Abstract
The emplacement of the Karoo Large Igneous Province (LIP) occurred synchronously with the Toarcian crisis (ca. 183 Ma), which is characterized by major carbon cycle perturbations. A marked increase in the atmospheric concentration of CO2 (pCO2) attests to significant input of carbon, while negative carbon isotope excursions (CIEs) in marine and terrestrial records suggest the involvement of a 12C-enriched source. Here we explore the effects of pulsed carbon release from the Karoo LIP on atmospheric pCO2 and δ13C of marine sediments, using the GEOCLIM carbon cycle model. We show that a total of 20,500 Gt C replicates the Toarcian pCO2 and δ13C proxy data, and that thermogenic carbon (δ13C of −36 ‰) represents a plausible source for the observed negative CIEs. Importantly, an extremely isotopically depleted carbon source, such as methane clathrates, is not required in order to replicate the negative CIEs. Although exact values of individual degassing pulses represent estimates, we consider our emission scenario realistic as it incorporates the available geological knowledge of the Karoo LIP and a representative framework for Earth system processes during the Toarcian.
Highlights
The emplacement of the Karoo Large Igneous Province (LIP) occurred synchronously with the Toarcian crisis, which is characterized by major carbon cycle perturbations
In order to investigate the consequences of carbon release from the Karoo LIP, it is essential to constrain the timing between the negative carbon isotope excursions (CIEs), pCO2 excursion, and Karoo activity
The emission scenario presented here replicates an overall global trend in δ13C, including several negative steps, and a total negative shift (δ13Cmax – δ13CpreT-OAE) of ~−4‰ for δ13Ccarb and ~−6‰ for δ13Corg. This is comparable to observed Toarcian carbonate δ13C records showing generally lower CIE magnitudes compared to those of organic matter, with mean total CIE values of ~−3 vs. ~−5‰, respectively[15]
Summary
The emplacement of the Karoo Large Igneous Province (LIP) occurred synchronously with the Toarcian crisis (ca. 183 Ma), which is characterized by major carbon cycle perturbations. Alternative suggestions include marine methane clathrate dissociation (e.g.,4,8,22,), permafrost melting[23], terrestrial organic matter[24,25], or thermogenic carbon release generated by sill emplacement into organic-rich sedimentary rocks of the Karoo Basin (e.g.,1,19,22,26,). Available high-precision U-Pb ages of the sills and thermogenic emission estimates from the Karoo Basin allows to identify detailed and realistic emission scenarios from the Karoo LIP; these have not yet been tested by carbon cycle modeling We test such scenarios using the GEOCLIM model[33,34], in order to explore whether pulsed carbon release from the Karoo LIP could explain the full extent of observed Toarcian pCO2 and δ13C changes. The emission scenarios are grounded by U-Pb geochronology, detailed evolution of the Karoo LIP, and estimates of both the mantle-derived and thermogenic carbon from volcanic and contact metamorphic degassing, respectively
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