Volcanic CO2 degassing postdates thermogenic carbon emission during the end-Permian mass extinction

Abstract

Massive carbon dioxide (CO₂) emissions are widely assumed to be the driver of the end-Permian mass extinction (EPME). However, the rate of and total CO₂ released, and whether the source changes with time, remain poorly understood, leaving a key question surrounding the trigger for the EPME unanswered. Here, we assimilate reconstructions of atmospheric Pco₂ and carbonate δ¹³C in an Earth system model to unravel the history of carbon emissions and sources across the EPME. We infer a transition from a CO₂ source with a thermogenic carbon isotopic signature associated with a slower emission rate to a heavier, more mantle-dominated volcanic source with an increased rate of emissions. This implies that the CO₂ degassing style changed as the Siberian Traps emplacement evolved, which is consistent with geochemical proxy records. Carbon cycle feedbacks from terrestrial ecosystem disturbances may have further amplified the warming and the severity of marine extinctions.

Fig. 1.. Two data assimilation constraints used in the double-inversion experiments using cGENIE Earth system model.

(A) Stacked carbonate δ¹³C_carb and continuous Pco₂ records (on a log scale) estimated from C₃ land plant δ¹³C in southwestern China during the EPME (9). (B) Stacked carbonate δ¹³C across the study interval. For two data assimilation constraints, the LOESS fit curve of δ¹³C_carb and the 16th percentiles, median, and 84th percentiles of the continuous Pco₂ records from (9) representing three kinds of CO₂ scenarios are applied in the double-inversion experiments. The conodont zones (27, 28) and radiometric ages (29) are from the GSSP Meishan section. ET, Early Triassic; C. c, Clarkina changxingensis; C. y, Clarkina yini; C. m, Clarkina meishanensis; H. c, Hindeodus changxingensis; C. t, Clarkina taylorae; H. p, Hindeodus parvus; I., Isarcicella.

Fig. 2.. The modeled time-varying carbon emissions and model-data comparisons of changes in SST and pH.

(A) Modeled time-varying δ¹³C_source of three standard experiments (experiments 1 to 3 in table S1) during the CIE divided into phase 1 (dominated by thermogenic carbon emissions) and phase 2 with mantle CO₂-dominated carbon emissions. Two ²⁰⁶Pb/²³⁸U ages of 251.907 ± 0.067 Ma characterized by sill intrusion and 251.901 ± 0.061 Ma marked by extrusive pyroclastic rocks are shown here (38). (B) Diagnosed rates of two-phase carbon emissions (1 Pg = 10¹⁵ g) compared with previously published modeling results (13, 17, 18). (C) Modeled cumulative amount of carbon emissions during the CIE compared with published model estimates (13, 17, 18). (D) Modeled changes in tropical (10°S to 10°N) SST (median value and lower and upper estimates are from three standard runs: experiments 1 to 3 in table S1) and comparison to low-latitude proxy SST change calculated based on conodont δ¹⁸O from South China (–3), Iran (36), and Armenia (4). (E) Modeled surface oceanic pH change (median value and lower and upper estimates are from three standard runs: experiments 1 to 3 in table S1) and comparison to proxy pH records based on boron isotopes of brachiopod from northern Italy and South China (13), and boron isotopes of marine carbonates from United Arab Emirates (12).

Fig. 3.. Spatial pattern of modeled global SST and pH changes during the EPME.

(A and C) SST and pH changes associated with mainly thermogenic CO₂ during phase 1 (251.942 Ma versus 252.02 Ma). This shows an average SST increase of 1° to 2°C and a pH decrease of 0.1- to 0.2-unit during phase 1. (B and D) SST and pH changes linked to volcanic CO₂-dominated emissions during phase 2 (251.902 Ma versus 251.942 Ma). A 4° to 7°C increase in SST and a 0.3- to 0.5-unit decrease in pH are shown. These model results are from the preferred experiment with median CO₂ scenario (experiment 2 in table S1). The absolute values of proxy-based estimates of SST and pH across the study interval are listed in table S4. The numbers in the map indicate locations of these proxy data: 1, South China; 2, Armenia; 3, Iran; 4, Italy; 5, United Arab Emirates.

Fig. 4.. Magnitudes of δ¹³C_carb excursion and proxy estimates of SST changes during the EPME.

(A) CIE magnitudes of δ¹³C_carb during phase 1 are similar to those during phase 2. (B) SST changes estimated based on conodont oxygen isotopes during phase 1 are substantially smaller than those during phase 2. Details of the seven sections shown here are listed in table S4 (Meishan, Liangfengya, and Daijiagou sections are from South China; Chanakhchi section is from Armenia; and Kuh-e-Ali Bashi, Zal, and Abadeh sections are from Iran).