Ocean biogeochemistry exhibits contrasting responses to a large scale reduction in dust deposition

Abstract. Dust deposition of iron is thought to be an important control on ocean biogeochemistry and air-sea CO₂ exchange. In this study, we examine the impact of a large scale, yet climatically realistic, reduction in the aeolian Fe input during a 240 year transient simulation. In contrast to previous studies, we find that the ocean biogeochemical cycles of carbon and nitrogen are relatively insensitive (globally) to a 60% reduction in Fe input from dust. Net primary productivity (NPP) is reduced in the Fe limited regions, but the excess macronutrients that result are able to fuel additional NPP elsewhere. Overall, NPP and air-sea CO₂ exchange are only reduced by around 3% between 1860 and 2100. While the nitrogen cycle is perturbed more significantly (by ~15%), reduced N₂ fixation is balanced by a concomitant decline in denitrification. Feedbacks between N₂ fixation and denitrification are controlled by variability in surface utilization of inorganic nitrogen and subsurface oxygen consumption, as well as the direct influence of Fe on N₂ fixation. Overall, there is relatively little impact of reduced aeolian Fe input (<4%) on cumulative CO₂ fluxes over 240 years. The lower sensitivity of our model to changes in dust input is primarily due to the more detailed representation of the continental shelf Fe, which was absent in previous models.