Fluxes of bio-available iron to the ocean

○ Akinori ItoResearch Institute for Global Change, JAMSTEC

Yan FengScripps Institution of Oceanography, University of California

[Ito and Kawamiya, GBC, 2010]

Modeled Chlorophylls Observed Chlorophylls(7.3 Tg OC a-1) (6.2 Tg OC a-1)

Oceanic emission of carbon-containing aerosols

High nitrate, low chlorophyll (HNLC) regionsSoutheast of New Zealand in the southwest Pacific sector of the Southern Ocean

Ocean iron fertilisation

[Wingenter et al., PNAS, 2004]

CO2 uptake

VOC emission

Aerosol iron solubility

Dust Sources Human Emissions

Insoluble Iron

Acidic Gases

Soluble Iron

Combustion Sources

Soluble iron emission

Dust

Combustion

Aerosol chemistry transport model [Ito and Feng, ACPD, 2010]

Meskhidze et al. (2005) and Solmon et al. (2009) predicted a significant deposition of soluble iron for smaller amounts of dust outflow during the transpacific transport.

(1)Iron internally mixed alkaline dust (Exp1)

McNaughton et al. (2008) and Fairlie et al. (2009) have argued that dust does not acidify in the free troposphere except for submicron particles, because the consumption of calcite alkalinity by uptake of acid gases is slow.

(2)Iron externally mixed alkaline dust (Exp2)

Sullivan et al. (2007) found that the submicron dust particles, which were likely associated with aluminosilicate- and iron-rich dust, could become very acidic due to mixing with sulphuric acid during the early stage of the transport.

Iron internally mixed alkaline dust

Iron, Alkaline minerals

Acidic Gases

Alkaline Gases

Dust Sources Human Emissions

Soluble Iron

Alkaline dust

Surface air

Free troposphere

Dissolved iron fraction (DIF) in dustFine mode

Coarse mode

Cruise measurement (Chen & Siefert, 2003)

Comparison of iron fractional solubility (%)

ModelObservation

Fine mode Coarse mode

Iron externally mixed alkaline dust

Acidic Gases

Alkaline Gases

Dust Sources Human Emissions

Soluble Iron

Insoluble Iron

DIF in the fine particles

Exp1

Exp2

Cruise measurement (Chen & Siefert, 2003)

Iron internally mixed alkaline dust (Exp1)

Iron externally mixed alkaline dust (Exp2)

Comparison of iron fractional solubility (%)

Iron internally mixed alkaline dust

Observation

Fine mode Coarse mode

Iron externally mixed alkaline dust

Aerosol supply of soluble ironExp1

Improved ModelHigh NitrateLow Chlorophyll(HNLC)

Iron internally mixed alkaline dust (Exp1)

Aerosol supply of soluble ironCombustion

DustHigh NitrateLow Chlorophyll(HNLC)

Key flux is the amount of the soluble or bio-available iron as for the biogeochemical response to the atmospheric deposition.

We propose that smaller dust particles may yield increased amounts of soluble iron relative to larger particles due to possible variations in mixing state of alkaline dust as a non-linear function of iron-containing aerosol particle size.

The acid mobilization of iron could be important process for input of bioavailable iron to the eastern North Pacific Ocean.

As global warming has been predicted to intensify stratification and reduce vertical mixing, air pollution might have a large impact on the marine phytoplankton production in the upper ocean. It may further influence the negative feedback of climate change through the ocean uptake of carbon dioxide as well as via aerosol-cloud interaction.

Take home messages