The significance of an Early Jurassic (Toarcian) carbon-isotope excursion in Haida Gwaii (Queen Charlotte Islands), British Columbia, Canada

During the Early Toarcian there was a significant disruption in the short-term active carbon reservoir as revealed by carbon-isotope records, which show a broad positive shift that is interrupted by a large 5–7‰ negative excursion (δ13Corg). Carbon-isotope excursion co-occurs with the deposition of organic-rich shales in many areas. This perturbation in carbon isotopes is thought to be indicative of severe climate change and marine anoxia. The two leading hypotheses as to the cause of this event invoke either global or regional controls. Here we present carbon-isotope data from Haida Gwaii, British Columbia, Canada showing a significant perturbation within a temporally constrained Early Toarcian succession that was deposited in the northeastern paleo-Pacific Ocean. These data reinforce the concept that the short-term active carbon reservoir was affected globally, and assist with the correlation of ammonite zonal schemes between western North America and Europe. The δ13Corg data show a broad positive shift that is interrupted by a sharp and pronounced negative excursion of 7‰ (8.5‰ in δ13Cwood) in the Early Toarcian Kanense Zone. This negative excursion also coincides with increasing total organic carbon (TOC) from ~ 0.4% to ~ 1.2%. These data suggest that the Early Toarcian carbon-isotope perturbation was indeed global and imprinted itself on all active global reservoirs of the exchangeable carbon cycle (deep marine, shallow marine, atmospheric).