High-resolution Al and Fe data from the Atlantic Ocean CLIVAR-CO2

Repeat Hydrography A16N transect: Extensive linkages between

atmospheric dust and upper ocean geochemistry

C. I. Measures,1 W. M. Landing,2 M. T. Brown,1,3 and C. S. Buck2

Received 21 June 2007; revised 19 October 2007; accepted 16 November 2007; published 5 February 2008.

[1] Trace element sampling and shipboard flow injection analysis during the June–August 2003 Climate Variability and Predictability (CLIVAR)-CO2 Repeat HydrographyA16N transect has produced a high-resolution section of dissolved Fe and Al in theupper 1000 m of the Atlantic Ocean between 62�N and 5�S. Using the surface waterdissolved Al and the Model of Aluminum for Dust Calculation in Oceanic Waters(MADCOW) model we have calculated the deposition of mineral dust to the surfaceocean along this transect and compare that to dissolved Fe concentrations. The lowest meanmineral dust depositions of�0.2 g m�2 a�1 are found to the north of 51�N; a region whichalso exhibits characteristics of biological Fe limitation through its low dissolved surfacewater Fe (�0.1 nM) and residual macronutrients, e.g., nitrate >2 mM. To the south of thisregion, mean dust deposition increases by an order of magnitude reaching�3 g m�2 a�1 at10�N, underneath the Saharan dust outflow. Surface water Fe values also increase along thissection to >1 nM. Distinct minima in Fe concentrations at the depth of the chlorophyllmaximum in the vertical profiles between 18 and 4�N illuminate the role that activebiological uptake plays in Fe cycling. An extensive subsurface zone of enhanceddissolved Fe concentrations (>1.5 nM) underlying this region is a result of the biologicalvertical transport and remineralization of the surface water Fe and is coincident with theintermediate nutrient maximum and oxygen minimum of this region. Elevatedconcentrations of dissolved Al in subsurface waters seen between 30 and 20�Ncoincide with the domain of the subtropical mode waters (STMW) which result from thesinking of surface waters in late winter in regions imprinted by dust deposition. Themagnitude of the Al enrichment observed in this water mass implies that the predominantsource to the STMW is from the more dust-impacted western Atlantic, with only limitedcontributions from the STMW formation region near Madeira. A deeper subsurfaceAl enrichment (30–45�N) is associated with the outflow from the Mediterranean,another heavily dust-impacted basin. These two regions of Al enrichment show thewidespread geochemical connection between atmospheric transport processes and theNorth Atlantic and underscore its susceptibility to imprinting by atmospherically bornematerials, natural as well as anthropogenic.

Citation: Measures, C. I., W. M. Landing, M. T. Brown, and C. S. Buck (2008), High-resolution Al and Fe data from the Atlantic

Ocean CLIVAR-CO2 Repeat Hydrography A16N transect: Extensive linkages between atmospheric dust and upper ocean

geochemistry, Global Biogeochem. Cycles, 22, GB1005, doi:10.1029/2007GB003042.

1. Introduction

[2] Understanding the linkages between atmospheric andoceanic processes is crucial to developing realistic global

models of climate feedback [Falkowski et al., 1998, 2000;Fung et al., 2000]. Nowhere is this need more apparent thanin the necessity to incorporate the factors that control theavailability of Fe in surface ocean waters, the lack of whichhas been shown to limit biological processes and carbonsequestration in large areas of the North, South and equa-torial Pacific Ocean [Martin and Fitzwater, 1988; Martin etal., 1990a]. It is well recognized that the atmosphericdelivery of continental mineral dust to, and its partialdissolution within, remote surface ocean waters is animportant vector of Fe delivery [Jickells et al., 2005].However, there are no actual dust deposition data over theopen ocean, only estimates extrapolated from adjacent land-

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1Department of Oceanography, University of Hawaii at Manoa,Honolulu, Hawaii, USA.

2Department of Oceanography, Florida State University, Tallahassee,Florida, USA.

3Now at Institute of Marine Sciences, University of California, SantaCruz, California, USA.

Copyright 2008 by the American Geophysical Union.0886-6236/08/2007GB003042$12.00

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based sampling sites [Duce et al., 1991]. Dissolved Alconcentrations in surface waters can be used to estimatedust deposition to the surface of the open ocean [Measuresand Brown, 1996; Measures and Vink, 2000], and these canthen be compared to dissolved Fe distributions to inferfluxes and to characterize processes.[3] However, our ability to investigate and model these

processes has been severely limited by our ability to obtaindata for these, and other trace elements, in sufficientquantity to resolve features and map out their geographicalextent. Part of the reason for the dearth of information is dueto the labor and time-intensive methods that have been usedhistorically to collect trace element samples at sea andwhich has made this work incompatible with the large-scalehydrography programs that provide global sampling oppor-tunities. We report here the first set of data from the NorthAtlantic, using a rosette-based sampling system for traceelements that was designed to enable the collection of ahigh-resolution section (approximately 1� spacing) of thedissolved trace elements Fe and Al in the upper 1000 m, aspart of the international Climate Variability and Predictabil-ity (CLIVAR)-CO2, Repeat Hydrography program duringthe A16N cruise.[4] The principal motivation for developing sections

specifically for Fe and Al is related to the apparent rolethat the availability of the micronutrient Fe in surface watersplays in moderating oceanic biological processes, and therole that atmospheric deposition of mineral dust in surfacewaters, traced by dissolved Al, plays in delivering Fe to thesurface waters of the remote oceans. Since the recognitionthat limited Fe availability in some oceanic surface watersmight be an important component of the glacial-interglacialcarbon dioxide feedback loop [Martin et al., 1990b], therehas been an impetus to understand the systematics ofoceanic Fe geochemistry, and to account for these processesin global models. In particular, this requires a better under-standing of how Fe, and other biogeochemically importanttrace elements enter the ocean through atmospheric deposi-tion processes. To accomplish this requires detailed traceelement sections across a wide range of oceanic biogeo-chemical regimes and atmospheric deposition gradients.The work reported here is the first CLIVAR trace elementcontribution to that global goal.

2. Methods

[5] The CLIVAR-CO2 Repeat Hydrography program’sA16N cruise was conducted aboard the NOAA researchvessel Ron Brown which left Reykjavik, Iceland on 19 June2003 and terminated in Natal, Brazil, on 12 August 2003.Details of the cruise track and ancillary data associated withthe program can be found at http://cchdo.ucsd.edu/data_access?ExpoCode=33RO200306_01.[6] Seawater samples were collected using 12L GO-FLO

bottles (General Oceanic) mounted on a conventional ro-sette frame, containing commercially available CTD andoxygen sensors (SeaBird SBE 911and SBE 42), and afluorometer (WetLabs FL1). The aluminum rosette framewas completely painted to eliminate most bare metal surfa-ces. The package was suspended on a 4 conductor Kevlar

cable (Cortland Cable), which passed through a Nylatronblock (General Oceanic) and was deployed using a SeaMacwinch with nylon rollers and level wind. To prevent traceelement contamination of the sampled water from the fewuncoated metal surfaces that remained, sample bottles wereclosed by electrical signal from the ship only while therosette frame was moving upward through the water columnat 5–10 m min�1 into water that had not been in contactwith the frame. A 12-depth profile in the upper 1000 m wasroutinely collected in approximately 1 h. Problems with theweight handling ability of the winch during the first leg ofthe cruise (Stations 5–78; 62 to 27�N) resulted in deploy-ment of only 10 or 11 bottles on the rosette, and samplingwas restricted to the upper 750 m. At Station 120, 6�N, asecond pattern of sampling depths was introduced. The twosampling patterns were then alternated to more closelymatch the sampling depths used by the main hydrographyCTD and also to improve the contouring of the TM datasets. Immediately after package recovery the 12 L bottleswere removed from the rosette frame and carried into a 20 ftlaboratory container van equipped with a HEPA filtered airsystem (Mac 10, ENVIRCO) where subsampling wascompleted under trace element clean conditions. A detaileddescription of the sampling system and its construction willbe provided elsewhere (C. I. Measures et al., A rosette systemfor trace metal clean sampling, submitted to LimnologyOceanography: Methods, 2007).[7] Seawater subsamples were filtered through 0.4 mm

47 mm polycarbonate track-etched filters (GE Poretics partnumber K04CP04700) held in a MFS polypropylene filterholder. Subsequent to this cruise, the combined rosette andsubsampling scheme was compared with other Fe samplingmethods, during the NSF-sponsored SaFe intercalibrationcruise (October 2004) and was found to produce samplesthat were essentially identical to those from other samplingsystems [Johnson et al., 2007].[8] Subsamples were run on board ship within 24 h of

collection using the flow injection analysis (FIA) methodsfor dissolved Al and Fe [Resing and Measures, 1994;Measures et al., 1995]. Detection limits and precisionsduring this cruise were approximately 0.5 and 0.1 nM,and 3.0 and 2.5%, respectively. Shipboard dissolved Fedata sets have been validated and corrected by shore-baseddissolved Fe determinations on replicate dissolved samplesreturned to FSU determined by WML using the inductivelycoupled plasma-mass spectrometry (ICP-MS) method ofWuand Boyle [1998]. The correction consisted of quantifyingand then subtracting the daily variation of a previouslyunidentified blank in the shipboard FIA method. This blankwas estimated by correlating the shore based ICP-MS Fedeterminations of a subset of each day’s shipboard run ofsamples against their shipboard values and subtracting thederived offset from each sample run on that particular day atsea.

3. Results and Discussion

[9] The data, which consist of 659 samples from 62profiles spaced at approximately 1 degree intervals between62�N and 5�S, are presented as three contour plots produced

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using Ocean Data View (http://odv.awi-bremerhaven.de),Figure 1. Each of the panels is color-coded to the parameterconcentration, and each is overlaid with contour linesdepicting either potential density (Figure 1, top and bottom)or oxygen concentration (Figure 1, middle).

3.1. Al Distributions

[10] We will first describe and interpret the dissolved Aldata set and use the insights that this provides to interpretthe dissolved Fe section. We will concentrate our attentionon the three major features that the Al section shows. Theseare the surface water values that can be used to infer themagnitude of dust deposition and the two subsurfaceregions of enriched Al between 20� and 40�N.[11] Various authors, working in many different ocean

basins, have concluded that the principal source of dis-solved Al to the surface waters of the noncoastal oceans isfrom the partial dissolution of atmospheric dust [Hydes,1979, 1983; Measures et al., 1984, 1986; Orians andBruland, 1986; Measures and Edmond, 1990; Moran andMoore, 1991; Yeats et al., 1992; Helmers and Rutgers vander Loeff, 1993; Measures, 1995; Powell et al., 1995;Measures and Vink, 1999; Bowie et al., 2002; Kramer etal., 2004; Measures et al., 2005]. Thus its distribution in

surface waters can be used as a tracer of the magnitude andlocus of dust deposition [Measures and Brown, 1996]. Insurface waters dissolved Al has an estimated residence timeof �5 years [Jickells et al., 1994; Orians and Bruland,1986] and is removed principally by scavenging processes[Moran and Moore, 1992]. As will be demonstrated anddiscussed below, scavenged Al, unlike Fe, does not appearto be released during biological remineralization processeswithin the water column. Thus in the open ocean, Al isintroduced into subsurface waters predominantly by thesubduction of water masses that have been labeled withAl by dust deposition at the ocean’s surface. The subsurfaceresidence time of dissolved Al is estimated to be �150 years[Orians and Bruland, 1986].[12] In order to calculate dust deposition from dissolved

Al, the Model of Aluminum for Dust Calculation in OceanicWaters (MADCOW) model assumes that between 1.5 and5% of mineral dust aerosols dissolve in the surface ocean.This range is derived from several published laboratorystudies of partial solubilities [Maring and Duce, 1987;Prospero et al., 1987; Chester et al., 1993; Lim and Jickells,1990] which are also close to the average solubility of Al inAsian aerosol (4.6%) sampled in Hawaii and reported bySato [2002]. Thus the absolute values of our calculated

Figure 1. Property distributions between 62�N and 5�S, contoured using Ocean Data View of (top)dissolved Al, (nM) overlain with potential density contours in kg m�3; (middle) Fe, (nM) overlain withoxygen contours in mM; (bottom) salinity, (PSS78) overlain with potential density contours in kg m�3.

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deposition vary by a factor of �3 depending on thefractional solubility of the mineral aerosol we assume. Inthe data below, we generally will use the term ‘‘mean dustdeposition,’’ which uses an assumed solubility of 3.3%, themid point of our expected range. C. S. Buck et al. (Thesolubility and deposition of aerosol Fe and other traceelements in the North Atlantic Ocean: Data from theA16N CLIVAR-CO2 Repeat Hydrography Section, manu-script in preparation, 2008) have made DI water andseawater aerosol solubility measurements along the A16Ntrack that are consistent with the values used for theMADCOW calculations.

3.2. Surface Waters

[13] The dissolved Al in the mixed layer samples alongwith the estimated dust depositions for 1.5% and 5%solubility (high and low deposition, respectively) are shownin Figure 2. Additionally, in Figure 2 we superimposedeposition estimates interpolated from the Group of Expertson Scientific Aspects of Marine Environmental Protection(GESAMP) model output [Duce et al., 1991] at selectedlatitudes along our cruise track. It should be noted that thelarge range of the GESAMP estimates at these latitudes inFigure 2 reflects the spacing between adjacent contourintervals which are 1 order of magnitude apart. This spacingdoes not represent any inherent inaccuracy in that data setbut merely reflects the factor of 2–3 uncertainty in theGESAMP estimates, which is similar to our factor of �3uncertainty due to atmospheric mineral solubility and otherassumptions. In addition, because of the multiyearresidence time of Al in surface waters, our dust depositionestimates are not instantaneous estimates, but instead rep-resent a 5-year running average; that is, any seasonal effectsat the time of our cruise should be muted significantly.

[14] Along the A16N cruise track the dissolved Al rangeswidely, from �2–37 nM, implying mean dust depositions(assuming 3.3% solubility) that vary by more than 1 orderof magnitude, from �0.2–�3.0 g mineral dust m�2 a�1

(hereinafter referred to simply as g m�2 a�1). The lowestmean dust deposition, �0.2 g m�2 a�1, is found in thenorthern part of the section between 48� and 60�N. Esti-mates rise rapidly to the south reaching a maximum in meandust deposition (�2 g m�2 a�1) at 30�N. Values thendecline to �1 g m�2 a�1 by 20�N. Estimates then increaseonce again reaching a maximum mean deposition of �3 gm�2 a�1 at 8�N. To the south of 8�N values decline to ca.1.5 g m�2 a�1 at 5�S. This N-S trend, with the exception ofthe maximum at 30�N (discussed below), is consistent withthe estimates from the Duce et al. [1991] GESAMP model,which were based on extrapolation of data gathered at land-based sites in the North Atlantic. Comparison of theabsolute values of our estimates with Duce et al.’s [1991]estimates, indicates that our values agree with those esti-mates best when lower dust solubilities, yielding higherdeposition, are assumed. Nevertheless, the agreement be-tween the sets of estimates, both in trend and absolute value,are within the factor of three uncertainty of each of theseapproaches. While satellite imagery of suspended dust loadwill not necessarily mirror dust deposition, it is interestingto note that our maximum deposition region at 8�N isconsistent with the average location of the large dust plumeemanating from the Sahara. This plume oscillates betweenthe equator and 20�N as it follows the migration of theIntertropical Convergence Zone during the period of max-imum dust transport, which is between December andAugust [Husar et al., 1997].[15] The maximum in deposition that we see at 30�N,

23�W though, is not apparent in either the GESAMP or the3-month averaged satellite imagery of Husar et al. [1997].Our sampling resolution across this feature (14 stations) andthe consistency of the trends we see within it, discounts thepossibility that this is a sampling or other kind of artifact.We note that our observed maximum lies approximately 370nmiles to the west and north of the Izana, Canary Islandssampling site at 28�N and approximately 780 nmiles to thenorth of the Sal Island sampling site in the Cape VerdeIslands at �17�N, the two eastern Atlantic sampling sitesthat were used in the GESAMP model at these latitudes.Thus it is possible to argue that the large physical separationof these two GESAMP sites might preclude observation ofthis feature. We also note that of all the data comparisonsbetween GESAMP and our data set, the estimates at thelatitude of Izana show the poorest agreement.[16] Another, and more interesting, explanation is that the

source of the mineral dust that produces this enhanceddeposition we see at 30�N is propagated from the west,rather than the east, in which case the material we seeentering the surface waters at this location (30�N, 23.4�W)from the marine boundary layer would not have traversed,nor be recorded at, the Izana sampling site (28�N, 16.5�W,2360 m above sea level [Prospero, 1996]). We will attemptto provide evidence for this argument when we discuss thesources of Al to the North Atlantic Sub tropical mode waterformation regions, below.

Figure 2. Dissolved Al in surface waters along the transectand the estimated dust deposition from applying the Modelof Aluminum for Dust Calculation in Oceanic Waters(MADCOW) model. Low dust is estimated by assuming 5%of the mass of the dust dissolves in surface waters, and highdust assumes 1.5% dissolution. The Group of Experts onScientific Aspects of Marine Environmental Protection(GESAMP) estimates are taken from Duce et al. [1991]with range bars set to represent the adjacent contour lines.

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3.3. Enhanced Al in the Subsurface Waters

[17] The elevated Al (>20 nM) seen in our deepestsamples (�750 m) at 37/36�N, is coincident with the upperpart of the Mediterranean outflow water, the presence ofwhich is also reflected in the salinity maximum seen inFigure 1, bottom. The Al labeling of this water results fromthe fact that the Mediterranean basin receives upward of 10 gmineral dust m�2 a�1 resulting in highly elevated dissolvedAl in the subducted surface waters that exit the Strait ofGibraltar [Hydes, 1983; Measures and Edmond, 1988].While the distribution of the enhanced salinity signal fromthe Mediterranean outflow is a well documented feature ofthe tropical North Atlantic, our section shows that thegeochemical imprint that accompanies this signal is alsounmistakable, influencing Al concentrations at this depthfrom �5� to 55�N [Hall and Measures, 1998].[18] The third region of elevated Al concentrations, is

centered at �300 m, between 35 and 20�N with Al valuesgenerally >18 nM. Again, as with the Mediterranean signal,the ultimate origin of the enrichment is from the subductionof surface waters that have been enriched in Al by atmo-spheric dust deposition. In this case the water mass is theSubtropical ‘‘mode’’ water (STMW) of the North Atlanticwhich is found at a potential density of approximately26.5 kg m�3. Two forms of STMW have been reportedfor the Atlantic [Hanawa and Talley, 2001], the classical18� water which forms in the western basin on the boundaryof the Gulf Stream [Worthington, 1959] and the Madeiramode water which forms in the eastern basin to the Northand West of Madeira [Siedler et al., 1987]. Both forms ofSTMWare believed to form in late winter/early spring whenwinter cooling increases surface water densities and themixed layers are at their deepest. The CLIVAR cruise trackpassed through the Madeira sub tropical mode water for-mation region in early July and we observed surface waterAl concentrations of 10–17.5 nM between 32� and 36�N,which are considerably below the values seen in the core ofthe STMW where maximum values of �24 nM were seen atdepth in our section. However, although our transit was notduring the STMW formation period, the �5-year residencetime of dissolved Al in surface waters implies seasonalvariations should be �20%. In contrast, in the western basin

of the North Atlantic, between 31� and 36�N, surface waterAl values ranging from 28 to 43 nM have been reported[Measures et al., 1984, 1986; Jickells et al., 1994], morethan sufficient to supply the observed Al concentrations inthe STMW. The concentrations that we see in the easternAtlantic are consistent with previous reports for dissolvedAl in the surface waters of this region [Hydes, 1983;Kramer et al., 2004]. A contour plot showing these datasets and the east-west gradient across the Atlantic at thislatitude is presented in Figure 3. Thus the dissolved Alconcentrations that we observed in surface waters of theeastern basin, when combined with reported values for thewestern basin surface waters, suggests that the main volu-metric contribution to the STMWof the Atlantic is from thewestern basin, i.e., the classic 18� variety. This is alsoconsistent with the conclusion of Siedler et al. [1987].[19] The apparent paradox of greater Al concentrations in

the surface waters of the western Atlantic at these latitudes(30–36�N) when the dust source providing the Al is on theeastern side of the Atlantic at more southerly latitudes (0–20�N), brings us back to the question of the origin andsystematics of the Al concentration maximum that weobserve in surface waters at 30�N shown in Figure 2.[20] Our surface water data above shows clearly that there

is significant deposition of mineral dust from the Saharanplume as it exits the African coast between 0 and 20�N intothe adjacent surface ocean. However, much larger amountsof material are transported rapidly at elevations that arewell above the marine boundary layer across the Atlantic[Prospero, 1996]. In summer months the strong circulationaround the Bermuda-Azores high coupled with the subsid-ing air masses can bring this high-altitude material into themarine boundary layer from which it can more easilysediment out or be removed by wet deposition, and sendit back across the Atlantic with an eastward trajectory. Thusthe predominant westerlies at this latitude are seeded in theirmarine boundary layer with dust on the western side of thebasin and appear as a point source that is progressivelydepleted through deposition along the advective track of theair mass, imprinting the surface ocean beneath them with asignal that is also progressively depleted from west to east.[21] While this is a speculative argument, it should be

noted that the Bermuda dust record shows elevated dust

Figure 3. Distribution of dissolved Al in the surface waters of the N Atlantic between 30 and 40�N.Data are from A16N, this manuscript; Kramer, Kramer et al. [2004]; EN 107, Measures et al. [1984]; EN157, C. I. Measures, unpublished data, 1986; EN120, Measures et al. [1986]; Hydes, Hydes [1983];Jickells, Jickells et al. [1994]; WBEX, C. I. Measures, unpublished data, 1986; IOC 91,Measures [1995].

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deposition during summer when the high-pressure system isstabilized, and facilitating downward transport of high-altitude Saharan material [Prospero, 1996]. Additionally,Sedwick et al. [2005] have reported increases in surfacewater dissolved Fe in Sargasso Sea surface waters associ-ated with enhanced summertime atmospheric deposition.This argument is also consistent with the plot of thedistribution of dissolved Al in surface waters over thislatitude range (Figure 3) which shows much higher valuesin the western basin than the east. Finally, the speculation isalso consistent with the GESAMP model which also showshigher deposition estimates in the western basin of theAtlantic, than in the eastern basin between �30–45�N.[22] It may also be coincidental, but enhanced deposition

of atmospheric mineral dust has also been seen in the NorthPacific at a similar latitude, 30�N, 150�W; this depositionalfeature is also believed to be the result of the transfer ofhigh-altitude dust from the Gobi Desert into the MBL by thehigh-pressure zones of this region [Hiscock et al., 2006].[23] If, as we suggest in the case of the Atlantic, the

STMW are labeled with Al derived from atmosphericdeposition, then there exists the potential to develop apaleotracer based on Al (or another atmospherically deliv-ered trace element) that might record the intensity of modewater formation, and/or dust transport to the STMW for-mation region of the North Atlantic.[24] Overall, it is evident from the Al distributions that a

large part of the upper 1000 m of this part of the AtlanticOcean is clearly imprinted by the process of atmosphericdeposition, either directly under the Saharan plume orindirectly through the deeper water masses that are alsolabeled by atmospheric deposition processes in their forma-tion regions. It is also important to note that below the Al-enriched surface waters underlying the Saharan dust plumethere is no evidence of subsurface remineralization ofsurface water scavenged Al in contrast to the distributionfor Fe (discussed below).[25] While our data show the atmospheric imprinting

through the distribution of dissolved Al, a natural compo-nent of continental materials, it is also likely that anthropo-

genic materials with continental sources and atmospherictransport vectors, are similarly imprinting the upper watersof this ocean basin as has been shown in the Sargasso for Pb[Boyle et al., 1986; Shen and Boyle, 1988; Veron et al.,1993; Wu and Boyle, 1997]. This suggests that the upperwater of the North Atlantic is one of the most susceptibleregions of the global oceans to atmospheric input and that itshould be a sensitive recorder of both historical and futurechanges in atmospheric inputs.

3.4. Fe Distributions

[26] Surface water Fe distributions to a large degreefollow the surface water Al concentrations, reflecting theimportance of the role of atmospheric deposition in supply-ing Fe to the surface waters of much of the North Atlantic(Figure 4). The correlation is strongest (R = 0.82) between51.5 and 9�N, roughly coinciding with the boundaries of theN Atlantic gyre. Within the gyre between 25 and 32�N, thedissolved Fe concentrations we observe are slightly higherthan the October surface water values reported by Sarthouet al. [2007] further to the east. The range and pattern of ourdissolved Fe values are similar to the winter/spring concen-trations reported by Bergquist and Boyle [2006] between30�N and 5�S to the west. To the north of the gyre, deepwinter mixing (mixed layers >300 m) supplies the majorityof the Fe to the surface waters from the subsurface layersthat are relatively enriched in Fe through biological remi-neralization. In contrast, Al concentrations in both thesurface waters and subsurface waters are low because oflow dust deposition and also because of lack of remineral-ization, thus decoupling the Al:Fe correlation.[27] To the south of 9�N, the degradation of the correla-

tion is most likely a result of the North Equatorial Coun-tercurrent (NECC), which is visible in the shipboardacoustic Doppler current profiler (E. Firing and J. Hummon,unpublished data, 2003). Between July and December, theNECC which has its origins in the seasonal retroflection ofthe North Brazil Current, advects surface waters across theAtlantic from the west [Richardson and Walsh, 1986;Wilson et al., 1994; Tsuchiya et al., 1992]. We speculatethat the entrainment of nutrients from the shelf and theAmazon outflow stimulates productivity in this waterresulting in the preferential removal of Fe.

3.5. Fe Systematics in the Subarctic Gyre

[28] The observation that in general the surface water Feconcentrations north of �51�N were extremely low (0.02–0.16 nM; average 0.09 nM) and that there were significantquantities of unutilized nutrients in the surface waters (up to5 mM nitrate), naturally poses the question of whether thisregion might be Fe-limited. This combination of nutrientproperties could of course simply be a result of the timing ofour cruise; that is, the spring bloom had not yet reached itsclimax, removing all available macronutrients. However,inspection of the World Ocean Circulation Experimentdatabase, even when restricting data sets to two summermonths (June and July), also indicates a very clear demar-cation in surface waters with remnant macronutrients (NO3

� 2–6 mM) extant in the surface waters of north of �50�N.Additionally, the Joint Global Ocean Flux Study (JGOFS),

Figure 4. Distribution of dissolved Al and Fe in thesurface waters along the transect.

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May/June 1989 North Atlantic Bloom Experiment (NABE),also reported surface water nitrate concentrations of 2–9mM between 50 and 60�N [Garside and Garside, 1993].Thus there appears to be abundant evidence that small butsignificant levels of macronutrients persist in the surfacewaters north of �50�N during the summer months, and thatthis is coincident with our observations of this being aregion of low dust input.[29] Martin et al. [1993] concluded from their JGOFS

NABE Fe and chlorophyll data that the growth capacity inthis northern part of their sampling region was much greaterthan was observed in the subarctic and equatorial Pacificand thus did not appear to be similarly Fe-limited as thoseregions were. We agree, this is not a classic HNLC regionsince chlorophyll is not particularly low, and remnantnutrients are not particularly high. However, this does notpreclude that Fe-limitation exists, but merely the degree towhich the surface waters may be Fe-limited. To examinethis concept in more detail, we wish to revisit the Fesufficiency argument at this latitude from the standpointof macronutrient availability and Fe supply from deepwinter mixing and mineral deposition. We estimate that latewinter deep mixing to �500 m would raise surface water Fevalues in this region to a maximum of �0.5 nM. Addition-ally, an annual mineral dust deposition of 0.2 g m�2 a�1

containing 800 mmole Fe g�1 of which 3% dissolves, wouldprovide an additional 4.8 mmoles Fe m�2 a�1 to the surfacewaters. Over a 50 m mixed layer (the value in summerduring plankton growth) this would be equivalent to anadditional 0.1 nM Fe/L, yielding a maximum of 0.6 nM.Using a N:Fe ratio of 15,000:1 for biological uptake underFe limiting conditions (derived from a C:Fe of 1 � 105 andC:N = 6.67 [Measures and Vink, 1999]), would imply that�9 mM nitrate could be removed, leaving a residual of�6 mM nitrate from the �15 mM surface water winternitrate values. While these estimates could be varied sig-nificantly by choosing different aerosol solubilities andN:Fe uptake ratios etc., the published values we have usedare reasonable, and yield results that are remarkably close tothe observed residual surface water nitrate values. Thus thiscalculation serves as test of concept, rather than a definiteproof of occurrence Alternatively, using these same values,we can calculate that full utilization of the winter nitratelevels would require �0.4 nM additional Fe, which could besupplied by the deposition and partial dissolution of afurther �1.0 g m�2 a�1 of mineral dust to the surfacewaters of the region, approximately 5 times that of our meandeposition estimate.[30] Thus it would appear from these calculations, and the

persistence of macronutrients in surface waters, that theregion north of 50�N is on the borderline of Fe limitation. Ifso, then this would be a particularly interesting region forrepeat studies over longer periods of time, since its Fesufficiency may vary significantly from year to year as aresult of natural, or climatically induced changes in dustflux. Thus we suggest that this would be a valuable, andlogistically feasible region in which to study the systematicchanges in oceanic biology and chemistry that accompanyan oceanic region as it transitions between Fe sufficiencyand Fe limitation.

3.6. Subsurface Fe Maximum

[31] The much shorter surface residence time and remi-neralization of Fe leads to significantly different subsurfacedistributions of this element from those of Al. Thus theMediterranean and STMW do not show visible Fe maxima,above the background concentrations that develop frombiological vertical transport and remineralization.[32] Instead, the most dramatic feature of the subsurface

Fe distribution is the large Fe maximum between �18 and4�N at depths between 200 and 800 m, where Fe valuesreach up to 2 nM. The latitude range of the subsurface Femaximum corresponds closely with the surface water Almaximum indicating atmospheric deposition of mineral dustis possibly fueling this process. Dissolved Fe in surfacewaters in this region also reach values of up to 1.5 nM.These enriched surface waters are separated from thesubsurface maximum by a minimum at �50 m, whichbecomes more pronounced toward 12�N. The dissolvedFe minimum at 50 m corresponds very closely to thechlorophyll maximum, recorded by the uncalibrated fluo-rometer that was mounted on the TM rosette (Figure 5).Thus we can see over a significant latitude range the directpartitioning of dissolved Fe into a living biological partic-ulate phase. The depletion between the surface mixed layerand the dissolved Fe minimum is typically 0.4 to 0.6 nM,significantly higher than the �0.2 nM depletion observedby Bruland et al. [1984] in the lower-euphotic zone of thestratified waters of the North Pacific gyre or the 0.2 to0.3 nM depletion reported by Bergquist et al. [2007] at10�N in the western Atlantic. While these differences mayreflect greater production in the eastern versus the westernNorth Atlantic region or the stratified North Pacific gyre,the biological implications of this are beyond the scope ofthis paper, or the data sets available for interpretation of thebiological signal. Clearly the ability to close budgets oncalculations like this would benefit greatly from the avail-ability of a suite of basic biological parameters.[33] The latitude range of the subsurface Fe maximum (18

to 4�N) corresponds very closely to the oxygen minimumand is most pronounced where O2 values are less than80 mM, as shown in Figure 1, middle. This implies thatbiological remineralization of surface derived Fe suppliedby the Saharan mineral dust plume plays an important rolein sustaining this feature. Alternatively, the oxygen mini-mum could be an advected feature originating whereoverlying seawater contacts coastal reducing sediments.However, we discount this latter possibility as there is littledissolved Mn, which also emanates from reducing sedi-ments, associated with this low-oxygen feature [Landing etal., 2006] In addition, south of the equator, where dustinputs and surface Fe concentrations are much lower, analmost equally intense low-oxygen zone shows little or nosubsurface Fe enrichment, reinforcing the notion that thesubsurface Fe enrichment north of the equator is predom-inantly supported by the vertical transport and reminerali-zation of the local surface Fe enrichment. The matchbetween the enhanced surface and subsurface Fe concen-trations starts to break down around �7�N which is mostlikely a result of the NECC bringing relatively low Fewaters from the east at this latitude during the time of our

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cruise. Presumably elevated Fe concentrations in the poorlyventilated subsurface waters integrate over a much longerperiod of time than those in the relatively rapidly moving,and seasonally flow reversing, surface waters.[34] If the Fe maximum is sustained by in situ reminer-

alization, then it is instructive to look at the ratio of N:Fewithin the oxygen minimum to develop some insights intothe water column Fe remineralization process. Under Felimiting conditions, the C:Fe ratio in phytoplankton is�105:1 [Sunda and Huntsman, 1995], which, using atypical C:N ratio of 6.7:1, translates to a N:Fe ratio of15 � 103. Thus one would expect that the subsurfaceremineralization of biological material delivered fromFe-rich surface waters to have a ratio of N:Fe � 15 �103. However, within the boundaries of the oxygen mini-mum, the N:Fe ratio varies from �15–30 � 103, at or wellabove the ‘‘limitation’’ ratio. This surprisingly high valuesuggests that during the biological remineralization processthere is a large relative loss of Fe compared to N, presum-ably through active scavenging of this particle reactiveelement. Bergquist et al. [2007] also observed elevated totaldissolved Fe values of >1.0 nM between 200 and 1000 m,associated with the oxygen minimum, at their 10�N, 45�Wstation. Through careful speciation determinations theywere also able to attribute the bulk of that Fe at thesedepths as being present in the form of colloids, a form of Fethat is probably more prone to scavenging than organicallycomplexed forms of Fe. The elevated subsurface concen-trations that we see are clearly much higher than the steadystate maximum value of 0.6 nM Fe postulated by Johnson etal. [1997] for deep waters and discussed by Boyle [1997].While our single survey of this region prevents us fromaddressing the seasonal variation directly, it would seemlikely that in this poorly ventilated region the elevated Feplume is a steady state feature where biological remineral-ization processes are balanced by scavenging of released Fe.

If a large fraction of the remineralized Fe is present in acolloidal form as suggested by Bergquist et al. [2007] thenthe relatively low concentration of Fe observed relative to Nmay be a result of continual scavenging by verticallytransported of eolian dust.[35] Previous observers have noted that in regions with

low surface water Fe, the subsurface waters were deficientin Fe relative to N. For example, Martin and Fitzwater[1988] reported N:Fe ratios in upwelling water of the HNLCregion of the North Pacific of �57,000:1, and reported aneven more limiting ratio of >100,000:1 in the Drake Passageregion of the Southern Ocean [Martin et al., 1990b]. Toobserve this same phenomenon in the water column thatunderlies a region of relatively high surface water Fe,indicates that there must be strong geochemical fraction-ation of these two elements during remineralization, andthat this ubiquitous loss of Fe develops the relative defi-ciency of Fe to N in subsurface waters. It should be notedthat Measures and Vink [1999] also found a relatively highN:Fe ratio of 23,000:1in the waters upwelling in theArabian Sea, a region that also receives significant dustdeposition 2.2–7.4 g m�2 a�1.[36] Thus all subsurface waters, regardless of the source

of biological material that fuels nutrient remineralizationwithin them, will be deficient in Fe to some degree. Theupwelling or deep mixing processes that returns the macronutrients to surface waters always requires a complementaryprocess that delivers an additional source of Fe to thesurface waters. In the open ocean, far from continentalshelves, the atmospheric transport of continental materials,its deposition and partial dissolution in surface waters, isusually the only available vector.[37] The high-resolution sections of dissolved Al and Fe

in the Atlantic Ocean along the CLIVAR A16N sectionclearly show the role that atmospheric deposition of conti-nental mineral dust plays in the geochemical cycle of these

Figure 5. Vertical profiles of dissolved Fe in the upper 200 m between 18 and 4�N (right) and theuncalibrated fluorometer signal between 18 and 4�N (left).

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two elements. The effect is manifested in surface waters byhigh concentrations of both elements in regions of highdeposition. Subsurface waters are also enriched with Al bysubduction of surface labeled waters in the Mediterraneanand in the western basin where the classical 18� subtropicalmode water forms. Together these water masses transmit theatmospheric signal into a large part of the upper 1000 mbetween 60�N and 5�S. This widespread distribution indi-cates the close chemical connection between atmosphericand oceanic processes in the North Atlantic. That thisrelationship is visible in a tracer with a relatively shortresidence time indicates the potential rate at which anthro-pogenic materials with eolian transport vectors can directlyaffect a large proportion of the upper waters of this basin.This clear labeling of the mode water by Al might indicatethat this tracer, or another with an atmospheric flux, mightbe developed into a tracer of the history of atmosphericdeposition and mode water formation.[38] In addition, a large region of enhanced Fe concen-

trations between 18� and 4�N is maintained by biologicaluptake in enriched surface waters, vertical transport andremineralization in the oxygen minimum zone. Despite thecopious quantities of Fe supplied to the surface waters ofthis region, the remineralized ratio of Fe:N appears to bebelow the Fe-limiting uptake ratio for photosynthetic organ-isms.[39] Also significant is that to the north of 51�N, where

atmospheric deposition is dramatically lower, the surfacewaters appear to be on the borderline of Fe-limitation,possibly explaining why this region has unused macro-nutrients in surface waters and suggesting the presence ofa previously unrecognized ‘‘HNLC’’ region, in the sense ofunutilized surface layer macronutrients after the phyto-plankton growth season.

[40] Acknowledgments. This work was supported by NSF grantsOCE 0223397 to CIM and OCE 223378 to WML. We thank John Bullisterand Nicky Gruber for the many kindnesses they showed in helping us tointegrate our new program into their larger shipboard commitments. Wealso thank the Captain and the crew of the RV Brown for their help with thedeployment and retrieval of our package, and particularly Mike Gowan,Chief Engineer, for his help with our troublesome winch. We also thank allthe members of the CLIVAR-CO2 Repeat Hydrography oversight commit-tee, cochaired by Dick Feely and Lynne Talley, for their enthusiasticsupport in adding the TM component to the CLIVAR Repeat Hydrographyand providing us with the ship time required for our work. This iscontribution 7251 of the School of Ocean Earth Science and Technology,University of Hawaii.

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�������������������������M. T. Brown, Institute of Marine Sciences, A317 Earth and Marine

Sciences Building, University of California, Santa Cruz, 1156 High Street,Santa Cruz, CA 95064, USA.C. S. Buck and W. M. Landing, Department of Oceanography, Florida

State University, 325 Oceanography-Statistics Building, Tallahassee, FL32306-3048, USA.C. I. Measures, Department of Oceanography, University of Hawaii at

Manoa, 1000 Pope Road, Honolulu, HI 96822, USA. (chrism@soest.hawaii.edu)

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