Efficiency of small scale carbon mitigation by patch iron fertilization

Biogeosciences Discussions 01/2009; 7(11). DOI: 10.5194/bg-7-3593-2010
Source: DOAJ


While nutrient depletion scenarios have long shown that the high-latitude High Nutrient Low Chlorophyll (HNLC) regions are the most effective for sequestering atmospheric carbon dioxide, recent simulations with prognostic biogeochemical models have suggested that only a fraction of the potential drawdown can be realized. We use a global ocean biogeochemical general circulation model developed at GFDL and Princeton to examine this and related issues. We fertilize two patches in the North and Equatorial Pacific, and two additional patches in the Southern Ocean HNLC region north of the biogeochemical divide and in the Ross Sea south of the biogeochemical divide. We obtain by far the greatest response to iron fertilization at the Ross Sea site. Here the CO2 remains sequestered on century time-scales and the efficiency of fertilization remains almost constant no matter how frequently iron is applied as long as it is confined to the growing season. The second most efficient site is in the Southern Ocean. Here the biological response to iron fertilization is comparable to the Ross Sea, but the enhanced biological uptake of CO2 is more spread out in the vertical and thus less effective at leading to removal of CO2 from the atmosphere. The North Pacific site has lower initial nutrients and thus a lower efficiency. Fertilization of the Equatorial Pacific leads to an expansion of the suboxic zone and a striking increase in denitrification that causes a sharp reduction in overall surface biological export production and CO2 uptake. The impacts on the oxygen distribution and surface biological export are less prominent at other sites, but nevertheless still a source of concern. The century time scale retention of iron in these models greatly increases the long-term biological response to iron addition as compared with models in which the added iron is rapidly scavenged from the ocean.

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    • "The adverse effects of low Fe concentrations on primary production are well established in ∼30% of the world's oceans, the so-called High Nitrate Low Chlorophyll (HNLC) regions [5]–[7]. The widespread Fe limitation of phytoplankton in HNLC waters has major implications for the ocean C cycle and has led to modelling efforts to link the cycling and bioavailability of Fe to atmospheric draw-down of CO2 into the ocean [8], [9]. More fundamental research into the biochemical basis of long-term physiological acclimation used by diatoms to survive in low Fe environments provides researchers with more accurate information with which to better model global ocean biogeochemistry. "
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    • "Low-Chlorophyll (HNLC) waters, where a number of natural and artificial iron fertilisation experiments have shown that low ambient iron concentrations limit phytoplankton growth. In situ iron fertilisation experiments have yielded enhanced biological production in all major HNLC areas such as the equatorial Pacific, the Southern Ocean and the subpolar North Pacific (de Baar and others 2005), but paleodata (Röthlisberger and others 2004) and modelling studies (Joos and others 1991; Sarmiento and others 2010 "
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    • "This long lifetime produces warming which is nearly uniform globally and other indirect consequences regionally Increased vegetation on land (likely targeting temperate or tropical regions), but the CO 2 response will be global Minimal expected changes to land or ocean regions as this method could remove CO 2 quickly, countering a lot of CO 2 emissions, with relatively little impact on local resources Imposed biogeochemical changes in ocean regions by fertilizing with nutrients. In the Southern Ocean, direct iron addition capitalizes on large inventories of unused plant nutrients such as nitrogen and phosphate (Sarmiento et al. 2010). In low-latitude waters, indirect ocean fertilization by pumping up nutrients in ocean pipes (Karl and Letelier 2008) Most studies to date have considered sulfate aerosols. "
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