Technical Comments

Response to 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.1209422

Abstract

Rampino and Caldeira argue that the first pulse of the Central Atlantic Magmatic Province would increase the concentration of atmospheric carbon dioxide (Pco2) by only 400 parts per million if erupted over 20,000 years, whereas we observed a doubling within this interval. In the absence of any data to the contrary, we suggest that a more rapid (≤1000-year) eruption is sufficient to explain this observation without relying on thermogenic degassing.

Our observations from the Newark Basin indicate that the first pulse of the Triassic-Jurassic Central Atlantic Magmatic Province (CAMP), represented by the Orange Mountain Basalt, was emplaced within a precession cycle and resulted in a doubling of the atmospheric partial pressure of CO2 (Pco2) above pre-eruptive background levels. A simple model with instantaneous degassing [<1 thousand years (ky), within the time scale of ocean overturning] of 2.5 × 1017 moles of CO2 (~1.2 × 1016 kg), roughly the efflux potential of the first volcanic pulse, gives a ~1400 parts per million (ppm) increase in Pco2 above the ~2000-ppm background level (1). This estimate is compatible with and (admittedly, barely) within the error of the doubling from ~2000 to 4400 ± 1200 ppm observed in the Newark Basin. Rampino and Caldeira (2) present a model whereby a 20-ky release of the same magnitude produces only a ~400-ppm atmospheric Pco2 increase, which they take as an indication that an additional source of CO2 is necessary to explain the observed Pco2 increase. We do not dispute this point, but it begs qualification.

The cycle stratigraphic record from the Newark Basin provides a constraint on the maximum duration (<20 ky) of the first pulse of magmatism, but we are not aware of any data (e.g., weathering at the tops of individual lava flows or accumulation of sediments between flows) that preclude a much more rapid release. Therefore, these release-time constraints provide two useful end-member scenarios to explain the observed changes in Pco2: Either the CO2 release was rapid and could be almost exclusively volcanogenic, or it was more protracted, which would require nearly 10 times as much CO2 [e.g., see (3, 4)] [1017 moles atmospheric reservoir versus 1018 moles atmosphere-ocean reservoir (58)], opening the possibility that it may be thermogenic in origin.

Because thermogenic evolution of CO2 from CaCO3 sediments is an unlikely source [e.g., see (9)], the next largest reactive carbon pool in Earth’s crust is organic, which implies that the extra CO2 needed for a protracted release would be relatively depleted in 13C. However, the organic carbon δ13C measurements from the Newark Basin (1) do not indicate a substantially larger 13C-depleted component in the overall atmospheric Pco2 increase, although there is a slight δ13C decrease (~0.5 per mil) above each volcanic unit. We note that some marine sections record a potential light carbon-isotope excursion at about this time (10); however, the exact relationship of the marine δ13C decrease to the CAMP eruptions remains unclear (e.g., see 11). Moreover, our observation of comparable Pco2 and δ13C changes after the second and third volcanic events would require a similar thermogenic input if the duration of each pulse was ~20 ky, which would represent a substantial repeated flux of thermogenic CO2 to the atmosphere at discrete intervals.

Therefore, we are left to speculate on the precise source of the CO2 pulse recorded in the Newark Basin, which is essentially an argument of release duration versus size. In the absence of any data to the contrary, we favor a rapid release that allows the majority of each perturbation to be volcanogenic but that does not preclude a metamorphic carbon source. The doubling of Pco2 observed after each volcanic unit in the Newark Basin is broadly consistent with other lower-resolution studies that indicate a tripling to quadrupling through the interval (1214). The continued challenge to the modeling community is to devise a scenario that conforms to these observations.

References and Notes

  1. Acknowledgments: This work was supported by National Science Foundation grant 0958867. This is Lamont-Doherty Earth Observatory Contribution 7497.
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