Antimony and arsenic biogeochemistry in the western Atlantic Ocean

Deep-Sea Research II 48 (2001) 2895–2915

Antimony and arsenic biogeochemistry in the western Atlantic Ocean Gregory A. Cuttera,*, Lynda S. Cuttera, Alison M. Featherstoneb, Steven E. Lohrenzc a

Department of Ocean, Earth, and Atmospheric Sciences, Old Dominion University, Norfolk, VA 23529-0276, USA b School of Chemistry, University of Tasmania, Hobart, Tasmania 7001, Australia c Department of Marine Science, University of Southern Mississippi, Stennis Space Center, MS 39529-0001, USA

Abstract The subtropical to equatorial Atlantic Ocean provides a unique regime in which one can examine the biogeochemical cycles of antimony and arsenic. In particular, this region is strongly affected by inputs from the Amazon River and dust from North Africa at the surface, and horizontal transport at depth from highlatitude northern (e.g., North Atlantic Deep Water) and southern waters (e.g., Antarctic Bottom and Intermediate Waters). As a part of the 1996 Intergovernmental Oceanographic Commission’s Contaminant Baseline Survey, data for dissolved As(III+V), As(III), mono- and dimethyl arsenic, Sb(III+V), Sb(III), and monomethyl antimony were obtained at six vertical profile stations and 44 sites along the 11,000 km transect from Montevideo, Uruguay, to Bridgetown, Barbados. The arsenic results were similar to those in other oceans, with moderate surface depletion, deep-water enrichment, a predominance of arsenate (>85% As(V)), and methylated arsenic species and As(III) in surface waters that are likely a result of phytoplankton conversions to mitigate arsenate ‘‘stress’’ (toxicity). Perhaps the most significant discovery in the arsenic results was the extremely low concentrations in the Amazon Plume (as low as 9.8 nmol/l) that appear to extend for considerable distances offshore in the equatorial region. The very low concentration of inorganic arsenic in the Amazon River (2.8 nmol/l; about half those in most rivers) is probably the result of intense iron oxyhydroxide scavenging. Dissolved antimony was also primarily in the pentavalent state (>95% antimonate), but Sb(III) and monomethyl antimony were only detected in surface waters and displayed no correlations with biotic tracers such as nutrients and chlorophyll a. Unlike As(III+V)’s nutrient-type vertical profiles, Sb(III+V) displayed surface maxima and decreased into the deep waters, exhibiting the behavior of a scavenged element with a strong atmospheric input. While surface water Sb had a slight correlation with dissolved Al, it is likely that atmospheric Sb is delivered with combustion byproducts and not from mineral aerosols. In the Amazon Plume, antimony concentrations dropped substantially, and an Amazon River sample had a concentration (0.25 nmol/l) that was less than one-fourth those found in other major rivers. Using these river data, and estimates of atmospheric fluxes based on *Corresponding author. Tel.: 757-683-4929; fax: 757-683-5303. E-mail address: [email protected] (G.A. Cutter). 0967-0645/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 1 ) 0 0 0 2 3 - 6

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shipboard measurements and collections from Barbados, the atmospheric deposition of antimony to the equatorial Atlantic (28S–88N) is twice the Amazon flux, while the atmospheric deposition of arsenic is only 10% of the river’s flux. # 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction As members of Group 5 in the Periodic Table, one might expect the behavior of the dissolved trace elements antimony and arsenic to be similar to that of phosphate, namely in the pentavalent state as oxyanions and showing nutrient-like depth profiles. Indeed, numerous studies of arsenic biogeochemistry (e.g., Andreae, 1979; Middelburg et al., 1988; Cutter and Cutter, 1998) have demonstrated that dissolved arsenic is primarily in the 5+ oxidation state (arsenate) and that it shows minor surface depletion and deep-water enrichment like phosphate (concentration range of 14–24 nmol/l). The similarity in chemical properties to phosphate is also largely responsible for the toxicity of arsenate to phytoplankton. The behavior of dissolved arsenic diverges from phosphate by the presence of the trivalent form, arsenite, and the methylated arsenicals (e.g., monomethyl and dimethyl arsenic) that may be produced by phytoplankton as a means of detoxification (Andreae and Klumpp, 1979; Sanders and Windom, 1980; Benson, 1984; Sanders et al., 1994). However, the uptake of arsenate and its bioconversions are by no means straightforward since different phytoplankton species perform different reactions (reduction, methylation; Andreae and Klumpp, 1979; Sanders and Windom, 1980; Sanders and Riedel, 1993), the concentration of phosphate appears to affect uptake (e.g., competitive inhibition; Sanders, 1979), higher molecular weight arsenic compounds may be produced (e.g., arsenobetaine; Howard and Comber, 1989), and bacteria also may methylate arsenic (Apte et al., 1986). Since the stability (residence time) of the different arsenic compounds varies from days to years, interpreting source/ sink relationships in vertical or horizontal profiles can be difficult. In contrast to arsenic, oceanographic distributions for dissolved antimony are far less numerous, but results from the Atlantic Ocean demonstrate that antimony is also predominantly pentavalent (antimonate), showing either conservative or mildly scavenged profiles (surface maxima of ca. 1.9 nmol/l, decreasing in deep waters, ca. 1.1 nmol/l; Middelburg et al., 1988; Cutter and Cutter, 1995, 1998; Takayanagi et al., 1996). While these distributions are indicative of a passive, abiotic cycle (i.e. adsorption/desorption), the existence of low concentrations (0.05 nmol/ l) of Sb(III) and monomethyl antimony (0.2 nmol/l) suggest a biotic production term like that of arsenic (Andreae and Froelich, 1984; Cutter, 1992). In support of biotic production, Andreae and Froelich (1984) found Sb(III), but not methylated Sb, in samples of marine phytoplankton. Most studies of arsenic and antimony have focused on the mid- to high latitudes, but the oceans at lower latitudes, particularly the Atlantic, represent dynamic regimes in which to examine the biogeochemical cycling of these elements because of inputs from equatorial upwelling, mineral aerosol (dust) deposition from North Africa, and discharges from extremely large rivers such as the Amazon. With respect to dust, it has been demonstrated that atmospheric deposition is the primary flux of antimony to the high-latitude North Atlantic (Cutter and Cutter, 1998) and Sargasso Sea (Cutter, 1993), while Takayanagi et al. (1996) also argued that the surface maxima in dissolved antimony profiles from the Mediterranean were likely due to inputs from the

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atmosphere. As the largest freshwater input to the ocean (Limeburner et al., 1995), the Amazon River represents a potentially large input of dissolved trace elements such as arsenic and antimony. However, studies of arsenic in sediments near the mouth of the Amazon, suggest substantial removal from the water column by iron oxyhydroxides (Sullivan and Aller, 1996); one might expect a similar trend for antimony. On a larger scale, deep waters of the western Atlantic contain the relatively young water masses that flow into the eastern basins of the Atlantic through the Romanche Fracture zone (Schlitzer, 1987) and the components of deep water (North Atlantic Deep Water and Antarctic Bottom Water) that eventually flow into the Pacific Ocean (Reid, 1989). Thus, there are numerous reasons for studying antimony and arsenic in the low-latitude Atlantic Ocean, particularly since these elements have substantial anthropogenic inputs that rival natural sources (Nriagu and Pacyna, 1988; Nriagu, 1989). This paper examines the biogeochemistry of antimony and arsenic in the western and equatorial Atlantic Ocean using samples from six vertical profile stations and surface waters along an 11,000 km transect from Uruguay to Barbados.

2. Methods 2.1. Sampling sites Samples for antimony and arsenic were collected as part of the 1996 Intergovernmental Oceanographic Commission’s Contaminant Baseline Survey Cruise from 18 May to 20 June 1996. The Research Vessel Knorr transited from Montevideo, Uruguay, to Bridgetown, Barbados, along a cruise track described in Cutter and Measures (1999). A total of six vertical profile stations were occupied, and the hydrographic conditions at each site are discussed thoroughly elsewhere (Cutter and Measures, 1999). Station 10 (33.08S, 40.08W; 4650 m depth) was just south of the Vema Channel in the northern-most Argentine Basin; Station 8 (17.08S, 25.08W; 5150 m) was in the center of the Brazil Basin, Station RF (0.598S, 20.038W; 6560 m) was in the Romanche Fracture zone of the Mid-Atlantic Ridge; Station 6 (8.08N, 45.08W; 4610 m) was located midway between the Mid-Atlantic Ridge and the continental rise of Brazil; and Stations A1 (5.728N, 48.098W; 3635 m) and A2 (6.498N, 48.88W; 4030 m) were in the offshore extension of the Amazon River plume. In addition to these vertical profiles stations, surface samples were taken on a twice daily basis along the 11,000 km transect from Uruguay to Barbados (but only in international waters). The major water masses sampled along the transect are described in Cutter and Measures (1999). 2.2. Sampling methods Vertical profile samples were collected using 12- and 30-l Go Flo bottles attached to a Kevlar hydrographic cable and tripped with plastic messengers. After recovery, the Go Flo’s were transferred to positive pressure clean labs for sample processing; waters from the 12-l bottles were directly sampled, while the 30-l bottles were pressurized with 8 psi filtered nitrogen and the water passed through 0.4-mm Nuclepore filters prior to collection. Samples for shipboard antimony and arsenic determinations were placed in 1-l Teflon bottles and refrigerated prior to analysis (512 h),

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while samples for methylated arsenic species were transferred to 250-ml polyethylene bottles and acidified to pH 1.6 with HCl. To collect underway surface samples, a towed 10 kg ‘‘fish’’ was positioned ca. 8 m from the after quarter of the ship, and water peristaltically pumped from the fish to the ship’s clean lab through Teflon-lined polyethylene tubing. Surface water samples were processed in the same fashion as those from the vertical profiles.

2.3. Analytical methods Shipboard determinations of As(III), Sb(III), As(III+V), and Sb(III+V) utilized the methods of Cutter et al. (1991). In brief, a sample was placed in a glass stripping vessel, adjusted to pH 6.2 with TRIS-HCl, and a solution of sodium tetrahydridoborate (NaBH4) added to generate arsine and stibine exclusively from the trivalent ions. The generated hydrides were stripped from solution using helium, collected in a liquid-nitrogen-cooled trap, and subsequently revolatilized and determined simultaneously with a gas chromatograph/photoionization detector (GC/PID). These hydrides also were generated from a separate sample that was acidified to 0.5 M HCl, and KI and NaBH4 added to simultaneously determine As(III+V) and Sb(III+V), hereafter referred to as Asi and Sbi, respectively. The standard additions method of calibration was utilized to assure accuracy, and determinations were made in triplicate to assess precision. The observed precision was better than 5% (relative standard deviation) for all species at concentrations >0.5 nmol/l, and the detection limit for As was 1.0 pmol/l, while that for Sb was 0.5 pmol/l. The exact identity of methylated arsenic and antimony forms in the water column is somewhat ambiguous (e.g., could be trivalent or pentavalent; Hasegawa et al., 1994) since the hydride generation method used here cannot differentiate them. To acknowledge this, we will only refer to the methylated species in generic terms (e.g., monomethyl arsenic rather than monomethyl arsonate, etc.). Shipboard determinations of monomethyl arsenic (MMAs), dimethyl arsenic (DMAs), monomethyl antimony (MMSb), and dimethyl antimony (DMSb) used the methods described by Andreae (1983), modified for simultaneous detection using the inorganic As and Sb GC/PID system (Cutter et al., 1991), except that a 4 m 15% OV-3 on Chromosorb W/AW DMCS (80/100 mesh) column was employed to separate the methyl hydrides. These hydrides were generated quantitatively using the Asi conditions (Andreae, 1983), with a detection limit of 5 pmol/l, and precision of 6% (RSD) at 0.1 nmol/l. Unfortunately, the GC/PID system for the methyl species failed half-way through the cruise. However, the methylated arsenic species were determined on stored samples using the selective hydride generation/atomic fluorescence procedure described by Featherstone et al. (1998), with a precision of 3.5% (RSD) and detection limits of 30 pmol/l for MMAs and 50 pmol/l for DMAs. Where methyl As data from shipboard and stored determinations overlapped, the concentrations were within 16% of each other, suggesting minimal storage artifacts. Thus, methylated As data are available for the entire cruise, while the methyl Sb results are limited to the first half of the data set, and the last vertical profile station (A-2) and transect samples (when the inorganic system was used). Samples for chlorophyll a analyses were filtered onto 2.5-cm Whatman GF/F glass fiber filters. The filters were subsequently extracted in 5 ml 90% acetone : 10% deionized water in darkness at
208C for 24 h. Following extraction, the filters were removed and the samples were centrifuged at 1500 rpm in a Damon HN-SII centrifuge for 5 min. Chlorophyll a concentrations were

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determined fluorometrically (Holm-Hansen et al., 1965) using a Turner Designs Model 10 Fluorometer.

3. Results and discussion 3.1. Vertical profiles At Station 10, the southern-most of the IOC vertical profiles stations and hence the one with the youngest Antarctic Bottom Water (AABW), the profile for Asi in (Fig. 1) shows moderate surface depletion (ca. 4 nmol/l, relative to underlying deep waters), a slow increase into the North Atlantic Deep Water (NADW; 2200–3200 m; average Asi of 21.2  1.7 nmol/l), and nearly identical concentrations in the AABW (3950–4460 m; average Asi of 20.4  0.4 nmol/l). Arsenite has its maximum (0.35 nmol/l, 2% of total As) in the 80 m deep mixed layer and then rapidly decreases into the deeper waters, with an average of 0.09 nmol/l (0.4% of total As) from 700 m to the bottom (Fig. 1); this deep water arsenite is identical to the concentration (0.10 nmol/l) found by Andreae (1979) in the Pacific. Both MMAs and DMAs were only detectable in the upper 200 m of Station 10 (Fig. 1), with DMAs having a maximum concentration of 0.69 nmol/l (3.9% of total As) and MMAs a maximum of 0.1 nmol/l (0.5% of total As). These distributions are similar to those found in other ocean basins (e.g., Andreae, 1979; Middelburg et al., 1988; Cutter and Cutter, 1998) and may be attributed to the uptake of arsenate by phytoplankton and subsequent conversion to arsenite and methylated arsenic in these low-phosphate (0.09 mmol/l) surface waters (e.g., Sanders and Windom, 1980; Benson, 1984). Indeed, the depth profiles of arsenite and MMAs species are similar to that of chlorophyll a (Fig. 1); this subject will be explored below in more detail using the horizontal transect data set. The Asi concentrations in the deeper water masses allow a comparison with those in other basins and hence the evolution of the Asi signal. In the northern water masses that combine to form NADW, Asi is 17.6  2.0 nmol/l (i.e., ‘‘preformed’’ Asi; Cutter and Cutter, 1998), while in the terminus of deep ocean circulation, the North Pacific, Asi averages 24.2  0.4 nmol/l (Andreae, 1979). The NADW at Station 10 has a 75% Northern Water signature (Cutter and Measures, 1999) using the calculations described in Broecker et al. (1991), and the observed Asi in this water mass indicates it has evolved to approximately half of its Atlantic/Pacific interbasin fractionation. This might seem faster than expected given that the 14C age for NADW near Station 10 is ca.160 years (Broecker et al., 1991), but the composition of North Pacific deep waters is only comprised of 25–30% Northern water (Broecker et al., 1985), and the dominant Southern waters (AAIW, UCDW, AABW) have lower Asi as shown in the data at Station 10 (Fig. 1). In contrast to Asi, Sbi displays no surface depletion (Fig. 2), a modest increase into the NADW (average Sbi of 0.91  0.12 nmol/l), and subsequent decrease into the AABW (average Sbi of 0.53  0.05 nmol/l). The other species of dissolved antimony, Sb(III) and monomethyl antimony are only found in the mixed layer (Fig. 2), and dimethyl antimony was not detectable in any samples for the entire cruise; DMSb thus will be omitted from the remaining discussions. Nevertheless, in the mixed layer MMSb is 14% of the total dissolved Sb, while Sb(III) is 9% of the total, much larger fractions than observed for arsenic. The existence of Sb(III) and MMSb in these oligotrophic surface waters suggests a relationship with biotic uptake and conversion like

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Fig. 1. Vertical profiles of As(III+V), As(III), mono- and dimethyl arsenic, and chlorophyll a at IOC Stations 10 (closed symbols) and 8 (open symbols). Note that the upper 300 m are plotted on an expanded scale.

that of As, but the chlorophyll maximum is at 102 m while those of the Sb species are at 5 m. However, studies of As conversions by marine phytoplankton (e.g., Andreae and Klumpp, 1979; Sanders and Riedel, 1993) show significant differences in the ability of specific phytoplankton species to reduce or methylate As. Thus, the depth distributions of phytoplankton species may obscure simple correlations between Chl a, Sb(III), and MMSb; this topic will be explored later using the horizontal transect data set. In the deep waters, the Sbi concentrations in the NADW are substantially lower than the ‘‘preformed’’ value of 1.32 nmol/l reported by Cutter and Cutter (1998), but consistent with the behavior of a mildly scavenged element that is removed to particulate matter during the transit of this water mass. Interestingly, the young AABW has a low Sbi concentration, and given that this water mass travels along the bottom, it may accumulate Sbi from sediment-water fluxes like other scavenged elements (e.g., Al; Orians and Bruland, 1986); the stations to the north will be examined for this prediction. At Station 8 in the Brazil Basin, Asi only displays very minor surface depletion (ca. 1.6 nmol/l, relative to deep waters), while arsenite has a well-developed subsurface maximum in the upper thermocline (0.37 nmol/l; 1.8% of total As), and the methylated As species both have their maxima at the surface (0.15 nmol MMAs/l or 0.9% of total; 0.52 nmol DMAs/l or 3% of total As; Fig. 1). With respect to the minimal surface depletion, it is noteworthy that phosphate has its highest surface water concentration at Station 8 (0.24 mmol/l; Cutter and Measures, 1999), suggesting a competitive inhibition effect (lower arsenate uptake in the presence of phosphate) as originally documented by the laboratory studies of Sanders (1979). However, elevated phosphate

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Fig. 2. Vertical profiles of Sb(III+V), Sb(III), and monomethyl antimony at IOC Stations 10 (closed symbols) and 8 (open symbols). Note that the upper 300 m are plotted on an expanded scale.

concentrations apparently do not preclude the production of arsenite and methyl arsenic species, as shown by the data in Fig. 1. Arsenite and methylated arsenic production also were observed in the presence of 0.4–2.4 mmol/l phosphate during the phytoplankton culturing experiments of Andreae and Klumpp (1979). Unfortunately, the differences in surface-water residence times for arsenate (ca. 490 years; note that the residence time in Cutter and Cutter, 1998, was in error), methylated arsenicals (days; see below), and arsenite (months; see below) make a mass balance calculation to quantify depletion versus enrichment difficult. In the deeper waters, Asi displays remarkably constant concentrations (19.4  0.3 nmol/l in the NADW, 1900–3300 m, and 19.3  0.3 nmol/l in the AABW, 4700–5200 m), slightly lower than those at Station 10 (Fig. 1). Perhaps the most interesting subsurface distribution is that of arsenite, which displays a prominent maximum in the Antarctic Intermediate Water (AAIW, ca. 700–850 m; As(III) of 0.09 nmol/l or 0.5% of total As) and Upper Circumpolar Deep Water (UCDW, ca. 900–1500 m; As(III) of 0.10 nmol/l or 0.6% of total). These same concentrations were found in the AAIW and UCDW at Station 10 and may reflect the recent surface origins of these water masses (Reid, 1989), but at Station 8 arsenite drops below detection limits in the upper NADW, creating the apparent arsenite maximum. The depth profile of Sbi at Station 8 (Fig. 2) shows highest concentrations in the mixed layer (1.46 nmol/l), a decrease into the AAIW (average Sbi of 0.60 nmol/l), and then a gradual increase through the UCDW (0.68 nmol/l), NADW (0.72  0.15 nmol/l), and finally the AABW

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(0.80  0.13 nmol/l). This type of profile is one that largely resembles that of a scavenged element with an atmospheric source, as has been suggested in the eastern basins of the Atlantic (Cutter and Cutter, 1995), high-latitude North Atlantic (Cutter and Cutter, 1998), and Mediterranean (Takayanagi et al., 1996). The elevated Sbi in the deepest samples of AABW supports the prediction that AABW should have increasing concentrations of Sb as this water mass travels northward and receives inputs from sediment fluxes. The Sb(III) profile is similar to that at the other stations, with detectable concentrations only in the mixed layer (maximum of 0.02 nmol/l or 1.3% of total Sb; Fig. 2). The MMSb distribution is also restricted to the upper 200 m (Fig. 2), but it has a maximum (0.32 nmol/l or 21% of total Sb) in the upper thermocline like that of Chl a (Fig. 1), suggesting a relationship between MMSb production and phytoplankton. Station RF was placed in the middle of the deep Romanche Fracture zone since it is the primary means for deep NADW and AABW to enter the eastern basins of the North and South Atlantic (Schlitzer, 1987), allowing a comparison with our earlier As and Sb data in the eastern Atlantic (Cutter and Cutter, 1995). The depth profile of Asi (Fig. 3) shows considerable surface depletion (6 nmol/l relative to deep waters) and a regenerative-type increase with depth. Although there are some variations with increasing depth, the concentrations of Asi are relatively constant in the AAIW (600–800 m, 20.3  1.7 nmol/l), NADW (1700–4000 m, 19.9  1.6 nmol/l), and AABW (4800–5300 m, 20.4  0.8 nmol/l). NADW at Station RF has a 66% northern composition (Cutter

Fig. 3. Vertical profiles of As(III+V), As(III), mono- and dimethyl arsenic, and chlorophyll a at IOC Stations RF (closed symbols) and 6 (open symbols). Note that methylated arsenic species were not determined at Station 6 and the upper 300 m are plotted on an expanded scale.

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and Measures, 1999), indicating considerable mixing with southern waters (i.e., AABW, UCDW), but Asi in the NADW at Station RF is nearly identical to that in the NADW of the eastern North Atlantic (20.9  0.5 nmol/l) and South Atlantic (18.6  1.1 nmol/l; Cutter and Cutter, 1995). Arsenite (0.29 nmol/l, 2% of total As), MMAs (0.15 nmol/l, 1% of total), and DMAs (0.55 nmol/l, 3.8% of total) all have maxima in the shallow 20-m mixed layer, in proximity to the Chl a maximum at 50 m (Fig. 3). While the methyl species are restricted to the upper 200 m, arsenite is found throughout the deep-water masses, with an average concentration of 0.06 nmol/l (0.3% of total As), and one cannot discern any relationship between water mass and arsenite concentration as was noted at Stations 10 and 8. Sbi shows extreme surface enrichment (2.2 nmol/l), so much so that the concentration scale had to be expanded compared to those at the other stations (Fig. 4). This station is in the equatorial region that receives considerable input of aerosols from North Africa and southern Europe (Duce et al., 1991; Prospero, 1996; Vink and Measures, 2001), and the high concentration of surfacewater Sbi is likely a result; this topic will be explored later using the surface transect data set. Below this surface maximum, Sbi has its lowest concentrations in the AAIW (0.87  0.21 nmol/l) and increases into the NADW (1.07  0.27 nmol/l) and AABW (1.13  0.07 nmol/l). The comparison between the Sbi concentration in the NADW within the Romanche Fracture zone

Fig. 4. Vertical profiles of Sb(III+V) and Sb(III) at IOC Stations RF (closed symbols) and 6 (open symbols). Note that monomethyl antimony was not determined at either of these stations and the upper 300 m are plotted on an expanded scale.

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and those in the eastern North Atlantic (1.12  0.19 nmol/l) and South Atlantic (1.08  0.09 nmol/l; Cutter and Cutter, 1995) is quite favorable and shows little change during the 580-year transport time between these sites (Broecker et al., 1991). Sb(III) was only detectable in three samples from the upper 200 m (maximum of 0.01 nmol/l or 1.7% of total Sb; Fig. 4), and no MMSb data were generated due to instrument failure. Station 6 was the vertical profile site located furthest northwest, and hence had the youngest NADW (ca. 80 years; Broecker et al., 1991) and AABW that had undergone considerable dilution (Cutter and Measures, 1999). The depth profile of Asi is remarkably simple (Fig. 3), with moderate surface depletion (4.4 nmol/l relative to waters below) and then very uniform concentrations in the deep water masses (20.2  0.6 nmol/l in the AABW, 4500–4600 m, and NADW, 1700–4200 m). Within the observed errors, this NADW value is identical to those found at the other stations, and is 2.5 nmol/l higher (14% increase) than the ‘‘preformed’’ value in the high-latitude North Atlantic (Cutter and Cutter, 1998). Arsenite (Fig. 3) has a large surface maximum (1.2 nmol/l, 7.8% of total As), a rapid decrease into the major thermocline, and then a slight increase into the AAIW (600–900 m; 0.08 nmol/l, 0.4% of total), like those found in the southern hemisphere stations. No determinations of arsenite were made in the deeper waters, and samples for methylated As were not collected at Station 6. Like Station RF, Sbi displays a pronounced surface maximum (1.52 nmol/l; Fig. 4), a decrease in the major thermocline, and then unlike the southern stations, an increase in the AAIW (1.11 nmol/l). Below the AAIW maximum (and subsequent decrease into the UCDW), Sbi increases in the NADW (1.07  0.07 nmol/l) and AABW (1.27  0.01 nmol/l). While the AABW was restricted to the bottom 100 m at Station 6 (Cutter and Measures, 1999), it had the highest Sbi concentration of the four previous stations, consistent with an increase from south to north along the AABW flow path due to sediment-water fluxes. Such behavior is identical to other scavenged elements such as aluminum (Orians and Bruland, 1986). Like the other stations, Sb(III) was only detectable in the upper portion of the water column (3 depths; Fig. 4). The two Amazon stations (A-1 and A-2) were located in the plume of the Amazon River, which extends hundreds of kilometers offshore during the high flow season (Lentz, 1995). Sampling only covered the upper 500 m of the water column, but the profiles of Asi (Fig. 5) are remarkable because of their low concentrations, the lowest to our knowledge of any in the open ocean (9.83 nmol/l at 2 m). This depletion of Asi appears to be due in part to dilution by the Amazon River whose waters are low in Asi because of intense iron oxyhydroxide scavenging (Sullivan and Aller, 1996). In support of this assertion, a river sample (pH 1.6 HCl preserved) collected by P. Lechler (Nevada Bureau of Mines and Geology) at 38130 S, 5980.20 W on 27 May 1997 (i.e., a year later, but also during high flow) had an Asi concentration of 2.8 nmol/l, much lower than the geometric mean of 5.5 nmol/l for rivers in North America and Asia (Byrd, 1990) or 46 nmol/l for European rivers (Andreae and Froelich, 1984); the extent of this Amazon dilution will be apparent in the horizontal data presented below. At the Amazon stations, arsenite (A-1, 1.9 nmol/ l or 19.5% of total; A-2, 0.7 nmol/l or 5.3% of total) and the methyl As species (maximum of 0.2 nmol MMAs/l or 1.7% of total at A-1; maximum of 1.1 nmol DMAs/l or 8% of total at A-1) showed well-developed maxima in the mixed layer and upper thermocline (Fig. 5). The significant enrichment of arsenite in the Amazon Plume also is found in the horizontal transect data and will be discussed below. At Station A-2 we examined the relationship between the chlorophyll and arsenite maxima more closely using high resolution sampling with the CTD/rosette. The results in

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Fig. 5. Vertical profiles of As(III+V), As(III), mono- and dimethyl arsenic, and chlorophyll a at IOC Stations A-1 (closed symbols) and A-2 (open symbols) in the Amazon Plume.

Fig. 6 show an arsenite maximum at the surface that is not associated with a corresponding Chl a maximum, but the deeper maxima in these two parameters are closely matched r ¼ 0:915, n ¼ 10). Andreae (1979) only observed loose correlations between arsenite and chlorophyll in the Pacific (albeit at lower sampling resolution), but the data in Fig. 6 indicate two potential mechanisms of production, one in surface waters and one related to the deep chlorophyll maximum. Arsenite production in surface waters may be by a photochemical means, or associated with a distinct biological community (e.g., different phytoplankton species than those at the deep Chl a maximum). The antimony data from the Amazon stations are also noteworthy for several reasons. First, although there is a near-surface maximum in Sbi at A-1 (Fig. 7), Station A-2 contains no Sbi maximum, and the subsurface concentrations are quite low (average of 0.5 nmol/l) at both sites. Secondly, the Sb(III) concentrations are the highest measured on the cruise (maximum of 0.06 nmol/l or 18% of total Sb at Stn. A-2) and penetrate much deeper than at the other IOC stations (Fig. 7). Indeed, the Sb(III) values approach those found by Middelburg et al. (1988) in the North Atlantic. MMSb data were obtained at Station A-2, and the depth profile (Fig. 7) is similar to the other offshore stations (i.e. maximum of 0.09 nmol/l or 16% of total Sb, and restricted to the mixed layer). We determined Sbi in the 1997 Amazon River sample, and like Asi, Sbi was very depleted (0.25 nmol/l) compared to the geometric mean of 1.0 nmol/l for 40 rivers in Asia and North America (Byrd, 1990) or 2.04 nmol/l for nine European rivers (Andreae and Froelich, 1984), suggesting removal by iron oxyhydroxides in the Amazon system. Thus, the

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Fig. 6. High resolution vertical profiles of As(III) and chlorophyll a at IOC Station A-2 taken using a CTD/rosette.

surface enrichment of Sbi typically observed at other oceanic stations could have been diluted with these low Sbi Amazon waters. 3.2. Horizontal transect The horizontal transect from Uruguay to Barbados between the vertical profile stations crossed a number of oceanographic features, including the Brazil Current, the South Atlantic Subtropical Gyre, the South Equatorial Current (SEC), the equatorial upwelling, the Intertropical Convergence Zone (ITCZ), and the Amazon Plume; for a discussion of the hydrographic conditions in these regions, see Cutter and Measures (1999) and Vink and Measures (2001). The arsenic data are plotted as a function of distance along the transect, and Asi displays its highest concentration in the Brazil Current (19.9  2.5 nmol/l), and with the exception of the SEC (17.8  1.1 nmol/l), decreases to the north and northwest (Fig. 8). The lowest Asi concentrations are found in the low-salinity Amazon Plume (11.3  2.9 nmol/l; Fig. 8; salinity data in Fig. 9), demonstrating the large areal extent of the Amazon’s influence. In general, the concentrations of Asi in surface waters of the western and equatorial Atlantic (average of 16.3  2.1 nmol/l, with the exception of the Amazon Plume data) are slightly higher than those in the eastern Atlantic (12.9  1.8 nmol/l; Cutter and Cutter, 1995), but identical to those in the high-latitude North Atlantic (15.7  1.3 nmol/l; Cutter and Cutter, 1998). Arsenite has much larger changes along the transect than Asi (Fig. 8), with elevated concentrations in the Brazil Current and Subtropical Gyre

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Fig. 7. Vertical profiles of Sb(III+V), Sb(III), and monomethyl antimony at IOC Stations A-1 (closed symbols) and A-2 (open symbols) in the Amazon Plume. Note that monomethyl antimony was not determined at Station A-1.

(averages of 0.62 and 0.67 nmol/l, respectively), very low concentrations (0.13 nmol/l) in the SEC where phosphate levels are elevated (0.21 mmol/l; Fig. 8 and Cutter and Measures, 1999), and increasing concentrations through the equatorial upwelling (0.36 nmol/l), ITCZ (0.55 nmol/l), and Amazon Plume (average of 0.89 nmol/l). Thus, arsenite averages 1–4% of the total dissolved As in these regimes with the exception of the Amazon Plume, where it is up to 9% of the total. An examination of the horizontal distributions of phosphate (Cutter and Measures, 1999) and arsenite (Fig. 8) shows a definite trend of increasing arsenite with decreasing phosphate, and a correlation coefficient ðrÞ of
0.64 (n ¼ 43; slope=
4.4  10
3 mol As/mol P, y intercept=1.0 nmol/l As III) is obtained for a linear fit. Based on the lab and field observations of Andreae (1979), Andreae and Klumpp (1979) and Sanders and Riedel (1993), as well as the biochemical arguments of Benson (1984), such a relationship would be expected (i.e. reduction to arsenite to mitigate ‘‘arsenate stress’’), but the linear fit is surprisingly good considering the two compounds have short surface water residence times (e.g., arsenite’s is 25–100 days with respect to oxidation; Johnson and Pilson, 1975). Arsenite on the horizontal transect only has a weakly positive correlation with Chl a ðr ¼ 0:286Þ, which is consistent with previous investigations (Andreae, 1979) since chlorophyll is only a gross measure of phytoplankton abundance and arsenite production varies with phytoplankton species and productivity. While the data strongly support a biotic source for surface-water arsenite, it should be acknowledged that photochemical reduction (e.g., as documented for the production of Fe(II), Miller et al., 1995) also could contribute.

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Fig. 8. Surface water concentrations of As(III+V), As(III), mono- and dimethyl arsenic, and phosphate as a function of distance along the 1996 IOC transect from Montevideo, Uruguay (see Cutter and Measures (1999) for a chart with the actual cruise track). Major oceanographic features are noted in the lower panel, while the location of the vertical profile stations where major course changes occurred are identified just below the distance axis.

The horizontal distributions of MMAs and DMAs (Fig. 8) are considerably different than that of arsenite, with MMAs remaining relatively constant (0.15  0.05 nmol/l or 1% of total As), while DMAs generally increases as the transect proceeds northward (e.g., 0.21 nmol/l, 1% of total As, in the Brazil Current to 1.29 nmol/l, 8% of total As, in the ITCZ). In contrast to arsenite, MMAs and DMAs do not display any simple relationship with phosphate (Fig. 8), and in fact are weakly positive (r ¼ 0:10 for DMAs; r ¼ 0:26 for MMAs), while MMAs and DMAs have relatively strong negative correlations with chlorophyll a (r ¼
0:843 for MMAs; r ¼
0:510 for DMAs); the data of Andreae (1979) for the oligotrophic North Pacific show similar behavior. These complicated relationships are likely due to a variety of factors, including phytoplankton species-specific production (e.g., Andreae and Klumpp, 1979; Sanders and Riedel, 1993); primary production, not chlorophyll a concentration, being a better predictor of methylated arsenic concentrations (i.e., the rate of production, not biomass, is more important; Andreae, 1979); increased methylation with increased water temperature (i.e. the transect started in the Southern

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Hemisphere fall and ended in the Northern Hemisphere summer; Howard and Apte, 1989); and bacteria, as well as phytoplankton, can methylate arsenic (Apte et al., 1986). Any or all of these processes would lead to concentrations that do not correlate with conventional hydrographic parameters such as the concentrations of nutrients and chlorophyll a. Antimony results from the horizontal transect (Fig. 9) are much more variable than those for arsenic (Fig. 8). Sbi concentrations decrease from elevated values in the Brazil Current (1.24  0.35 nmol/l), but then increase in the southernmost Subtropical Gyre before returning to low values in these oligotrophic waters (0.75  0.20 nmol/l). Proceeding northward, Sbi (Fig. 9) steadily increases in the South Equatorial Current (1.32  0.12 nmol/l), the Equatorial upwelling zone with the highest values measured on the transect (average of 1.44  0.57 nmol/l), and then the ITCZ (1.25  0.36 nmol/l). These Sbi concentrations are similar to those found in surface

Fig. 9. Surface water concentrations of Sb(III+V), Sb(III), monomethyl antimony, and salinity as a function of distance along the 1996 IOC transect from Montevideo, Uruguay (see Cutter and Measures (1999) for a chart with the actual cruise track). Major oceanographic features are noted in the lower panel, while the location of the vertical profile stations where major course changes occurred are identified just below the distance axis. Note the scale breaks on the Sb species and salinity axes.

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waters of the eastern Atlantic (1.12  0.25 nmol/l; Cutter and Cutter, 1995), but somewhat lower than those in the high latitude North Atlantic (1.87  0.38 nmol/l; Cutter and Cutter, 1998). The Amazon Plume has the lowest concentrations measured on the 1996 IOC transect (Fig. 9), but due to high variability in this region (in or out of the plume; see salinity data in Fig. 9), the average concentration (0.79  0.28 nmol/l) is about the same as that in the Subtropical Gyre. Like arsenic, the Amazon discharge has a major influence on the concentration of Sbi in the western Equatorial Atlantic, in this case lowering its concentrations (see A-1, A-2, and river data above). However, a strong input also must be operating in these low latitudes since the highest values are found from about 28S to 88N (Fig. 9), the region encompassing the northern portions of the SEC, the Equatorial Upwelling, and the ITCZ. Earlier data from the eastern Equatorial Atlantic (Cutter and Cutter, 1995) also found elevated Sbi in surface waters from 38S to 28N, which corresponded to waters where surface aluminum was elevated. In a companion paper, (Vink and Measures, 2001), they show that Al increased approaching the Equator, largely due to dust inputs from North Africa. It has been argued that atmospheric deposition elevates surface water Sbi (Cutter, 1993; Cutter and Cutter, 1995; 1998; Takayanagi et al., 1996), and while mineral aerosols are low in Sb, Al can be used as an indicator of enhanced atmospheric flux from North Africa and perhaps entrained southern European air masses (i.e. fossil fuel combustion is a more likely source of Sb to the atmosphere than mineral aerosols; Nriagu, 1989; Nriagu and Pacyna, 1988; Arimoto et al., 1991; 1995). However, the surface-water residence times for these elements are sufficiently different (ca. 3 years for Al, Orians and Bruland, 1986; ca. 240 yr for Sbi; note there was a calculation error in the residence time reported by Cutter and Cutter, 1998) that the input signal could be decoupled, obscuring any correlation (e.g., Sb can mix much farther than Al). Plotting the Sbi vs. Al (data from Vink and Measures, 2001) from the IOC transect in the Equatorial Upwelling and ITCZ regions yields a correlation coefficient ðrÞ of 0.3214 (m ¼ 4:12, b ¼ 0:03), not a robust fit, but one that supports an atmospheric source. The strength of the atmospheric source will be evaluated quantitatively in the section to follow. Sb(III) is uniformly low across the entire transect (50.03 nmol/l, 3% of total Sb; Fig. 9), with highest values in the Amazon Plume, and MMSb is relatively constant on the 11,000-km transect (0.13  0.07 nmol/l or 10% of total Sb), with the exception of one high value in the Subtropical Gyre that corresponded with equivalent MMAs and DMAs maxima (Fig. 8). The distribution of MMSb is nearly identical to that of MMAs, and since the phytoplankton studies of Andreae and Klumpp (1979) documented MMAs production, the implication is that MMSb is also produced by phytoplankton. However, neither Andreae and Froelich (1984) nor Kantin (1983) detected MMSb in phytoplankton or macro algae (respectively), with the former workers suggesting methylation by bacteria as the likely source of water column MMSb. The existing transect data cannot resolve the actual source of MMSb, but its widespread distribution suggests either very uniform production, or more likely, a long surface-water residence time to allow mixing. Similarly, the very low concentrations of Sb(III) display no obvious correlations with hydrographic parameters (e.g., phosphate in Fig. 8 and salinity in Fig. 9), making it difficult to identify sources for a transient chemical species with a residence time with respect to oxidation of at least 125 days (Cutter, 1992; probably an underestimate as this was calculated at the suboxic– anoxic interface of the Black Sea where the presence of manganese and iron oxides is likely to increase the rate). Kantin (1983) detected Sb(III) in one of three macro algae tested, while Andreae and Froelich (1984) found Sb(III) in phytoplankton from the Baltic Sea (0–80% of the

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total inorganic Sb), indicating potential biotic production. However, one would expect a more variable distribution like that for arsenite (Fig. 8) if bioreduction is occurring (i.e. with a short residence time, concentrations should closely mirror production). Given these considerations, an abiotic process such as photochemical reduction (i.e., Fe(II) production; Miller et al., 1995) may better explain Sb(III) in surface waters. 3.3. Biogeochemical cycling of antimony and arsenic in the western and Equatorial Atlantic Data from the Atlantic vertical profiles and horizontal transects do not give us a profoundly different view of the biogeochemical cycle of arsenic than was revealed in the earlier work of Andreae (1979). They do, however, provide some refinements, particularly with respect to the role of phosphate in arsenate depletion (uptake) and the production of arsenite. In this respect, the horizontal data set (Fig. 8) suggests that when phosphate levels drop below ca. 0.05 mmol/l (observed As/P atomic ratio of 0.25  0.04, n ¼ 17), arsenate uptake and the production of arsenite increases. This As/P ratio is substantially lower than the value of 1.0 in the North Pacific where Andreae (1979) found evidence of arsenate stress. While this production of arsenite is in the upper water column and positively correlated with phytoplankton biomass (e.g., Fig. 6), the presence of arsenite in deep waters with 14C ages older than 160 years (Broecker et al., 1991) requires an additional explanation. Potential reasons include that its residence time is much longer than that calculated simply from its oxidation rate (e.g., Johnson and Pilson, 1975; Cutter, 1992), it is delivered to deeper waters on sinking detritus and released (As(III) found in sediments and phytoplankton; Andreae and Froelich, 1984; Cutter, 1992), or bacterial reduction of arsenate occurs in deep waters (Johnson, 1972). The present data set does not provide compelling evidence for any of these possibilities. The methylated arsenicals also have biotic sources in the upper water column, but their residence times must be much less than that of arsenite (i.e., weeks; Andreae, 1979) as they do not penetrate into the deep or intermediate water masses (e.g., Figs. 1, 3 and 5). In contrast to arsenic, the biogeochemical cycle of antimony is still being revealed and the western and Equatorial Atlantic data provide considerable verification for the notion of this element behaving as a scavenged element like aluminum (Middelburg et al., 1988; Takayanagi et al., 1996; Cutter and Cutter, 1998), but with a much longer residence time (a factor of ca. 20). The surface Sbi maxima at most stations and increasing concentrations along the AABW flow path (Figs. 2 and 4) are the best examples of this type of behavior. Unlike aluminum, antimony has multiple chemical species. The similarity between methyl antimony and arsenic profiles is indicative of a largely phytoplanktonic source. Unfortunately, existing phytoplankton data (e.g., Andreae and Froelich, 1984) do not support a direct causal relationship, and bacteria may be responsible for antimony methylation. Similarly, while it is tempting to ascribe Sb(III) production to phytoplankton as for the case of As(III), the lack of any correlation with bio-indicators such as chlorophyll, and the presence of Sb(III) only in the shallowest depths (except in the 500-m profiles at the two Amazon stations) are inconsistent with a phytoplankton source. Future studies should investigate whether Sb(III) can be produced by photochemical reduction. For both antimony and arsenic, perhaps the most interesting findings are the extremely low concentrations of these elements in the Amazon River and plume samples, and the low arsenic, but elevated antimony, concentrations in equatorial waters (Figs. 8 and 9); the area of interest is roughly from 28S to 88N on the transect, and includes the vertical profile stations RF, 6, A-1, and

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A-2. This is a very dynamic physical and biotic regime, and for most trace elements two primary allochthonous inputs are Amazon discharge (e.g., DeMaster et al., 1983; Nittrouer and DeMaster, 1996) and atmospheric deposition from North Africa (e.g., Duce et al., 1991). With respect to the Amazon, studies have shown that the river plume extends hundreds of kilometers offshore (Lentz, 1995), and between June to December can be entrained into the North Brazil Current retroflection (but, not continuously) and travel in the North Equatorial Counter Current as far east as Africa (Limeburner et al., 1995). Based on Coastal Zone Color Scanner images and drifter studies, Muller-Karger et al. (1988) have argued that the lowered salinities found between 58N and 108N (e.g., Fig. 9 and Cutter and Measures, 1999) are not just due to precipitation in the ITCZ, but have an Amazonian source as well. While Amazon water ultimately circulates to the Carribean, the work of Muller-Karger et al. (1988) suggests that up to 60% of the Amazon’s 6  1012 m3 yearly discharge of fresh water travels in this fashion (the remaining 40% travels northeast and directly into the Carribean). Therefore, the Amazon may affect trace element budgets across the tropical Atlantic. The importance of another trace element input to this region, atmospheric deposition, was recognized over two decades ago by Buat-Menard and Chesselet (1979). In this respect, we can compare the relative importance of Amazon input versus atmospheric deposition using our data set for the Amazon River, data from the Atmosphere–Ocean Chemistry Experiment (AEROCE) sampling site on Barbados (Arimoto et al., 1991), and the early data of Buat-Menard and Chesselet (1979). The region of interest, 28S to 88N, has an approximate area of 6  106 km2, and applying 60% of the Amazon’s discharge times the measured concentrations of As (2.8 nmol/l) and Sb (0.25 nmol/l), yields an antimony flux of 0.9  106 mol yr
1 and an arsenic flux of 9.6  106 mol yr
1. We have a very limited ðn ¼ 9Þ set of precipitation samples from the AEROCE site on Barbados collected in 1989, with an volume weighted average concentration of 0.26 nmol/l for Sb and 0.17 nmol/l for As. Using the average annual rainfall on Barbados of 127 cm, the As wet depositional flux is 211 nmol m2 yr
1 and the antimony flux is 327 nmol m2 yr
1. Although it might seem questionable to apply these fluxes to the rest of the tropical Atlantic, Buat-Menard and Chesselet (1979) estimated that the atmospheric flux of antimony to the tropical North Atlantic (0–108N) is 287 nmol m2 yr
1, within 12% of our measurements. While they did not measure arsenic, comparisons of selenium flux measured at Barbados (Cutter and Cutter, 2001) and that of Buat-Menard and Chesselet (1979) are within 20%, suggesting that it is not unreasonable to apply these Barbados fluxes to the entire region. In this fashion, the atmospheric input of antimony over the 28S to 88N area is 2  106 mol yr
1 and the atmospheric arsenic flux is 1.3  106 mol yr
1. Thus, atmospheric deposition delivers twice as much antimony to this region than does the Amazon, consistent with the observed profiles and findings from other oceanic regions (e.g., Cutter, 1993; Cutter and Cutter, 1998). Atmospheric deposition is only about 10% of that from the Amazon for arsenic, and therefore plays a much smaller role. Of course, these flux calculations are based on a very limited data set, and seasonal measurements and process studies in the Amazon, as well as extensive atmospheric deposition studies, would increase their accuracy and mechanistic underpinnings. Moreover, the source of elevated Sb in North African air masses remains elusive since mineral aerosols have low Sb content; it is likely that some combustion inputs are entrained in the air masses from North Africa (Nriagu, 1989; Arimoto et al., 1991, 1995). Overall, the biogeochemical cycles of arsenic and antimony in the western Atlantic have these allochthonous inputs superimposed upon the biotic (uptake, reduction,

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methylation, and demethylation) and abiotic (i.e. scavenging) processes that result in the observed distributions and speciation. Acknowledgements The authors thank the IOC Go Flo team lead by W. Landing and J. Dalziel for sample acquisition, P. Ganguli for tireless sample analyses at sea, P. Clement for nutrient data, K. Grembowicz for chlorophyll analyses, R.C. Kidd for salinities, the master and crew of the R.V. Knorr, and A. Howard and an anonymous reviewer for helpful comments on the original submission. A. Featherstone also thanks E. Butler for his advice and guidance. This work was supported by the United States National Science Foundation (Grants OCE-9523159 for the IOC cruise and ATM-8701710 for the Barbados measurements, both to G. Cutter), the Intergovernmental Oceanographic Commission, and the Habitat Chemistry Section of the Marine Environmental Science Division, Department of Fisheries and Oceans, Bedford Institute of Oceanography.

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