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Total Fe Flux. Al (derived from Ti) Flux. Al Flux. Figure 2. LWCC set up for the analysis of labile Fe species using the ferrozine method. Figure 3 . Particle dry deposition velocity data for deposition on a water surface in a wind tunnel (Slinn et al., 1978). R/V Kilo Moana. Fe (II) Flux.

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Total Fe Flux

Al (derived from Ti) Flux

Al Flux

Figure 2. LWCC set up for the analysis of labile Fe species using the ferrozine method.

Figure 3. Particle dry deposition velocity data for deposition on a water surface in a wind tunnel (Slinn et al., 1978)

R/V Kilo Moana

Fe (II) Flux

Figure 5. Al fluxes based on measured Al (left) and Ti concentrations (right). Assumes a crustal average Ti/Al ratio of ~0.063 [Taylor and McLennan, 1985]

Easily reducible Fe(III) Flux

NO3- Flux

Figure 1: From left: high volume cascade impactor, R/V Kilo Moana, EUCFe 2006 Cruise track.

Total Labile Fe Flux

Figure 6. NO3- fluxes versus Sample ID.

Figure 4. Fe fluxes to the three distinct regions of the cruise vs. Sample ID. Top: total Fe; middle left Fe(II); middle right: Fe(III)hydrox, and bottom: total labile Fe.

Atmospheric trace metal and labile iron deposition fluxes to the equatorial Pacific during EUCFe2006

Lindsey M. Shankand Anne M. Johansen, Department of Chemistry, Central Washington University, 400 East University Way, Ellensburg, WA 98926, [email protected], [email protected]

Abstract

In the equatorial Pacific Ocean iron limitation persists despite the existence of the equatorial undercurrent which carries high loads of nutrients. Deposition of atmospheric particles may constitute another important pathway for which iron is delivered to this remote ocean, where only sparse atmospheric data exists. We collected aerosols over the Equatorial Pacific Ocean between Hawaii and Papua New Guinea during the EUCFe cruise (R/V Kilo Moana in Aug-Oct 2006) as part of the larger campaign to characterize the various iron sources to this region. Ahigh-volume cascade impactor operating at 760 L min-1 was used to resolve aerosols in four size fractions. Samples were digested and analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and long pathlength absorbance spectroscopy for trace metals and iron speciation respectively. The high-flow rate and particle size fractionation enabled greater temporal and spatial resolution that allows us to estimate fluxes with unprecedented precision.

  • Methods

  • Sample Collection

  • Aerosol sampling was conducted over the equatorial Pacific Ocean as part of the Equatorial Undercurrent Iron (EUCFe) field sampling campaign from Aug.-Oct. 2006 aboard the R/V Kilo Moana (see Figure 1 for cruise track). Six different types of collectors were used to chemically characterize aerosols. Focus here will be on the high volume cascade impactor (ChemVol 2000, R&P) used to collect atmospheric aerosols into four size fractions: ultrafine (da≤0.1 μm), fine (0.1≤da<1 μm), coarse (1≤da<10 μm) and large (da≥10μm) onto polyurethane foam (PUF) substrates (fine, coarse, large) and polypropylene filters (ultrafine). Sample pretreatment, handling and storage were performed following strict trace metal clean procedures. Samples were stored at -20 oC until analysis was feasible.

  • Analysis

  • We collected 152 samples with the ChemVol collector and the following analyses have been performed:

  • Long pathlength absorbance spectroscopy with a long waveguide capillary cell (LWCC, WPI, 200 cm) and portable spectrometer (TIDAS) was used to analyze labile iron species, Fe(II)(aq) and easily reducible Fe(III). Samples were extracted in pH 4.0 formate buffer followed by shaking on a stir-plate to release particles from the substrates. Quantification was accomplished by complexation with ferrozine and subsequent absorbance measurements at 562 nm (Figure 2). All Fe analyses were performed inside a clean laminar flow hood.

  • Inductively Coupled Mass Spectrometry (ICP-MS, Thermo X Series) was employed for the analysis of 37 trace metal species. H2O2, HNO3, HF, and HCl were used in a three day digestion procedure that allowed for the complete recovery of all metals from alumino-silicate lattices.

  • Flux determination

  • Fluxes were determined using the equation: F = Ci vd , where Ci is the concentration of species i and vd is the deposition velocity. vd was estimated using the average diameter for each size bin and applying a model for particle deposition on a water surface in a wind tunnel (Figure 3). Settling velocities used to estimate fluxes were 0.02, 0.03, 1.0, and 12 cm s-1 for ultrafine, fine, coarse, and large particle regimes, respectively.

Introduction

Iron (Fe) is an essential micronutrient necessary for the metabolic processes (e.g., photosynthesis and cellular respiration) of marine phytoplankton in remote ocean regions considered High-Nutrient-Low-Chlorophyll (HNLC). The equatorial Pacific Ocean (EPO) is unique in that it contains the world’s most expansive HNLC region. [Chavez and Barber, 1987] suggested that because of its vastness and large proportion, the equatorial Pacific could account for up to 50% of global new production.

The equatorial Pacific is characterized as an upwelling zone where the supply of major nutrients from below leads to some enhancement of biological production [Turner and Hunter, 2001]. Yet iron limitation persists despite the existence of the equatorial undercurrent, and it is thought that the main delivery pathway for iron to this region is the atmosphere. Measurements of iron flux to this particular part of the world’s ocean remains limited, and therefore atmospheric influence on phytoplankton productivity is unknown in this region with high global production potential. Although assumed to be small [Gao et al., 2001; Raemdonck et al., 1986] this atmospheric contribution of Fe may control productivity in this vast oceanic region, analogous to results of a Southern Ocean study by [Cassar et al., 2007]. Therefore, the present study addresses the delivery and deposition of nutrients, with a particular focus on iron and its speciation, to this unique ocean basin, providing a much needed data set to the oceanography and atmospheric scientific communities.

Results

  • Conclusions

  • PCA and HYSPLIT results identify three regions of the cruise track where aerosol chemistry is distinctly different. The three regions are the North Pacific, Equatorial Pacific, and Bismarck Sea.

  • Table 1 summarizes the average fluxes for NO3-, total Fe, total labile Fe, measured Al, and Al derived from Ti concentrations for the three distinct regions. Ti appears to under predict Al fluxes in regions influenced by land masses, however it over predicts Al concentrations in the pristine regions suggesting crustal material from different sources. NO3- fluxes are significantly larger in the North Pacific and Bismarck Sea regions, indicating that these regions are more heavily influenced by anthropogenic emissions than the more pristine equatorial Pacific Ocean.

  • In aerosols collected over the equatorial Pacific most of the labile iron was found in the fine size fraction (0.1-1 μm in diameter), with significant amounts also found in the coarse modes, consistent with predictions that the fine mode, or accumulation mode, is the dominant size fraction which also accounts for the largest surface area in regions far from dust sources, such as the remote equatorial Pacific. The deposition of iron seems to be controlled by the large fraction, as particles in this regime have the highest settling velocities, however PCA results indicate that large Fe is a recycled component, and may not constitute a new flux of iron to the ocean surface.

  • Total iron, Fe(II), easily reducible Fe(III) (termed Fe(III)hydrox), total labile Fe, Al and NO3- fluxes were determined by estimating dry deposition velocities for each of the four size fractions separately, offering a better estimate than models in which a single settling velocity is assumed for all size particles.

  • Values for dry deposition and dust fluxes are comparable to previous estimates (Table 2) [Duce, 1991; Jickells et al., 2005].

Acknowledgements

This research was supported by National Science Foundation Grant ATM-0137891 and Central Washington University.

People: Dr. Jim Murray, CWU Atmospheric Chemistry Group

References

Boyd, P.W., A.J. Watson, C.S. Law, E.R. Abraham, T. Trull, R. Murdoch, D.C.E. Bakker, A.R. Bowie, and e. al., A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization, Nature, 407, 695-702, 2000.

Cassar, N., M.L. Bender, B.A. Barnett, S.M. Fan, W.J. Moxim, H. Levy, and B. Tilbrook, The Southern Ocean Biological Respnse to Aeolian Iron Deposition, Science, 317 (5841), 1067-1070, 2007.

Charlson, R.J., J.E. Lovelock, M.O. Andreae, and S.G. Warren, Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate, Nature, 326, 655-661, 1987.

Chavez, F.P., and R.T. Barber, An estimate of new production in the equatorial Pacific, Deep-Sea Research, 34, 1229-1243, 1987.

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Duce, R.A., and N.W. Tindale, Atmospheric transport of iron and its deposition in the ocean, Limnology and Oceanography, 36 (8), 1715-1726, 1991.

Jickells, T.D., Z.S. An, K.K. Andersen, A.R. Baker, G. Bergametti, N. Brooks, J.J. Cao, P.W. Boyd, R.A. Duce, K.A. Hunter, H. Kawahata, N. Kubilay, J. laRoche, P.S. Liss, N. Mahowald, J.M. Prospero, A.J. Ridgwell, I. Tegen, and R. Torres, Global iron connections between desert dust, ocean biogeochemistry, and climate, Science, 308 (5718), 67-71, 2005.

Johansen, A.M., and M.R. Hoffmann, Chemical characterization of ambient aerosol collected during the northeast monsoon season over the Arabian Sea: Anions and cations, J. Geophys. Res., 109 (D5), 2004.

Johansen, A.M., R.L. Siefert, and M.R. Hoffmann, Chemical characterization of ambient aerosol collected during the southwest-monsoon and inter-monsoon seasons over the Arabian Sea: Anions and cations, J. Geophys. Res., 104, 26325-26347, 1999.

Johansen, A.M., R.L. Siefert, and M.R. Hoffmann, Chemical composition of aerosols collected over the tropical North Atlantic Ocean, J. Geophys. Res., 105, 15277-15312, 2000.

Raemdonck, H., W. Maenhaut, and M.O. Andreae, Chemistry of marine aerosol over the tropical and equatorial Pacific, J. Geophys. Res., 91, 8623-8636, 1986.

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Taylor, S.R., and S.M. McLennan, The Continental Crust: Its Composition and Evolution, Blackwell Scientific Publ., Oxford, England, 1985.

Turner, D.R., and K.A. Hunter, The Biogeochemistry of Iron in Seawater, John Wiley & Sons, LTD, Chichester, 2001.


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