Rapid but Short-Lived Carbon Release as a Key Driver of the Toarcian Oceanic Anoxic Event (jenkyns Event)

The Toarcian Oceanic Anoxic Event (or Jenkyns Event, ~183 Ma) was a major hyperthermal event in the Jurassic that was associated with seawater deoxygenation, mass extinction, and marked perturbations to the hydrological cycle. The event is marked in the stratigraphic record by a negative carbon isotope excursion (CIE). This CIE is indicative of a substantial release of 12C-enriched carbon, and the magnitude and morphology of the CIE provide first-order constraints on the source of carbon, its release rate, and the relationship between carbon release and environmental changes/extinction. However, the stratigraphic expression of the CIE is highly variable in different sections and substrates, complicating efforts to assess its precise morphology and the true pattern and rate of carbon release. We present a statistical analysis of carbon isotope data from >100 globally distributed records of the event. When sampled at sufficiently high resolution, a majority of records show that the decline to minimum carbon isotope values during the CIE is characterised by up to 3 stratigraphically abrupt negative shifts. The thicknesses of these shifts scale with sedimentation rate, and their relative timing is consistent. These observations indicate that the shifts are not artefacts caused by stratigraphic incompleteness, and that each shift represents a pulse of rapid carbon release. A probabilistic analysis of the shifts, encompassing stratigraphic uncertainties and the effects of stratigraphic time-averaging, suggests that these carbon release pulses occurred over shorter timespans and at potentially faster rates than carbon release during most other hyperthermals. Nevertheless, carbon release rates during the Toarcian were slower than Anthropogenic carbon emissions when measured on comparable time scales.