The impact on atmospheric CO₂ of iron fertilization induced changes in the ocean's biological pump

X. Jin (1) , N. Gruber (2, 3) , H. Frenzel (1) , S. C. Doney (4) , J. C. Mcwilliams (3)

1 IGPP - Institute of Geophysics and Planetary Physics [Los Angeles]
2 IBP - Institute of Biogeochemistry and Pollutant Dynamics [ETH Zürich]
3 IGPP and Department of Atmospheric and Oceanic Sciences
4 WHOI - Department of Marine Chemistry and Geochemistry

Using numerical simulations, we quantify the impact of changes in the ocean's biological pump on the air-sea balance of CO₂ by fertilizing a small surface patch in the high-nutrient, low-chlorophyll region of the eastern tropical Pacific with iron. Decade-long fertilization experiments are conducted in a basin-scale, eddy-permitting coupled physical biogeochemical ecological model. In contrast to previous studies, we find that most of the dissolved inorganic carbon (DIC) removed from the euphotic zone by the enhanced biological export is replaced by uptake of CO₂ from the atmosphere. Atmospheric uptake efficiencies, the ratio of the perturbation in air-sea CO₂ flux to the perturbation in export flux across 100 m, are 0.75 to 0.93 in our patch size-scale experiments. The atmospheric uptake efficiency is insensitive to the duration of the experiment. The primary factor controlling the atmospheric uptake efficiency is the vertical distribution of the enhanced biological production. Iron fertilization at the surface tends to induce production anomalies primarily near the surface, leading to high efficiencies. In contrast, mechanisms that induce deep production anomalies (e.g. altered light availability) tend to have a low uptake efficiency, since most of the removed DIC is replaced by lateral and vertical transport and mixing. Despite high atmospheric uptake efficiencies, patch-scale iron fertilization of the ocean's biological pump tends to remove little CO₂ from the atmosphere over the decadal timescale considered here.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Interfaces continentales, environnement; Océan, Atmosphère; Sciences de la Terre

X. Jin, N. Gruber, H. Frenzel, S. C. Doney, J. C. Mcwilliams. The impact on atmospheric CO₂ of iron fertilization induced changes in the ocean's biological pump. Biogeosciences Discussions, 2007, 4 (5), pp.3863-3911. ⟨hal-00297930⟩