Calibrating the magnitude of the Toarcian carbon cycle perturbation

Abstract

Sedimentary rocks recording the Toarcian oceanic anoxic event (T-OAE) are marked by a negative carbon isotope excursion (CIE) reaching up to À7‰ and ranking among the largest known in the Phanerozoic. These records suggest a dramatic perturbation of the carbon cycle that has been linked to the transfer of juvenile carbon from the endogenic to the exogenic reservoirs. Nevertheless, the magnitude of the Toarcian CIE varies significantly from one substrate to another, hence complicating mass balance evaluation of the potential driving mechanisms. Here we show, using high-resolution, paired records of δ 13 C total organic carbon (TOC) and hydrogen index from the Denkingen core and Dotternhausen quarry (Germany), that the amplitude of the negative CIE in marine organic matter is considerably exaggerated by changes in organic matter sourcing. Our corrected 3-4‰ CIE implies a constant carbon isotope fractionation between organic and carbonate carbon across the T-OAE and may point to a prominent role of cyanobacteria as organic matter producers during the Early Jurassic. These results also suggest that the difference between inorganic and organic carbon isotope values in marine strata constitutes a poor proxy of pCO 2 levels across the T-OAE. Moreover, this corrected 3-4‰ CIE, together with evidence for >5°C seawater warming across the T-OAE, point to the input of >25,000 Gt C with much more 13 C-enriched signature than previously modeled. Our results thus allow reconciling inorganic and organic δ 13 C records of the T-OAE and have important implications for the evaluation of their causal mechanisms.

Carbon sequestration in an expanded lake system during the Toarcian oceanic anoxic event

“…Figure S7). On the other hand, compound-specific carbon-isotope analysis on long-chain n-alkanes, sourced from terrestrial higher-plant leaf waxes or freshwater algae, also shows a 3-4‰ negative excursion, similar in magnitude to that observed in the bulk δ 13 C TOC record of the same core and coeval marine successions [85][86][87] . Aquatic and terrestrial organic matter (e.g.…”

“…Figure S8. Comparison of the δ 13 C TOC , δ 13 C compound-specific and HI records from Yorkshire, Dotternhausen, Denkingen and the Sichuan Basin [85][86][87]89 , showing elevated HI values in all marine and lacustrine records, in concert with the T-OAE negative CIEs of ~4-7‰ in δ 13 C TOC record and of ~3-4‰ in δ 13 C compound-specific records. Figure S9.…”

“…Figure S9. The cross-plot relationships between bulk organic δ 13 C and Hydrogen Index (HI) from Yorkshire, Dotternhausen, Denkingen and the Sichuan Basin [85][86][87]89 , showing correlations between bulk-organic δ 13 C and Hydrogen Index for all the marine and lacustrine records during the T-OAE negative CIE intervals with, however, a varying magnitude of correlation between different geological settings.…”

Sinemurian–Pliensbachian calcareous nannofossil biostratigraphy and organic carbon isotope stratigraphy in the Paris Basin: Calibration to the ammonite biozonation of NW Europe

“…It may represent a local event only, due to the potential mixing of various sources of continental and marine organic matter, whose average values differ considerably (Suan et al, 2015), as well as to potential diagenetic alteration of the organic matter during dolomitization.…”

Late Permian–earliest Triassic high-resolution organic carbon isotope and palynofacies records from Kap Stosch (East Greenland)

“…44 | P a g e be taken into account (e.g., the transgression on top of interval 3 or interval 5) (Suan et al 2015).…”