Impact of iron fertilisation on Southern Ocean ecosystems and global carbon cycle during the last glacial cycle

Rising atmospheric CO₂ concentration is a major driver of climate change. One of the several processes proposed to explain the lower atmospheric CO₂ concentration during the last glacial period is an increase in aeolian iron flux into the Southern Ocean. As the Southern Ocean is a high-nutrient-low-chlorophyll region, increased iron deposition can impact Southern Ocean marine ecosystems,  increase export production, and reduce surface Dissolved Inorganic Carbon (DIC) concentration. Here, we investigate the responses of Southern Ocean marine ecosystems to changes in iron flux and their impact on ocean biogeochemistry and atmospheric CO₂ during the last glacial period. We use a recently developed complex ecosystem model that includes four different classes of phytoplankton functional types and fully incorporated iron, silica and calcium carbonate cycles. 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 %. We also show that the decrease in CO₂ as a function of Southern Ocean iron input follows an exponential decay relationship, which arises due to the saturation of the biological pump efficiency and levels out at ∼21 ppm in our simulations.

We also investigate the role of iron flux changes on the abrupt atmospheric CO₂ increase during Heinrich Stadials, which are associated with a near collapse of the Atlantic Meridional Overturning Circulation (AMOC), a sudden decrease in Greenland temperature and warming in the Southern Ocean. Previous modelling studies have investigated the role of the ocean circulation in driving changes in atmospheric CO₂ concentration during these abrupt events, while the role of reduced aeolian iron input during Heinrich stadials remained poorly constrained. We show that a weakened iron fertilisation during Heinrich Stadials can lead to ~6 ppm rise in CO₂ out of the total increase of 15 to 20ppm as observed. This is caused by a 5% reduction in nutrient utilisation in the Southern Ocean, leading to reduced export production and increased carbon outgassing from the Southern Ocean.