The role of atmospheric iron deposition in driving carbon uptake over the Indian Ocean

Abstract

The Indian Ocean (IO) is characterized by large gradients in atmospheric aerosols and is also an important sink region for atmospheric CO₂. However, there are major gaps in our understanding of the linkages between micronutrient dissolved iron (DFe) derived from aerosol over the IO, and the responses of phytoplankton, uptake of CO₂ by the upper ocean, and subsequent CO₂ sequestration. The present study carries out simulations with an Earth system model to assess the importance of atmospheric sources of DFe in driving the uptake of CO₂ over the IO. The results reveal that along with a general increase in upper ocean phytoplankton following atmospheric DFe addition, there are important shifts in phytoplankton functional types. Such shifts impact the dominant biomineral flux and alter the efficiency of export of particulate organic carbon out of the surface mixed layer. The shifts in phytoplankton functional types also largely offset ocean CO₂ uptake from positive biological response by reducing surface alkalinity owing to increase in calcium carbonate flux. The dominance of calcium carbonate flux over the Southern Ocean sector of the IO results in net outgassing of CO₂ over this region in response to present day atmospheric DFe addition. While the deep ocean carbon sequestration, as indicated by the remineralization length scale of particulate organic carbon, is controlled by ballasting effects of mineral dust over the high-dust northern IO, it is a function of ecosystem structure over the low-dust southern IO. The magnitude and the type of phytoplankton response to atmospheric DFe plays a crucial role in driving the surface-to-deep ocean gradients in dissolved inorganic carbon and alkalinity with the greatest contribution to subsurface dissolved inorganic carbon (alkalinity) response originating from the gas-exchange (carbonate) pump. At interannual timescales, the phytoplankton response to atmospheric sources of DFe depends on availability of nitrate over the northern IO, while phytoplankton response over the southern IO is strongly coupled to the variability of atmospheric iron flux. These results help disentangle the relation between atmospheric deposition of DFe, availability of major nutrients, and phytoplankton functional types and spatial and temporal variation in the net ocean CO₂ uptake.