Dust and pollution: A recipe for enhanced ocean fertilization?

Abstract

For Fe contained in aeolian dust to act as a micronutrient for oceanic phytoplankton, some fraction of it must first be transformed (mobilized) into a form soluble in ocean water. Fe solubilization in deliquesced mineral dust aerosols emanating from East Asia during the springtime outflow conditions is investigated here with a Lagrangian box model of the gas and aqueous-phase chemistry. The model simulates the scavenging of soluble and reactive gaseous species by mineral dust aerosols, chemical reactions of these species and mineral dust ions in deliquesced solution, and dissolution of Fe-III oxides that occurs as a result of the acid mobilization. The calculations indicate that mineral calcite (CaCO3) strongly buffers deliquesced dust aerosols with a pH that remains close to 8 until the amount of acid added to the aerosol solution exceeds CaCO3 alkalinity. SO2 pollutant emissions are a potential source of acidity to advecting dust from East Asia. The model is used to simulate the chemical evolution of dust plumes formed from two contrasting documented Gobi-desert storms that advected dust to Fe-limited regions of the North Pacific Ocean (NPO). These calculations indicate that only plumes with relatively high initial SO2-to-dust ratios are capable of delivering significant amounts of bioavailable Fe to the NPO. The estimated change in phytoplankton population inferred from the model-calculated inputs of bioavailable Fe for these two episodes is consistent with the satellite-measured chlorophyll a concentrations in the NPO. Model simulations indicate that (1) large dust advection episodes, that can export vast amounts of mineral dust to the open ocean, should have insignificant dissolved iron fraction (DIF) as the amount of SO2 required to acidify such dust plumes is about an order of magnitude higher than what can typically be entrained in the plume during its advection; (2) smaller dust plumes will generally have higher DIFs because they require lower amounts of SO2 and, even if such small plumes may not cause algae blooms, they could still be important sources of dissolved Fe to the NPO. The calculations suggest that future changes in SO2-pollutant emissions from East Asia, either up or down, may affect the productivity of the NPO. Copyright 2005 by the American Geophysical Union.