The TICE event: Perturbation of carbon–nitrogen cycles during the mid-Tournaisian (Early Carboniferous) greenhouse–icehouse transition

Abstract

Carbonate carbon (δ13Ccarb) and bulk nitrogen (δ15N) isotopic variation during the mid-Tournaisian was analyzed in the Malanbian and Long'an sections of South China. Their C-isotope profiles document a large positive excursion, herein termed the ‘mid-Tournaisian carbon isotope excursion’ (TICE), during the Siphonodella isosticha conodont Zone, although its magnitude differed between the two sections (>6‰ at Malanbian versus ~3‰ at Long'an). The TICE event coincided with sedimentologic and oxygen-isotopic evidence of climatic cooling and glaciation during the mid-Tournaisian. It was probably triggered by an increase in organic carbon burial rates linked to changes in global-ocean circulation. The study sections also document a large positive shift in δ15N, from 1.7‰ to 4.2‰ at Malanbian and from 1.5‰ to 3.8‰ at Long'an. The N-isotope shift shows no termination within the study sections and is likely to mark the onset of an extended interval of 15N-enriched marine nitrate that lasted for the duration of the Late Paleozoic Ice Age. Its initiation coincided with TICE and thus may have been linked to ocean-circulation changes that resulted in intensified upwelling and an increase in water-column denitrification. The continuation of the N-isotope shift over millions of years may have been linked to glacio-eustatic fall and a long-term shift in the locus of denitrification from continental-shelf sediments to continent-margin oxygen-minimum zones. The TICE event thus marks the onset of sustained continental glaciation during the Late Paleozoic Ice Age.