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Marine Chemistry

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Sources, fluxes and residence times of trace elements measured during theU.S. GEOTRACES East Pacific Zonal TransectDavid Kadkoa,⁎, Ana Aguilar-Islasb, Clifton S. Buckc, Jessica N. Fitzsimmonsd,William M. Landinge, Alan Shillerf, Claire P. Tillg, Kenneth W. Brulandh, Edward A. Boylei,Robert F. Andersonja Applied Research Center, Florida International University, Miami, FL, USAb College of Fisheries and Ocean Sciences, University of Alaska Fairbanks, Fairbanks, AK, USAc Skidaway Institute of Oceanography, University of Georgia, Savannah, GA, USAdDepartment of Oceanography, Texas A&M University, College Station, TX, USAe Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL, USAfDepartment of Marine Science, Stennis Space Center, University of Southern Mississippi, MS, USAg Department of Chemistry, Humboldt State University, Arcata, CA 95521, USAhOcean Sciences Department, UC Santa Cruz, Santa Cruz, CA 95064, USAiDepartment of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USAj Lamont-Doherty Earth Observatory, Columbia University, NY, USA

A R T I C L E I N F O

Keywords:Trace elementsResidence timesEast Pacific OceanGEOTRACES

A B S T R A C T

Trace element (TE) fluxes and their residence times (Fe, Mn, Cu, Pb, Cd, and V) within the surface ocean weredetermined along the GEOTRACES East Pacific Zonal Transect (GP16/EPZT) and found to reflect the diversephysical and geochemical conditions encountered across the track. The TE flux from atmospheric deposition,vertical mixing, and upwelling into the mixed layer and into the particle production zone (PPZ) along theGEOTRACES EPZT transect were evaluated with 7Be-based methods developed in earlier works. A horizontalinput flux is driven from east to west by the South Equatorial Current (SEC), and estimated advection velocitieswere applied to horizontal gradients in the distributions of several TEs to approximate this term. There is aminimum in atmospheric deposition in the central gyre, with higher fluxes to the east due to large near-shoreaerosol TE loadings, and higher to the west due to greater precipitation-driven deposition velocities (Vb). The7Be-derived vertical diffusion (Kz) values range from 2.5 to 39 m2/d (0.29 × 10−4 to 4.5 × 10−4 m2/s) withhigher values generally within the nearshore upwelling region and the lowest values within the stratified centralgyre. Manganese displayed a well-defined gradient extending from the nearshore stations into the central gyresuch that the advective term is a major component of the total input flux, particularly within the central gyre.Relative to other inputs the atmospheric input of soluble Mn is only of minor importance. Unlike Mn, there is nodiscernable horizontal gradient in the dissolved Fe data and therefore, there is no horizontal component of flux.Nearshore removal processes are more intense for dissolved Fe than for dissolved Mn and as a result, dissolvedMn remains elevated much farther offshore than does dissolved Fe. For the stratified mid-ocean gyre stations, thetotal input of Fe from all sources is relatively small compared to the inshore stations, and atmospheric depositionbecomes the dominant mode of input. Aerosol Fe solubility determined by a 25% acetic acid leach with hy-droxylamine hydrochloride was much greater than that derived from a leach using ultra-pure deionized water.This led to significant differences in the residence time of Fe calculated for the mid-ocean gyre using thesedifferent solubilities. Generally, each element displays relatively short (weeks–months) residence times withinthe nearshore region of robust upwelling, reflecting large input terms and rapid removal. Moving offshore, totalinput fluxes decrease and the residence times of the TEs increase markedly until the western edge of the transect.There, relaxation of ocean stratification permits greater upward turbulent flux and greater rainfall leads togreater atmospheric input of TEs.

https://doi.org/10.1016/j.marchem.2020.103781Received 8 September 2019; Received in revised form 6 March 2020; Accepted 12 March 2020

⁎ Corresponding author at: Applied Research Center, Florida International University, 10555 W. Flagler St., Suite 2100, Miami, FL 33174, USA.E-mail address: dkadko@fiu.edu (D. Kadko).

Marine Chemistry 222 (2020) 103781

Available online 19 March 20200304-4203/ © 2020 Elsevier B.V. All rights reserved.

T

1. Introduction

The International GEOTRACES Program was established to identifyprocesses and quantify fluxes that control the distributions of key traceelements (TEs) in the ocean as these chemical species play importantroles as nutrients, as tracers of current and past oceanographic pro-cesses, and as contaminants derived from human activity (GEOTRACESPlanning Group, 2006). Their biogeochemical cycling has relevance tothe carbon cycle, climate change, and ocean ecosystems. To accomplishthese goals, the Program established a global series of basin-scaletransects to undertake comprehensive TE measurements in the watercolumn and air. One of these was the 2013 U.S. GEOTRACES East Pa-cific Zonal Transect (EPZT, US GEOTRACES cruise GP16) (Moffett andGerman, 2018) which originated within the highly productive upwel-ling region of the Peru Margin and extended westward across theCentral South Pacific Ocean to Tahiti, crossing some of the most remoteand oligotrophic open-ocean waters on Earth (Fig. 1).

The Southeast Pacific Ocean along the EPZT transect (10–18°S) is anespecially interesting region to study trace metal fluxes to the surfaceocean because low dust input supports only a very low supply of tracemetals to the open ocean (Wagener et al., 2008), and it has been sug-gested that mixing processes might therefore dominate metal fluxes tothe surface ocean (Fitzsimmons et al., 2016; Buck et al., 2019). In theupwelling region near South America horizontal mixing and advectionare especially important, and upwelling drives the ETSP (Eastern Tro-pical South Pacific) OMZ (Oxygen Minimum Zone) (Karstensen et al.,2008) which results in unique metal signatures (Scholz et al., 2011,2014) and biological communities (Ohnemus et al., 2016).

Metal fluxes to the surface ocean are critical drivers of primaryproduction, and the EPZT transect is situated across several biologicalregimes: 1) the lowest productivity waters in the entire global ocean inthe South Pacific subtropical gyre (centered at 25–30°S; Claustre et al.,2008), 2) some of the highest productivity waters in the ocean along theSouth American continent (e.g. Carr, 2001), and 3) the Fe-limitedEquatorial Pacific Region (Martin et al., 1994). Sampling across suchdiverse oceanographic settings would be expected to manifest differentsources, sinks, distributions and residence times of trace elements,which must be well constrained to predict or estimate biological re-sponses.

The GEOTRACES program affords the opportunity to extensivelysample TEs in the water column and aerosols, including the determi-nation of aerosol solubilities and tracer measurements such as the iso-tope 7Be, a cosmic-ray producted isotope (half-life = 53.3 days) that isdeposited into the surface ocean and used to study the atmospheric

fluxes of TEs and their transport in the upper ocean (e.g. Young andSilker, 1980; Kadko and Olson, 1996; Kadko and Johns, 2011; Haskellet al., 2015; Kadko et al., 2015; Shelley et al., 2017). In this work, weutilize the extensive dissolved TE, 7Be tracer, and aerosol concentrationand solubility datasets from the EPZT to calculate TE fluxes into theocean mixed layer and the Particle Production Zone (PPZ; depth wherefluorescence reaches 10% of maximum fluorescence) across thetransect; from this, we evaluate the residence time of TEs in the upperocean of this region.

2. Methods

2.1. Sample collection

Seawater and aerosol samples were collected during the U.S. GEO-TRACES EPZT (Eastern Pacific Zonal Transect -GEOTRACES cruiseGP16) expedition on the R/V Thomas G. Thompson (TGT303, 25October 2013–20 December 2013, Manta, Ecuador - Papeete, Tahiti)along the cruise track shown in Fig. 1.

2.1.1. AerosolsDetails of the aerosol collection methods have been presented in

prior work (Buck et al., 2019). Briefly, bulk aerosol samples were col-lected on 12-replicate acid-washed 47 mm Whatman 41 (W41) ash-lessfilter discs mounted in Advantec-MFS polypropylene inline filterholders (PP47). When the wind was directed from the bow, air waspumped through the filters using a high-volume aerosol sampler (model5170 V-BL, Tisch Environmental) at approximately 100 L/min througheach filter. The sampler was mounted on the starboard rail of the 03-deck approximately 16 m above sea level and forward of both the ship'ssuperstructure and exhaust stacks. Each collection period lasted ap-proximately 3 days.

2.1.2. Water samples for trace elementsSeawater was collected following established GEOTRACES sample

collection protocols (Cutter and Bruland, 2012) using a trace metal-clean CTD mounted on an epoxy-coated aluminum rosette housing 24GO-Flo bottles (12 L each) on a Vectran conducting cable. Seawater wasfiltered in a clean sampling van under ~0.5 atm of filtered air through0.2 μm Acropak-200 polyethersulfone capsule filters (Pall) into pre-cleaned bottles following three 10% volume rinses. Samples wereacidified to pH 2 with ultrapure hydrochloric acid (Optima grade,Fisher Scientific) or pH ~1.7 with quartz-distilled hydrochloric acid(Bruland lab) and stored at least 3 months prior to analysis.

Fig. 1. The 2013 US GEOTRACES EPZT cruise track with aerosol sample deployment locations marked as alternating black and white lines; recovery location of eachsample coincides with the deployment location of the next sample (details in Buck et al., 2019). The water sampling stations are indicated by a blue star and thestation number; these did not always coincide with aerosol collection deployments. (For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

D. Kadko, et al. Marine Chemistry 222 (2020) 103781

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2.1.3. Water column 7BeDetails of sample collection for this expedition have been presented

in Kadko (2017). Briefly, samples were collected at selected depths bypumping 400–700 L of seawater via a ~4 cm hose into large plasticbarrels on deck. From these barrels, the seawater was then pumpedthrough iron impregnated acrylic fibers at ~10 L/min (Lal et al., 1988;Krishnaswami et al., 1972; Lee et al., 1991) to extract the 7Be fromseawater. The efficiency of the fiber for extraction of Be from seawaterhas been determined by adding 500 mL of a 1000 ppm Be atomic ab-sorption standard to a drum containing seawater. The seawater waspumped through an iron fiber cartridge and at every 100 L the Becontent of the cartridge effluent was measured by atomic absorption.From this data, the integrated Be extraction efficiencies were calcu-lated. For sample volumes in the range 400–700 L, the extraction effi-ciencies were respectively 82–76% (Kadko and Johns, 2011).

2.2. Analytical methods

2.2.1. Aerosol samplesTotal trace metal concentrations in the samples were determined at

the University of Alaska Fairbanks by inductively coupled plasma massspectrometry (ICPMS) using a Thermo Element-2 ICP mass spectro-meter following digestion with concentrated nitric and hydrofluoricacids. Method fitness was tested by digesting three reference materialsand quantifying elemental recoveries. Details can be found in Bucket al. (2019). For 7Be, the Whatman-41 aerosol filters were stackedthree-high in a plastic Petri dish and counted by gamma spectroscopy.This configuration was calibrated with a commercially prepared mixedsolution of known gamma activities.

Soluble TE concentrations were measured in both ultrapure deio-nized water (DI water; 18.2 MΩ∙cm) after Buck et al. (2006), and inammonium acetate solution with a reducing agent. Three filters fromeach deployment were extracted by each treatment producing triplicateDI water soluble aerosol samples and triplicate acetic acid solubleaerosol samples which will be called “HAc” samples from this pointforward.

The DI water method produces a lower-limit of elemental solubilityand the HAc method an upper limit such that a “solubility window” foreach element is provided (Shelley et al., 2018). The HAc leach treat-ment is designed to extract TEs associated with biogenic material,carbonates, most forms of Fe and Mn (oxyhydr)oxides and those ad-sorbed to clay minerals. The reducing conditions may simulate theconditions that particles experience during passage through a zoo-plankton gut or inside a fecal pellet or organic aggregate while the lowpH (2.1) serves to prevent any reprecipitation or adsorption artifacts.

All sample handling was performed under Class-100 laminar flowconditions and all filters were stored frozen prior to processing.Extractions with DI water took place while at sea, while extractionswith HAc took place at the University of Alaska Fairbanks.

The W41 filter discs were extracted with 100 mL of ultrapure DIwater (>18 MΩ cm) under a vacuum. The extract flowed through aTeflon-PFA filter holder (Savillex) loaded with an acid-washed 0.4 μmNuclepore backing filter and directly into an acid-washed 100 mL 100%LDPE receiving bottle. Samples were immediately acidified with Teflon-distilled 6 M hydrochloric acid to a final concentration of 0.024 M HCl.DI water soluble aerosol TE concentrations were determined by in-ductively coupled plasma mass spectrometry (Perkin Elemer Nexion300D) at the Skidaway Institute of Oceanography. This quadrupole ICP-MS utilizes NH3 as a reaction gas to eliminate polyatomic interferences.Additionally, the analytes were concentrated on an in-line chelatingresin (Nobias PA-1) prior to introduction to the plasma (ESI seaFAST 3system). Instrument detection limits were determined by measuring thereagent blank of the system multiple times during each run and cal-culating three times the standard deviation of those analyses.Deployment filter blank extracts were prepared at sea with W41 discsthat had been deployed in the aerosol samplers for 1 h while not in

operation. Three replicate blanks were extracted each time a group ofsamples was extracted, thereby accounting for any variability in thequality of the ship-provided DI water.

For the determination of HAc soluble aerosol trace element load-ings, the thawed W41 filter discs were folded twice into a wedge andtransferred to acid cleaned centrifuge tubes. Extractions were carriedout following the “HAcT⁎+R,” procedure described in Berger et al.,2008, which includes a reducing agent and a heating step. We modifiedthe method by adding a centrifugation step to prevent transfer of un-extracted particulate trace elements. Briefly, 1 mL of 25% acetic acidwith 0.02 M hydroxlamine hydrocholoride was added to the foldedfilters, which were immediately placed in a 90 °C hot water bath for10 min, then left to cool for 2 h. Samples were centrifuged, and thesolution was carefully transferred into Teflon beakers. Filters were thenrinsed three times with 500 μl of ultra-pure DI water, with a cen-trifugation step after each rinse and careful transfer into the corre-sponding Teflon beaker. The solutions on the beakers were set on a hotplate to dry down to a pearl. The residue was then reconstituted with1 mL of Optima grade concentrated nitric acid and decanted into anacid-washed, wide-mouth, 30 ml polyethylene bottle (Nalgene). Eachbeaker was rinsed 3 times with 5 ml of 0.1% v/v Optima grade nitricacid, with rinses decanted into the corresponding 30 ml bottle. Allsample handling was performed under Class 100 laminar flow condi-tions. Deployment blanks were processed in the same manner as sam-ples. HAc soluble aerosol trace metal concentrations were determinedby inductively coupled plasma mass spectrometry (Thermo Element 2)using external standard curves. Indium was added to standards andsamples to track instrument fluctuations. Prior to direct injection,sample solutions were diluted as needed by a factor of 2 to 20 with 1 MOptima grade nitric acid, and deployment blank solutions were dilutedby a factor of 2. Analysis took place at the University of Alaska Fair-banks.

2.2.2. Dissolved trace metalsThe dissolved metal data used in this paper have been culled from

the GEOTRACES Intermediate Data Product (IDP) 2017 (Schlitzer et al.,2018) and from several published papers (Resing et al., 2015 for Mn;Fitzsimmons et al., 2017 and John et al., 2018 for Fe; Rashan and Wu,2018 for Cu; Ho et al., 2018 for V); the details of these methods can befound in these publications. Dissolved Fe, Mn, Cu, Pb, and Cd were alsomeasured using established methods (Biller and Bruland, 2012; Parkeret al., 2016) after UV irradiation and preconcentration onto Nobias PA1chelating resin, followed by ICP-MS analysis and comparison to stan-dard addition curves. Dissolved Pb was also analyzed across thetransect using isotope dilution ICP-MS with 206Pb on a quadrupole ICP-MS (Fisons PQ+; Lee et al., 2011). These datasets were all rigorouslyintercalibrated by the originating labs, and the intercalibration proce-dures were reviewed and approved by the GEOTRACES Standards &Intercalibration Committee before being published in the GEOTRACESIDP 2017.

2.2.3. Water column 7BeDetails of the 7Be analysis for this expedition are presented in Kadko

(2017). On land, the fibers were dried and then ashed. The ash wassubsequently pressed into a pellet (5.8 cm diameter) and placed on alow background germanium gamma detector. The isotope 7Be has areadily identifiable gamma peak at 478 keV. The detector was cali-brated for the pellet geometry by adding a commercially preparedmixed solution of known gamma activities to an ashed fiber, pressingthe ash into a pellet, and counting the activities to derive a calibrationcurve. The uncertainty of the extraction efficiency (4%) and the de-tector efficiency (2%) was in all cases smaller than the statisticalcounting error and the uncertainty in the blank.

D. Kadko, et al. Marine Chemistry 222 (2020) 103781

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2.3. Flux calculations

The flux of TEs into the mixed layer and the PPZ along the GEOT-RACES EPZT transect are composed of four components (SupplementalFig. S1): i) atmospheric deposition ii) vertical mixing from below, iii)upwelling if present, through a vertical concentration gradient and iv)horizontal advection from east to west driven by the South EquatorialCurrent (SEC) if horizontal gradients in the TE distribution exist. In theNorthern Equatorial Pacific, Landing and Bruland (1987) reportedrapid off- shore advective transport from the eastern boundary by theNorth Pacific Equatorial Current, indicating the importance of thistransport term in any TE mass budget calculation.

2.3.1. Atmospheric depositionThe bulk atmospheric flux for aerosol trace elements (Fatm) is esti-

mated from the concentration of trace elements in the aerosols (CTE)and the bulk deposition velocity (Vb) which includes dry + wet de-position, such that

= ×F C Vatm TE b (1)

It has been shown that Vb can be derived from the upper oceaninventory of 7Be and the aerosol 7Be activity (Young and Silker, 1980):

= ×V Inventory Be Be( )/( )b aerosol7 7 (2)

where λ is the 7Be decay constant (0.013 d−1). This equation holds inthe absence of upwelling, and when horizontal transport and particleexport of 7Be are negligible. These considerations have been discussedelsewhere (Kadko and Johns, 2011; Kadko, 2017).

This method has been used to derive the TE atmospheric flux forsites in the Atlantic (Kadko et al., 2015; Anderson et al., 2016; Shelleyet al., 2017), the Pacific (Buck et al., 2019), and the Arctic (Kadko et al.,2016, 2019; Marsay et al., 2018) oceans.

The most biogeochemically relevant fraction of atmospheric flux isthat which is operationally defined as soluble. We adjust Eq. (1) toaccount for the elemental aerosol fractional solubility, β such that theatmospheric flux of soluble TE equals the product of the bulk atmo-spheric flux and the measured fractional solubility determined for eachelement:

= × ×F C Vatm TE b (3)

In this paper we will consider the atmospheric flux of soluble TEsbased on the solubilities determined by both the DI and HAc leachapproaches.

2.3.2. Vertical diffusionDiffusive transport below the mixed layer is described by FD = -Kz

dC/dz., where Kz is the turbulent diffusion coefficient and dC/dZ is thevertical gradient in TE concentration. The shape of the 7Be profilebelow the ocean mixed layer can be used to derive vertical diffusionrates (Kadko and Olson, 1996; Kadko and Johns, 2011; Haskell et al.,2015; Kadko, 2017):

=C z C e( ) oz H( ) (4a)

where C(z) is the 7Be activity at depth z, H is the depth of the mixedlayer, Co is the 7Be activity at depth H, and.

= + +wK

wK K2

12

4z z z

2 12

(4b)

where w is the upwelling rate (m/d). In the absence of upwelling(w=0),

=C(z) C exp[ ( /K ) ·Z]o z1/2 (5)

where Z is the depth below the mixed layer H.

2.3.3. UpwellingWithin regions of upwelling the dilution of the water column 7Be

inventory by deep, 7Be-free upwelled water provides a means to inferupwelling rates quantitatively (Kadko and Johns, 2011; Haskell et al.,2015; Kadko, 2017). The net upwelling flux is calculated using Eq. (6),where w is the upwelling rate either at the base of the mixed layer or thebase of the PPZ, C is the trace element concentration at those horizons,and Cml is the mixed layer concentration:

=F w [C C ]U ML (6)

The results and discussion for the upwelling stations of the EPZTtransect (stations 1–15) have been presented elsewhere (Kadko, 2017).

2.3.4. Horizontal advectionNear-surface east-to-west currents, calculated from satellite-tracked

drogued drifter velocities, are in the range 0.1–0.5 m/s for the EPZTregion (Lumpkin and Johnson, 2013). The timescale of transport fromnearshore to the central gyre (~5000 km) would then be in the range580–116 days respectively, with the upper end of this range more likelyas the velocity for the entire mixed layer would be lower than thatdetermined for the near-surface. Surface 228Ra data (Sanial et al., 2018)provide additional insight into the rate of offshore transport from thecontinental margin. The continental shelf is a source of 228Ra whichsubsequently decays with a 5.75 y radioactive half-life upon transportoffshore. The open water activities of the central gyre are ~ half thenear-shore values. Based on radioactive decay, the transport timescalewould be ~ 2000 days, equivalent to a horizontal velocity of 0.03 m/s.This is not consistent with the Lumpkin and Johnson (2013) observa-tions and likely is an underestimate, as 228Ra dilution and loss due tosurface uptake and particle settling were ignored in this simple calcu-lation. In the discussions that follow, the effect of horizontal transportupon the TE distributions will be considered in detail, using a range ofvelocities (Sections 3.1 and 3.2).

The net horizontal flux into the mixed layer is given by:

=F H u dCdXHZ (7)

Where H is the height of the mixed layer, u is the horizontal velocityand dC/dX is the horizontal concentration gradient between profiles.Meridional transport is assumed to be small compared to the zonaltransport in this section. For water below the mixed layer and withinthe PPZ, an estimate of 0.05 m/s, based on a high-resolution oceangeneral circulation model, was used (Masumoto et al., 2004). Includingthe mixed layer as part of the PPZ, the net horizontal flux into the PPZis:

= +F H u dCdX

H H u dCdX

( )PPZ PPZ (8)

Where Hppz is the base of the PPZ and u′ is the horizontal velocitybetween the mixed layer and base of the PPZ. It is assumed that withinthese layers, u, u′ and dC

dX are invariant with depth.These calculations will depend on the depths of the MLD and the

PPZ, and these parameters may naturally vary in ways that cannot al-ways be predicted or observed. However, as shown below this does nothave a great effect on the calculated residence times.

2.4. Residence time calculations

Residence time is a useful concept that informs us of several aspectsof trace element biogeochemical behavior in the upper ocean, includingdissolution and scavenging processes, biological utilization, and theseasonal relationship of surface concentrations to atmospheric input(e.g. Jickells, 1999; Croot et al., 2004; Hayes et al., 2015; Bridgestocket al., 2016; Kadko et al., 2019). For the mixed layer it can be definedby the dissolved TE inventory divided by the sum of the input fluxes.

D. Kadko, et al. Marine Chemistry 222 (2020) 103781

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= + + +[mixed layer TE inventory]/[F F F F ]ML atm D U HZ (9)

The residence time can also be calculated using the sum of the ex-port fluxes (e.g. Black et al., 2019), since at steady state, the sum of theinput fluxes should equal the sum of the output fluxes.

The horizontal mixing fluxes and the upwelling fluxes we calculateare net flux terms, representing the differences between physical mixinginput and output at each station. The atmospheric input fluxes and thevertical turbulent mixing fluxes are gross input fluxes. When the sum ofthese net and gross input fluxes is positive this suggests that thereshould be a biogeochemical output flux to keep the dissolved traceelement concentrations at steady-state. The probable candidate for abiogeochemical removal term would be biotic and abiotic conversion toparticulate forms. If the sum of all terms is negative, a biogeochemicalinput flux is required, likely remineralization from particulate to dis-solved form, either from particles settling from the mixed layer to thePPZ depth, from particles advected horizontally, or slow dissolutionfrom deposited aerosols. Such terms would be added to the other po-sitive fluxes to calculate the residence time. The residence times wereport here thus represent the residence times with respect to the netinput from physical mixing, atmospheric input, and potential biogeo-chemical input fluxes. A schematic showing the various fluxes is shownin supplemental Fig. S1.

While the effect of variable mixed layer depth (MLD) would affectthe horizontal flux term (FHZ), the effect on the residence time calcu-lation is mitigated because the mixed layer TE inventory changes in thesame sense of the flux term. For example, an increase in MLD mightproduce a higher mixed layer TE inventory (in the numerator) but alsoa higher FHZ (Eq. (7)) in the denominator. In this way, the effects ofvariable MLD on residence times tend to cancel out.

While residence times are often reported for the mixed layer, it isalso useful to consider the geochemical behavior of TEs within theslightly deeper particle production zone (PPZ), defined from the surfaceto its base level where the fluorescence signal approaches 10% of thefluorescence maximum. This has recently been adopted as defining theeuphotic zone (Owens et al., 2015; Ohnemus et al., 2016; Kadko, 2017).Then

= + + +[PPZ TE inventory]/[F F F F ]PPZ atm D U PPZ (10)

Both calculations will be considered here.

3. Results and discussion

3.1. Flux calculations

3.1.1. Atmospheric fluxThe GEOTRACES transect crossed a significant gradient in atmo-

spheric deposition driven by dust input into the eastern transect nearthe continental shelf (stations 1–7), coincident with the area of up-welling (Table 1, Buck et al., 2019). Moving westward over the openocean (stations 9–36), the aerosol and TE concentrations were sub-stantially lower, although this is not as apparent for 7Be. Here the re-gion is characterized by the strong stratification and oligotrophic con-ditions of the subtropical gyre (Fig. 2A).

Continuing west of 140oW, the rainfall is considerably higher thanover the mid-gyre stations which is manifested by decreasing surfacesalinity and increasing 7Be inventories (Fig. 2B); it has been observedthat 7Be deposition rates correlate with the rate of precipitation (e.g.

Young and Silker, 1980; Olsen et al., 1985; Uematsu et al., 1994; Kimet al., 1999; Kadko and Prospero, 2011; Peng et al., 2019). Accordingly,the higher 7Be inventories result in higher deposition velocities towardsthe western stations compared to stations to the east (Eq. (2)). Thisresults in a minimum in atmospheric TE deposition in the central gyre,with higher fluxes to the east due to higher aerosol TE loads (Fig. 2A),and higher to the west due to higher Vb values resulting from increasedprecipitation (Fig. 2B, Table 2). Our bulk atmospheric flux estimates ofMn (based on the 7Be method) across the transect range between 1 and16.5 nmol/m2/d which is comparable to the range (0.1–16 nmol/m2/d)of atmospheric particulate flux presented in Black et al. (2019) whoused dust-model estimates combined with upper crust ratios for eachelement. Buck et al. (2019) estimated bulk aerosol Mn deposition torange between 0.45 and 68 nmol/m2/d by assuming a constant de-position velocity of 1500 m/d applied to observed aerosol TE con-centrations but those estimates are susceptible to the short-termvariability of aerosol concentrations and the episodic nature of dusttransport.

Note that the low salinity in the eastern stations (Fig. 2B) is not theresult of high precipitation but of upwelling of low salinity water(Fig. 2C). Because of upwelling, Eq. (2) does not hold for the eastern-most stations 1–13, as the observed 7Be inventory does not reflect at-mospheric input, but rather upwelling of deep, 7Be deficient water. Forthis reason, the inventories of these stations (for use in Eq. (2)) areassigned that of station 15, the nearest station not affected by the up-welling process (Kadko, 2017; Buck et al., 2019). With a 7Be aerosolconcentration of 0.211 ± 0.092 dpm/m3, stations 1–15 have a constantdeposition velocity (1210 m/d, Table 2), which is consistent with therelatively uniform rain rate across these stations (Fig. 2B).

The fractional solubility for several TEs, determined by the DI andHAc leaches, are presented in Tables 3a and 3b. These were then ap-plied to Eq. (3) to derive the atmospheric fluxes of soluble (aerosoldissolvable) TEs.

3.1.2. Vertical diffusionThe turbulent diffusion coefficients for the upwelling stations of the

EPZT transect (stations 1–15) based on 7Be profiles have been derivedpreviously (Kadko, 2017) and are summarized in Table 4. The 7Beprofiles for stations 18–36 are shown in supplemental Fig. S2, and thederived diffusion coefficients presented in Table 4. The 7Be-derived Kzvalues range from 2.5 to 39 m2/d (0.29 × 10−4 to 4.5 × 10−4 m2/s)with higher values generally within the nearshore upwelling region andthe lowest values within the central gyre. This pattern is consistent withthe greater surface ocean stratification of the gyre which is reflected ina plot of Kz vs. Brunt- Väisälä frequency (N) derived for the watercolumn between the base of the mixed layer and the depth of the PPZ(Fig. 3A). In addition, the 7Be-derived Kz generally decreases as themixed layer depth increases (Fig. 3B) suggesting that the deeper mixedlayers here penetrate into more stratified water. In these figures thestations with the most robust upwelling (stations 1, 7, and 9) manifestKz values greater than the trend for the other stations, which is con-sistent with the energetic environment of the upwelling regime.

3.1.3. Upwelling fluxThe upwelling rates for the eastern stations of the EPZT transect

(stations 1–15) have been derived elsewhere (Kadko, 2017) and aresummarized in Table 4. The upwelling rate is modeled to linearly de-crease from the base of the mixed layer to zero at 100 m below the

Table 1Average aerosol trace element concentrations.

Sta no. Aerosol no. V (pmol/m3) Fe (nmol/m3) Mn (nmol/m3) Cu (nmol/m3) Cd (pmol/m3) Pb (pmol/m3) Al (nmol/m3) Ti (nmol/m3)

1–7 1–4 11.583 ± 6.918 1.158 ± 0.811 0.0231 ± 0.0156 0.0161 ± 0.011 0.635 ± 0.432 1.392 ± 1.667 4.260 ± 3.036 0.131 ± 0.0889–36 5–17 0.280 ± 0.145 0.056 ± 0.023 0.0008 ± 0.0003 0.00105 ± 0.0003 0.026 ± 0.021 0.377 ± 0.160 0.139 ± 0.051 0.014 ± 0.006

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mixed layer. The upwelling rates are significant for these stations, butthe net upwelling TE fluxes (defined in Eq. (6)) are not the dominantsource of a TE into the mixed layer and the PPZ. This is because the TEconcentrations in these reservoirs are high, indicating that the bio-geochemical removal processes do not keep up with the upwellingsupply of TEs. The TEs are thus removed out of the box by outflow,rather than by biogeochemical removal (supplemental Fig. S1). A

summary of all the input fluxes for the mixed layer and the PPZ areshown in Tables 5a and 5b.

3.1.4. Horizontal advectionHorizontal velocities discussed above were combined with hor-

izontal gradients along the EPZT transect to derive horizontal fluxesaccording to Eq. (7) and (8). These are summarized in Tables 5a and 5b.

Fig. 2. A) Aerosol TE and 7Be concentrations along the EPZT transect. Note the high TE concentrations towards the eastern portion of the transect. This is not asapparent for 7Be. B) Rainfall (from Global Precipitation Climatology Project, GPCP) for the cruise period across the transect. Note the coincident increase inprecipitation, decrease in salinity and increases in 7Be inventory and bulk deposition velocity (Vb) at the westernmost portion of the transect. Station numbers areindicated. C) 7Be inventory plotted against salinity. Stations within the upwelling region are plotted separately (squares). The linear regression applies to non-upwelling region stations (black dots). Despite low rainfall, salinity is low at the eastern portion of the transect because of upwelling of low salinity water. Note thatthe 7Be inventory is also lower because of upwelled low-7Be water (Kadko, 2017).

Table 2Bulk atmospheric TE fluxa,b.

Sta 7Be inventory(dpm/m2)

7Be flux Vbc (m/d)

Fe flux (nmol/m2/d)

Mn flux (nmol/m2/d)

Cu flux (nmol/m2/d)

Cd flux pmol/m2/d

Pb flux pmol/m2/d

Al flux (nmol/m2/d)

V flux (nmol/m2/d)

1 19,670 256 1210 1400 28 19.5 770 1690 5160 147 19,670 256 1210 1400 28 19.5 770 1690 5160 149 19,670 256 1210 68 1 1.3 32 460 170 0.3411 19,670 256 1210 68 1 1.3 32 460 170 0.3413 19,670 256 1210 68 1 1.3 32 460 170 0.3415 19,670 256 1210 68 1 1.3 32 460 170 0.3418 12,520 163 1220 68 1 1.35 30 460 175 0.3423 12,930 168 1265 71 1 1.3 33 480 186 0.3526 15,085 196 1475 83 1.2 1.55 38 560 205 0.4130 14,910 194 1460 82 1.2 1.5 38 550 200 0.4132 18,260 237 1785 100 1.4 1.9 46 670 250 0.5034 27,230 354 2660 150 2.1 2.8 69 1000 370 0.7536 34,980 455 3420 190 2.7 3.6 89 1290 475 0.96

a TE fluxes calculated using average aerosol concentrations reported in Table 1.b Soluble atmospheric flux = bulk atmospheric flux (shown here) x solubility (see Tables 3a and 3b).c Aerosol 7Be for stations 1–15 = 0.211 ± 0.092 dpm/m3; for stations 18–36 = 0.133 ± 0.039 dpm/m3.

D. Kadko, et al. Marine Chemistry 222 (2020) 103781

6

3.1.4.1. Manganese. In both the mixed layer and at the depth of thePPZ, Mn shows a clear east-to-west gradient (Fig. 4). For thiscalculation, we choose a current speed of 0.1 m/s which is at thelower end of the range of Lumpkin and Johnson (2013) and consistentwith the 228Ra data of Sanial et al. (2018). The data in Tables 5a and 5bindicate that despite a large atmospheric dust input to the near shorestations, the greatest flux of Mn is upwelled, suggesting a shelf-sedimentporewater origin (e.g. Sanial et al., 2018). A similar observation wasmade off NW Africa, where high Mn occurs in a region associated withboth upwelling and dust input, but the dominant source was upwelling(Shiller, 1997). We also note that some dissolved Mn at the easternmoststations is derived from upwelling of waters where in situ reduction ofparticulate Mn(IV) to dissolved Mn(II) is favored due to the oxygendeficient conditions (Cutter et al., 2018; Lee et al., 2018), but this islikely less than the porewater-diffused Mn flux.

For the offshore gyre stations, horizontal transport is the dominantflux term. Dissolved Mn profiles of most stations display a maximumbelow the mixed layer suggestive of this advective input (supplementalFig. S3). The horizontal Mn concentration gradient between stations issteepest for the near-shore stations, indicating removal as the Mn, dif-fused from sediments, is transported offshore and begins to be oxidized(Fig. 4). Towards the open ocean (west of station 15 at 104oW) the

Table 3aTE aerosol solubilities in dionized water.

Deployment Start date Start latitudeos Start longitude oW Fe% % Error Mn% % Error Cu% % Error

Aer1 26-Oct 4.07 81.99 2.42 0.12 43.7 2.2 42.8 4.0Aer2 29-Oct 12.01 79.20 1.99 0.59 37.0 2.2 37.7 2.0Aer3 1-Nov 12.05 77.66 1.27 0.54 32.1 2.0 38.2 2.6Aer4 4-Nov 12.00 81.50 1.94 0.50 34.2 2.7 36.6 3.9Aer5 7-Nov 12.00 86.50 4.75 0.50 61.7 32.4 46.0 20.8Aer6 10-Nov 12.00 94.00 1.55 0.61 44.4 14.2 29.9 4.1Aer7 13-Nov 14.00 99.00 0.94 0.52 35.5 4.4 24.7 6.4Aer8 16-Nov 16.00 104.00 0.69 0.13 25.1 4.7 18.8 8.5Aer9 19-Nov 15.00 109.19 0.56 0.11 26.0 8.6 10.3 4.1Aer10 22-Nov 14.99 112.75 1.47 0.13 48.8 8.4 17.8 7.3Aer11 25-Nov 14.77 115.00 0.83 0.15 30.2 4.6 12.8 5.7Aer12 28-Nov 14.00 120.00 0.49 0.21 20.5 5.5 56.8 13.8Aer13 1-Dec 12.54 125.00 0.35 0.27 18.2 7.5 15.7 6.2Aer14 4-Dec 11.67 128.00 1.27 0.32 26.5 8.1 16.9 3.3Aer15 7-Dec 11.60 135.00 1.23 0.25 51.8 18.0 17.3 1.7Aer16 10-Dec 11.31 140.00 1.68 0.84 NA NA 24.9 10.8Aer17 13-Dec 11.03 142.95 0.69 0.34 38.8 3.5 24.7 5.4Average 1.42 ± 1.04 35.9 ± 11.9 27.7 ± 13.2

Table 3bTE aerosol solubilities by HAc method.

Deployment Start date Start latitude oS Start longitude oW Fe% % Error Mn% % Error Cu% % Error

Aer1 26-Oct 4.07 81.99 12.52 0.88 46.59 2.22 55.35 4.06Aer2 29-Oct 12.01 79.20 8.45 0.48 40.25 2.05 53.44 2.33Aer3 1-Nov 12.05 77.66 8.32 0.85 43.42 5.18 56.95 6.63Aer4 4-Nov 12.00 81.50 10.45 2.40 39.27 6.01 65.57 4.82Aer5 7-Nov 12.00 86.50 31.63 2.63 93.65 2.17 85.71 8.16Aer6 10-Nov 12.00 94.00 16.54 3.14 52.78 19.16 60.98 6.43Aer7 13-Nov 14.00 99.00 22.86 2.90 37.78 4.75 87.50 23.63Aer8 16-Nov 16.00 104.00 6.88 0.97 12.89 2.55 22.06 8.93Aer9 19-Nov 15.00 109.19 19.45 0.00 31.11 0.00 45.21 0.00Aer10 22-Nov 14.99 112.75 60.66 12.92 55.56 10.80 72.55 36.92Aer11 25-Nov 14.77 115.00 26.72 6.62 53.33 9.75 68.00 25.25Aer12 28-Nov 14.00 120.00 13.03 5.41 28.00 9.01 65.38 14.37Aer13 1-Dec 12.54 125.00 22.96 3.96 33.33 14.92 80.00 25.62Aer14 4-Dec 11.67 128.00 16.08 2.29 24.19 18.95 36.96 6.80Aer15 7-Dec 11.60 135.00 – – – – – –Aer16 10-Dec 11.31 140.00 68.21 33.06 123.91 57.45 126.67 53.00Aer17 13-Dec 11.03 142.95 12.88 3.83 42.37 7.67 69.09 18.58Average 22.35 ± 17.9 47.4 ± 27.0 65.7 ± 23.6

Table 4Vertical transport parameters.

Station upwelling rate Diffusion

W (ML)a W (PPZ)b Kzc

(m/d) (m/d) (m2/d)

1 4.4 3.3 387 1.3 0.75 399 1.5 0.68 30.6511 0.04 0.005 14.713 −0.12 −0.007 24.115 0.15 0.01 2618 0 0 2823 0 0 2426 0 0 630 0 0 2.532 0 0 2.534 0 0 2036 0 0 7

a Upwelling at the base of the mixed layer (Kadko, 2017).b Upwelling at the base of the PPZ (Kadko, 2017).c Turbulent diffusion coefficient. Values for stations 1–15 from Kadko

(2017); values for stations 18–36 from this work.

D. Kadko, et al. Marine Chemistry 222 (2020) 103781

7

gradient is diminished suggesting a slower removal rate. This will beapparent in the residence time calculations discussed below. Sanialet al. (2018) found a similar discontinuity in the upper 200 m 228Radistribution across the transect but noted that even in the gyre stations228Ra was still above the detection limit which suggested these surfacewaters had recently (years) been in contact with the continentalmargin. They suggested that the correlation between dissolved Mn and228Ra over the transect indicates that Mn was transported over con-siderable length scales into the open ocean. They also used 228Ra as a“flux gauge” to derive a Mn flux from the shelf into the open water of9000 nmol/m2/d from the upper 200 m, but they noted that estimatesof Mn benthic flux (the ultimate source of Mn for offshore transport)have been as low as 1100 nmol/m2/d from this area (Scholz et al.,2011). Black et al. (2019) derived an offshore flux of 1960 nmol/m2/d.Normalizing our PPZ water column to 200 m, we derive an averageoffshore Mn flux of ~750 nmol/m2/d.

3.1.4.2. Iron. Unlike dissolved Mn, there is no discernable horizontalgradient in the dissolved Fe concentration field. (Fig. 5). Therefore,there is no horizontal component of flux. Nearshore removal processesare more intense for dissolved Fe than for dissolved Mn (Landing andBruland, 1987). Fe(II), supplied by porewater diagenesis, is rapidlyoxidized via abiotic reactions in the presence of oxygen (Millero et al.,1987) and is thus rapidly attenuated nearshore to the South Americancontinent, while the kinetically inhibited dissolved Mn must await

N (sec-1)

0.006 0.008 0.010 0.012 0.014 0.016

Kz

m(2

)d/

0

10

20

30

40

50Stations 18-36 (gyre)Stations 1-15 (nearshore)

Mixed Layer Depth (m)

30 40 50 60 70 80 90

Kz

m(2

)d/

0

10

20

30

40

50Stations 18-36 (gyre)Stations 1-15 (nearshore)

Fig. 3. A). The 7Be-derived Kz plotted against Brunt- Väisälä frequency. B). The7Be-derived Kz plotted against mixed layer depth. Nearshore stations 1–15(squares) and offshore stations 18–36 (black dots) are indicated separately.Nearshore stations 1,7, and 9 with the greatest rates of upwelling are circledand fall off the trend set by the rest of the stations.

Table5a

TEfluxesinto

themixed

layera.

Sta

MLfluxFe

nmol/m

2 /d

MLFlux

Mnnm

ol/m

2 /d

MLFlux

Cunm

ol/m

2 /d

MLFlux

Pbnm

ol/m

2 /d

Diffuse

Upw

ell

Atmos

Horizontal

Sum

Diffuse

Upw

ell

Atmos

Horizontal

Sum

Diffuse

Upw

ell

Atmos

Horizontal

Sum

Diffuse

Upw

ell

Atmos

Horizontal

Sum

(BULK)

(BULK)

(BULK)

(BULK)

1200

440

1400

02040

257

−620

28234

−97

367

−1410

19.5

96−926

−3.2

3.08

1.7

1.2

2.8

70

131400

01410

−119

6528

253

227

109

019.5

104

232

−0.3

1.85

1.7

1.3

4.5

90.0

068

068

392

180

1289

863

−38.0

−22.5

1.3

119

59−3.1

−1.95

0.5

1.4

−3.1

118.4

068

076

122

1.2

1245

369

−1.3

0.7

1.3

100

101

−1.8

0.06

0.5

1.2

013

12.7

−1.2

680

7938.1

01

234

273

622

−26.4

1.3

96692

1.3

−0.08

0.5

1.2

2.9

1534.8

668

0109

0.0

10.5

1406

415

4.4

−18

1.3

167

154

−0.8

−0.01

0.5

2.0

1.7

18−12.0

068

054

178

01

217

425

−7.0

01.35

4−2

−1.12

0.00

0.5

0−0.7

23−40.1

071

031

64.1

01

3398

0.0

01.3

45

−0.5

0.00

0.5

00

260.0

083

083

8.6

01.2

4454

−4.1

01.55

53

0.07

0.00

0.6

00.6

30−9.8

082

072

17.9

01.2

6079

−4.2

01.5

75

−0.04

0.00

0.6

00.5

324.6

0100

0105

13.0

01.4

6983

−1.1

01.9

89

−0.05

0.00

0.7

00.6

3425.6

0150

0176

−6.4

02.1

3430

0.0

02.8

47

−0.44

0.00

1.0

00.6

3642.3

0190

0232

43.0

02.7

4591

−3.8

03.6

55

−0.1

0.00

1.3

01.2

D. Kadko, et al. Marine Chemistry 222 (2020) 103781

8

microbially-catalyzed oxidation and thus persists farther offshore.

3.1.4.3. Copper. For the mixed layer, as with Mn, there is a relativelysteep gradient in concentrations leaving the coastal zone, with adiminished gradient further offshore into the open ocean (Fig. 6). Atthe depth of the PPZ there is also a steep decline in concentrationnearshore, although the gradient is not as well-defined as in the mixedlayer. Further offshore, there appears to be a small increase in Cuconcentration going westward, which may be related to the fact that theseaward transect is bearing somewhat northwest, back above more Cu-rich subsurface waters.

3.1.4.4. Lead. As with Mn and Cu, within the mixed layer there is arelatively steep gradient in dissolved Pb concentrations leaving thecoastal zone, with a diminished gradient further offshore into the openocean (Fig. 7). At the depth of the PPZ there is also a steep near-shoredecline in Pb concentration although the gradient is less well-defined.

3.1.4.5. Cadmium. Dissolved Cd concentrations (and inventories)within the mixed layer and the PPZ show a relatively steep gradientleaving the coastal zone, consistent with its association with major algalnutrients in upwelling zones (e.g. Takesue et al., 2004; Janssen et al.,2014). There is no discernible gradient offshore into the open ocean(Supplemental Fig. S4, Supplemental Table S2).

3.1.4.6. Vanadium. The dissolved V concentration field has nodiscernable horizontal gradient (Supplemental Fig. S5, SupplementalTa

ble5b

TEfluxesinto

thePPZa.

Sta

PPZFlux

Fenm

ol/m

2 /d

PPZFlux

Mnnm

ol/m

2 /d

PPZFlux

Cunm

ol/m

2 /d

PPZFlux

Pbnm

ol/m

2 /d

Diffuse

Upw

ell

Atmos

Horizontal

Sum

Diffuse

Upw

ell

Atmos

Horizontal

Sum

Diffuse

Upw

ell

Atmos

Horizontal

Sum

Diffuse

Upw

ell

Atmos

Horizontal

Sum

(BULK)

(BULK)

(BULK)

(BULK)

1144

661400

01610

−558

028

307

−222

250

−950

19.5

123

−555

120

−3.35

1.7

2121

7700

218

1400

02320

−2050

−52.5

28405

−1670

72.5

56.25

19.5

158

307

2.80

1.64

1.7

28.4

9445

235

680

747

910

−367

1471

1018

380

105

1.3

119

674

−0.31

4.06

0.5

36.9

1150.0

0.1

680

118

71.9

−4.7

1514

582

−55.3

0.765

1.3

100

144

0.26

0.00

0.5

33.7

1384.1

−0.14

680

152

337

61

559

903

−13.3

−4

1.3

96198

3.28

0.02

0.5

37.1

15109

168

0178

−410

−5

1436

18−22

−1.75

1.3

277

255

−1.22

−0.03

0.5

43.2

18−33.3

068

035

−230

01

259

312.8

01.35

246

251

0.76

0.00

0.5

01.2

2312.0

071

083

−270

01

53.7

−213

−2.0

01.3

43

−0.41

00.5

00.1

260.0

083

083

−21.5

01.2

62.7

42−7.2

01.55

5.4

−0.3

−0.09

00.6

00.5

30−1.6

082

080

−4.8

01.2

92.1

88−4.8

01.5

7.3

4.0

0.03

00.6

00.6

320.935

0100

0101

−6.1

01.4

80.3

76−1.1

01.9

8.4

9.2

−0.05

00.7

00.6

345.12

0150

0155

−59.4

02.1

55.3

−2

−10.6

02.8

4.1

−3.7

−0.26

01.0

00.7

3613.0

0190

0203

−32.3

02.7

65.0

35−7.2

03.6

5.5

1.9

−0.17

01.3

01.1

aHorizontalfl

uxes

basedon

aneast-to

-westmixed

layervelocity

of0.1m/s.Sum

includes

bulkatmosphericinputnotcorrectedforsolubilityandisthereforean

upperlim

it.

Distance (Km)0200040006000

)gk/l

om

n(n

M

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Mn =-9.855x10-5 x Km +1.22 r ² = 0.95

Mn =-6.44x10-4 x Km +3.29

18

7

13

2632

PPZ

Distance (km)

02000400060000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Mixed Layer

7

13

26

3218

Mn =-3.64x10-5 x Km +0.829 r ² = 0.22

Mn =-7.25x10-4 x Km +2.99 r ² = 0.84

)gk/l

om

n(n

M

(A)

(B)

11

1115

15

23

23

30

30

Fig. 4. A) Dissolved Mn plotted against distance from station 1 for the mixedlayer across the EPZT. B) Dissolved Mn plotted against distance from station 1for the PPZ across the EPZT. Station numbers are indicated.

D. Kadko, et al. Marine Chemistry 222 (2020) 103781

9

Table S2).

3.2. Residence time calculations

The residence time of the TEs were calculated using Eq. (9) and(10), the flux data from Table 5a, 5b, the aerosol solubility data ofTables 3a and 3b, and the TE inventories in Table 6. The results arepresented in Table 7, and Figs. 8, 10, 11, and 12. Note that only thepositive flux values in Tables 5a and 5b are used to calculate the re-sidence times with respect to input fluxes. For the relatively few caseswhere the sum of all terms is negative, an equal input flux is assumedand added to the other positive fluxes to calculate the residence time, asdiscussed in Section 2.4. A discussion of the uncertainty of these cal-culations is given in the supplementary material. We consider the re-sidence time of each element within the mixed layer and PPZ, usingsolubilities based on both the DI and HAc leaching methods.

3.2.1.1. Manganese. There is not a significant difference between thesolubility of aerosol Mn determined by the DI and HAc leach methods,and as discussed below, the atmospheric input of soluble Mn along thistransect is minor compared to the other inputs. Consequently, theresidence times based on solubilities determined by either method arenearly identical (Table 7, Fig. 8).

For nearshore stations 1–15, the dissolved Mn residence timeswithin the mixed layer range from 197 to 380 days and average269 ± 67 days. This is comparable to the 0.4 y nearshore scavenging

residence time derived by Landing and Bruland (1987) for the CentralCalifornia coast using a one-dimensional advection-diffusion approachand to the 0.22–1.8 y residence time for the upper 150 m of the centralNorth Pacific calculated by comparing surface inventories to sedimenttrap fluxes (Martin and Knauer, 1980).

However, residence times based on the 234Th-derived particulateexport flux of Mn at 100 m depth along the EPZT transect ranged from1.5 to 3 y, considerably longer than this (Black et al., 2019). This islikely because the Mn flux determined by the 234Th method cannotaccount for lateral removal of Mn from upwelling zones which wouldlead to a lower apparent total Mn export flux. It has been suggested forexample that the export flux of POC determined by the 234Th methodunderestimates primary production (Kadko, 2017), as export produc-tion can become spatially decoupled from new production. This is be-cause a fraction of the newly produced organic material can be trans-ported laterally before leaving the euphotic zone, a process known to beparticularly important in dynamic coastal upwelling systems (Plattneret al., 2005). Here we calculate the input flux (upwelling, atmosphericdeposition, and diffusion) of Mn at station 1 to be ~13,300 nmol/m2/d(Table 5a) while Black et al. (2019) report an export flux of only1000 nmol/m2/d. Clearly, there is a decoupling between surface pro-duction and export flux at these eastern sites. These authors acknowl-edge that if net dissolved TE removal via offshore transport exceeds theparticulate TE export flux, then their residence times would be an upperlimit. Their residence time applies only to the specific removal processarising from the vertical export of particles.

Mixed layer

Distance (km)

02000400060000.00

0.05

0.10

0.15

0.20

PPZ

Distance (km)

0200040006000

)gk/l

om

n(e

F

0.0

0.2

0.4

0.6

0.8

)gk/l

om

n(e

F

(A)

(B)

26

23

30

13

11

15

189

7

26

2330 13

11

15

18

9

7

Fig. 5. A) Dissolved Fe plotted against distance from station 1 for the mixedlayer across the EPZT. B) Dissolved Fe plotted against distance from station 1for the PPZ across the EPZT. Station numbers are indicated.

Mixed Layer

Distance (km)

02000400060000.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

PPZ

Distance (km)

02000400060000.2

0.4

0.6

0.8

1.0

1.2

1.4

Slope ~-2.7x10-4

15

15

23

30

30

23

)gk/l

om

n(u

C)

gk/lo

mn(

uC

Cu =-2.64x10-4 x Km +1.36

7

7

Cu =3.84 x10-5 x Km +0.27

Cu =-1.17x10-5 x Km +0.61 r ² = 0.16

(A)

(B)

18

18

9

9

13

13

Fig. 6. A) Dissolved Cu plotted against distance from station 1 for the mixedlayer across the EPZT. B) Dissolved Cu plotted against distance from station 1for the PPZ across the EPZT. Station numbers are indicated.

D. Kadko, et al. Marine Chemistry 222 (2020) 103781

10

For the offshore stations 18–36, dissolved Mn residence times in-crease to the range of 89 to 595 days with an average of447 ± 211 days. This is comparable to the 1.9 y reported by Black et al.(2019) for the gyre region of this study where decoupling of new pro-duction and export flux does not occur. This estimate is shorter than the19 y residence time presented in Landing and Bruland (1987) where thelack of strong westward advection off the Central California coast intothe North Pacific results in much longer Mn residence times. Our resultsare consistent with Fig. 4A, where the steeper gradient for the near-shore stations suggests more rapid Mn removal within the upwelling

environment. The longer Mn residence time at the open ocean stationsis comparable to the advective timescale of 580 days for the transport5000 km offshore to the central gyre (with a velocity of 0.1 m/s) and isconsistent with the diminishment of the 228Ra signal (Sanial et al.,2018).

Over the PPZ, nearshore stations 1–15, residence times range from129 to 569 days, with an average of 341 ± 153 days. For gyre stations18–36, residence times of the PPZ range from 600 to 2070 days with anaverage of 1294 ± 540 days. The average residence times of Mn for theopen ocean mixed layer (1.2 y) and the PPZ (3.5 y) are comparable tothe lower end of the 5–20 year Mn residence times discussed in Shiller(1997) and the 5–6 year range modeled by Wu et al. (2014) for a depthof 200 m (~depth of the PPZ in this study) in the N. Pacific and the N.Atlantic based on Mn oxidation rates. These estimates are lower thanthe 9.6–11 y residence times estimated by comparing dissolved Mnseawater inventories with aerosol fluxes (Jickells, 1999).

As described in Section 3.1, a value of 0.1 m/s has been chosen forthe ocean horizontal velocity (hence the horizontal flux) calculations.The sensitivity of the residence times to this parameter is illustrated inFig. 9. For current speeds between 0.05 and 0.5 m/s (range of a factor of10), the Mn residence times for the mixed layer (Fig. 9A) of stations1–15 range between a factor of 2.8–9.7. For stations to the west(18–36), the residence times range between a factor of 2.8–9.5. Ascurrent speed increases the residence time decreases for all stations,particularly when the horizontal flux term has the greatest relativeimportance. For the PPZ (Fig. 9B), the Mn residence times for upwellingstations 1–15 range between a factor of 1.6–8.8, and for stations to thewest (11–36) the range is between a factor of 1.6–7.7.

Relative to other inputs the atmospheric input of soluble Mn is onlyof minor consequence in these calculations, even nearshore where theatmospheric input of TEs is greatest. This result is similar to that de-rived from calculations made for the region off the NW African coast(Shiller, 1997). Within the gyre, the total input flux decreases, andvertical diffusion and/or horizontal flux dominates Mn input into thesesurface waters (Sanial et al., 2018; Black et al., 2019). Zheng et al.(2019) suggest sources other than aerosols are more significant con-tributors to Mn in the N. Pacific as well, and Kadko et al. (2019) showthat atmospheric deposition of Mn is insignificant relative to other in-puts in the Arctic Ocean. In the Atlantic, aerosol input is generally muchgreater than that of other ocean basins, and away from shelf areas thesurface Mn distribution is very similar to the flux of mineral aerosol tothe surface ocean (Shiller, 1997).

3.2.1.2. Iron. The observed Fe characteristics differ from those of Mn intwo distinct ways. First, the DI and HAc aerosol Fe solubilities areappreciably different. Second, within the gyre, atmospheric Fe input issignificant relative to other input terms.

For the nearshore station 1, the Fe residence time within the mixedlayer using the two leaching methods are similar and quite short

Mixed layer

Distance (km)0200040006000

0

5

10

15

20

25

30

PPZ

Distance (km)

0200040006000

)gk/l

om

p(b

P

0

5

10

15

20

25

30

)gk/l

om

p(b

P

(A)

(B)

2632

2330

2632

2330

13

11

15

13

11

15

18

18

9

9

7

7

Pb = -4.87 x10-3 x Km +25.08

Pb = -3.20 x10-3 x Km +23.68 r ² = 0.88

Fig. 7. A) Dissolved Pb plotted against distance from station 1 for the mixedlayer across the EPZT. B) Dissolved Pb plotted against distance from station 1for the PPZ across the EPZT. Station numbers are indicated.

Table 6Mixed layer and PPZ TE inventories (nmol/m2).

Station Fe ML Fe PPZ Mn ML Mn PPZ Cu ML Cu PPZ Pb ML Pb PPZ

1 6145 10,860 131,375 202,680 52,140 73,870 970 15207 3800 10,610 124,790 269,040 41,170 84,230 950 20209 8290 23,900 169,880 341,950 53,470 112,360 990 222511 3155 9090 86,440 250,780 43,450 132,170 735 215013 3785 15,260 81,530 267,500 27,790 122,890 640 231015 7650 18,670 115,687 248,070 55,085 107,900 1090 228018 2635 17,190 37,990 196,395 21,610 85,930 645 238023 4810 12,780 32,916 160,500 24,720 100,810 550 222026 8585 26,790 36,231 130,590 32,650 95,030 740 225030 3970 8330 43,677 100,200 34,760 68,030 990 192032 8575 18,020 47,165 95,115 44,670 85,740 1195 222034 3145 17,440 20,710 99,550 18,920 92,410 590 252536 3220 25,220 26,210 105,635 29,500 98,935 820 2780

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(<10d, Table 7, Fig. 10) because the upwelling and diffusive inputs arehigh and more significant than the atmospheric input. The short re-sidence time indicates intense removal processes in the nearshore up-welling zone. Despite the relatively large dust input for this station, theDI soluble atmospheric Fe input is only ~ 2%, and the HAc solubleinput is ~30% of the total input from all sources. A similar conclusionwas drawn by Buck et al. (2019) for the nearshore of this transect andby Fitzsimmons et al. (2016) for stations farther south but equally closeto the South American continent.

Moving offshore, but still in the zone of upwelling (stations 7–15),the average residence time increases to 230 ± 60 and 105 ± 60 dayswith the DI and HAc solubilities, respectively (the anomalously highresidence time of station 9 is not included in the average). Upwelling isdiminished in importance, and the atmospheric soluble input is as highas ~60% of the total Fe input flux using the DI solubility and as high as95% with the HAc solubility. For the stratified mid-gyre stations 18–32upward turbulent diffusion is limited (Fig. 3) and the total Fe input isrelatively small compared to the inshore stations. The choice of solu-bility in the residence time calculation is therefore critical as the at-mospheric delivery of Fe becomes the dominant input term. The

resulting average residence time using the DI solubility ~ 11 y which ismuch longer than the 295 ± 110 days based on the HAc solubility. Theshorter HAc-based residence time is more consistent with most surfacedissolved Fe residence times reported in the literature (e.g. Jickells,1999; Croot et al., 2004; Boyle et al., 2005; Bergquist and Boyle, 2006;Hayes et al., 2015) except for that reported in the Arctic (Kadko et al.,2019) which was based on the DI solubility and where low particlescavenging likely occurs.

As discussed elsewhere (e.g., Aguilar-Islas et al., 2010; Sholkovitzet al. 2012), provenance, more than leaching methodology, determinesthe fractional solubility of aerosol Fe, with anthropogenic Fe beingmore readily soluble relative to mineral Fe. In the gyre area of thisregion characterized by minimal mineral dust deposition, the soluble Feis likely dominated by highly soluble anthropogenic or pyrogenicsources (Conway et al., 2019; Ito et al., 2019). Higher solubilities ob-tained from the HAc method might be more representative of the actualsolubility of aerosol Fe in seawater, as the heating step, reducing con-ditions, and lower pH of the HAc method may artificially account fortime-dependent Fe dissolution pathways, such as those supported bynaturally occurring organic Fe-binding ligands in seawater, and the

Table 7Trace element residence times (days).

Station Mixed Layerb PPZb

Fe Mna Cua Pba Fe Mna Cua Pba

DI HAc DI HAc DI HAc DI HAc DI HAc DI HAc DI HAc DI HAc

1 9.4 6.5 213 213 37 37 203 183 47 21 363 363 78 78 12 127 115 12 380 377 189 183 259 228 11 8.6 127 128 288 281 278 2609 8585 545 197 197 449 448 198 196 35 34 247 247 167 167 323 31611 337 134 235 235 428 426 413 410 178 139 428 428 667 666 616 59113 277 136 300 300 38 39 245 231 179 154 296 296 574 573 338 33115 183 137 279 279 322 321 499 467 169 150 569 569 389 388 545 52618 2730 173 89 89 3680 3180 575 576 17,800 1130 757 756 522 520 2500 218023 4770 303 337 337 5610 5050 1060 1050 980 460 601 601 22,890 20,580 2590 307026 7280 463 680 678 5610 5090 2880 1740 22,700 1440 2070 2060 13,200 13,200 11,340 594030 3410 217 560 559 4510 4195 5430 2850 7150 450 1080 1080 8870 8250 8390 469032 1420 318 572 571 5000 4630 5405 2830 7760 775 1180 1170 9610 8890 9630 504034 113 53 595 590 3840 3160 1370 937 2410 450 1680 1680 8730 15,435 7650 401036 71 38 294 293 4540 3750 1920 1005 1610 455 1600 1590 13,740 12,590 6520 3420

a Based on horizontal mixed layer velocity of 0.1 m/sb For each element residence times are shown based on the DI and HAc solubility.

Fig. 8. The residence time of dissolved Mn plotted against distance from station 1 for the mixed layer and the PPZ across the EPZT, using both DI and HAc aerosolsolubilities.

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conditions particles experience during passage through a zooplanktongut or inside a fecal pellet or organic aggregate (Shelley et al., 2018).

In the westernmost stations, moving out of the gyre, atmosphericinput increases, and stratification is relaxed resulting in enhanced tur-bulent diffusion. These factors contribute to shorter dissolved Fe re-sidence times for stations 34 and 36. Using the DI solubility, these are113 and 71 days, respectively, and with the HAc solubility, 53 and38 days.

For the PPZ, using the DI solubility, the Fe residence times for thenearshore stations 1,7, and 9 range from 11 to 47 days, and average31 ± 18 days. Using the HAc solubility, they range from 9 to 34 days,and average 21 ± 13 days. These residence times are similar because ofthe dominance of the upwelling and diffusive terms compared to theatmospheric input. Further offshore, the residence time increases forstations 11, 13, and 15, averaging 175 ± 6 days using the DI solubilityand 148 ± 8 days using the HAc solubility. For the stratified mid-gyrestations 18–32 the average residence time increases to 30 ± 24 y usingthe DI solubility and to 2.3 ± 1.2 y using the HAc solubility. Movingout of the gyre, the residence times for stations 34 and 36 are 6.6 and

4.4 y, respectively using the DI solubility. Both stations have a re-sidence time of 1.2 y with the HAc leach. As discussed above, the HAcsolubilities appear to be more appropriate for Fe in these calculations.

3.2.1.3. Copper. The HAc solubility is approximately twice that basedon the DI leach, but the soluble atmospheric flux across the transect isrelatively small compared to other inputs into the mixed layer,contributing <1% to 16% of the total Cu input into the mixed layerusing the DI solubility, and < 1% to 31% with the HAc solubility,increasing in both cases towards the western edge of the transect. Thusthe choice of aerosol leach solubility is not critical in these calculations(Table 7, Fig. 11).

The average mixed layer residence time of Cu for nearshore stations1–15 is 240 ± 185 days using both the DI and HAc solubilities. For thestratified mid-gyre stations 18–36 the average residence time increasesto 12.8 ± 2.1 y using the DI solubility, which is not significantly greaterthan the 11.4 ± 2.3 y using the HAc solubility. The increase of re-sidence time within the gyre reflects diminished input from upwarddiffusion and a smaller advective input term (smaller gradient, Fig. 6A).

Fig. 9. The relationship of the residence time of dissolved Mn to surface horizontal current velocity for A) the mixed layer and B) the PPZ. Station numbers areindicated.

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For comparison, Bruland (1980) derived a dissolved Cu surface re-sidence time with respect to atmospheric deposition of 10 years, andBoyle et al. (1977) estimated 2.1–50 years with respect to scavenging,both for surface waters of the North Pacific.

For the PPZ, there is generally a similar pattern, with the nearshorestations 1–15 manifesting an average Cu residence time of360 ± 230 days, using both solubilities. For stations 18–36, using theDI solubility, the residence times range from 1.4 to 63 y, averaging30 ± 19y. For the HAc solubility, the residence times range from 1.4 to56 y, averaging 28 ± 17y.

The Cu concentrations at stations 1–9 were the highest along thetransect. High concentration suggests that net upwelling flux (Eq. (6)) isnot the dominant source of Cu into the mixed layer and the PPZ, as Cu isremoved by outflow (Fig. S1). This implies biogeochemical removalrates cannot keep up with the upwelling input, and/or there are addi-tional, unaccounted input sources. The concentration of Cu ligands atthe nearshore stations were also the highest along the EPZT transect,

and the stability constant for Cu-binding ligands were an order ofmagnitude greater than those of stations further offshore (Boiteau et al.,2016). It has been suggested that the ligands are derived from shelfsediments and upwelled into the surface waters (Boiteau et al., 2016). Aquestion then is the role of these ligands in the Cu cycling of the area.

3.2.1.4. Lead. The concentrations of aerosol Pb across the EPZT weretoo low to allow reliable determination of solubilities. We use insteadthe aerosol solubilities of marine background samples presented inShelley et al. (2018). Average DI and HAc solubilities were respectively,33% and 63%. The average mixed layer residence time of Pb fornearshore stations 1–15 was 303 ± 124 days and 286 ± 121 days usingthe DI and HAc solubilities respectively (Table 7, Fig. 12). Upwellingand horizontal advection were the dominant input components forthese stations. Moving out to the open ocean gyre stations 18–32, thetotal input of Pb to the upper ocean was small compared to the inshorestations (Tables 5a and 5b) with atmospheric input generally dominant.

Fig. 10. The residence time of dissolved Fe plotted against distance from station 1 for the mixed layer and the PPZ across the EPZT, using both DI and HAc aerosolsolubilities.

Fig. 11. The residence time of dissolved Cu plotted against distance from station 1 for the mixed layer and the PPZ across the EPZT, using both DI and HAc aerosolsolubilities.

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With the HAc solubility, average residence times were 5.0 ± 2.8 ywhich is consistent with estimates of 2–3 y presented elsewhere(Bruland, 1980; Nozaki et al., 1976). With the DI solubility theaverage residence time was 8.4 ± 6.3 y. For the western edgestations 34 and 36, the atmospheric input increases, and the averageresidence time was 2.7 ± 0.1 y and 4.5 ± 1.1 y, respectively for theHAc and DI solubilities.

For the PPZ, there is generally a similar pattern, with the residencetime of Pb for nearshore stations 1–15 manifesting an average of352 ± 214 days and 339 ± 206 days using the DI and HAc solubilitiesrespectively. The residence times increase to an average of 18.9 ± 11.2y for offshore stations 18–32 using the DI solubility and 11.5 ± 4.2 yusing the HAc solubility. For the western edge stations 34 and 36, theaverage residence time was 19.4 ± 2.1 y and 10.1 ± 1.1 y respectively,for the HAc and DI solubilities.

3.2.1.5. Cadmium and vanadium. The relevant information for theseelements are shown in Supplemental Tables 1–3. The cadmiumresidence time pattern for the DI and HAc solubilities are similar(Supplementary Fig. S6) and resembles that of Mn. Considering just theHAc solubilities, there are short residence times over the nearshorestations 1–15, (44 ± 45 d, ML and 71 ± 124 d, PPZ) and longerresidence times for offshore stations 18–30, (209 ± 170 d, ML and619 ± 398 d, PPZ). For the western edge stations 34 and 36 the averageresidence times shorten again (57 ± 21 d, ML and 317 ± 25 d, PPZ) aswas observed for other elements. The residence times for V are shownin Fig. S7. The pattern is similar to other elements discussed here, withgenerally low residence times near shore, increasing in the central gyreand decreasing towards to western edge of the transect.

4. Conclusions

The TE fluxes from atmospheric deposition, vertical mixing, andupwelling into the mixed layer and the PPZ along the GEOTRACESEPZT were evaluated with 7Be-based methods developed in earlierworks. Horizontal advection velocity from east to west, driven by theSouth Equatorial Current, was estimated from literature values, andwhen horizontal gradients in the TE distributions were observed, fluxesdue to horizontal advection were determined. Atmospheric depositionis lowest in the central gyre, with higher fluxes to the east due to largenear-shore aerosol TE loads, and higher to the west due to higher

deposition velocities (Vb) driven by greater precipitation. The 7Be-de-rived Kz values range from 2.5 to 39 m2/d (0.29 × 10−4 to4.5 × 10−4 m2/s) with higher values generally within the nearshoreupwelling region and the lowest values within the stratified centralgyre. Mn displayed a well-defined gradient extending from the near-shore stations into the central gyre. Even with a modest choice ofhorizontal velocity the advective term was a major component of thetotal input flux, particularly within the central gyre. Relative to otherinputs the atmospheric input of soluble Mn was minor. Unlike man-ganese, there was no discernable horizontal gradient in the Fe con-centration field and therefore there is no horizontal component of flux.Nearshore removal processes were more intense for dissolved Fe thanfor dissolved Mn and as a result, dissolved Mn remained elevated muchfarther offshore than dissolved Fe. For the stratified mid-ocean gyrestations upward turbulent diffusion of Fe was limited and atmosphericdeposition became dominant, although the total input from all sourceswas relatively small compared to the inshore stations. Aerosol Fe so-lubility determined by a 25% acetic acid leach with hydroxylaminehydrochloride was much greater than that derived from a leach usingultrapure deionized water. This led to significant differences in theresidence time of Fe calculated for the mid-ocean gyre depending on theaerosol fractional solubility chosen for the calculation. For Mn, Cu, Cd,and Pb, there is a relatively steep gradient in mixed layer concentra-tions leaving the coastal zone.

The residence times of the TEs calculated with the measured watercolumn inventories and the input fluxes discussed above reflected thesignificantly different physical and geochemical conditions encounteredacross the zonal transect. Generally, each element displayed relativelyshort (weeks–months) residence times within the nearshore region ofintense upwelling, reflecting large input terms and rapid removal.Moving offshore, total input fluxes decreased and the residence times ofthe TEs increased markedly until the western edge of the transect.There, relaxtion of ocean stratification permited greater upward tur-bulent flux and greater rainfall led to greater atmospheric input of thetrace elements. These results suggest that one size does not fit all whenconsidering ocean residence times as there is a range of scavengingremoval pathways for different elements as well as natural gradients inoceanic fluxes. Regional difference should be expected.

As discussed in Twining and Baines (2013), trace metals can influ-ence the growth and structure of natural phytoplankton communitiesand, conversely, the composition and structure of phytoplankton

Fig. 12. The residence time of dissolved Pb plotted against distance from station 1 for the mixed layer and the PPZ across the EPZT, using both DI and HAc aerosolsolubilities.

D. Kadko, et al. Marine Chemistry 222 (2020) 103781

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communities influence the distribution of metals in the ocean. Themetal contents of phytoplankton reflect biochemical demands and en-vironmental availability. The ambient trace metal distributions reflectthe complex interplay between external sources, microbial uptake andrecycling, and physical mixing. Studies of the sources and residencetimes of these elements across diverse oceanic settings, as discussedhere, will provide further insight into the relationship between traceelement abundance and phytoplankton community structure.

Acknowledgements

The authors thank the captain and crew of R/V Thomas G.Thompson, and the Chief Scientists Dr. Chris German and Dr. JimMoffett for their support. Dr. Mark Stephens provided important at-seaand laboratory technical assistance. Comments provided by two anon-ymous reviewers are appreciated. Data has been submitted to TheBiological and Chemical Oceanography Data Management Office (BCO-DMO) and is accessible at: http://www.bco-dmo.org/dataset/665158.This work was supported by the Chemical Oceanography Program ofthe NSF, grants OCE-1232794 (DK), OCE-1454368 (CB), OCE-1234417(AAI), OCE-1234646 (WML) and OCE 1233502 (KWB).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.marchem.2020.103781.

References

Aguilar-Islas, A.M., Wu, J., Rember, R., Johansen, A.M., Shank, L.M., 2010. Dissolution ofaerosol-derived iron in seawater: leach solution chemistry, aerosol type, and colloidaliron fraction. Mar. Chem. 120, 25–33. https://doi.org/10.1016/j.marchem.2009.01.011.

Anderson, R.F., Cheng, H., Edwards, R.L., Fleisher, M.Q., Hayes, C.T., Huang, K.-F.,Kadko, D., Lam, P.J., Landing, W.M., Lao, Y., Lu, Y., Measures, C.I., Morton, P.L.,Moran, S.B., Ohnemus, D.C., Robinson, L.F., Shelley, R.U., 2016. How well can wequantify dust deposition to the ocean? Phil. Trans. R. Soc. A 374. https://doi.org/10.1098/rsta.2015.0285.

Berger, C.J.M., Lippiatt, S.M., Lawrence, M.G., Bruland, K.W., 2008. Application of achemical leach technique for estimating labile particulate aluminum, iron, andmanganese in the Columbia River plume and coastal waters off Oregon andWashington. J. Geophys. Res. 113https://doi.org/10.1029/2007JC004703. C00B01.

Bergquist, B.A., Boyle, E.A., 2006. Dissolved iron in the tropical and subtropical AtlanticOcean. Glob. Biogeochem. Cycles 20, GB1015. https://doi.org/10.1029/2005GB002505.

Biller, D.V., Bruland, K.W., 2012. Analysis of Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb in sea-water using the Nobias-chelate PA1 resin and magnetic sector inductively coupledplasma mass spectrometry (ICP-MS). Mar. Chem. 130–131, 12–20.

Black, E.E., Lam, P.J., Lee, J.-M., Buesseler, K.O., 2019. Insights from the 238U-234Thmethod into the coupling of biological export and the cycling of cadmium, cobalt,and manganese in the Southeast Pacific Ocean. Glob. Biogeochem. Cycles 33, 15–36.https://doi.org/10.1029/2018GB005985.

Boiteau, R.M., Till, C.P., Ruacho, A., Bundy, R.M., Hawco, N.J., AM, McKenna, Barbeau,K.A., Bruland, K.W., Saito, M.A., Repeta, D.J., 2016. Structural characterization ofnatural nickel and copper binding ligands along the US GEOTRACES Eastern PacificZonal Transect. Front. Mar. Sci. 3, 243.

Boyle, E.A., Sclater, F.R., Edmond, J.M., 1977. The distribution of dissolved copper in thePacific. Earth Planet. Sci. Lett. 37, 38–54.

Boyle, E., Bergquist, B., Kayser, R., Mahowald, N., 2005. Iron, manganese and lead atHawaii Ocean Time-Series Station ALOHA: temporal variability and intermediatewater hydrothermal plume. Geochim. Cosmochim. Acta 69, 933–952.

Bridgestock, L., van de Flierdt, T.V., Rehkamper, M., Paul, M., Middag, R., Milne, A.,Lohan, M.C., Baker, A.R., Chance, R., Khondoker, R., Strekopytov, S., Humphreys-Williams, E., Achterberg, E.P., Rijkenberg, M.J.A., Gerringa, L.J.A., de Baar, H.J.W.,2016. Return of naturally sourced Pb to Atlantic surface waters. Nat. Commun. 7.https://doi.org/10.1038/ncomms12921.

Bruland, K., 1980. Oceanographic distributions of cadmium, zinc, nickel, and copper inthe north Pacific. Earth Planet. Sci. Lett. 47, 176–198.

Buck, C.S., Landing, W.M., Resing, J.A., Lebon, G.T., 2006. Aerosol iron and aluminumsolubility in the northwest Pacific Ocean: results from the 2002 IOC Cruise. Geochem.Geophys. Geosyst. 7https://doi.org/10.1029/2005GC000977. Q04M07.

Buck, C.S., Aguilar-Islas, A., Marsay, C., Kadko, D., Landing, W.M., 2019. Trace elementconcentrations, elemental ratios, and enrichment factors observed in aerosol samplescollected during the US GEOTRACES eastern Pacific Ocean transect (GP16). Chem.Geol. 511, 212–224.

Carr, M.-E., 2001. Estimation of potential productivity in Eastern Boundary Currentsusing remote sensing. Deep-Sea Res. II Top. Stud. Oceanogr. 49, 59–80.

Claustre, H., Sciandra, A., Vaulot, D., 2008. Introduction to the special section bio-opticaland biogeochemical conditions in the South East Pacific in late 2004: the BIOSOPEprogram. Biogeosciences 5, 679–691.

Conway, T.M., Hamilton, D.S., Shelley, R.U., Aguilar-Islas, A.M., Landing, W.M.,Mahowald, N.M., John, S.G., 2019. Tracing and constraining anthropogenic aerosoliron fluxes to the North Atlantic Ocean using iron isotopes. Nat. Commun. 10 (1).https://doi.org/10.1038/s41467-019-10457-w.

Croot, P.L., Streu, P., Baker, A.R., 2004. Short residence time for iron in surface seawaterimpacted by atmospheric dry deposition from Saharan dust events. Geophys. Res.Lett. 31. https://doi.org/10.1029/2004GL020153.

Cutter, G.A., Bruland, K.W., 2012. Rapid and noncontaminating sampling system for traceelements in global ocean surveys. Limnol. Oceanogr. Methods 10, 425–436.

Cutter, G.A., Moffett, J.W., Nielsdóttir, M.C., Sanial, V., 2018. Multiple oxidation statetrace elements in suboxic waters off Peru: in situ redox processes and advective/diffusive horizontal transport. Mar. Chem. 201, 77–89.

Fitzsimmons, J.N., Conway, T.M., Lee, J.-M., Kayser, R., Thyng, K.M., John, S.G., Boyle,E.A., 2016. Dissolved iron and iron isotopes in the Southeastern Pacific Ocean. Glob.Biogeochem. Cycles 30.

Fitzsimmons, J.N., John, S.G., Marsay, C.M., Hoffman, C.L., Nicholas, S.L., Toner, B.M.,German, C.R., Sherrell, R.M., 2017. Iron persistence in a distal hydrothermal plumesupported by dissolved-particulate exchange. Nat. Geosci. 10 (3), 195.

GEOTRACES_Planning_Group, 2006. Science Plan, Scientific Committee on OceanicResearch. Baltimore, Maryland.

Haskell, W.Z., Kadko, D., Hammond, D.E., Knapp, A.N., Prokopenko, M.G., Berelson,W.M., Capone, D.G., 2015. Upwelling velocity and eddy diffusivity from 7Be mea-surements used to compare vertical nutrient flux to export POC flux in the EasternTropical South Pacific. Mar. Chem. 168, 140–150.

Hayes, C.T., Fitzsimmons, J.N., Boyle, E.A., McGee, D., Anderson, R.F., Weisend, R.,Morton, P.L., 2015. Thorium isotopes tracing the iron cycle at the Hawaii OceanTime-series Station ALOHA. Geochim. Cosmochim. Acta 169, 1–16. https://doi.org/10.1016/j.gca.2015.07.019.

Ho, P., Lee, J., Heller, M., Lam, P., Shiller, A.M., 2018. The distribution of dissolved andparticulate Mo and V along the U.S. GEOTRACES East Pacific Zonal Transect (GP16):the roles of oxides and biogenic particles in their distributions in the oxygen deficientzone and the hydrothermal plume. Mar. Chem. 201, 242–255. https://doi.org/10.1016/j.marchem.2017.12.003.

Ito, A., Myriokefalitakis, S., Kanakidou, M., Mahowald, N., Scanza, R., Hamilton, D.,Baker, A., Jickells, T., Sarin, M., Bikkina, S., Gao, Y., Shelley, R., Buck, C.S., Landing,W., Bowie, A., Perron, M., Guieu, C., Meskhidze, N., Johnson, M., Feng, Y., Kok, J.,Nenes, A., Duce, R., 2019. Pyrogenic iron: The missing link to high iron solubility inaerosols. Sci. Adv. 5 (5). https://doi.org/10.1126/sciadv.aau7671. eaau7671.

Janssen, D.J., Conway, T.M., John, S.G., Christian, J.R., Kramer, D.I., Pedersen, T.F.,Cullen, J.T., 2014. Undocumented water column sink for cadmium in open oceanoxygen-deficient zones. Proc. Natl. Acad. Sci. U. S. A. 111, 6888–6893. https://doi.org/10.1073/pnas.1402388111.

Jickells, T.D., 1999. The inputs of dust derived elements to the Sargasso Sea; a synthesis.Mar. Chem. 68, 5–14.

John, S.G., Helgoe, J., Townsend, E., Weber, T., DeVries, T., Tagliabue, A., Moore, K.,Lam, P., Marsay, C.M., Till, C., 2018. Biogeochemical cycling of Fe and Fe stableisotopes in the Eastern Tropical South Pacific. Mar. Chem. 201, 66–76.

Kadko, D., 2017. Upwelling and primary production during the U.S. GEOTRACES EastPacific zonal transect. Glob. Biogeochem. Cycles 31. https://doi.org/10.1002/2016GB005554.

Kadko, D., Johns, W., 2011. Inferring upwelling rates in the equatorial Atlantic using 7Bemeasurements in the upper ocean. Deep-Sea Res. I 58, 247–257. https://doi.org/10.1016/j.dsr.2011.03.004.

Kadko, D., Olson, D., 1996. Be-7 as a tracer of surface water subduction and mixed layerhistory. Deep-Sea Res. 43, 89–116.

Kadko, D., Prospero, J., 2011. Deposition of 7Be to Bermuda and the regional ocean:environmental factors affecting estimates of atmospheric flux to the ocean. J.Geophys. Res. 116, C02013. https://doi.org/10.1029/2010JC006629.

Kadko, D., Landing, W.M., Shelley, R.U., 2015. A novel tracer technique to quantify theatmospheric flux of trace elements to remote ocean regions. J. Geophys. Res. Oceans119. https://doi.org/10.1002/2014JC010314.

Kadko, D., Galfond, B., Landing, W.M., Shelley, R.U., 2016. Determining the pathways,fate, and flux of atmospherically derived trace elements in the Arctic Ocean/icesystem. Mar. Chem. 182, 38–50. https://doi.org/10.1016/j.marchem.2016.04.006.

Kadko, D., Aguilar-Islas, A., Bolt, C., Buck, C.S., Fitzsimmons, J.N., Jensen, L.T., Landing,W.M., Marsay, C.M., Rember, R., Shiller, A.M., Whitmore, L.M., Anderson, R.F., 2019.The residence times of trace elements determined in the surface Arctic Ocean duringthe 2015 US Arctic GEOTRACES expedition. Mar. Chem. 208, 56–69. https://doi.org/10.1016/j.marchem.2018.10.011.

Karstensen, J., Stramma, L., Visbeck, M., 2008. Oxygen minimum zones in the easterntropical Atlantic and Pacific oceans. Prog. Oceanogr. 77, 331–350.

Kim, G., Alleman, L., Church, T., 1999. Atmospheric depositional fluxes of trace elements,210Pb and 7Be into the Sargasso Sea. Glob. Biogeochem. Cycles 13, 1183–1192.

Krishnaswami, S., Lal, D., Somayajulu, B.L.K., Dixon, F.S., Stonecipher, S.A., Craig, H.,1972. Silicon, radium, thorium and lead in seawater: in-situ extraction by syntheticfibre. Earth Planet. Sci. Lett. 16, 84–90.

Lal, D., Chung, Y., Platt, T., Lee, T., 1988. Twin cosmogenic radiotracer studies ofphosphorus recycling and chemical fluxes in the upper ocean. Limnol. Oceanogr. 336,1559–1567.

Landing, W.M., Bruland, K.W., 1987. The contrasting biogeochemistry of iron and man-ganese in the Pacific Ocean. Geochim. Cosmochim. Acta 51 (1), 29–43.

Lee, T., Barg, E., Lal, D., 1991. Studies of vertical mixing in the Southern California Bightwith cosmogenic radionuclides 32P and 7Be. Limnol. Oceanogr. 365, 1044–1053.

D. Kadko, et al. Marine Chemistry 222 (2020) 103781

16

Lee, J.-M., Boyle, E.A., Echegoyen-Sanz, Y., Fitzsimmons, J.N., Zhang, R., Kayser, R.A.,2011. Analysis of trace metals (Cu, Cd, Pb, and Fe) in seawater using single batchNitrilotriacetate resin extraction and isotope dilution inductively coupled plasmamass spectrometry. Anal. Chim. Acta 686, 93–101.

Lee, J.-M., Heller, M.I., Lam, P.J., 2018. Size distribution of particulate trace elements inthe U.S. GEOTRACES Eastern Pacific Zonal Transect (GP16). Mar. Chem. 201,108–123.

Lumpkin, R., Johnson, G.C., 2013. Global Ocean surface velocities from drifters : mean,variance, El Nino–Southern Oscillation response, and seasonal cycle. J. Geophys. Res.Oceans 118, 2992–3006. https://doi.org/10.1002/jgrc.20210.

Marsay, C.M., Kadko, D., Landing, W.M., Morton, P.L., Summers, B.A., Buck, C.S., 2018.Concentrations, provenance and flux of aerosol trace elements during USGEOTRACES Western Arctic cruise GN01. Chem. Geol. https://doi.org/10.1016/j.chemgeo.2018.06.007.

Martin, J.H., Knauer, G.A., 1980. Manganese cycling in Northeast Pacific waters. EarthPlanet. Sci. Lett. 51, 266–274.

Martin, J.H., Coale, K.H., Johnson, K.S., Fitzwater, S.E., Gordon, R.M., Tanner, S.J.,Hunter, C.N., Elrod, V.A., Nowicki, J.L., Coley, T.L., Barber, R.T., Lindley, S., Watson,A.J., Van Scoy, K., Law, C.S., Liddicoat, M.I., Ling, R., Stanton, T., Stockel, J., Collons,C., Anderson, A., Bidigare, R.R., Ondrusek, M., Latasa, M., Millero, F.J., Lee, K., Yao,W., Zhang, J.Z., Friederich, G.E., Sakamato, C., Chavez, F.P., Buck, K., Kolber, Z.,Greene, R., Falkowski, P.G., Chisholm, S.W., Hoge, F., Swift, R., Yungel, J., Turner, S.,Nightingale, P.D., Hatton, A., Liss, P.S., Tindale, N.W., 1994. Testing the iron hy-pothesis in ecosystems of the equatorial Pacific Ocean. Nature 371, 123–129.

Masumoto, Y., Sasaki, H., Kagimoto, T., Komori, N., et al., 2004. A fifty-year eddy-re-solving simulation of the world ocean–preliminary outcomes of OFES (OGCM for theEarth Simulator). J. Earth Simul. 1, 35–56.

Millero, F.J., Sotolongo, S., Izaguirre, M., 1987. The oxidation kinetics of Fe(II) in sea-water. Geochim. Cosmochim. Acta 51, 793–801.

Moffett, J.W., German, C.R., 2018. The US GEOTRACES Eastern tropical Pacific transect(GP16). Mar. Chem. 201, 1–5. https://doi.org/10.1016/j.marchem.2017.12.001.

Nozaki, Y., Thomson, J., Turekian, K.K., 1976. The distribution of 210Pb and 210Po in thesurface waters of the Pacific Ocean. Earth Planet. Sci. Lett. 32, 304–312.

Ohnemus, D.C., Rauschenberg, S., Cutter, G.A., Fitzsimmons, J.N., Sherrell, R.M.,Twining, B.S., 2016. Elevated trace metal content of prokaryotic communities asso-ciated with marine oxygen deficient zones. Limnol. Oceanogr. 62, 3–25. https://doi.org/10.1002/lno.10363.

Olsen, C.R., Larsen, I.L., Lowry, P.D., Cutshall, N.H., Todd, J.F., 1985. Atmospheric fluxesand marsh-soil inventories of 7Be and 210Pb. J. Geophys. Res. 90 (10), 487–495.

Owens, S.A., Pike, S., Buesseler, K.O., 2015. Thorium- 234 as a tracer of particle dynamicsand upper ocean export in the Atlantic Ocean. Deep-Sea Res., Part II 116, 42–59.https://doi.org/10.1016/j.dsr2.2014.11.010.

Parker, C., Brown, M., Bruland, K., 2016. Scandium in the open ocean: a comparison withother group 3 trivalent metals. Geophys. Res. Lett. 43, 2758–2764.

Peng, A., Liu, G., Jiang, Z., Liu, G., Liu, M., 2019. Wet depositional fluxes of 7Be and210Pb and their influencing factors at two characteristic cities of China. Appl. Radiat.Isot. 147, 21–30.

Plattner, G.-K., Gruber, N., Frenzel, H., McWilliams, J.C., 2005. Decoupling marine exportproduction from new production. Geophys. Res. Lett. 32, L11612. https://doi.org/10.1029/2005GL022660.

Rashan, S., Wu, J., 2018. Dissolved and colloidal copper in the tropical South Pacific.Geochim. Cosmochim. Acta 233, 81–94.

Resing, J.A., Sedwick, P.N., German, C.R., Jenkins, W.J., Moffett, J.W., Sohst, B.M.,Tagliabue, A., 2015. Basin-scale transport of hydrothermal dissolved metals acrossthe South Pacific Ocean. Nature 523, 200–203. https://doi.org/10.1038/nature14577.

Sanial, V., Kipp, L.E., Henderson, P.B., van Beek, P., Reyss, J.-L., Hammond, D.E., Hawco,N.J., Saito, M.A., Resing, J.A., Sedwick, P., Moore, W.S., Charette, M.A., 2018.Radium-228 as a tracer of dissolved trace element inputs from the Peruvian con-tinental margin. Mar. Chem. 201, 20–34.

Schlitzer, R., Anderson, R.F., Dodas, E.M., Lohan, M., Geibert, W., Tagliabue, A., Bowie,A., Jeandel, C., Maldonado, M.T., Landing, W.M., Cockwell, D., Abadie, C.,Abouchami, W., Achterberg, E.P., Agather, A., Aguliar-Islas, A., van Aken, H.M.,Andersen, M., Archer, C., Auro, M., de Baar, H.J., Baars, O., Baker, A.R., Bakker, K.,Basak, C., Baskaran, M., Bates, N.R., Bauch, D., van Beek, P., Behrens, M.K., Black, E.,Bluhm, K., Bopp, L., Bouman, H., Bowman, K., Bown, J., Boyd, P., Boye, M., Boyle,E.A., Branellec, P., Bridgestock, L., Brissebrat, G., Browning, T., Bruland, K.W.,Brumsack, H.-J., Brzezinski, M., Buck, C.S., Buck, K.N., Buesseler, K., Bull, A., Butler,E., Cai, P., Mor, P.C., Cardinal, D., Carlson, C., Carrasco, G., Casacuberta, N.,Casciotti, K.L., Castrillejo, M., Chamizo, E., Chance, R., Charette, M.A., Chaves, J.E.,Cheng, H., Chever, F., Christl, M., Church, T.M., Closset, I., Colman, A., Conway,T.M., Cossa, D., Croot, P., Cullen, J.T., Cutter, G.A., Daniels, C., Dehairs, F., Deng, F.,Dieu, H.T., Duggan, B., Dulaquais, G., Dumousseaud, C., Echegoyen-Sanz, Y.,Edwards, R.L., Ellwood, M., Fahrbach, E., Fitzsimmons, J.N., Russell Flegal, A.,Fleisher, M.Q., van de Flierdt, T., Frank, M., Friedrich, J., Fripiat, F., Fröllje, H.,Galer, S.J.G., Gamo, T., Ganeshram, R.S., Garcia-Orellana, J., Garcia-Solsona, E.,Gault-Ringold, M., George, E., Gerringa, L.J.A., Gilbert, M., Godoy, J.M., Goldstein,

S.L., Gonzalez, S.R., Grissom, K., Hammerschmidt, C., Hartman, A., Hassler, C.S.,Hathorne, E.C., Hatta, M., Hawco, N., Hayes, C.T., Heimbürger, L.-E., Helgoe, J.,Heller, M., Henderson, G.M., Henderson, P.B., van Heuven, S., Ho, P., Horner, T.J.,Hsieh, Y.-T., Huang, K.-F., Humphreys, M.P., Isshiki, K., Jacquot, J.E., Janssen, D.J.,Jenkins, W.J., John, S., Jones, E.M., Jones, J.L., Kadko, D.C., Kayser, R., Kenna, T.C.,Khondoker, R., Kim, T., Kipp, L., Klar, J.K., Klunder, M., Kretschmer, S., Kumamoto,Y., Laan, P., Labatut, M., Lacan, F., Lam, P.J., Lambelet, M., Lamborg, C.H., LeMoigne, F.A.C., Le Roy, E., Lechtenfeld, O.J., Lee, J.-M., Lherminier, P., Little, S.,López-Lora, M., Lu, Y., Masque, P., Mawji, E., McClain, C.R., Measures, C., Mehic, S.,Barraqueta, J.-L.M., van der Merwe, P., Middag, R., Mieruch, S., Milne, A., Minami,T., Moffett, J.W., Moncoiffe, G., Moore, W.S., Morris, P.J., Morton, P.L., Nakaguchi,Y., Nakayama, N., Niedermiller, J., Nishioka, J., Nishiuchi, A., Noble, A., Obata, H.,Ober, S., Ohnemus, D.C., van Ooijen, J., O’Sullivan, J., Owens, S., Pahnke, K., Paul,M., Pavia, F., Pena, L.D., Peters, B., Planchon, F., Planquette, H., Pradoux, C.,Puigcorbé, V., Quay, P., Queroue, F., Radic, A., Rauschenberg, S., Rehkämper, M.,Rember, R., Remenyi, T., Resing, J.A., Rickli, J., Rigaud, S., Rijkenberg, M.J.A.,Rintoul, S., Robinson, L.F., Roca-Martí, M., Rodellas, V., Roeske, T., Rolison, J.M.,Rosenberg, M., Roshan, S., Rutgers van der Loeff, M.M., Ryabenko, E., Saito, M.A.,Salt, L.A., Sanial, V., Sarthou, G., Schallenberg, C., Schauer, U., Scher, H., Schlosser,C., Schnetger, B., Scott, P., Sedwick, P.N., Semiletov, I., Shelley, R., Sherrell, R.M.,Shiller, A.M., Sigman, D.M., Singh, S.K., Slagter, H.A., Slater, E., Smethie, W.M.,Snaith, H., Sohrin, Y., Sohst, B., Sonke, J.E., Speich, S., Steinfeldt, R., Stewart, G.,Stichel, T., Stirling, C.H., Stutsman, J., Swarr, G.J., Swift, J.H., Thomas, A., Thorne,K., Till, C.P., Till, R., Townsend, A.T., Townsend, E., Tuerena, R., Twining, B.S.,Vance, D., Velazquez, S., Venchiarutti, C., Villa-Alfageme, M., Vivancos, S.M.,Voelker, A.H.L., Wake, B., Warner, M.J., Watson, R., van Weerlee, E., AlexandraWeigand, M., Weinstein, Y., Weiss, D., Wisotzki, A., Woodward, E.M.S., Wu, J., Wu,Y., Wuttig, K., Wyatt, N., Xiang, Y., Xie, R.C., Xue, Z., Yoshikawa, H., Zhang, J.,Zhang, P., Zhao, Y., Zheng, L., Zheng, X.-Y., Zieringer, M., Zimmer, L.A., Ziveri, P.,Zunino, P., Zurbrick, C., 2018. The GEOTRACES intermediate data product 2017.Chem. Geol. 493, 210–223.

Scholz, F., Hensen, C., Noffke, A., Rohde, A., Liebetrau, V., Wallmann, K., 2011. Earlydiagenesis of redox-sensitive trace metals in the Peru upwelling area—response toENSO-related oxygen fluctuations in the water column. Geochim. Cosmochim. Acta75 (22), 7257–7276. https://doi.org/10.1016/j. gca. 2011.08.007.

Scholz, F., Severmann, S., McManus, J., Hensen, C., 2014. Beyond the Black Sea para-digm: the sedimentary fingerprint of an open-marine iron shuttle. Geochim.Cosmochim. Acta 127, 368–380.

Shelley, R.U., Roca-Martí, M., Castrillejo, M., Sanial, V., Masqué, P., Landing, W.M., vanBeek, P., Planquette, H., Sarthou, G., 2017. Quantification of trace element atmo-spheric deposition fluxes to the Atlantic Ocean (> 40 N; GEOVIDE, GEOTRACESGA01) during spring 2014. Deep-Sea Res. I 119, 34–49. https://doi.org/10.1016/j.dsr.2016.11.010.

Shelley, R.U., Landing, W.M., Ussher, S.J., Planquette, H., Sarthou, G., 2018. Regionaltrends in the fractional solubility of Fe and other metals from North Atlantic aerosols(GEOTRACES cruises GA01 and GA03) following a two-stage leach. Biogeosciences15, 2271–2288.

Shiller, A.M., 1997. Manganese in surface waters of the Atlantic Ocean. Geophys. Res.Lett. 24, 1495–1498.

Sholkovitz, E.R., Sedwick, P.N., Church, T.M., Baker, A.R., Powell, C.F., 2012. Fractionalsolubility of aerosol iron: Synthesis of aglobal-scale data set. Geochim. Cosmochim.Acta 89, 173–189. https://doi.org/10.1016/j.gca.2012.04.022.

Takesue, R.K., van Geen, A., Carriquiry, J.D., Ortiz, E., Godı’nez-Orta, L., Granados, I.,Saldı’var, M., Ortlie, L., Escribano, R., Guzman, N., Castilla, J.C., Varas, M.,Salamanca, M., Figueroa, C., 2004. Influence of coastal upwelling and ElNin˜o–Southern Oscillation on nearshore water along Baja California and Chile:shore-based monitoring during 1997–2000. J. Geophys. Res. 109, C03009. https://doi.org/10.1029/2003JC001856.

Twining, B.S.B., Baines, S.B.S., 2013. The trace metal composition of marine phyto-plankton. Annu. Rev. Mar. Sci. 5, 191–215. https://doi.org/10.1146/annurev-marine-121211-172322.

Uematsu, M., Duce, R.A., Prospero, J.M., 1994. Atmosphere beryllium-7 concentrationsover the Pacific Ocean. Geophys. Res. Lett. 21, 561–564.

Wagener, T., Guieu, C., Losno, R., Bonnet, S., Mahowald, N., 2008. Revisiting atmosphericdust export to the Southern Hemisphere Ocean: biogeochemical implications. Glob.Biogeochem. Cycles 22, GB2006.

Wu, J., Roshan, S., Chen, G., 2014. The distribution of dissolved manganese in the tro-pical–subtropical North Atlantic during US GEOTRACES 2010 and 2011 cruises. Mar.Chem. 166, 9–24. https://doi.org/10.1016/j.marchem.2014.08.007.

Young, J.A., Silker, W.B., 1980. Aerosol deposition velocities on the Pacific and AtlanicOceans calculated from 7Be measurements. Earth Planet. Sci. Lett. 50, 92–104.https://doi.org/10.1016/0012-821X(80)90121-1.

Zheng, L., Minami, T., Konagaya, W., Chan, C.-Y., Tsujisaka, M., Takano, S., Norisuye, K.,Sohrin, Y., 2019. Distinct basin-scale-distributions of aluminum, manganese, cobalt,and lead in the North Pacific Ocean. Geochim. Cosmochim. Acta 254, 102–121.https://doi.org/10.1016/J. GCA. 2019. 03.038.

D. Kadko, et al. Marine Chemistry 222 (2020) 103781

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