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Δ13c of Terrestrial Vegetation Records Toarcian CO2 and Climate Gradients Wolfgang Ruebsam1*, Matías Reolid2 & Lorenz Schwark1,3

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Δ13c of Terrestrial Vegetation Records Toarcian CO2 and Climate Gradients Wolfgang Ruebsam1*, Matías Reolid2 & Lorenz Schwark1,3 www.nature.com/scientificreports OPEN δ13C of terrestrial vegetation records Toarcian CO2 and climate gradients Wolfgang Ruebsam1*, Matías Reolid2 & Lorenz Schwark1,3 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 diferent substrates. Based on diferences 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 12 CO2 levels resulted from the injection of C-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 refected by latitudinal climate gradients recorded in land plant carbon isotopes. Anthropogenic fossil carbon emissions steadily increase atmospheric CO2 levels and thereby impact on Earth’s climate and carbon cycle1. As a consequence rising global temperatures can led to a reactivation of carbon stored in permafrost regions that upon its release to the atmosphere will further accelerate global warming2. Melting polar ice caps and sea level rise, climate extremes and enhanced stress for marine and continental ecosystems have been proven to be direct consequences of global warming3–5. Predictions on the evolution of Earth’s climate system, the carbon cycle and the response of ecosystems are, however, problematic. Tus, investigation of sedi- ment archives that record ancient climate perturbation can serve as analogues for recent climate change and can thereby guide in predicting consequences of global warming and its cascade of consequences. Here, we address changes in Earth’s climate and carbon cycle that occurred in conjunction with the early Toarcian Oceanic Anoxic Event (Early Jurassic; ∼183 Ma). Tis study utilizes stable carbon isotopes recorded in diferent substrates, facilitating the reconstruction of changes in the global carbon cycle, atmospheric CO2 levels and latitudinal climate gradients during the early Toarcian global warming. Background Around the globe, sediment archives that span the early Toarcian record profound environmental changes. A rapid high-amplitude sea level rise paralleled by a decline in oxygen isotope values of macrofossil calcite, has been interpreted to refect a rise in sea water temperatures that was potentially accompanied by a reduction in the 6–9 volume of land-based ice caps . Rising global temperatures evolved parallel to an increase in atmospheric CO2 level inferred from stomata data10. In the marine realm global warming led to expansion of marine death zones and triggered the genesis of the Toarcian Oceanic Anoxic Event (T-OAE)11, whereas on land it caused substantial shifs in foral assemblages10,12,13. A hallmark of the early Toarcian is a negative carbon isotope excursion (CIE) that is interpreted to refect a global carbon cycle perturbation, caused by injections of 12C-enriched carbon into Earth’s hydro-atmosphere 1Department of Organic and Isotope Geochemistry, Institute of Geoscience, University of Kiel, Kiel, Germany. 2Departamento de Geología and CEACT, Universidad de Jaén, Jaén, Spain. 3WA-OIGC, Curtin University, Perth, Australia. *email: [email protected] SCIENTIFIC REPORTS | (2020) 10:117 | https://doi.org/10.1038/s41598-019-56710-6 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. Earth’s paleogeography and distribution of climate belts during the Early Jurassic (modifed afer Rees (ref. 27)). Paleogeographic map generated with Adobe Illustrator CC 2019, http://www.adobe.com/products/ illustrator.html. Locations mentioned in the text are indicated (CB: Cleveland Basin, UK; LB: Lusitanian Basin, Portugal; LC: La Cerradura, Iberian Basin, Spain; SB: Sichuan Basin, China). 9,14–18 system . Carbon sources are debated controversially and comprise a volcanic CO2 and/or thermogenic 10,19 CH4 associated with the emplacement of the Karoo-Ferrar Large Igneous Province of southern Gondwana , destabilization of methane hydrates14,16, increased rates of wetland methanogenesis17, or permafrost decay and thermokarst blowout events during global warming9. Te CIE has been reported in marine and terrestrial organic matter as well as in marine carbonates10,12,14,15, suggesting that the carbon cycle perturbation afected the entire exchangeable carbon reservoir. A decline in δ13C documented in land plant-derived lipids indicates atmospheric 13C depletion and substantiates a perturbation of the atmospheric carbon cycle18,20,21. However, 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. Moreover, infor- mation 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 10,22 CO2 concentration may further be complicated by stratigraphic gaps and methodological limitation . Here we utilize compound-specifc carbon isotope data of land plant wax lipids to reconstruct changes in the atmospheric carbon reservoir across the early Toarcian carbon cycle perturbation and the associated climate event. Te δ13C analysis of land plant-derived wax lipids, compounds not afected by the diferential preservation of fossilized wood fragments15, provide a robust method for reconstructing changes in the isotopic composi- tion of the atmospheric carbon reservoir. Te compound-specifc δ13C record is complemented by δ13C data from marine calcite that refect changes in the oceanic carbon reservoir. Te reconciliation of δ13C excursions in land plant and marine substrates allows reconstruction of changes in the entire exchangeable carbon reservoir. Moreover, parallel evaluation of marine and terrestrial carbon isotope excursions provide information not only on changes in atmospheric CO2 concentration but also on absolute atmospheric CO2 levels prior to and during the early Toarcian carbon cycle perturbation23,24. Study site. In this study we investigated upper Pliensbachian to lower Toarcian sediments, represented by the Emaciatum to Serpentinum ammonite zones and the NJT5b to NJT6 nannofossil zones, cropping out at La Cerradura (Subbetic, southern Spain)25. Ammonite assemblages in combination with coccolithophore-based biostratigraphic data indicate that the sediments can be correlated with the T-OAE26, which is further supported by paleontological and geochemical data25. Sediments, mainly marlstone-limestone alternation, were deposited in a fragmented marine platform with hemipelagic sedimentation at a paleolatitude of about 26°N at the southern Iberian paleomargin. Floral assemblages suggest that during the Early Jurassic (183 Ma) the study site was located in the semi-arid climate belt27 (Fig. 1). Results and Discussion An atmospheric record of the toarcian carbon cycle perturbation. The early Toarcian car- 13 13 bon cycle perturbation is expressed in δ Corg and δ Ccarb data by negative excursions of −3.4‰ and −1.2‰, respectively (Table 1 in the SI). A shif towards lower δ13C values occurred in a stepwise manner at SCIENTIFIC REPORTS | (2020) 10:117 | https://doi.org/10.1038/s41598-019-56710-6 2 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 2. Stable carbon isotopes determined on fossilized land plant lipids, bulk organic and carbonate carbon from the La Cerradura section (southern Spain) show a stepped negative CIE at the Polymorphum-Serpentinum zonal transition, confrming that the early Toarcian carbon cycle perturbation afected the entire exchangeable 13 carbon reservoir. At La Cerradura terrigenic lipids record a magnitude in Δ Cn-alkane of −3.7‰, which is comparable to the magnitude documented for the CIE recorded in plant wax alkanes from the Cleveland (UK)20 and Sichuan Basins (China)18. Diferences in the absolute values of plant wax alkane δ13C refect environmental conditions in diferent climate belts (see Fig. 1). the Polymorphum-Serpentinum zonal transition (Fig. 2). Stratigraphic position as well as pattern and pac- ing of the CIE at La Cerradura match trends from other locations documenting a multiphasic carbon cycle perturbation9,15,16. Te δ13C signatures of terrestrial n-alkanes recording a negative CIE with a magnitude of −3.7‰ (−3.1‰ on average) (Fig. 2) parallel the high-resolution δ13C bulk data (Table 2 in the SI). Te stepped CIE character is doc- 13 umented in the δ Cn-alkane record, confrming that the CIE refects multiple re-occurring carbon
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