Intensified oceanic circulation during Early Carboniferous cooling events: Evidence from carbon and nitrogen isotopes

Abstract

The Early Carboniferous was a climatic transition interval from a mid-Paleozoic greenhouse world into the Late Paleozoic Ice Age. It was marked by a long-term cooling trend that was punctuated by short glaciations in the Tournaisian and Visean (early to mid-Early Carboniferous). 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 climatic transition interval. The carbon and nitrogen isotope profiles of both study sections record major perturbations during the mid-Tournaisian and Visean. A mid-Tournaisian event (TICE) is marked by increases in δ13Ccarb, δ13Corg, and δ15Nbulk at Arrow Canyon (from +0.6 to +7.0‰, −25.2 to −23.7‰, and –0.6 to +8.9‰, respectively) and Namur-Dinant (from −0.3 to +4.7‰, −27.9‰ to −22.2‰, and –5.7 to +5.2‰, respectively). An early Visean event (VICE; named herein) is marked by increases in δ13Ccarb, δ13Corg, and δ15Nbulk at Arrow Canyon (from −0.6 to +3.1‰, −28.4 to −23.6‰, and +1.4 to +9.4‰, respectively) and Namur-Dinant (from +2.1 to +4.3‰, −26.7 to −25.0‰, and –1.4 to +3.3‰, respectively). The positive excursions in all three isotopic records during these events are consistent with increased fractional burial of organic carbon 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.