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

•Paired δ13Ccarb and δ15N of mid-Tournaisian were analyzed at two sections in South China.•Both δ13Ccarb and δ15N underwent distinct positive shifts, with spatial variation in the δ13Ccarb shift.•The positive shifts of δ13Ccarb and δ15N preceded that in apatite δ18O.•High and low δ15N values correspond to icehouse and greenhouse climate intervals, respectively.•Water-column denitrification increased during icehouse climate conditions.

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.