Sarmiento & Gruber, 2006) by exporting carbon from the surface into the deep ocean via the biological pump. The net efficiency of the biological carbon export depends partly on the relative competitive fitness of a variety of phytoplankton functional types (Kvale et al., 2015b;Kvale et al., 2019). Major groups of these plankton include diazotrophs, coccolithophores, and diatoms. These groups differ in size, shape, and cell wall composition, which affect their sinking velocities (Collins et al., 2014;Klaas & Archer, 2002;Miklasz & Denny, 2010) and thus the amount of carbon exported into the deep ocean (DeVries et al., 2012). Calcifiers (e.g., coccolithophores) and silicifiers (e.g., diatoms) are two functional types thought to exert a dominant influence on global carbon cycling (Matsumoto et al., 2002) due to both their relatively efficient carbon export properties as well as their dominance in the Southern Ocean. Both diatoms and coccolithophores photosynthesize, which leads to oceanic CO 2 uptake. However, coccolithophores also produce calcite platelets, which leads to a decrease in surface alkalinity, that induces a net CO 2 outgassing (a mechanism also called the calcium carbonate counter-pump). Diatoms tend to dominate export production (EP) in the High Nutrient Low Chlorophyll (HNLC) regions. As EP is limited by the availability of Fe in HNLC regions, it has been hypothesized that iron fertilization of HNLC regions might be an efficient mechanism for enhancing ocean sequestration of carbon both in the modern climate as well as in the past (Martin, 1990).It is therefore important to better constrain the response of ecosystems to climatic changes, and in particular the response of diatoms and coccolithophores in the Southern Ocean. One example of significantly
Abstract. While several processes have been identified to explain the decrease in atmospheric CO2 during glaciations, a better quantification of the contribution of each of these processes is needed. For example, enhanced aeolian iron input into the ocean during glacial times has been suggested to drive a 5 to 28 ppm atmospheric CO2 decrease. Here, we constrain this contribution by performing a set of sensitivity experiments with different aeolian iron input patterns and iron solubility factors under boundary conditions corresponding to 70 000 years before present (70 ka), a time period characterised by the first observed peak in glacial dust flux. We show that the decrease in CO2 as a function of Southern Ocean iron input follows an exponential decay relationship. This exponential decay response arises due to the saturation of the biological pump efficiency and levels out at ∼21 ppm in our simulations. We show that the changes in atmospheric CO2 are more sensitive to the solubility of iron in the ocean than the regional distribution of the iron fluxes. If surface water iron solubility is considered constant through time, we find a CO2 drawdown of ∼4 to ∼8 ppm. However, there is evidence that iron solubility was higher during glacial times. A best estimate of solubility changing from 1 % during interglacials to 3 % to 5 % under glacial conditions yields a ∼9 to 11 ppm CO2 decrease at 70 ka, while a plausible range of CO2 drawdown between 4 to 16 ppm is obtained using the wider but possible range of 1 % to 10 %. This would account for ∼12 %–50 % of the reconstructed decrease in atmospheric CO2 (∼32 ppm) between 71 and 64 ka. We further find that in our simulations the decrease in atmospheric CO2 concentration is solely driven by iron fluxes south of the Antarctic polar front, while iron fertilisation elsewhere plays a negligible role.
Abstract. While several processes have been identified to explain the decrease in atmospheric CO2 during glaciations, a better quantification of the contribution of each of these processes is needed. For example, enhanced aeolian iron input into the ocean during glacial times has been suggested to drive a 5 to 28 ppm atmospheric CO2 decrease. Here, we constrain this contribution by performing a set of sensitivity experiments with different aeolian iron input patterns and iron solubility factors under boundary conditions corresponding to 70 thousand years before present (70 ka BP), a time period characterised by the first observed peak in glacial dust flux. We show that the decrease in CO2 as a function of the Southern Ocean iron input follows an exponential decay relationship. This exponential decay response arises due to the saturation of the biological pump efficiency and levels out at ∼21 ppm in our simulations. Using a best estimate of surface water iron solubility between 3 and 5 %, a ∼9 to 11 ppm CO2 decrease is simulated at 70 ka BP, while a plausible range of CO2 draw-down between 4 to 16 ppm is obtained using the wider but possible range of 1 to 10 %. This would account for ∼12–50 % of the reconstructed decrease in atmospheric CO2 (∼32 ppm) between 71 and 64 ka BP. We further find that in our simulations the decrease in atmospheric CO2 concentrations is solely driven by iron fluxes south of the Antarctic polar front, while iron fertilization elsewhere plays a negligible role.
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