IS INCREASING ATMOSPHERIC CO₂ SUFFICIENT TO DRIVE BIOTIC TURNOVER IN THE TERRESTRIAL AND MARINE REALMS DURING THE LATE TRIASSIC?

About 80% of all species went extinct at the close of the Triassic. Atmospheric CO₂ increased significantly at the T-J boundary as a result of the emplacement of the Central Atlantic Magmatic Province flood basalts. CO₂-induced global warming due to volcanic outgassing has been invoked as a mechanism for the end-Triassic mass extinction. While previous modeling studies have shown that changes in atmospheric CO₂ can significantly alter the climate system, the relationship between increasing pCO₂ and biotic turnover has not been fully explored.

Our aim is to 1) examine the impact of increasing pCO₂ on the Late Triassic world, 2) explore the climatic stress on marine and terrestrial organisms resulting from an increase in CO₂, and 3) evaluate if the resulting environmental stresses would have been sufficient to drive biotic turnover on land and in the ocean during the Late Triassic. A fully coupled ocean-atmosphere GCM was used to simulate the Late Triassic world. Sensitivity experiments were run to assess the effects of changing CO₂ on the Triassic climate system. A Late Triassic paleogeography, reduced solar luminosity, and uniform land surface characteristics were used to simulate Triassic conditions. Simulations differed only in prescribed pCO₂, which was set at values varying between 600 and 2400 ppmv.

Model results indicate that terrestrial organisms would have been exposed to severe heat stress with increasing CO₂. Doubling pCO₂ from 1200 to 2400 ppmv raises the average surface temperature on land by 3.7°C, compared to 2.4°C in the ocean. The land area where temperatures exceed 35°C and 40°C for more than 30 days annually increases by more than 4 and 6 times, respectively, when CO₂ is doubled. Elevated CO₂ increases the number of high temperature days in all areas. In contrast, the marine biota would not have been as severely stressed by temperature increases. However, increased CO₂ results in greater ocean stratification in all basins, which combined with warmer ocean temperatures, produce conditions that enhance oxygen depletion in the water column. These differing responses of the Triassic climate system (severe heat stress on land and oxygen depletion in the ocean) to increased CO₂ may have been sufficient to drive biotic turnover at the end Triassic.