Technical Comments

Comment on “Atmospheric Pco2 Perturbations Associated with the Central Atlantic Magmatic Province”

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Science  04 Nov 2011:
Vol. 334, Issue 6056, pp. 594
DOI: 10.1126/science.1208653

Abstract

Schaller et al. (Research Article, 18 March 2011, p. 1404) proposed that carbon dioxide (CO2) released by the Central Atlantic Magmatic Province eruptions over periods of about 20,000 years led to substantial increases of up to 2000 parts per million (ppm) in the concentration of atmospheric carbon dioxide (Pco2) near the Triassic-Jurassic boundary. Use of an atmosphere-ocean model coupled to a carbon-cycle model predicts Pco2 increases of less than 400 ppm from magmatic volatiles, with only a small climatic impact.

Schaller et al. (1) proposed that large amounts of CO2 released by the Central Atlantic Magmatic Province (CAMP) (2.6 × 106 km3 of magma) eruptions contributed to substantial increases in atmospheric CO2 near the Triassic-Jurassic boundary. With an estimated volcanic efflux of 1.4 × 1010 kg of CO2 per km3 of basaltic magma (2), the total CO2 release for the CAMP basalts is about 3.4 × 1016 kg of CO2. There are three lava flow events in the Newark and Hartford Basins (1), so that each of the three lava flows could represent an erupted volume about 0.8 × 106 km of magma releasing 1.1 × 1016 kg CO2. Schaller et al. (1) estimate that the release of this much CO2 over a period of ~20 thousand years (ky) (the resolution of orbital precession) would directly increase atmospheric partial pressure of CO2 (Pco2) by ~1400 parts per million (ppm) from a base of about 2000 ppm (using a conversion factor of 7.82 × 1012 kg of CO2 per ppm CO2). This is less than a doubling of Pco2 and hence a global climatic warming estimated as less than ~3°C (3).

An eruption rate producing 0.8 × 106 km3 of basaltic magma in only 20 ky would be much greater than the commonly inferred 105- to 106-year duration for flood basalt volcanism (4, 5). However, if one scales up from the eruption of Laki in 1783, which produced 12 km3 of basaltic lava over about 2 months (2) at a rate of 4 × 103 m3 s−1 (or 1.1 × 107 kg s−1), then it would be possible to produce ~106 km3 of lava in about 20 ky of semicontinuous eruption.

The time over which the magma is erupted affects the amount of CO2 that stays in the atmosphere. We used an ocean-atmosphere box model coupled to a carbon-cycle model (6, 7) to simulate increases in Pco2 resulting from the CAMP eruptions for two cases: an unrealistic instantaneous release, and release over a period of 20 ky (Fig. 1). Instantaneous release of 1016 kg of CO2 results in an increase of Pco2 of about 1300 ppm over a high early Jurassic background of about 2000 ppm, similar to results of Schaller et al. (1). If the release is accomplished over a substantial time period, then one must take into account uptake of CO2 by the oceans, and if long enough, interactions with the solid earth through rock weathering. When released over 20 ky, the increase results in a peak Pco2 value of only about 400 ppm over early Jurassic background (Fig. 1). Thus, magmatic CO2 release alone, even for eruptions producing a million cubic kilometers in periods as short as 20 ky, is probably not sufficient to cause major climatic changes and mass extinction (Fig. 1).

Fig. 1

Increase in atmospheric Pco2 caused by magmatic volatiles from CAMP eruption of 1016 kg of CO2 added to the atmosphere instantaneously (blue line) and over 20 ky (red line).

Schaller et al. (1) estimated Pco2 up to 4000 to 5000 ppm after CAMP eruptions based on pedogenic carbonates. Palaeobotanical evidence suggests that Pco2 may have increased by a factor of four across the Triassic-Jurassic boundary (8), and palynological studies have been interpreted as indicating an atmospheric Pco2 at least 10 times present levels, with temperatures rising by about 10°C (9). If these estimates are correct, then an additional source of CO2 from interactions between CAMP magma and country rock (10) or release of marine hydrate deposits (11) seems to be required to explain the evidence for very high early Jurassic Pco2 at the time of the CAMP eruptions.

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