DENITRIFICATION IN THE EASTERN TROPICAL PANTHALASSIC OCEAN THERMOCLINE DURING THE LATE PENNSYLVANIAN

Core black shales of Late Pennsylvanian cyclothems of the Midcontinent region of North America exhibit large positive nitrogen (N) isotope excursions (to +14‰) relative to background values (+5 to +6‰) (Algeo et al., 2008; Herrmann et al., 2012). These excursions are inferred to represent denitrification of thermoclinal waters in the eastern tropical Panthalassic Ocean (ETPO), with subsequent lateral transport of the denitrified waters through a deepwater channel in the Permian Basin region and upwelling onto the Midcontinent Shelf in Kansas (where the N-isotope signal has been analyzed). The N-isotope excursion peaks are correlative with the most rapid phase of eustatic rise during deglaciations following Gondwanan glacial stages. In this study, we use an oceanic box model to investigate the marine nitrogen cycle in ETPO in order to place constraints on the origin of this N-isotope signal. Specifically, we varied the rate and locus of denitrification (i.e., water-column versus sediment porewater) as well as the rate of nitrogen fixation to simulate the magnitude and duration of the observed N-isotope excursion. Organic carbon isotopes covary strongly with d¹⁵N and thus provide an additional constraint on viable interpretations. Our results indicate that the N-isotope excursions were only weakly related to eustatic elevation, and that the increase in d¹⁵N during deglacial sea-level rise is inconsistent with a sea-level control (higher sea-level should favor sedimentary denitrification and, thus, lower d¹⁵N; Algeo et al., 2014). N-isotopic variation was probably driven by a combination of wind-shear and upwelling rates, and lateral advection of open-ocean watermasses, as in the late Pleistocene eastern tropical Pacific Ocean. Based on this scenario, our model yields an estimated increase in upwelling rates in ETPO by a factor of 2 to 4× during deglacial eustatic rises.