Abstract
Throughout Earth’s history, variations in atmospheric CO2 concentration modulated climate. Understanding changes in atmospheric carbon cycle is therefore pivotal in predicting consequences of recent global warming. Here, we report stable carbon isotopes (δ13C) of molecular land plant fossils complemented by bulk organic and inorganic carbon fractions for early Toarcian (Early Jurassic) sediments that coincided with global warming and a carbon cycle perturbation. The carbon cycle perturbation is expressed by a negative excursion in the δ13C records established for the different substrates. Based on differences in the magnitude of the carbon isotope excursion recorded in land plants and marine substrates we infer that the early Toarcian warming was paralleled by an increase in atmospheric CO2 levels from ~500 ppmv to ~1000 ppmv. Our data suggest that rising atmospheric CO2 levels resulted from the injection of 12C-enriched methane and its subsequent oxidation to CO2. Based on the cyclic nature of the CIE we concluded that methane was released from climate sensitive reservoirs, in particular permafrost areas. Moderate volcanic CO2 emissions led to a destabilization of the labile permafrost carbon pool triggering the onset of Toarcian climate change only. The main carbon cycle perturbation then subsequently was driven by a self-sustained demise of a carbon-rich cryosphere progressing from mid to high latitudes as reflected by latitudinal climate gradients recorded in land plant carbon isotopes.
Highlights
Around the globe, sediment archives that span the early Toarcian record profound environmental changes
Our data unequivocally demonstrates that the Toarcian carbon cycle perturbation affected the marine and the atmospheric carbon reservoir, as previously shown by Pienkowski et al and Hesselbo et al
The −3.7‰ magnitude of the carbon isotope excursion (CIE) at La Cerradura is similar to that reported in long chain n-alkanes from the Sichuan Basin (China)[18], but is slightly lower than the −5.3‰ CIE (−4.2‰ on average) determined for terrigenic n-alkanes from the Cleveland Basin (UK)[20] (Fig. 2)
Summary
Sediment archives that span the early Toarcian record profound environmental changes. A hallmark of the early Toarcian is a negative carbon isotope excursion (CIE) that is interpreted to reflect a global carbon cycle perturbation, caused by injections of 12C-enriched carbon into Earth’s hydro-atmosphere system[9,14,15,16,17,18]. Current δ13C records of land plant-derived lipids cover only a brief stratigraphic interval and provide no information on the recovery phase of the CIE and on the long-term evolution of the atmospheric carbon reservoir. Information on atmospheric CO2 concentration and its absolute change during the early Toarcian warming event are based on stomata data from a single section only and span the onset of the CIE10. Reconstruction of atmospheric CO2 concentration may further be complicated by stratigraphic gaps and methodological limitation[10,22]
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