclear if this negative nitrogen isotope excursion was coeval with the transient ‘negative’ excursion in the carbon isotope record. Here, we produced all three isotope records from the Hawarden Core and can now demonstrate that, at least in this portion of the Western Interior Seaway, that the peak of the negative Nitrogen isotope excursion was coincident with the transient ‘negative’ excursion in the carbon isotope record during the onset of OAE2.