INTENSIFIED OCEANIC CIRCULATION DURING EARLY CARBONIFEROUS COOLING EVENTS: EVIDENCE FROM CARBON AND NITROGEN ISOTOPES

The Early Carboniferous climate transition to the Late Paleozoic Ice Age was marked by a long-term cooling trend punctuated by short glaciations in the mid-Tournaisian and Visean. Here, we generated organic carbon and nitrogen isotope profiles for two widely separated Lower Carboniferous sections (Arrow Canyon Range, Nevada, and Namur-Dinant Basin, Belgium) in order to assess the global nature and timing of changes in the carbon-nitrogen cycles linked to oceanic productivity and redox conditions during this climate transition. Both sections record major carbon- and nitrogen-cycle perturbations during the mid-Tournaisian and Visean. The mid-Tournaisian event (TICE) is marked by increases of 5-7 ‰ in δ¹³C_carb, 1-5 ‰ in δ¹³C_org, and 9-11 ‰ in δ¹⁵N_bulk, and the Visean event (VICE) is marked by increases of 2-4 ‰ in δ¹³C_carb, 2-5 ‰ in δ¹³C_org, and 5-8 ‰ in δ¹⁵N_bulk. The positive excursions in all three isotopic records are consistent with increased organic carbon burial and enhanced denitrification, implying intensification of marine productivity and expansion of hypoxia in the global ocean. Given that each event coincided with lowering of sea-surface temperatures and increased glaciation (as documented from published conodont δ¹⁸O and sea-level records), we hypothesize that global cooling led to intensified oceanic circulation and upwelling on continental margins, triggering increased marine productivity and attendant redox changes within the affected upwelling zones. Our results provide new insights into changes in Early Carboniferous oceanic conditions in response to the initial stages of cooling leading into the Late Paleozoic Ice Age.