Combined Effects of Atmospheric and Seafloor Iron Fluxes to the Glacial Ocean

<jats:title>Abstract</jats:title><jats:p>Changes in the ocean iron cycle could help explain the low atmospheric CO<jats:sub>2</jats:sub> during the Last Glacial Maximum (LGM). Previous modeling studies have mostly considered changes in aeolian iron fluxes, although it is known that sedimentary and hydrothermal fluxes are important iron sources for today's ocean. Here we explore effects of preindustrial‐to‐LGM changes in atmospheric dust, sedimentary, and hydrothermal fluxes on the ocean's iron and carbon cycles in a global coupled biogeochemical‐circulation model. Considering variable atmospheric iron solubility decreases LGM surface soluble iron fluxes compared with assuming constant solubility. This limits potential increases in productivity and export production due to surface iron fertilization, lowering atmospheric CO<jats:sub>2</jats:sub> by only 4 ppm. The effect is countered by a decrease in sedimentary flux due to lower sea level, which increases CO<jats:sub>2</jats:sub> by 15 ppm. Assuming a 10 times higher iron dust solubility in the Southern Ocean, combined with changes in sedimentary flux, we obtain an atmospheric CO<jats:sub>2</jats:sub> reduction of 13 ppm. The high uncertainty in the iron fluxes does not allow us to determine the net direction and magnitude of variations in atmospheric CO<jats:sub>2</jats:sub> due to changes in the iron cycle. Our model does not account for changes to iron‐binding ligand concentrations that could modify the results. We conclude that when evaluating glacial‐interglacial changes in the ocean iron cycle, not only surface but also seafloor fluxes must be taken into account.</jats:p>