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Deep-Sea Research I

Deep-Sea Research I 56 (2009) 2115–2128

0967-06

doi:10.1

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journal homepage: www.elsevier.com/locate/dsri

Biogeochemical impact of a model western iron source in the PacificEquatorial Undercurrent

Lia Slemons a,�, Thomas Gorgues b, Olivier Aumont c, Christophe Menkes d, James W. Murray a

a School of Oceanography, University of Washington, Seattle WA 98195-5351, USAb JISAO-School of Oceanography, University of Washington, Seattle, WA 98195-5351, USAc Laboratoire d’Oc�eanographie et de Climatologie: Exp�erimentation et Approches Num�eriques (LOCEAN), IRD/IPSL, Plouzan�e, Franced Laboratoire d’Oc�eanographie et de Climatologie: Exp�erimentation et Approches Num�eriques (LOCEAN) IRD/CNRS/UPMC/MNHR, Noum�ea Cedex, New Caledonia

a r t i c l e i n f o

Article history:

Received 15 May 2008

Received in revised form

24 July 2009

Accepted 10 August 2009Available online 19 August 2009

Keywords:

Iron

Ecosystem model

Bioavailability

Equatorial Pacific

Limitation

High-nitrate low-chlorophyll

Transport

Export

Biological pump

37/$ - see front matter & 2009 Elsevier Ltd. A

016/j.dsr.2009.08.005

responding author. Tel.: +1206543 0632; fax:

ail addresses: ossianla@u.washington.edu (L.

he.menkes@noumea.ird.nc (C. Menkes), jmur

a b s t r a c t

Trace element distributions in the source waters of the Pacific Equatorial Undercurrent

(EUC) show the existence of elevated total acid-soluble iron concentrations. This region

has been suggested to contribute enough bioavailable iron to regulate interannual and

interglacial variability in biological productivity downstream in the high-nitrate low-

chlorophyll upwelling zone of the eastern equatorial Pacific. We investigated the

advection and first-order biogeochemical impact of an imposed, data-based iron

maximum in the western Pacific EUC using an ecosystem model forced by a global

dynamical model. We imposed two source profiles of iron constrained by total acid-

soluble iron measurements. Though the data for total acid-soluble iron included both

dissolved and acid-soluble particulate iron species, we treated all of the total acid-

soluble iron as if it was dissolved and bioavailable. A deeper (270 m) source was

centered in the density horizon of the observed iron maximum and a shallower (180 m)

source was located in the core of our model’s EUC, where a dissolved iron maximum has

been frequently postulated. These source runs were compared with a control run that

contained no specific source of iron associated with the EUC. In the source runs elevated

iron concentrations were simulated in the EUC across its entire zonal span, evident as a

subsurface plume of dissolved iron slightly below the core of the EUC. In the control run

there was no iron maximum associated with the EUC. Upwelling of iron-replete water in

the central and eastern equatorial Pacific increased integrated primary productivity in

the Wyrtki box (1801W:901W, 51S:51N, 0:200 m) by 41% and 66% for the deeper and

shallower iron perturbation, respectively. The source runs increased the realism of the

zonal extent of HNLC conditions and the meridional distributions of biological

productivity, relative to the control run. However, in the source simulations surface

chlorophyll concentrations were too high by a factor of two and maximum surface

nitrate concentrations were too low, relative to climatologies. The relative abundance of

diatoms roughly doubled upon the input of additional iron, exceeding field observations.

Though biogeochemical data are limited and we did not adjust parameters to optimize

the model fits to observations, these results suggest that acid-soluble particulate iron

supplied to the EUC in the western equatorial Pacific is unlikely to be entirely

bioavailable.

& 2009 Elsevier Ltd. All rights reserved.

ll rights reserved.

+1206 685 3351.

Slemons), thomas.gorgues@ifremer.fr (T. Gorgues), olivier.aumont@ird.fr (O. Aumont),

ray@u.washington.edu (J.W. Murray).

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1. Introduction

1.1. Regulation of the equatorial Pacific high-nitrate low-

chlorophyll regime

The eastern equatorial Pacific is one of the world’slargest high-nitrate low-chlorophyll (HNLC) ocean regions(Chavez and Toggweiler, 1995). Equatorial upwellingproduces between 20% and 30% of global new production(Chavez and Barber, 1987; Chavez and Toggweiler, 1995).The most important findings from the US JGOFS ProcessStudy (EqPac) and subsequent studies in the equatorialPacific were (1) that this region has all the biologicalcharacteristics of HNLC regimes (Murray et al., 1994); (2)that iron is a major limiting nutrient for phytoplanktongrowth rate, biomass and new production (Martin et al.,1994; Price et al., 1994; Kolber et al., 1994; Coale et al.,1996; Behrenfeld et al., 1996; Landry et al., 1997;Aufdenkampe et al., 2002); (3) that there is a maximumof particulate and arguably of dissolved iron in or slightlybelow the Equatorial Undercurrent (EUC) along 1401W(Coale et al., 1996; Gordon et al., 1997); (4) that upwellingfrom the EUC is the primary source of iron to the euphoticzone in this region (Coale et al, 1996; Fung et al., 2000);and (5) that variations in productivity are driven byvariability in upwelling and input of iron from the EUC(Chavez et al., 1999; Friedrichs and Hofmann, 2001; LeBorgne et al., 2002; Ryan et al. 2006).

The ecumenical iron hypothesis states that the twoprimary regulators of HNLC conditions in the equatorialPacific are iron supply and grazing (Chisholm and Morel,1991; Landry et al., 1997). Nanoplankton biomass iscontrolled primarily by rapidly growing microzooplankton(Frost and Frazen, 1992; Verity et al., 1996; Landry et al.,1997), whereas iron limitation affects a broader range ofplankton size classes, particularly diatoms, but includingpicoplankton (Landry et al., 1997; Mann and Chisholm,2000) and biomass response in iron fertilization experi-ments (Fitzwater et al., 1996).

1.2. Previous biogeochemical modeling studies

Given the apparent paradox of a highly productive, yetnutrient-limited, HNLC system, numerous modeling stu-dies have investigated the factors controlling equatorialPacific productivity. Toggweiler and Carson (1995) pro-posed that export production is kept low by efficientnutrient recycling within three-dimensional meridionaloverturning cells. Using an ocean circulation modelcoupled to a biogeochemical model with nitrate andammonia as limiting nutrients, they showed that althoughnew nitrate was advected eastward in the EUC, primaryproduction in the eastern equatorial Pacific was largelyfueled by regenerated nitrogen. In a similar approach,Jiang et al. (2003) investigated nutrient fluxes andincluded silicate as a limiting nutrient. In their study,iron was not explicitly modeled; instead, its influence wasparameterized by modification of the rate constant forplankton growth. Loukos et al. (1997) examined theimpact of including iron as a colimiting nutrient with

nitrate and ammonia in one-dimensional biogeochemicalmodels and showed the importance of iron in controllingboth biomass and primary production. Leonard et al.(1999) found similar results in an expanded one-dimen-sional iron-based model that included two each ofphytoplankton, zooplankton, and detrital size fractions.Christian et al. (2002a) confronted the paucity of iron fielddata by testing the response of various subsurface andsurface (i.e. aeolian iron supply with 10% solubility)western iron sources and showed that western ironsources can propagate east in the EUC (their Fig. 7). Thewestern-iron-source simulations of Christian et al.(2002a) showed that (1) the depth of a western ironsource had significant impact on the magnitude andlocation of the ecosystem response, (2) iron distributionswere very sensitive to the parameterization of scavenging,and (3) iron cycle parameters such as aerosol solubilityand Fe:N biological uptake ratios had to be tuned to nearthe limits of published ranges to represent the measuredextent of HNLC conditions (upper limit for solubility andlower limit for Fe:N ratios). The influence of the depthdifference between the ferricline and the depth of thewind-driven upwelling in the eastern Pacific was found tobe critical by Gorgues et al. (2007) in a model setupsimilar to the one presented here. Vichi et al. (2008) alsofound that the ferricline depth modulated the ecosystemresponse to tropical instability waves when a westernPacific shelf iron source was included.

1.3. Candidate western iron sources

It is not clear that a maximum of dissolved ironconcentration exists in the EUC. The dissolved ironmaximum highlighted by Gordon et al. (1997) consistedprimarily of a single depth measurement located �50 mdeeper in the water column than their particulate ironmaximum. Both maxima were beneath the core of the EUCas indicated by a bulge in the isotherms. Subsequent ironprofiles at 1401W have been inconclusive as to theexistence of a subsurface maximum in dissolved ironlocated in the EUC (Measures et al., 2006). Nakayama et al.(1995) presented data for dissolved iron at 1501E;however, two profiles collected 3 d apart were strikinglydifferent. Measurements of total acid-soluble iron andmanganese, which includes both dissolved metal speciesand particulate species soluble under pHo2 conditions, inthe far western equatorial Pacific (143–1561E) by Mackeyet al. (2002) and Slemons et al. (submitted) support awestern origin for iron in the thermocline. They found asubsurface maximum of total acid-soluble iron slightlybelow the core of the EUC and elevated values in the NewGuinea Coastal Undercurrent (NGCUC), which, along withcontributions from the New Ireland Coastal Undercurrent(NICU), has been demonstrated to be the primary pathwayfor water feeding the EUC from the South Pacific (Tsuchiyaet al., 1989; Fine et al., 1994; Butt and Lindstrom, 1994;Ueki et al., 2003). Despite uncertainty as to the existence,magnitude, and variability of a bioavailable iron max-imum in the EUC, the hypothesis of a western thermoclineiron source has been widely utilized in studies of

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equatorial productivity on monthly to geological time-scales (Wells et al., 1999; Ryan et al., 2006; Vichi et al.,2008), but the biogeochemical impact of such an ironsource has not been assessed using iron observations(Christian et al., 2002a).

Hydrothermal, riverine and sedimentary sources haveall been proposed as sources of iron to the EUC in thewestern equatorial Pacific (Coale et al., 1996; Gordon et al.,1997; Wells et al., 1999; Milliman et al., 1999; Mackeyet al., 2002). This region is volcanically active, andalthough it is not thoroughly surveyed, known hydro-thermal sources in the Bismarck Sea occur either deeperthan 1000 m or in shallow water, well separated from thecore of the NGCUC (Gamo et al., 1993; Obata et al., 1993;Pichler et al., 1999). The coincidence of maxima inparticulate Fe, Mn, and Al at 1401W on the equator areevidence of a lithogenic source of trace elements to theEUC around the New Guinea platform (Gordon et al.,1997). Neodymium isotopic signatures at 1401W indicatelithogenic input from young continental sources, such asthe northern New Guinea shelf and slope (Lacan andJeandel, 2001). Continental-shelf iron sources are increas-ingly recognized as important sources of iron to the openocean (e.g. Elrod et al., 2004; Chase et al., 2007;Planquette et al., 2007; Nishioka et al., 2007; Lam andBishop, 2008). Sediment flux to the New Guinea slope isespecially high because of the large fluxes of particulatematter from the island’s mountainous rivers (Millimanet al., 1999; Sholkovitz et al., 1999). A significant fractiondescends down the slope to the depth of the NGCUC as ahypopycnal flow (Kineke et al., 2000; Burns et al., 2008).The location of such a potentially large iron source in theformation region of the EUC suggests a significantbiogeochemical role.

In this study, we utilize the existing data of the totalacid-soluble iron maximum in the western equatorialPacific to test the conditions necessary to produce thesuggested EUC iron maximum at 1401W (Gordon et al.,1997) and its potential biogeochemical role.

1.4. Inclusion of a data-based candidate iron source to the

EUC

In this study we evaluated the impact of a western ironsource using the biogeochemical model PISCES (Aumontand Bopp, 2006). We prescribed a source based on theMackey et al. (2002) and Slemons et al. (submitted) (laterreferred to as M&S) total acid-soluble iron data. Our aimwas to project the resulting iron distribution, andinvestigate the biogeochemical impact to the easternHNLC region. We employed three model scenarios: (1) acontrol with default iron sources as defined by Aumontand Bopp (2006), (2) a western source with a maximum indissolved (bioavailable) iron matching the data of M&Swith a subsurface dissolved iron maximum below thedensity core of EUC, and (3) a western-source ironmaximum of magnitude equal to the values reported byM&S but moved upward to be located in the core densityof the EUC. Our study differed from that of Christian et al.(2002a, b) in three key ways: (1) The circulation model is a

global ocean, not limited to the Pacific basin. Modelstudies (e.g. Rodgers et al., 1999; Gorgues et al., 2007)have shown the importance of an adequate representationof the Indonesian Throughflow in achieving the correctratio of northern and southern source waters to the EUCand depth of the EUC in the eastern Pacific. (2) Our ironsources are based on measured iron profiles from fourcruises (M&S). Our western iron boundary condition witha maximum at 270 m resulted in an annually averagedflux of 2.8 mmol Fe m�2 d�1 from the model grid cellswhere the source iron profile was imposed, which wasconsiderably greater than the Christian et al. (2002a)continuous injection of 8mmol Fe m�2 d�1 at 125 m. Ironconcentrations decreased rapidly downstream from ourimposed iron flux. The iron source of Vichi et al. (2008)was also based on profiles from Mackey et al. (2002), butthey investigated temporal variability in biological pro-ductivity whereas we examine the impact of a westerniron supply in controlling annual mean nutrient fields andproductivity patterns. (3) The PISCES parameterization ofiron includes ligand complexation of dissolved iron, aparticulate iron pool, and iron scavenging.

1.5. Bioavailability of particulate iron

Although advances in clean measurement techniqueshave catalyzed the categorization of iron measurementsinto soluble, colloidal (together making up ‘‘dissolved’’iron), small particulate and large particulate iron (e.g. Wuet al., 2001) size classes, definition and measurement ofbioavailable iron has remained elusive (e.g. Wells andTrick 2004; Sunda, 2001; Castruita et al., 2008). Free,inorganic iron is known to be readily accessible tophytoplankton, but labile particulate iron may becomebioavailable through processes such as photoreduction,phagotrophy (Barbeau and Moffet, 2000), reduction at cellsurfaces (Sunda and Huntsman, 1995; Kuma et al., 2000),and perhaps interaction with various iron-complexingligands (Kustka et al., 2005; Chen and Wang, 2008).

Total acid-soluble iron measurements in the westernequatorial Pacific (M&S) as well as repeated observationsin the central equatorial Pacific of both dissolved andparticulate iron are available, but it is difficult to assess infield studies the biological significance of the observediron, particularly nonlocal effects. By employing a globalbiogeochemical model with a robust iron cycle, we cantest the distribution of realistic total acid-soluble ironsources across the Pacific basin and constrain thebioavailability of the iron source by evaluating theresulting climatological shifts in nutrient and biologicalfields. We do not aim in these experiments to optimize thefit of model fields to observations, but rather to apply themodel to the following questions: (1) Would the additionof a data-based total acid-soluble iron source located inthe western EUC (as dissolved iron) result in a subsurfacedissolved iron concentration maximum in the centralequatorial Pacific? (2) Would such a concentrationmaximum imposed in the western equatorial Pacificremain in the EUC as it was transported eastward? (3)What constraints on the bioavailability of this iron source

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are implied by the resulting shifts in nutrient andbiological fields in the eastern equatorial Pacific?

2. Methods

2.1. Model configuration

The biogeochemical PISCES model (Aumont and Bopp,2006) was run in its offline configuration forced by thedynamical fields from the ocean general circulation modelOPA (Madec et al., 1998) in a manner similar to that ofGorgues et al. (2005). Both models were described inprevious publications, and the most relevant details ofthis configuration are highlighted here. The OPA domainwas global with 21 zonal resolution and variable mer-idional resolution of 1/21 near the equator and lowerpoleward. This global configuration maintains an openIndonesian Throughflow. There were 31 vertical levels,with 12 in the upper 120 m. Temperature and salinityfields were initialized with World Ocean Atlas 1998(WOA98) climatologies (Antonov et al., 1998; Boyer etal., 1998). The model was forced by National Centers forEnvironmental Prediction (NCEP) climatological windsand daily heat and water fluxes (Kalnay et al., 1996).

PISCES includes colimitation of biological productionby nitrate, ammonia, phosphate, silicate and iron, each ofwhich are explicitly cycled. Phytoplankton, zooplankton,particulate organic carbon (POC) and particulate iron eachexist in two size classes. Diatoms have higher nutrienthalf-saturation constants than the smaller phytoplankton,referred to hereafter as nanoplankton, although theyrepresent all non-silicifying phytoplankton. Biogenic silicasinks at the same rate as large particulate organic carbon,both of which sink faster than small particulate organiccarbon. Uptake and remineralization of C, N, and P occurwith a constant Redfield ratio. PISCES was initialized usingnutrient climatologies from Conkright et al. (1998) and aniron field from the model climatology employed byAumont and Bopp (2006). PISCES was spun up for 50years to stabilize the biogeochemical fields.

2.2. Iron cycle in PISCES

Several components of the iron cycle were explicitlymodeled in PISCES (Fig. 1). Dissolved iron consists of freeand ligand-complexed species, both of which were

Fig. 1. Schematic of PISCES iron cycle. Prognostic model va

considered bioavailable. The Fe:C ratio in zooplanktonwas fixed, but varied in nanoplankton and diatomsaccording to dissolved iron availability, light andnutrient stress (Sunda et al., 1991; Marchetti et al.,2006). Particulate iron was remineralized in proportionto the particulate iron pool at the same temperature-dependent rate as POC, slowing at lower temperatures(and greater depths).

In our model, free dissolved iron was scavenged inproportion to the total concentration of large and smallPOC and biogenic silica. Moore et al. (2004) employed asimilar parameterization, but included sinking dust flux asa scavenging surface, in part to balance greater prescribediron solubility. Observations suggest abrupt increases iniron scavenging rates when [Fe]4�0.6 nM, because ligandcomplexation keeps iron in solution up to that concentra-tion (Johnson et al., 1997; Rue and Bruland, 1997).Concentrations in seawater are usually greater thanpredicted by the thermodynamic solubility of Fe(OH)3

(�0.01 nM) (Liu and Millero, 2002). To mimic thisphenomenon, our model included a ligand pool ofuniform concentration throughout the oceans with avariable iron complexation equilibrium constant. Theequilibrium constant was calculated based on the for-mulation of Liu and Millero (2002) such that the ironscavenging rate increased dramatically when dissolvediron concentrations exceeded the 0.6 nM threshold. Mod-els without ligand complexation, employing a linearscavenging rate, rapidly deplete iron (Archer and Johnson,2000; Christian et al., 2002a; Parekh et al., 2004).However, the single-ligand parameterization used in ourmodel was likely an oversimplification. Deepwater ironconcentrations are not globally uniform, ranging between�0.5 and 1.5 nM although rarely exceeding 0.6 nM,suggesting a range of ligand-binding capacities (Parekhet al., 2004; Moore and Braucher, 2008).

Dissolved iron sources in PISCES include atmosphericdeposition, river runoff, and coastal sediments. Atmo-spheric iron fluxes are based on Tegen and Fung (1995)and assume a constant 1% solubility of deposited iron.Coastal sources of iron are parameterized as an imposed0.5 nM iron concentration in each model grid cell alongthe coast throughout the upper 18 vertical levels(0–275 m) with adjustment for bathymetric variability.To account for sub-model-gridscale bathymetric varia-tions, which are especially significant near continentalslopes, the OPA model grid was mapped to the

riables are shown in boxes with a grey background.

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0

500

1000

1500

2000

0 1 2 3 4 5 6 7 8

Total Dissolvable Iron (nM) (<0.45um)

Dep

th (m

)

Fig. 2. Vertical profiles of iron in the western equatorial Pacific. Blue circles represent total dissolvable iron concentrations of Mackey et al. (2002) at

equatorial stations between 147 and 1551E. Green squares show total acid-soluble iron concentrations of Slemons et al. (submitted) at equatorial stations

between 145 and 1561E. The solid lines show dissolved iron concentrations along the equator between 154 and 1581E in the three model simulations:

control (pink), 270-m source (red), and 180-m source (yellow). The purple line indicates the depth of the density of the EUC core (25.0osyo26.0) during

the Mackey et al. (2002) surveys. The shorter green line shows the depth of the same density surface in the model at this location. For interpretation of the

references to color in this figure legend, the reader is referred to the web version of this article.

Fig. 3. Zonal sections of iron (nM) (top) and nitrate (mM) (bottom) concentrations (color shading) along the equator. The control run fields are shown on

the left, and anomaly plots for the two source runs minus the control are shown in the middle and right and labeled in the panel. Red (blue) shades

indicate an increase (a decrease) in the source run relative to the control. Note the nonlinear scale for the iron anomaly plots. Isopycnal surfaces contoured

in black in the top panel. Zonal velocity (m/s, solid is eastward flow, dashed is westward flow) is contoured in the bottom panel for the model (black) and

TAO array (1988–2006) observations (magenta).

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high-resolution ETOPO5 bathymetric dataset, and thefractional ocean bottom area shallower than 275 m wascalculated. The imposed iron concentration was thenmultiplied by this fractional area. Iron in PISCES isconserved.

2.3. Imposed western iron source scenarios

Each of our three model scenarios included thestandard PISCES atmospheric, riverine, and sedimentarysources described above. The control run included onlythese sources. Our two source experiments included anadditional western iron source based on observations(Fig. 2). We fit a curve to match the amplitude of thesubsurface maximum in total acid-soluble iron (all ironsolubilized after acidification: 1 ml HCl/l seawater, pHo2;3 month storage) measured by M&S from 143 to 1561Ealong the equator. The imposed maxima are upper boundestimates of bioavailable iron, since the measurements areof total acid-soluble iron. The source simulations eachincluded a dissolved iron concentration profile uniformlyimposed within 0.51 of the equator from 154 to 1581E withmaxima at 270 m (270 src) and 180 m (180 src). For the270 src simulation, we located the maximum ironconcentration at the depth of the iron maximumobserved by M&S. For the 180 src simulation, weimposed the maximum iron concentration in the centerof the model’s EUC core (defined as sy=25.5). At thelocation of the imposed maximum at 180 m, the velocitycore of the model’s EUC spanned 0.51S–11N. Both sourcesimulations were run for 30 years.

In this study, we evaluated the downstream biogeo-chemical impact of a realistic EUC-associated iron sourcein the western equatorial Pacific. We did not attempt tolocate the imposed iron source in the model NGCUC or

Fig. 4. Meridional sections of dissolved iron (nM) at 1401W for the three mode

(1997) measurements. Isopycnal surfaces are overlain in the model panels.

NICU even though they are the most likely entry points forsubsurface iron (e.g. Mackey et al., 2002), because our 21-resolution configuration limits the accuracy of the model’srepresentation of coastal currents. By 1551E the EUC isthought to have acquired most of its western sourcewaters (Tsuchiya et al., 1989), although the pathways ofthe sources from the North Pacific are poorly known. Thus,we chose our imposed source profile location where theEUC was well established in the model and in observa-tions (Fig. 3). Starting from a fully spun-up PISCESbiogeochemical state, both source profiles wereimplemented for 30 years.

3. Results

3.1. Iron and nitrate distributions

The additions of an elevated dissolved iron source inthe western equatorial thermocline altered the patternand quantity of model nutrients across the tropical Pacific.Zonal distributions of simulated iron and nitrate along theequator are shown in Fig. 3. Nutrient concentrations areshown for the control simulation, and nutrient concentra-tion anomalies, relative to the control, are shown for thesource simulations. In the control simulation, both ironand nitrate concentrations increased with depth withtypical nutrient-like profiles and shoaled eastward alongisopycnal surfaces (black contours, upper panels), espe-cially in the nitrate field. In the source simulations,enhanced iron concentrations were greatest within sev-eral hundreds of kilometers of the imposed source anddecreased exponentially eastward, primarily because ofloss by scavenging (Figs. 3 and 6). The additional ironreached surface waters west of 1201W for the sourceoriginating at 270 m and west of 1561W for the 180-m

l experiments (labeled in the panel) and measurements of Gordon et al.

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source. Model dynamics robustly represented the depthand gradient of the EUC, although the model zonalvelocities were slightly lower than TAO observations(Fig. 3, lower panels). This weak anomaly resulted fromthe weak bias in the mean state of the trade winds foundin NCEP compilations (Putnam et al., 2000).

Although the simulated iron perturbations were ad-vected eastward, maximum source-run iron concentra-tions occurred below the core of the EUC. Ironconcentrations at 1401W are shown alongside the dis-solved iron measurements of Gordon et al. (1997) fromJuly 1990, a non-ENSO year, in Fig. 4. During their samplingat this location, the depth of the core of the EUC was atapproximately 125 m, as indicated by the doming ofisotherms in their data (Gordon et al., 1997, their Fig. 1).The dissolved iron concentration ‘‘bulls-eye’’ maximumsuggested at 200 m by Gordon et al. (1997) was wellbeneath both the distinct maximum in particulate iron at�150 m (Gordon et al., 1997, their Fig. 2) and the core ofthe EUC. The core of the model’s EUC was at a similardepth during climatological July, indicated by the zonalvelocity and the doming of the isopycnals. In contrast tothe observations of Gordon et al. (1997), dissolved ironconcentrations followed isopycnal surfaces in the controlsimulation and showed no ‘‘bulls-eye’’ maximum.Both source simulations produced maxima in meridionaliron centered slightly north of the equator. The dissolvediron ‘‘bulls-eye’’ maximum highlighted by Gordon et al.(1997) was evident at this location only in the270-m source simulation. The 180-m source simulationgenerated a broader iron enrichment, 2–31 across at itswidest. Maximum iron concentrations in both sourcesimulations at this location were more than double theobserved dissolved iron maximum of 0.35 nM, but notsignificantly greater than the sum of dissolved andparticulate iron measurements (0.65–0.75 nM) at 200 m(Gordon et al., 1997). In both source runs, iron anomaliesrelative to the control were greatest below the core ofthe EUC.

Both source simulations resulted in negative anomaliesof surface nitrate in the central and eastern equatorialPacific. In addition, the source simulations produced

Fig. 5. Surface nitrate concentrations (mM) for the three model runs and WOA

delineates the equator. Contour lines show the vertically integrated (upper 12

control run. Negative values are dashed.

negative nitrate anomalies relative to the control below150 m east of 1001W (Fig. 3). At this location, nitrateconcentrations simulated by the control run exceededobservations from the World Ocean Atlas (Garcia et al.,2006) because of overly vigorous model denitrificationand incorrect model dynamics. As a result, this areaexhibited strong horizontal nitrate gradients and thus washighly sensitive to local negative nitrate anomalies. Thesource of these model errors (whether due to lowresolution or inadequate representation of tropical over-turning cells) is not clear (Cravatte et al., 2007). Thisnitrate depletion occurred too deep to have any immedi-ate biological impact in the eastern equatorial Pacific andwas not investigated further in our study. The sourcesimulations resulted in positive anomalies of nitrate,relative to the control, below the zone of increasedproductivity in the equatorial upwelling zone.

Simulated and observed (2005 World Ocean Atlas,Garcia et al., 2006) surface nitrate concentrations areshown in Figs. 5 and 6. The magnitude of the observedmaximum concentrations of surface nitrate in the easternPacific, located at 1001W, were most closely matched inthe control simulation. The control simulation also betterreproduced the meridional location of the observedsurface nitrate maximum, located a few degrees south ofthe equator. However, the source simulations betterrepresented surface nitrate concentrations along theequator in the central and eastern equatorial region.West of �1051W, surface nitrate concentrations at theequator reached 10mM in the control run and reachedonly 7mM in the source runs and the WOA data. Thegreatest drawdown of surface macronutrients, in bothmagnitude and spatial extent, occurred in the 180-msource simulation.

3.2. Impacts on biology and carbon cycle

Both source runs resulted in increased primaryproduction relative to the control (41% and 66% forthe 270-m and 180-m source simulation, respectively)integrated across the entire Wyrtki box (1801W:901W,

2005 observations as labeled in each panel. The horizontal black line

0-m) primary productivity anomalies for each source run relative to the

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Fig. 7. Surface chlorophyll concentrations (mg/m3) for the three model runs and average SeaWiFS observations from 1997 to 2004 as labeled in each

panel. Note the nonlinear scale.

Fig. 6. Upper panel: Average iron concentrations (nM) along the equator on the sy=25.5(70.5) density surface for the control (x’s), 270-m source

(squares), and 180-m source (triangles) simulations. Lower panel: Surface nitrate concentrations (mM) along the equator for WOA 2001 observations (solid

line) and the control (x’s), 270-m source (squares), and 180-m source (triangles) model simulations.

L. Slemons et al. / Deep-Sea Research I 56 (2009) 2115–21282122

51S:51N, 0:120 m). The greatest increases in vertically integrated primary production from the sourcesimulations were generated just southeast of the max-imum in surface nitrate (Fig. 5). Visual inspectionindicates that primary production in the source simula-tions decreased where surface nitrate was drawn down(o1mM).

The increase in primary productivity in the sourcesimulations dramatically impacted the surface chlorophyllfields. Simulated and observed (SeaWiFS compilation,Yoder and Kennelly, 2005) surface chlorophyll concentra-tions are shown in Fig. 7. Our 21 zonal resolution model

simulated overly broad surface chlorophyll concentrationsin all simulations, but this bias was exacerbated in thesource simulations.

Climatological estimates of equatorial primary produc-tion have classically been calculated over the Wyrtki box(1801W:901W, 51S:51N). The vertically integrated modelprimary productivity of 904 mg C m�2 d�1 in the 270-msource run closely matched the non-El Nino compilation(field studies from 1990 to 1992) of 900 mg C m�2 d�1

reported by Chavez et al. (1996). Primary productivitywas less than Chavez’s estimate in the control run(643 mg C m�2 d�1) and greater in the 180-m source

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run (1066 mg C m�2 d�1), although we should note thatthis average is very sensitive to the meridional widthselected. Integrated primary productivity (0–120 m) at1401W over meridional bands of varying width from ourmodel simulations are compared with observations from anon-ENSO season (fall 1992) in Fig. 8 (Barber et al., 1996).Observed primary productivity was greatest within 21 ofthe equator (131 mmol C m�2 d�1) and generally decreasedpoleward (to 59 mmol C m�2 d�1 averaged over 101 of theequator, a difference of 72 mmol C m�2 d�1). The exceptionto this pattern (101 mmol C m�2 d�1 within 11 of theequator) was likely due to a passing TIW at 21N (seedescription of Fig. 9). The source simulations producedsimilar meridional gradients (differences of 26 and35 mmol C m�2 d�1 between the 11 and 101 meridionalaverages for 270 and 180 src, respectively). In the

Fig. 8. Model and observed average primary productivity (mmol C m�2 d�1) at

101S:101N, 4101 between 121S:121N). Observational data (dashed line, circles) a

from an August–September 1992 cruise (EqPac Survey II) during a non-El Nino

are mean August–September climatological values integrated over the upper 1

Fig. 9. Model and observed particulate organic carbon flux (mmol C m�2 d�1) at

same cruise as the data in Fig. 8 (Murray et al., 1996). Model values are the Au

control simulation, primary production was notably morehomogeneous meridionally (14 mmol C m�2 d�1 differencebetween the 11 and 101 band averages). Observed primaryproductivity averaged across 101S:101N band wasrepresented well in all three model simulation: allconverged within 13 mmol C m�2 d�1 of the observedprimary productivity along 1401W (59 mmol C m�2 d�1).

Among the model’s primary producers, diatoms ex-perienced a greater response to additional iron, althoughnanoplankton remained dominant in all simulations interms of both primary productivity and biomass. Diatombiomass increased more than 3-fold for the 270-m ironsource and approximately 4-fold for the 180-m source inthe upper 25 m of the far eastern equatorial upwellingzone (1101W:901W, 21N:21S) relative to the controlsimulation. The relative contribution of diatoms to

1401W for meridional bands of varying width (11S:11N, 21S:21N, 51S:51N,

re mean primary productivity vertically integrated to the 0.1% light depth

period reported by Barber et al. (1996). Model primary productivity data

20 m (control: pluses; 270 src: stars; 180src, diamonds).

120 m at 1401W. Observational data (line, circles) were collected from the

gust–September average; symbols as in Fig. 8.

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Table 1Relative phytoplankton biomass contribution of diatoms for the model

integrated over the euphotic zone in the eastern equatorial Pacific

(1351W:901W, 51N:51S) and for observations compiled between 1988

and 1992 (Chavez et al., 1996, their Table 4).

Model Observations

Control 25% 1401W 5–10%

270-m src 33% 1251W 15–31%

180-m src 36% 1101W 6–14%

L. Slemons et al. / Deep-Sea Research I 56 (2009) 2115–21282124

phytoplankton biomass was within or in excess of theupper end of the range of measured values (Table 1)(Chavez et al., 1996).

All simulations generated the greatest open-oceanexport flux in the eastern equatorial Pacific from 61N to81S with a local minimum at the equator. Although limitedmeasurements exist of export production for this region,a similar pattern of a near-equatorial minimum in POCexport and a northern maximum in export around 61Nwas observed during non-ENSO conditions (Survey II)across 1401W (Fig. 9) (Murray et al., 1996). Our simula-tions reproduced the zonal gradient, meridional patternand magnitude of POC flux measurements, and therelatively low ratio of POC export to total primaryproductivity (the e-ratio) observed at the equator (Murrayet al., 1989, 1996; Dunne et al., 2000). Observed maxima inPOC export were located at 21N and 51S during Survey II(Murray et al., 1996), which may be associated withcoincident convergence zones (Johnson, 1996, their Fig. 1;Walsh et al., 1997). The convergent front at 21N wasshown to be the result of a passing tropical instabilitywave (Johnson, 1996). The meridional maxima andminima in POC export were likely to have been shiftedby the passing TIW and would not be expected to exactlymatch our model outputs, which were climatological.

4. Discussion

4.1. Constraints and fate of a western iron source

In our source runs we used measured profiles of totalacid-soluble iron from �1551E (M&S) and treated thesetotal iron concentrations as if they were entirely bioavail-able. Measurements of total acid-soluble iron includedboth truly soluble inorganic iron available for directbiological uptake and colloidal and labile particulate iron,some fraction of which may be available for biologicaluptake. We used the total acid-soluble iron profiles todefine the source because the dataset is robust and therewas clearly a maximum in total acid-soluble ironassociated with the EUC that existed in the western andcentral Pacific. There is much less data for dissolved ironand a distinct maximum of dissolved iron associated withthe EUC is not evident. Since all dissolved iron in themodel is treated as bioavailable, whether or not it isligand-bound, it is available for uptake by primaryproducers immediately upon entry into the euphoticzone. Thus, the source simulations employ an upperbound of bioavailable iron.

The prescribed western equatorial Pacific iron pertur-bations did not remain on their original isopycnal surfacesas they advected across the basin, but spread throughoutthe thermocline, particularly deeper than their originaldensity horizon. This spreading and sinking was likely dueto dilution from tropical recirculation cells (Lu et al., 1998)and sinking from scavenging (Figs. 3 and 6). Nonetheless,subsurface maxima in dissolved iron were propagatedacross the Pacific basin. In the central equatorial Pacific,the source simulations matched the observed distributionof dissolved and particulate iron fields (Gordon et al.,1997) better than the control simulation. The magnitudesof the maxima from the two source simulations at 1401Wwere double the observed dissolved iron concentration,but nearly matched (25% greater) the sum of dissolvedand particulate iron values, which is consistent with ourtreatment of total acid-soluble iron data as dissolved ironin the model. The source simulations generated iron fieldsin agreement with total iron concentrations for thisregion.

More recent measurements of dissolved iron at 1401Wreported a greater dissolved iron concentration of 0.7 nMin the core of the EUC (Measures et al., 2006). Variabilityin iron fields is expected, both from variations in transportof the EUC on interannual (ENSO) and decadal timescales(Chavez et al., 1999; Le Qu�er�e et al., 2003; Le Borgne et al.,2002; Rodgers et al., 2008) and from variations incandidate sources, like the strength and depth of theNGCUC (Ryan et al. 2006). Although residence times andcycling of dissolved and particulate iron species differ, ourcombination of the two appears to be a sensible assump-tion given the high advection rates associated with theEUC. The implications of iron speciation for interior oceaniron cycling can best be addressed in future modelingstudies that explicitly include cycling of both dissolvedand particulate iron. The inclusion of an explicit westernthermocline iron source improved the representation ofthe total iron field in the central equatorial Pacific.

Our model’s iron cycle included sediment, atmo-spheric, riverine, and desorption sources and scavengingloss dependent on dissolved iron and particle concentra-tions. It has been shown to be robust over a broad range ofiron concentrations (e.g. Bopp et al., 2003). As demon-strated in other models (Christian et al., 2002a; Moore andBraucher, 2008), changes to model iron sources often callfor optimization of iron cycle parameters. Significantuncertainties in published values of these parametersexist and preclude prognostic modeling of the iron cycle.Even given a robust distribution of total acid-soluble iron,the availability of various iron species for biologicaluptake remains a complicated question. In our simula-tions, broad shifts in nutrient and biological fields areused to evaluate the bioavailability of a realistic, data-based iron supply; no iron cycle parameters wereadjusted.

4.2. High-nitrate low-chlorophyll shift

The prescribed western iron sources used in this study ledto an intense localized increase in primary and newproduction and surface chlorophyll and a drawdown of

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surface nitrate. As expected from earlier studies (e.g. Christianet al., 2002a), the depth of iron injection determines the zonalextent of the biological response. A northward shift inmeridional extent of the elevated nitrate region occurred inthe source simulations (Fig. 5). The flow pathway of the lowerEUC thermostad waters, where most of the iron enrichmentin the source simulations occurred, brings nutrient-richwaters to the surface south of the equator off the SouthAmerican coast (Lukas, 1986; Toggweiler et al., 1991),so this shift was expected from the localized drawdown ofmacronutrients south of the equator. The control simulationbetter represented observations of both the location and themagnitude of nutrient and biological fields in the easternequatorial Pacific. This implies that a significant amount ofthe total acid-soluble iron from the western Pacific is not fullybioavailable in the eastern equatorial Pacific. We did notattempt to quantify what fraction of acid-soluble iron isbioavailable because the bioavailability of iron is not a linearfunction of iron supply.

Other interconnected factors besides iron bioavailabilitycould produce excessive productivity in our simulations ofequatorial Pacific productivity, including biases in equatorialdynamics or incorrect iron cycle parameterizations. Wecannot rule out the possibility that our PISCES simulationsmay be biased in the equatorial Pacific and the additional ironsimply revealed those biases. As observed in similar explicitiron cycle model studies, uncertainty and natural variabilityin published iron cycle parameters cause the representationof iron in ecosystem models to be an underdeterminedproblem (e.g. Christian et al., 2002a; Moore and Braucher,2008). A close dataset comparison is complicated by the factthat equatorial climatologies for many biological variables donot exist. Given the global robustness of PISCES productivityand our experimental design in which only the westerniron supply was changed, the most parsimonious explanationis that primary producers are not able to access a portion ofthe total acid-soluble iron transported eastward in theEUC.

Observations show that the dynamical (salinity) andbiological fronts (new production, nutrients) are usuallycoincident or, in the case of suspended upwelling, elevatedsurface nitrate is observed east of the salinity front (Stoenset al., 1999; Radenac et al., 2001; Le Borgne et al., 2002). Thezonal extent of low surface nitrate matched the dynamicalwarm pool best in the 180-m source simulation and worst inthe control simulation (not shown). The inclusion of westerniron sources better modulated the transition from oligo-trophic warm-pool to HNLC cold-tongue conditions. Over-estimation of the extent and magnitude of the equatorialHNLC region is common to many coarse-resolution modelsand is considered to be driven by an overly shallow EUC inthe eastern Pacific (Gnanadesikan et al., 2002). This bias wasalso present in our simulations (Figs. 3, 5 and 6), and itaffects both dynamical and nutrient fields similarly.

4.3. Impacts on biology and carbon cycle

Our results were in agreement with the ecumenicaliron hypothesis and demonstrate several key biologicalshifts in addition to increases in primary production andbiomass upon relief of iron limitation. Although physical

dynamics were the same for all simulations, the sourcesimulations produced a more realistic meridional distri-bution of primary production at 1401W, especially in180 src (Fig. 8). In a parallel result, the meridional patternin export production also improved in the source simula-tions. Recirculation cells known as Tropical Cells (Lu et al.,1998) advect settling POC poleward in surface waters, somaximum POC export coincides with maximum surfaceconvergence off the equator where water masses aredownwelling (Walsh et al., 1997). An equatorial minimumin carbon export existed in all of the simulations due tothe dynamics of particle advection, but was moreapparent in the source simulations. These results indicatethe inclusion of a subsurface iron maximum located in theEUC improves the distribution of biological fields in theequatorial upwelling zone. Integrated nutrient fields werebetter represented in the control simulation, but thesource simulations improved the nutrient and productiv-ity gradients in the eastern equatorial Pacific. Nonetheless,productivity was too high and nitrate too low in thesource simulations despite a better simulation of mer-idional and zonal gradients, respectively. Therefore, it isevident that the upper limit of bioavailable iron used hereis too high.

Our model appeared to overestimate diatom produc-tivity, although a notable shift in community structureoccurred in the source simulations, favoring largerdiatoms as expected from relief of iron limitation(e.g. Landry et al., 1997). Because datasets of large-scalecommunity structure are limited, this result is not suitableto evaluate the relative fitness of our simulations,but it does suggest a shift in particle size distributionshould be considered when predicting changes in exportflux with iron fertilization. An increase in the relativeabundance of diatoms and resultant biogenic silica ballasthas been proposed as a factor to explain increasesin export flux and the e-ratio (i.e. Armstrong et al.,2002; Dunne et al., 2007). Model experiments regularlyproduce increased export efficiency in response to anincrease in diatom abundance, although with a range ofsensitivities; the e-ratio of Moore et al. (2004) was lessresponsive than that of Aumont and Bopp (2006). InPISCES, the settling rate of POC increases with particlesize, and the aggregation rate increases with POC and DOCabundance. Model diatoms produce more large (and thusfaster sinking) particulate organic material, includingbiogenic silica. In the field, ballast production has beenshown to be highly variable and dependent on nutrientconditions: eastern equatorial Pacific diatoms incorporatefivefold less silica per volume than Southern Oceandiatoms (Baines et al., 2008). Despite the increasedfraction of diatoms (and their greater Si:N uptake due tolight limitation, not shown) in the source simulations, thee-ratio decreased when averaged across the tropicalPacific region east of the dateline. Export efficiencydecreased locally in the equatorial upwelling zone, wherethe greatest increase in diatom biomass and productionoccurred, and off the equator in zones of convergence anddownwelling.

Increased particle remineralization was the immediatecause for decreased export efficiency in the source

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simulations. Intensified surface productivity resulted inincreased biomass shallower in the water column andincreased the probability that a particle would beremineralized before export out of the euphotic zonebecause of the particle’s increased residence time in theupper water column. Measurements of carbon exportduring iron fertilization experiments have indicated thatiron fertilization is an inefficient means of vertical carbonexport, likely because of significant carbon remineraliza-tion (e.g. Boyd et al., 2004; Frew et al., 2005). A similarmechanism was apparent in our model simulations, but itis important to note our simulations were carried out for30 model years, sufficient time for grazer response.Periodic iron fertilization events, including seasonalintensifications of continental iron supply, could producea different community structure and export pattern.Grazing has been proposed to contribute to iron reminer-alization and influence patterns of iron and carbon export(e.g. Sato et al., 2007; Boyd et al., 2007).

5. Conclusions

Profiles of total acid-soluble iron in the westernequatorial Pacific (�1551E) have a maximum associatedwith the Equatorial Undercurrent (EUC) (Mackey et al.,2002; Slemons et al., submitted). This maximum is mostlyin the form of particulate iron that appears to originatefrom sediments along the northeastern continental mar-gin of Papua New Guinea (Slemons et al., submitted).

We conducted model simulations in which this EUC-associated western iron source was transported to the eastand we evaluated its impact on biogeochemical distribu-tions. In our model we treated all of the total acid-solubleiron as if it was 100% bioavailable. It is important toemphasize that achieving precise model realism is limitedboth by the availability of biogeochemical data and by thesignificant uncertainty in modeling biological processes.Nonetheless, the resulting biogeochemical fields demon-strated five primary conclusions: (1) As the iron max-imum in the EUC is advected from the western Pacific tothe east the concentrations of iron decrease. (2) Thiswestern iron source is consistent with the maxima in totaliron (dissolved plus particulate) observed at 1401Wslightly below the core of the EUC by Gordon et al.(1997). (3) The control run without additional iron sourcesresults in excessive surface nitrate and chlorophyll in thecentral equatorial Pacific (1801W:1101W). (4) The inclu-sion of a maximum of iron in the western EUC providednotable improvements to the shape of biogeochemicalfields for which localized equatorial dynamics are key:concentrations and zonal gradients of surface nitratealong the equator, the narrow band of enhanced primaryproductivity in the equatorial upwelling zone, andmeridional gradients of primary productivity and carbonexport production. (5) Nonetheless, the additional ironresulted in excessive depletion of surface nitrate andexcessive surface chlorophyll in the eastern equatorialPacific, suggesting that it is incorrect to assume that all ofthe acid-soluble iron is bioavailable. We do not diagnosehere what fraction of total acid-soluble iron is bioavail-

able. Future work should include both experimental andmodel studies of the bioavailability of different ironspecies besides dissolved iron.

Acknowledgements

Discussions with Keith Rodgers, William Kessler, andBruce Frost provided valued insight into the formulationof this paper. The Ferret program (http://ferret.pmel.noaa.gov/Ferret), developed by NOAA’s Pacific MarineEnvironmental Laboratory, was used for analysis andgraphics in this paper. Data from the NOAA/PMEL TAOarray (http://www.pmel.noaa.gov/tao/index.shtml) andthe Ocean Color Processing Group (http://oceancolor.gsfc.nasa.gov) were used in model evaluation and presenta-tion. This work was funded by the National ScienceFoundation (Grant no. NSF-OCE-0425721), the Universityof Washington Program on Climate Change, and the JointInstitute for the Study of the Atmosphere and Ocean(JISAO) under NOAA Cooperative Agreement No.NA17RJ1232, Contribution #1485.

References

Antonov, J., Levitus, S., Boyer, T.P., Conkright, M., O’Brien, T., Stephens, C.,1998. World Ocean Atlas (2) Temperature of the Pacific Ocean, NOAAAtlas NESDIS (28) NOAA, Silver Spring, MD.

Archer, D.E., Johnson, K., 2000. A model of the iron cycle in the ocean.Global Biogeochemical Cycles 14 (1), 269–279.

Armstrong, R.A., Lee, C., Hedges, J.I., Honjo, S., Wakeham, S.G., 2002. Anew, mechanistic model for organic carbon fluxes in the ocean basedon the quantitative association of POC with ballast minerals. Deep-Sea Research II 49 (1–3), 219–236.

Aufdenkampe, A.K., McCarthy, J.J., Navarette, C., Rodier, M., Dunne, J.P.,Murray, J.W., 2002. Biogeochemical and physical controls on newproduction in the tropical Pacific. Deep-Sea Research II 49 (13–14),2619–2648.

Aumont, O., Bopp, L., 2006. Globalizing results from ocean in situ ironfertilization studies. Global Biogeochemical Cycles 20 (2), GB2017,doi:10.1029/2005GB002591.

Baines, S.B., Twining, B.S., Vogt, S., 2008. Are all diatoms the same?Variations in cellular stoichiometry of diatoms from two HNLCregions and their implications for Si, Fe, and C export. AmericanSociety of Limnology and Oceanography (2008 Abstracts).

Barbeau, K., Moffet, J.W., 2000. Laboratory and field studies of colloidaliron oxide dissolution as mediated by phagotrophy and photolysis.Limnology and Oceanography 45 (4), 827–835.

Barber, R.T., Sanderson, M.P., Lindley, S.T., Chai, F., Newton, J., Trees, C.C.,Foley, D.G., Chavez, F.P., 1996. Primary productivity and its regulationin the equatorial Pacific during and following the 1991–1992 El Nino.Deep-Sea Research II 43 (4–6), 933–969.

Behrenfeld, M.J., Bale, A.J., Kolber, Z.S., Aiken, J., Falkowski, P.G., 1996.Confirmation of iron limitation of phytoplankton photosynthesis inthe equatorial Pacific Ocean. Nature 383, 508–511.

Bopp, L., Kohfeld, K.E., Le Qu�er �e, C., Aumont, O., 2003. Dust impact onmarine biota and atmospheric CO2 during glacial periods. Paleocea-nography 18 (2)10.1029/2002PA000810.

Boyd, P.W., Law, C.S., Wong, C.S., Nojiri, Y., Tsuda, A., Levasseur, M., Takeda,S., Rivkin, R., Harrison, P.J., Strzepek, R., Gower, J., McKay, R.M., Abraham,E., Arychuk, M., Barwell-Clarke, J., Crawford, W., Crawford, D., Hale, M.,Harada, K., Johnson, K., Kiyosawa, H., Kudo, I., Marchetti, A., Miller, W.,Needoba, J., Nishioka, J., Ogawa, H., Page, J., Robert, M., Saito, H., Sastri,A., Sherry, N., Soutar, T., Sutherland, N., Taira, Y., Whitney, F., Wong, S.E.,Yoshimura, T., 2004. Decline and fate of an iron-induced subarcticphytoplankton bloom. Nature 428, 549–553.

Boyd, P.W., Jickells, T., Law, C.S., Blain, S., Boyle, E.A., Buesseler, K.O., Coale,K.H., Cullen, J.J., de Baar, H.J.W., Follows, M., Harvey, M., Lancelot, C.,Levasseur, M., Owens, N.P.J., Pollard, R., Rivkin, R.B., Sarmiento, J.,Schoemann, V., Smetacek, V., Takeda, S., Tsuda, A., Turner, S., Watson,A.J., 2007. Mesoscale iron enrichment experiments 1993–2005: synth-esis and future directions. Science 315, 612–617.

ARTICLE IN PRESS

L. Slemons et al. / Deep-Sea Research I 56 (2009) 2115–2128 2127

Boyer, T.P., Levitus, S., Antonov, J., Conkright, M., O’Brien, T., Stephens, C.,1998. World Ocean Atlas (5) Salinity of the Pacific Ocean, NOAA AtlasNESDIS (30), NOAA, Silver Spring, MD, 166 pp.

Burns, K., Brunskill, G., Brinkman, D., Zagorskis, I., 2008. Organic carbonand nutrient fluxes to the coastal zone from the Sepik River outflow.Continental Shelf Research 28, 283–301.

Butt, J., Lindstrom, E., 1994. Currents off the east coast of New Ireland,Papua New Guinea, and their relevance to regional undercurrents inthe western equatorial Pacific Ocean. Journal of GeophysicalResearch 99 (6), 12503–12514.

Castruita, M., Shaked, Y., Elmegreen, L.A., Stiefel, E.I., Franc-ois, M.M., Morel,F.M.M., 2008. Availability of iron from iron-storage proteins to marinephytoplankton. Limnology and Oceanography 53 (3), 890–899.

Chase, Z., Strutton, P.G., Hales, B., 2007. Iron links river runoff and shelfwidth to phytoplankton biomass along the U.S. west coast.Geophysical Research Letters 34, L04607, doi:10.1029/2006GL028069.

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

Chavez, F.P., Buck, K.R., Service, S.K., Newton, J., Barber, R.T., 1996.Phytoplankton variability in the central and eastern tropical Pacific.Deep-Sea Research II 43 (4–6), 835–870.

Chavez, F.P., Strutton, P.G., Friederich, G.E., Feely, R.A., Wanninkhof, R.,Feldman, G., Foley, D., McPhaden, M.J., 1999. Biological and chemicalresponse of the equatorial Pacific Ocean to climatic forcing duringthe 1997–1998 El Nino. Science 286, 2126–2131.

Chavez, F.P., Toggweiler, J.R., 1995. Physical estimates of global newproduction: the upwelling contribution. In: Summerhayes, C.P. (Ed.),Upwelling in the Ocean: Modern Processes and Ancient Records.Wiley, New York, pp. 313–320.

Chen, M., Wang, W., 2008. Accelerated uptake by phytoplankton of ironbound to humic acids. Aquatic Biology 3, 155–166.

Chisholm, S.W., Morel, F.M.M., 1991. What controls phytoplanktonproduction in nutrient-rich areas of the Open Sea. Limnology andOceanography 36, U1507–U1511.

Christian, J.R., Verschell, M.A., Murtugudde, R., Busalacchi, A.J., McClain,C.R., 2002a. Biogeochemical modeling of the tropical Pacific Ocean.II: Iron Biogeochemistry. Deep-Sea Research II 49, 545–565.

Christian, J.R., Verschell, M.A., Murtugudde, R., Busalacchi, A.J., McClain,C.R., 2002b. Biogeochemical modeling of the tropical Pacific Ocean. I:Seasonal and interannual variability. Deep-Sea Research II 49,509–543.

Coale, K.H., Fitzwater, S.E., Gordon, R.M., Johnson, K.S., Barber, R.T., 1996.Control of community growth and export production by upwellediron in the equatorial Pacific Ocean. Nature 379, 621–624.

Conkright, M.E., Levitus, S., O’Brien, T., Boyer, T.P., Stephens, C., Johnson,D., Stathoplos, L., Baranova, O., Antonov, J., Gelfeld, R., Burney, J.,Rochester, J., Forgy, C., 1998. World Ocean dataset 1998, CD-ROMdata set documentation. National Oceanographic Data CenterInternal Report 14, 111 pp.

Cravatte, S., Madec, G., Izumo, T., Menkes, C., Bozec, A., 2007. Progress inthe 3-D circulation of the eastern equatorial Pacific in a climateocean model. Ocean Modelling 17 (1), 28–48.

Dunne, J.P., Murray, J.W., Rodier, M., Hansell, D.A., 2000. Export flux in thewestern and central equatorial Pacific: zonal and temporal varia-bility. Deep-Sea Research I 47, 901–936.

Dunne, J.P., Sarmiento, J.L., Gnanadesikan, A., 2007. A synthesis of globalparticulate export from the surface ocean and cycling through theocean interior and on the seafloor. Global Biogeochemical Cycles21101029/2006GB002907.

Elrod, V.A., Berelson, W.M., Coale, K.H., Johnson, K.S., 2004. The flux ofiron from continental shelf sediments: a missing source for globalbudgets. Geophysical Research Letters 31, L12307, doi:10.1029/2004GL020216.

Fine, R.A., Lukas, R., Bingham, F.M., Warner, M.J., Gammon, R.H., 1994. Thewestern equatorial Pacific: a water mass crossroads. Journal ofGeophysical Research 99 (12), 25063–25080.

Fitzwater, S.E., Coale, K.H., Gordon, R.M., Johnson, K.S., Ondrusek, M.E.,1996. Iron deficiency and phytoplankton growth in the equatorialPacific. Deep-Sea Research II 43 (4–6), 995–1015.

Frew, R.D., Hutchins, D.A., Nodder, S., Sanudo-Wilhelmy, S., Tovar-Sanchez, A., Leblanc, K., Hare, C.E., Boyd, P.W., 2005. Particulate irondynamics during Fe cycle in subantarctic waters southeast of NewZealand. Global Biogeochemical Cycles 20, GB1S93, doi:10.1029/2005GB002494.

Friedrichs, M.A.M., Hofmann, E.E., 2001. Physical control of biologicalprocesses in the central equatorial Pacific Ocean. Deep-Sea Research I48, 1023–1069.

Frost, B.W., Frazen, N.C., 1992. Grazing and iron limitation in the controlof phytoplankton stock and nutrient concentration—a chemostatanalog of the Pacific equatorial upwelling zone. Marine Ecology-Progress Series 83, 291–303.

Fung, I.Y., Meyn, S.K., Tegen, I., Doney, S.C., John, J.G., Bishop, J.K.B., 2000.Iron supply and demand in the upper ocean. Global BiogeochemicalCycles 14 (1), 281–295.

Gamo, T., Sakai, H., Ishibashi, J., Nakayama, E., Isshiki, K., Matsuura, H.,Shitashima, K., Takeuchi, T., Ohta, S., 1993. Hydrothermal plumes inthe eastern Manus Basin, Bismark Sea: CH4, Mn, Al and pHanomalies. Deep-Sea Research I 40, 2335–2349.

Garcia, H.E., Locarnini, R.A., Boyer, T.P., Antonov, J.I., 2006. In: Levitus, S.(Ed.), World Ocean Atlas 2005, Volume 4: Nutrients (Phosphate,Nitrate, Silicate), NOAA Atlas, 396 pp.

Gordon, R.M., Coale, K.H., Johnson, K.S., 1997. Iron distributions in theequatorial Pacific: implications for new production. Limnology andOceanography 42 (3), 419–431.

Gorgues, T., Menkes, C., Aumont, O., Vialard, J., Dandonneau, Y., Bopp, L., 2005.Biogeochemical impact of tropical instability waves in the equatorialPacific. Geophysical Research Letters 32101029/2005GL024110.

Gorgues, T., Menkes, C., Aumont, O., Dandonneau, Y., Madec, G., Rodgers,K., 2007. Indonesian throughflow control of the eastern equatorialPacific biogeochemistry. Geophysical Research Letters 34, L05609,doi:10.1029/2006GL028210.

Gnanadesikan, A., Slater, R.D., Gruber, N., Sarmiento, J.L., 2002. Oceanicvertical exchange and new production: a comparison betweenmodels and observations. Deep-Sea Research II 49 (1–3), 363–401.

Jiang, M.S., Chai, F., Dugdale, R.C., Wilkerson, F.P., Peng, T.H., Barber, R.T.,2003. A nitrate and silicate budget in the equatorial Pacific Ocean: acoupled physical–biological model study. Deep-Sea Research II 50,2971–2996.

Johnson, E.S., 1996. A convergent instability wave front in the centraltropical Pacific. Deep-Sea Research II 43 (4–6), 753–778.

Johnson, K.S., Gordon, R.M., Coale, K.H., 1997. What controls dissolved ironconcentrations in the world ocean?. Marine Chemistry 57, 137–161.

Kalnay, E.C. The NCEP/NCAR reanalysis project. Bulletin of the AmericanMeteorological Society 77, 437–471.

Kineke, G.C., Woolfe, K.J., Kuehl, S.A., Milliman, J.D., Dellapenna, T.M.,Purdon, R.G., 2000. Sediment export from the Sepik River, Papua NewGuinea: evidence for a divergent sediment plume. Continental ShelfResearch 20, 2239–2266.

Kolber, Z.S., Barber, R.T., Coale, K.H., Fitzwater, S.E., Greene, R.M., Johnson,K.S., Lindley, S., Falkowski, P.G., 1994. Iron limitation of phytoplanktonphotosynthesis in the equatorial Pacific Ocean. Nature 371, 145–149.

Kuma, K., Tanaka, J., Matsunaga, K., Matsunaga, K., 2000. Effect ofhydroxamate ferrisiderophore complex (ferrichrome) on iron uptakeand growth of a coastal marine diatom, Chaetoceros sociale.Limnology and Oceanography 45 (6), 1235–1244.

Kustka, A., Shaked, Y., Milligan, A.J., King, D.W., Morel, F.M.M., 2005.Extracellular production of superoxide by marine diatoms: contrast-ing effects on iron redox chemistry and bioavailability. Limnologyand Oceanography 50 (4), 1172–1180.

Lacan, F., Jeandel, C., 2001. Tracing Papua New Guinea imprint on thecentral equatorial Pacific Ocean using neodymium isotopic composi-tions and rare earth element patterns. Earth and Planetary ScienceLetters 186, 467–512.

Lam, P.J., Bishop, J.K.B., 2008. The continental margin is a key source ofiron to the HNLC North Pacific Ocean. Geophysical Research Letters35101029/2008GL033294.

Landry, M.R., Barber, R.T., Bidigare, R.R., Chai, F., Coale, K.H., Lewis, H.G.,McCarthy, J.J., Roman, M.R., Stoecker, D.K., Verity, P.G., White, J.R.,1997. Iron and grazing constraints on primary production in thecentral equatorial Pacific: an EqPac synthesis. Limnology andOceanography 42 (3), 405–418.

Leonard, C.L., McClain, C.R., Murtugudde, R., Hofmann, E.E., Harding, L.W.,1999. An iron-based ecosystem model of the central equatorialPacific. Journal of Geophysical Research 104 (C1), 1325–1341.

Le Borgne, R., Barber, R.T., Delcroix, T., Inoue, H.Y., Mackey, D.J., Rodier, M.,2002. Pacific warm pool and divergence: temporal and zonalvariations on the equator and their effects on the biological pump.Deep-Sea Research II 49 (13–14), 2471–2512.

Le Qu�er�e, C., Aumont, O., Monfray, P., Orr, J., 2003. Propagation of climaticevents on ocean stratification, marine biology, and CO2: case studiesover the 1979–1999 period. Journal of Geophysical Research108101029/2001JC000920.

Liu, X., Millero, F.J., 2002. The solubility of iron in seawater. MarineChemistry 77, 43–54.

ARTICLE IN PRESS

L. Slemons et al. / Deep-Sea Research I 56 (2009) 2115–21282128

Loukos, S., Frost, B., Harrison, E.E., Murray, J.W., 1997. Ecosystem modelwith iron limitation of primary production in the equatorial Pacific at1401W. Deep-Sea Research II 44, 2221–2249.

Lu, P., McCreary, J.P., Klinger, B.A., 1998. Meridional circulation cells andthe source waters of the Pacific Equatorial Undercurrent. Journal ofPhysical Oceanography 28, 62–84.

Lukas, R., 1986. The termination of the Equatorial Undercurrent in theeastern Pacific. Progress in Oceanography 16, 63–90.

Mackey, D.J., O’Sullivan, J.E., Watson, R.J., 2002. Iron in the westernPacific: a riverine or hydrothermal source for iron in the EquatorialUndercurrent?. Deep-Sea Research I 49, 877–893.

Madec, G., Delecluse, P., Imbard M., Levy, C., 1998. OPA 8.1 Ocean generalcirculation model reference manual. Notes Pole Model, 11. IPSL, Paris,France, 91 pp.

Mann, E.L., Chisholm, S.W., 2000. Iron limits the cell division rate ofProcholorococcus in the eastern equatorial Pacific. Limnology andOceanography 45 (5), 1067–1076.

Marchetti, A., Maldonado, M.T., Lane, E.S., Harrison, P.J., 2006. Ironrequirements of the pennate diatom Pseudo-nitzschia: comparison ofoceanic (high nitrate, low-chlorophyll waters) and coastal species.Limnology and Oceanography 51 (5), 2092–2101.

Martin, J.H., et al., 1994. Testing the iron hypothesis in ecosystems of theequatorial Pacific Ocean. Nature 371, 123–129.

Measures, C.I., Yang, J.J., Kaupp, L.J., Leonard, L.T., Henderson, G.M., 2006.Trace metal enrichments and their sources in the EquatorialUndercurrent of the central and eastern Pacific. GeophysicalResearch Abstracts 8, 06155.

Milliman, J.D., Farnsworth, K.L., Albertin, C.S., 1999. Flux and fate offluvial sediments leaving large islands in the East Indies. Journal ofSea Research 41, 97–107.

Moore, J.K., Doney, S.C., Lindsay, K., 2004. Upper ocean ecosystemdynamics and iron cycling in a global three-dimensional model.Global Biogeochemical Cycles 18, GB4028, doi:10.1029/2004GB002220.

Moore, J.K., Braucher, O., 2008. Sedimentary and mineral dust sources ofdissolved iron to the world ocean. Biogeosciences 5, 631–656.

Murray, J.W., Barber, R.T., Roman, M.R., Bacon, M.P., Feely, R.A., 1994.Physical and biological controls on carbon cycling in the equatorialPacific. Science 266, 58–65.

Murray, J.W., Downs, J.N., Strom, S., Wei, C., Jannasch, H.W., 1989.Nutrient assimilation, export production and 234Th scavenging in theeastern equatorial Pacific. Deep-Sea Research 36 (10), 1471–1489.

Murray, J.W., Young, J., Newton, J., Dunne, J., Chapin, T., Paul, B., McCarthy,J.J., 1996. Export flux of particulate organic carbon from the centralequatorial Pacific determined using a combined drifting trap-234Thapproach. Deep-Sea Research II 43 (4–6), 1095–1132.

Nakayama, E., Obata, H., Okamura, K., Isshiki, H., Kimoto, T., 1995. Ironand manganese in the atmosphere and oceanic waters. In: Sakai, H.,Nozaki, Y. (Eds.), Biogeochemical Processes and Ocean Flux in theWestern Pacific. Terra Scientific Publishing Co., Tokyo, pp. 53–68.

Nishioka, J., Ono, T., Saito, H., Nakatsuka, T., Takeda, S., Yoshimura, T.,Suzuki, K., Kuma, K., Nakabayashi, S., Tsumune, D., Mitsudera, H.,Johnson, W.K., Tsuda, A., 2007. Iron supply to the western subarcticPacific: importance of iron export from the Sea of Okhotsk. Journal ofGeophysical Research 112, C10012, doi:10.1029/2006JC004055.

Obata, H., Karatani, H., Nakayama, E., 1993. Automated determination ofiron in seawater by chelating resin concentration and chemilumi-nescence detection. Analytical Chemistry 65, 1524–1528.

Parekh, P., Follows, M., Boyle, E., 2004. Modeling the global ocean ironcycle. Global Biogeochemical Cycles 18, GB1002, doi:10.1029/2003GB002061.

Pichler, T., Veizer, J., Hall, G.E.M., 1999. The chemical composition ofshallow-water hydrothermal fluids in Tutum Bay, Ambitle Island,Papua New Guinea and their effect on ambient seawater. MarineChemistry 64, 229–252.

Planquette, H., Statham, P.J., Fones, G.R., Charette, M.A., Moore, C.M.,Salter, I., N�ed�elec, F.H., Taylor, S.L., French, M., Baker, A.R., Mahowald,N., Jickells, T.D., 2007. Dissolved iron in the vicinity of the CrozetIslands, Southern Ocean. Deep-Sea Research II 54, 1999–2019.

Putnam, W.M., Legler, D.M., O’Brien, J.J., 2000. Interannual variability ofsynthesized FSU and NCEP reanalysis pseudostress products over thePacific Ocean. Journal of Climate 13, 3003–3016.

Price, N.M., Ahner, B.A., Morel, F.M.M., 1994. The equatorial Pacific Ocean:grazer-controlled phytoplankton populations in an iron-limitedsystem. Limnology and Oceanography 39, 520–534.

Radenac, M.-H., Menkes, C., Vialard, J., Moulin, C., Dandonneau, Y.,Delcroix, T., Dupouy, C., Stoens, A., Deshamps, P.-Y., 2001. Modeledand observed impacts of the 1997–1998 El Nino on nitrate and newproduction in the equatorial Pacific. Journal of Geophysical Research106 (C11), 26879–26898.

Rodgers, K.B., Cane, M.A., Naik, N.H., 1999. The role of the Indonesianthroughflow in equatorial Pacific thermocline ventilation. Journal ofGeophysical Research 104 (C9), 20551–20570.

Rodgers, K.B., Aumont, O., Menkes, C., Gorgues, T., 2008. Decadalvariations in equatorial Pacific ecosystems and ferricline/pycnoclinedecoupling. Global Biogeochemical Cycles 22101029/2006GB002919.

Rue, E.L., Bruland, K.W., 1997. The role of organic complexation onambient iron chemistry in the equatorial Pacific Ocean and theresponse of a mesoscale iron addition experiment. Limnology andOceanography 42 (5), 901–910.

Ryan, J.P., Ueki, I., Chao, Y., Zhang, H., Polito, P.S., Chavez, F.P., 2006.Western Pacific modulation of large phytoplankton blooms in thecentral and eastern equatorial Pacific. Journal of GeophysicalResearch 111, GO2013, doi:10.1029/2005JG000084.

Sato, M., Takeda, S., Furuya, K., 2007. Iron regeneration and organic iron(III)-binding ligand production during in situ zooplankton grazingexperiment. Marine Chemistry 106, 471–488.

Sholkovitz, E.R., Elderfield, H., Szymczak, R., Casey, K., 1999. Islandweathering: river sources of rare earth elements to the WesternPacific Ocean. Marine Chemistry 68 (1–2), 39–57.

Slemons, L.O., Murray, J.W., Resing, J., Paul, B., Dutrieux, P., submitted,Global Biogeochemical Cycles. Western Pacific coastal sources ofiron, manganese and aluminum to the Equatorial Undercurrent.

Stoens, A., Menkes, C., Radenac, Y.D., Grima, N., Eldin, G., M�emery, L., Navarett,C., Andr�e, J., Moutin, T., Raimbault, P., 1999. The coupled physical-newproduction system in the equatorial Pacific during the 1992–1995 ElNino. Journal of Geophysical Research 104 (C2), 3323–3339.

Sunda, W.G., Swift, D.G., Huntsman, S.A., 1991. Low iron requirement forgrowth in oceanic phytoplankton. Nature 351, 55–57.

Sunda, W.G., Huntsman, S.A., 1995. Iron uptake and growth limitation inoceanic and coastal phytoplankton. Marine Chemistry 50, 189–206.

Sunda, W.G., 2001. Bioavailability and bioaccumulation of iron in the sea.In: Turner, D.R., Hunter, K.A. (Eds.), The Biogeochemistry of Iron inSeawater. Wiley, Chichester, pp. 41–84.

Tegen, I., Fung, I., 1995. Contribution to the atmospheric mineral aerosolload from land surface modification. Journal of Geophysical Research100 (D9), 18707–18726.

Toggweiler, J.R., Dixon, K., Broecker, W.S., 1991. The Peru Upwelling andthe ventilation of the south Pacific thermocline. Journal of Geophy-sical Research 96 (C11), 20467–20497.

Toggweiler, J.R., Carson, S., 1995. What are the upwelling systemscontributing to the ocean’s carbon and nutrient budgets. In:Summerhayes, C.P. (Ed.), Upwelling in the Ocean: Modern Processesand Ancient Records. Wiley, New York, pp. 337–360.

Tsuchiya, M., Lukas, R., Fine, R.A., Firing, E., Lindstrom, E., 1989. Sourcewaters of the Pacific Equatorial Undercurrent. Progress in Oceano-graphy 23, 101–147.

Ueki, I., Kashino, Y., Kuroda, Y., 2003. Observation of current variations offthe New Guinea coast including the 1997–1998 El Nino period andtheir relationship with Sverdrup transport. Journal of GeophysicalResearch 108 (36-1-17).

Verity, P.G., Stoecker, D.K., Sieracki, M.E., Nelson, J.R., 1996. Microzoo-plankton grazing of primary production at 140 degrees W in theequatorial Pacific. Deep-Sea Research II 43, 1227–1255.

Vichi, M., Masina, S., Nencioli, F., 2008. A process-oriented model studyof equatorial Pacific phytoplankton: the role of iron supply andtropical instability waves. Progress in Oceanography 78 (2), 147–162.

Walsh, I.D., Gardner, W.D., Richardson, M.J., Chung, S.P., Plattner, C.A., Asper,V.L., 1997. Particle dynamics as controlled by the flow field of theeastern equatorial Pacific. Deep-Sea Research II 44 (9–10), 2025–2047.

Wells, M.L., Vallis, G.K., Silver, E.A., 1999. Tectonic processes in PapuaNew Guinea and past productivity in the eastern equatorial PacificOcean. Nature 398, 601–604.

Wells, M.L., Trick, C.G., 2004. Controlling iron availability to phytoplank-ton in iron-replete coastal waters. Marine Chemistry 86 (2004), 1–13.

Wu, J., Boyle, E., Sunda, W., Wen, L., 2001. Soluble and colloidal iron in theoligotrophic North Atlantic and North Pacific. Science 293, 847–849.

Yoder, J., Kennelly, M., 2005. Live Access to US JGOFS SMP Data: GlobalSeaWiFS chlorophyll. U.S. JGOFS iPub. ‘April 9, 2009’ /http://usjgofs.whoi.edu/dods-bin/nph-dods/data/smp/yoder/SW19972004_8d_fsm.ncS.