Spatial and temporal distribution of iron in the surface water of the northwestern Atlantic Ocean

Geochimica et Cosmochimica Acta, Vol. 60, No. 15, pp. 2729-2741, 1996 Copyright © 1996 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/96 $15.00 + .00

Pergamon

P I I S0016-7037(96) 00135-4

Spatial and temporal distribution of iron in the surface water of the northwestern Atlantic Ocean JINGFENG WU * and GEORGE W. LUTHER III College of Marine Studies, University of Delaware, Lewes, DE 19958, USA (Received February 22, 1995; accepted in revised form April 15, 1996)

A b s t r a c t - - T h e spatial and temporal distribution of three size fractions ( < 0 . 2 # m , < 0 . 4 # m , and unfiltered) of total Fe were studied in four cruises (in October 1991, July 1992, M a r c h 1993, and M a y 1993) along a transect from the m o u t h of Delaware Bay across the G u l f Stream to the Sargasso Sea. Results show that Fe in all fractions decreased dramatically with increasing distance from shore and with increasing salinity ( 0 . 3 - 1 4 n M for < 0 . 2 # m Fe, 0 . 1 5 - 1 . 3 n M for 0 . 2 - 0 . 4 # m Fe, and 0 . 8 - 1 1 1 0 0 n M for > 0 . 2 # m Fe fraction). C o n c a v e < 0 . 2 # m Fe-salinity curves m a y result from several processes, including Fe removal by particle scavenging and phytoplankton uptake, shelf sedimentary Fe input, atmospheric Fe deposition, and multiple water e n d m e m b e r mixing. Significant removal of > 0 . 2 # m Fe in the shelf water may have important implications for estimating riverine Fe inputs to the open ocean. M a x i m u m concentrations of > 0 . 2 / z m Fe ( 1 . 5 - 3 . 5 n M ) and major nutrients were observed at the shelf/ slope front, probably due to e n h a n c e d vertical mixing and convergence circulation at the frontal zone. Significant temporal variations in Fe concentrations were also observed. In M a r c h 1993, e n h a n c e d riverine discharge, vertical mixing, and b o t t o m resuspension contribute to significantly higher Fe concentration in shelf waters than in other seasons. Seaward advective Fe transport from the continental shelf leads to elevated Fe concentrations in offshore waters at this time. 1. INTRODUCTION

ferent oceanographic e n v i r o n m e n t s (including shelf water, slope water, the G u l f Stream, and the Sargasso sea, Fig. 1 ), which provide us an opportunity to study the processes controlling Fe biogeochemical cycling in surface water.

The distribution of Fe in seawater is controlled by a combination of input, removal, and internal cycling processes, including atmospheric deposition, vertical and horizontal mixing and advection, photochemical reactions, phytoplankton uptake and biological recycling, and particle scavenging ( D u c e and Tindale, 1991; Landing and Bruland, 1987; Hutchins et al., 1993). These processes are expected to respond to seasonal variations of biological, physical, and atmospheric processes in marine e n v i r o n m e n t s (Steele, 1981), leading to variations in seawater Fe concentration. During the last decade, with the d e v e l o p m e n t of trace metal " c l e a n " techniques, reliable oceanic Fe data have been obtained ( S y m e s and Kester, 1985; Landing and Bruland, 1987; Martin and Gordon, 1988; Saager et al., 1989; Bruland et al., 1991, 1994; Martin et al., 1993; W u and Luther, 1994). However, these data are mainly from single vertical profiles and were collected at different times, from different locations. Few high quality Fe data exist to allow the description of the spatial and temporal variations of Fe in marine surface waters. Studies addressing these variations are essential to understanding marine Fe biogeochemical cycling and its relation to biological production in the sea. In this study, the distributions of Fe in three size fractions ( < 0 . 2 / ~ m , < 0 . 4 #m, and unfiltered) were investigated during four cruises (in October t991, July 1992, M a r c h 1993, and M a y 1993) along a surface water transect in the northwestern Atlantic. This transect e n c o m p a s s e s a variety of dif-

2. STUDY AREA Water circulation in our sampling area is generally characterized by a southwest alongshore flow of Delaware Bay water within 30 Km of the bay mouth (Garvine, 1991 ), a southwest alongshelf mean flow of shelf water near the shelf break (Beardsley and Boicourt, 1981 ), and a northeast flow of the Gulf Stream which separates cool, low-salinity slope water from warm, high-salinity Sargasso Sea water. These circulations are complex and seasonally variable and include the meandering of the Gulf Stream, the synoptic feature of wind stress and alongshelf flow (Beardsley and Boicourt, 1981), and the drift of warm core eddies in the slope water and the cold core eddies in the Sargasso Sea water. According to satellite surface temperature data from NOAA (National Oceanic and Atmospheric Administration) for oceanographic features analysis, our sampling stations can be placed in the following water masses: shelf water, slope water, the Sargasso Sea, and Gulf Stream water (Fig. 1 ). The characteristics of these water masses are also evident in T-S diagrams (Fig. 2). 3. SAMPLING AND ANALYTICAL METHODS Our procedures for contamination-free sampling and analysis for Fe have been described in detailed by Wu and Luther (1994). Seawater samples were collected at 5 - 1 0 m below sea surface with an acid clean 10 L GO-Flo bottle and nonmetallic Kevlar wire. Upon recovery, three subsamples (<0.2 #m, <0.4 #m, and unfiltered) were collected from the single GO-Flo bottle. Filtration was performed with 47 mm diameter Nuclepore filters under pressure, using high-purity 0.2 lzm filtered nitrogen inside a class-100 clean bench (about a 1.2-1.5 L sample was passed through each filter). After collection, the samples were acidified with 1 mL quartz doubledistilled HNO3 per 1 L sample, triple bagged, and transported to the

* Present address: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.

2729

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LONGITUDE (W) FIG. I. Sampling locations in the northwestern Atlantic Ocean. Station 9 (March 1993) and Stations 9 - I 1 (October 1991 ) were in Sargasso Sea/Gulf Stream water; Stations 10-17 (March 1993), I I - 1 7 (May 1993), 13-15 (July 1992),and 13-16 (October 1991) were in slope water; Stations 18- 23 (March 1993, May 1993 ), 16 21 (July 1992), and 17 23 (October 1991 ) were in shelf water.

land-based laboratory in Lewes for later analysis. We have observed no detectable blank (<0.1 riM) from the amount of HNO3 used for sample acidification and the acid washed filter used for filtration. In the laboratory, these samples were directly analyzed for Fe with an adsorptive cathodic stripping voltammetry method (CSV) (van den Berg et al., 1991; Wu and Luther, 1994) after UV irradiation for 6 h under 1 KW UV lamp. Iron measured by this procedure should include both inorganic and organic Fe fractions because of the acidification and UV irradiation prior to analysis. Quantification was made with a three-point standard addition method. The accuracy of the Fe measurement was daily calibrated with standard seawater NASS-4 (1.89 _+ 0.29 nM). Total reagent blank including Q-HNO3 was 0.05 _+ 0.03 nM. The detection limit for the method with 8 min deposition is 0.15 nM. The method has a precision of 10-15% at the 0.5 nM Fe level. Nutrients were analyzed by standard methods (Strickland and Parsons, 1972) on filtered and immediately frozen samples.

4. RESULTS AND DISCUSSION 4.1. Spatial Variation 4.1.1. Iron in <0.2 # m fraction The distribution of Fe in the < 0 . 2 # m fraction along the transect is s h o w n in Fig. 3 and Tables 1 - 4 . In all four cruises, Fe concentration showed an order of magnitude decrease from the m o u t h of Delaware Bay at station 23 (14.1 n M in M a r c h 1993, 5.0 n M in May 1993, and 7.2 n M in October 1991) to the shelf break at station 17 (1 n M in

M a r c h 1993, 0.5 nM in May 1993, and July 1992, 0.3 nM in October 1991 ) and was relatively constant from the shelf break to the G u l f Stream and the Sargasso Sea (stations 9 17) (Fig. 3). The high Fe concentration at station 23 for all transects indicated an Fe input from the Delaware Bay estuary. If this transect is considered to cross the mixing zone of two endmembers, the high-salinity waters of the Gulf Stream and the Sargasso Sea and the low-salinity shell: water near the mouth of Delaware Bay, one may plot Fe against salinity to examine its behavior during water transport from the river influenced coastal region to the open ocean. This kind of simplification has been applied by Bruland and Franks ( 1 9 8 3 ) in studying the distributions of Cd, Zn, Mn, Cu, and Ni along a surface water transect in the western north Atlantic. The Fe-S%,, relationship in Fig. 3 does not conform to a t w o - e n d m e m b e r Fe-salinity dilution line. If FeS%~ mixing curves can be used to predict element behavior for our sampling stations as in estuarine waters ( Sholkovitz, 1976; Boyle et al., 1977), then the concave Fe-S%c curves suggest an apparent " r e m o v a l " of < 0 . 2 /~m Fe during Fe transport from the continental shelf to the open ocean. Although dissolved Fe ( < 0 . 4 # m ) removal at low salinities ( < 10%~ ) during estuarine mixing has been well documented (e.g., Sholkovitz, 1976: Boyle et al., 1977; Church, 1986), few reports have been made on Fe removal in higher salinity ( > 2 9 % c ) waters. The assumption that the mixing of low salinity high Fe inner shelf water at station 23 and the high salinity low Fe Gulf Stream or Sargasso Sea water at station 9 produce shelf and slope waters at the other transect stations can be complicated by the existence of a southwesterly alongshore coastal current which transports low salinity inner shelf water away from our sampling stations (Garvine, 1991 ) and a southwesterly alongshelf mean flow at the shelf edge which brings intermediate salinity water masses from the north into our sampling area (Beardsley and Boicourt, 1981 ). In these cases, Fe removal cannot be unambiguously demonstrated from simple Fe-salinity plots for all our transect stations. An alternative treatment of the data would be the plot of Fe vs. salinity for stations on the linear region of temperature vs. salinity plot, if we assume that the air-sea heat exchange is insignificant during the mixing of water masses. The T-S diagrams (Fig. 2) show that our sampling stations can be generally divided into two mixing zones characterized by distinct linear lines in the T-S plots. The first mixing zone is characterized by the mixing of estuarine waters into the middle and outer shelf regions which occured at stations 21 23, 2 0 - 2 3 , and 1 9 - 2 3 for March 1993. May 1993, and October 1991 cruises, respectively. The second mixing zone is due to the mixing of outer shelf and slope waters with the Gulf Stream and the open ocean waters, which occured at stations 1 0 - 2 1 , 1 1 - 2 0 , and 1 1 - 1 9 , lk~r March 1993, May 1993, and October 1991 cruises, respectively. Firm conclusions for Fe b e h a v i o r in the first mixing region can not be obtained because of the poor sampling resolution of merely three data points for each of the cruises (Fig. 2 ), although these data in Fe-salinity plots may suggest a conservative mixing for < 0 . 2 p m Fe in this mixing zone. The Fe-Salinity plots for the second mixing region are shown in Fig. 4.

Geochemistry of iron in Atlantic surface water 30-

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The extent of apparent Fe "removal" follow the sequence: March 1993 > May 1993 > October 1991 > July 1992. While the destabilization and coagulation of Fe-organic colloids by seawater cations are the major mechanisms for dissolved Fe removal in the low-salinity estuarine waters (Boyle et al., 1977), the same mechanism should not be important for the observed Fe removal in the relatively high salinity coastal water. The sharp decline of the negative surface charge of suspended particles at < 10%o salinities results in the flocculation of riverine Fe-organic colloid (Hunter, 1983). In higher salinity coastal and open ocean waters, however, there is very little decrease of the particle surface charge with increasing salinity (Hunter and Liss, 1979; Hunter, 1983; Newton and Liss, 1989), suggesting that colloid coagulation should not be important. Thus, if there is a removal of <0.2 #m Fe in the high-salinity coastal water, it would be due to scavenging by marine inorganic/organic particles and living biota. The Fe removal in the surface euphotic zone by particle scavenging has been suggested from vertical Fe distributions in the Pacific and Atlantic oceans (Bruland et al., 1991, 1994; Wu and Luther, 1994). Martin et al. (1994) have reported the rapid Fe removal during Fe enrichment experiments in the equatorial Pacific Ocean. In their experiments, 3 - 4 nM dissolved Fe 2+ added

into surface seawater disappeared within days to below 0.2 nM. Most of the Fe(II) injected into the water can be attributed to ( 1) oxidation within minutes to Fe(III) and colloidal iron oxyhydroxides which precipitated with a first-order rate constant of 0.1 h ~(Johnson et al., 1994), (2) complexation by excess dissolved organic ligands (Rue and B ruland, 1995; van den Berg, 1995; Wu and Luther, 1995), and (3) active uptake by biota. The concentration of <0.2/zm Fe in shelf stations 17-23 (Fig. 3) are generally much higher than 0.2 nM. One would expect that Fe with such a high concentration in seawater should not stay long in dissolved form before it is removed onto marine particles and by sedimentation. Thus, it is logical to predict that removal of dissolved Fe should occur in the coastal zone during the Fe transport from the mouth of the bay to the open ocean. Like Fe, other strongly hydrolyzed and particle reactive metals such as A1 (Orians and Bruland, 1986), Pb (Schaule and Patterson, 1981), and Ti (Skrabal, 1994) also show removal in coastal surface waters. The distinct distributions of Fe, A1, Pb, and Ti significantly contrast with those for most other nonhydrolyzed trace metals (Mn, Ni, Cu, Cd), which usually behave conservatively during coastal mixing (Bruland and Franks, 1983; Kremling and Hydes, 1988). The seasonality of apparent "Fe removal" (Fig. 3) can

2732

J. Wu and G. W. Luther III 15-

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FIG. 3. Fe (<0.2 #m) vs. salinity plot for cruises in March 1993, July 1992, May 1993, and October 1991. Station numbers are shown in the figure.

be explained by temporal changes in particle scavenging. In March, significant vertical mixing indicated by the upward isopycnal surfaces (Fig. 5 ) generated high concentrations o f suspended particles from bottom sediment, which could

adsorb dissolved Fe onto surfaces and thus remove it from the dissolved phase. In May and October, water stratification prevented vertical mixing, but phytoplankton spring and fall blooms could produce high concentrations of biogenic parti-

Table 1. Hydrographic, nutrient and iron data on CHESS-8 (7-9 March 1993). Sample was taken from 10 m depth. NA - not analyzed Stn. No. 9 10 11 13 15 16 17 18 19 20 21 22 23

Latitude Longitude Theta (°C) 36.29°N 72.29°W 22.99 36.65°N 72.68°W 14.83 36.83°N 72.86°W 12.68 37.17°N 73.23°W 8.28 37.52°N 73.61°W 8.65 37.68°N 73.80~vV 7.18 37.85°N 73.98°W 7.11 38.03°N 74.17°W 6.02 38.21°N 74.36°W 5.29 38.37°N 74.54°W 4.64 38.54°N 74.73°W 4.06 38.72°N 74.91°W 3.23 38.88°N 75.09°W 3.16

Salinity (%o) 36.49 35.48 34.98 33.90 34.00 33.42 33.48 33.10 32.95 32.66 32.45 31.54 29.02

02 (p.M) 211 240 224 2'79 282 309 285 303 307 385 339 354 352

NO 3 (~M) 0.21 6.00 4.64 5.76 4.93 5.40 5.48 4.55 2.01 0.73 0.57 NA 0.64

P• 4 (p.M) 0.11 0.48 0.37 0.53 0.43 0.48 0.51 0.57 0.45 0.39 0.36 NA 0.41

Si <0.2 p~Fe <0.4 gFe (gM) (riM) (nM) 1.38 0.78_+0.03 0.71_+0.02 3.06 0.75_+0.05 0.81i-0.08 2.78 1.02!0.03 1.09i-0.05 3.41 0.89&0.07 1.29/,-0.05 2.13 1.08_+0.06 1.06_+0.05 2.7 NA 0.805.-0.06 2.27 1.00i-0.07 1.70"L-0.06 2.11 3.60-20.15 4.20~.08 1.38 4.80-L-0.09 4.87_+0.22 1.18 5.63-+0.20 5.58_+0.10 1.02 N A 7.30~.10 NA 10.8--~0.22 10.9~.10 7.28 14.1--~0.27 15.5_+0.15

Total Fe (nM) 3.05_+0.08 4.60L-0.20 11.20~.30 8.4-+0.22 NA 6.9-L-0.05 16.10&0.12 44.30",20.32 61.60-k0.87 54.201--1.50 450.0&11.0 190(~-43 11100k320

Geochemistry of iron in Atlantic surface water

2733

Table 2. Hydrographic, nutrient and iron data on CHESS-9 (18-20 May 1993). from 10 m depth. NA - not analyzed Stn No. 11 13 16 17 18 19 20 22 23

Latitude Longitude 36.83°N 37.17°N 37.68°N 37.85°N 38.03°N 38.21°N 38.37°N 38.72°N 38.88°N

72.86°W 73.23°W 73.80°W 73.98°W 74.17°W 74.36°W 74.54°W 74.91°W 75.09°W

Theta (°C) 15.77 16.51 11.95 13.69 9.73 10.80 10.25 11.10 12.60

Salinity (%~) 34.61 34.98 32.99 33.50 32.34 32.09 31.80 30.24 29.70

02 (}tM) 255 235 273 269 279 283 269 268 276

cles. In July, however, both surface stratification and low phytoplankton production provide relatively low concentrations of suspended particles for scavenging. Iron concentrations in the surface water are also affected by Fe sources other than riverine input. In the coastal zone, continental shelf and slope sediment input to the water column and atmospheric deposition are two major input sources for Fe. Since Fe is very abundant in the earth crust, the strength of both input processes generally decreases with increasing distance from the shore. If shelf and slope sediment and atmospheric Fe inputs decrease seaward, then an upward deviation of the linear Fe-S%~ curve at its low salinity end could result, and concave Fe-S%o curves (apparent " F e r e m o v a l " ) similar to those in Fig. 4 could be generated. In March, the diffusive/advective input of Fe from shelf sediment to the surface water could result in a positive deviation of Fe concentration from the conservative mixing line at nearshore stations. With the onset of water column stratification from May to July (Fig. 5), the sedimentary Fe input appeared to be greatly decreased. A decrease in the extent of the positive deviation of Fe concentration from the conservative mixing line was observed from March to July (Fig. 4). When the surface mix layer started to deepen from July to October, the Fe input increased again, leading to the apparent " r e m o v a l " in October (Fig. 4). Concave property-salinity curves for Cu, Ni, and Cd have been reported in the continental shelf of the northeastern Atlantic (Kremling and Pohl, 1989) and in the Gulf of Mexico (Boyle et al., 1984). Boyle et al. (1984) suggested that the trace element enrichments in the Gulf of Mexico could be due either to a shelf source or to a river source masked by evaporation.

NO 3 (~tM) 0.21 0.21 0.28 1.92 0.25 0.34 0.25 0.25 NA

PO 4 (btM) 0.14 0.15 0.20 0.35 0.22 0.18 0.13 0.14 NA

Si (IxM) NA 1.16 1.78 1.31 0.96 1.05 0.74 0.63 NA

<0.2 ~t Fe (nM) 0.48_+0.02 0.41!-0.04 0.34_+0.04 0.455:0.03 0.73_+0.05 1.13_+0.04 1.52_+0.07 4.30i-0.03 4.97_+0.10

Sample was taken

<0.4 Ix Fe (nM) 0.51_+0.03 0.60!-0.05 0.43_+0.03 0.65!-0.04 1.07_+0.05 1.23_+0.04 1.50-I-0.03 4.46_+0.10 6.20i-0.09

Total Fe (nM) 1.2_+0.05 1.50-20.03 3.21_+0.08 3.20!0.07 1.92_+0.05 2.31_+0.07 30.60!1.10 559.00+_2.4 775.00+_2.8

One may argue that the apparent " F e r e m o v a l " (Fig. 3) was actually due to the conservative mixing of more than two endmembers and that there was little real removal of Fe along the transect. This transect acrossed more than 250 Km and may encompass a variety of water masses. In addition, the air-sea heat exchange may affect the use of T-S diagrams in indicating the mixing of water masses in the surface water. The Fe data in Fig. 3 could be due to three endmembers mixing (the 29-30%o low-salinity water at the mouth of Delaware Bay, 33%o salinity water at the shelf water/slope water front, and Gulf Stream/Sargasso Sea water). The relatively low and constant Fe concentration at salinity >33%0 waters was maintained through the separation of low Fe offshore water from high Fe inshore water by shelf water/ slope water front. Therefore, futher studies are needed to unambiguously address Fe removal in coastal waters, since the extent of Fe removal in these waters may affect the estimation of Fe fluvial fluxes to the open ocean. 4.1.2. Iron in >0.2 #m and 0.2-0.4 #m fractions Iron in the > 0 . 2 # m fraction (the difference between unfiltered and < 0 . 2 # m Fe, Fig. 6) exhibited a two to three orders of magnitude decrease from the shelf water at station 23 (11.1 # M in March 1993, 0.6 # M in July 1992, and 0.8 # M in May 1993) to the slope, Gulf Stream, and Sargasso Sea waters at stations 9 - 1 6 ( 1 . 3 - 1 0 . 2 nM in March 1993, 0 . 1 - 3 . 4 nM in July 1992, and 0 . 7 - 2 . 8 nM in May 1993). This is similar to the dramatic inshore-offshore gradients of total Fe concentrations observed in the North Pacific ( 1 0 0 0.3 nM, Martin and Gordon, 1988) and the northwestern

Table 3. Hydrographic, nutrient and iron data on CHESS-7 (28 June-4 July 1992). Sample was taken from 10 m depth. Stn No. 9 13 15 16 17 18 21

Latitude Longitude 36.29°N 37.17°N 37.52°N 37.68°N 37.85°N 38.03°N 38.54°N

72.29°W 73.23°W 73.61°W 73.80°W 73.98°W 74.17°W 74.73°W

Theta (°C) 26.46 17.70 19.40 17.00 14.40 15.40 18.10

Salinity (?'~) 36.14 33.90 34.30 33.60 32.80 32.30 31.00

02 (}tM) 197 224 230 227 248 237 292

NO 3 (}tM) 0.07 0.66 0.25 0.21 2.75 0.31 0.31

PO 4 (~tM) 0.01 0.36 0.34 0.24 0.76 0.29 0.27

Si (IxM) 0.77 8.45 1.91 1.80 3.70 2.26 1.27

<0.2 ~t Fe Total Fe (nM) (nM) 0.58_+0.08 0.60!-0.03 0.53_+0.02 3.98_!0.04 0.55_-t-0.04 1.61_+0.04 0.56i-0.04 1.50i-0.05 0.36_+0.03 1.90!-0.12 0.59!-0.02 1.40i-0.06 4.15-I-0.06 600.00+_2.50

2734

J. Wu and G. W. Luther Ill TABLE 4. Hydrographic, nutrient and iron data on FAMINE-I ( 7 - 11 October 1991 ). Sample was taken from 5 m depth.

Stn Latitude Longitude Theta Salinity 0 2 NO3 PO4 Si <0.2~Fe No. (°C) (%0) (~M) (~tM) (~M) (~tM) (nM) 9 36.29°N 72.29°W 26.93 36.26 243 0.50 0.08 1.50 0.20-2-0.02 11 12 13 14 15 16 17 18 19 20 21 23

36.83°N 37.00°N 37.17°N 37.34°N 37.52°N 37.68°N 37.85°N 38.03°N 38.21°N 38.37°N 38.54°N 38.88°N

72.86°W 73.05°W 73.23°W 73.42°W 73.61°W 73.80°W 73.98°W 74.17°W 74.36°W 74.54°W 74.73°W 75.09°W

27.41 27.51 22.05 22.88 23.78 23.12 18.86 18.77 18.92 19.90 19.11 18.55

36.19 36.16 34.48 34.96 35.23 34.80 33.30 33.04 32.71 31.93 31.75 30.32

239 239 281 266 255 283 303 314 313 305 335 300

0.03 0.07 0.11 0.07 0.05 0.06 0.21 0.21 0.24 0.28 0.39 0.75

1.18 1.23 1.14 1.11 1.18 1.14 1.70 1.72 2.18 1.58 0.66 5.09

0.402-0.01 0.20-2-_0.03 0.20~.02 0.16_+0.03 0.16+0.03 0.30-L-0.06 0.40L-0.07 0.53_+0.05 1.10_-/-0.04 2.17+0.04 4.43_+0.20 7.15_+0.23

[ <0.2 # m Fe]-S%~ relationship at salinity <34%~ (Fig. 7 ). The dramatically decreasing [ >0.2 # m Fe]/[ <0.2 p,m Fe] ratio with increasing S%~ strongly suggests a significant removal of the >0.2 p m Fe fraction in coastal water of salini-

Atlantic ( 3 0 0 - 3 nM, Symes and Kester, 1985). The >0.2 ~m Fe decreased (two to three orders of magnitude, Fig. 6), with increasing S%~ much faster than < 0 . 2 p m Fe (one order of magnitude, Fig. 3), leading to a concave [ >0.2 # m Fe]/

6

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Geochemistry of iron in Atlantic surface water 2735

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-40. .,'-4

,.q

-60.

-80.

May

-loo. O.

1993 60.

120.

180.

C r o s s - s h e l f Distance (kin)

240.

D e n s i t y (at) 1993, Fzo.and 5. Contoured July 1992 cruises. cross-sections of potential density across the surface water transect during March ! 993, May

ties less than 34%o. The removal of >0.2 ,am Fe in the coastal water is important to the estimation of riverine Fe input to the open ocean. If one assumes that, on the global scale, 99% of particulate Fe supplied from estuaries is re-

moved in coastal zones as estimated from our >0.2 "am Fe data using the method of Boyle et al, (1974), the global riverine particulate Fe flux reported by GESAMP (1987) will be reduced to I X 1013g y-Z as compared to 1 X 1012 g

2736

J. Wu and G. W. Luther Ill Station

23

O.

21 ÷

÷

20

÷

1÷

÷

Number 19

÷

÷

18 ÷

I÷

17 ÷

÷

16 ÷

15

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÷

14 ÷

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÷

-20. ~D (D

-40.

-60.

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July2-4, 1992 // -100.

#

I

[

I

I

60.

120.

180.

240.

C r o s s - s h e l f D i s t a n c e (kin) D e n s i t y (et) FIG. 5. (Continued)

y - J for riverine dissolved Fe input. Thus, global atmospheric deposition of 3 × 10 ~2 g y ~ for dissolved Fe and 2.9 × 10 ~3 g y-~ for particulate Fe (Duce and Tindale, 1991) will be the predominant source of Fe to the open ocean. The possibility that [ > 0 . 2 # m F e ] / [ <0.2 # m Fe]-salinity relationship is controlled by a simple dissolution/precipitation equilibrium between particulate Fe minerals and dissolved Fe species in solution can be ruled out. Since marine particle surfaces are controlled by ubiquitous surface-bound organic matter (Hunter and Liss, 1979; Newton and Liss, 1989), Fe partitioning between <0.2 /~m and >0.2 # m phases should depend on the chemistry of the organic matter forming the surface film rather than that of the underlying solid matrix. However, sorption equilibrium may occur between dissolved Fe and Fe adsorbed on the surface of suspended particles in which Fe partitioning between the two size fractions is controlled by a constant Kt), [ > 0 . 2 # m Fe]/ ([ < 0 . 2 tim Fe] × [suspended particles]). In other words, a higher dissolved Fe concentration in the lower salinity region is accompanied by higher concentrations of Fe in the particulate matter and KD is independent of particle concentration. Alternatively, if most of <0.2 # m Fe was in < 0 . 2 # m colloids and not truly dissolved, the distribution of these two Fe fractions may reflect a quasi-equilibrium between Fe in large particles and small colloids (Morel and Gschwend, 1987; Honeyman and Santschi, 1989), where KD should decrease with increasing suspended particle concentration. Unfortunately, these hypotheses can not be tested since suspended particle concentration was not measured on our samples. However, if most of the < 0 . 2 / z m Fe were in < 0 . 2 / z m colloids and not truly dissolved, then the larger decrease

in >0.2 # m Fe concentrations compared to <0.2 # m Fe concentrations with increasing distance from shore is consistent with the basic principle that residence times of particles in the water increase with decreasing particle sizes. In addition, the existence of nanomolar levels of excess F e ( l l I ) complexing organic ligands in the 0.4 # m filtered seawater samples collected at our transect stations (#13 and 16 in May 1993, Wu and Luther, 1995) may affect the [ >0.2 # m Fe]/( [ < 0 . 2 # m Fe] ratio. At these higher salinity stations, the excess organic ligand and particulate Fe have similar concentrations. It is not clear if the formation of Feorganic complexes can increase or decrease Fe adsorption on suspended particles since organic material can adsorb to the surface. However, the formation of Fe-organic complexes may increase dissolved Fe removal into the biota, if the Fe complexing organic ligands observed at our sampling stations were siderophores synthesized by biota in response to Fe limitation and if the Fe-organic complexes are available for uptake. A few reports have been made on siderophore production by marine bacteria (Trick, 1989; Brown and Trick, 1992; Haygood et al., 1993) and on the determination of stability constants for Fe-siderophore complexes (Reid et al., 1993; Lewis et al., 1995). These siderophores have the ability to complex freshly precipitated Fe hydrolysis products. Another feature in the >0.2 # m Fe distributions is that there were maximum Fe concentrations ( 1 . 5 - 3 . 5 nM) at stations 13 and 17 during the July 1992 cruise and at stations 16 and 17 during the May 1993 cruise (Fig. 6). These maxima occurred over the salinity range of 32.8-34.0%0 and coincide with maxima of major nutrients (N, P, Si) (Fig.

Geochemistry of iron in Atlantic surface water 23

10000~

10000'

March 7-9, 1993

May 18-20, 1993

1000-

1000.

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0.1

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2737

July 2-4, 1992 21

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I

29

31

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I

33

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w

35

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37

Salinity (psu) FIG. 6. Fe (>0.2 ~zm) vs. salinity plots for cruises in March 1993, July 1992, May 1993. Station numbers are shown in the figure.

8). These stations were at the shelf water/slope water fronts as determined from satellite surface temperature data from NOAA. The existence of shelf/slope fronts is a well known continental shelf-slope circulation feature in the Middle Atlantic Bight (Beardsley and Boicourt, 1981 ). Enhanced nutrient concentration, phytoplankton biomass, and primary production have been well documented at frontal regions (Fournier et al., 1979; Houghton and Marra, 1983; Church et al., 1984; Marra et al., 1990). At the shelf-break front, as coastal water is drawn off the shelf, the water moves upward along isopycnal surfaces. The upward inclination of the isopycnals (25.5 O"t in May 1993 and July 1992, Fig. 5) indicates that turbid and nutrient-rich deep shelf waters were brought into the euphotic zone at the frontal boundary. In the Middle Atlantic Bight, the effective vertical velocities of upwelling are on the order of 10-20 m d -~ (Marra et al., 1990). Therefore, the concurrent maximum nutrient and >0.2 #m Fe concentrations (Figs. 6, 8) can result from this kind of enhanced vertical mixing and upwelling which bring particles and nutrients from bottom shelf and slope water to the surface euphotic zone. This is supported by the presence of a dissolved Ti maximum at station 16 in the October 1991

cruise (Skrabal, 1994) and is consistent with the data of Kremling (1983) who reports a dramatic increase in inorganic nutrients and trace metals (Cd, Cu, Mn, Ra 226) in the shelf break surface water off Scotland. Kremling and Hydes (1988) found that the elevated concentrations of trace metals and nutrients off Scotland were due to the upwelling at the frontal zone. Since strong surface convergence zones are always found at the shelf/slope front (Mooers et al., 1979; Houghton and Marra, 1983), the elevated >0.2 ~zm Fe concentrations may also be caused by the accumulation of particles due to the convergent circulation at the front. It has been found that this process could increase the concentrations of particulate Cu, Cd, and Zn observed in tidal frontal zones (Sick et al., 1978). Interestingly, no <0.2 ~m Fe maxima were observed at the frontal zone where high concentrations of >0.2 #m Fe, nutrient, and dissolved Ti occured (Figs. 3, 6, 8). It is possible that dissolved Fe upwelled along with particulates into the surface water was rapidly transformed into the particulate Fe fraction through Fe oxyhydroxide precipitation and by scavenging due to high concentrations of organic/inorganic particles and living biota at the frontal zone.

2738

J. Wu and G. W. Luther Ill 1000

1000'

May 18-20, 1993 100

100" c5

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e-i <5 10"

10 ¸

eq c5

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eq c5 &

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1

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Salinity (PSU)

Salinity (PSU) 1000'

July 2-4, 1992

100" c5 v

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Salinity (PSU)

FiG. 7. The plot of [>0.2 #rn Fe]/[<0.2 #m Fe] vs. salinity for cruises in March 1993, July 1992, and May 1993.

The distributions of 0 . 2 - 0 . 4 / ~ m Fe obtained by the difference between <0.4 # m Fe and < 0 . 2 # m Fe (Fig. 9) were characterized by relatively higher concentrations ( 1 . 2 - 1 . 5 nM) at the mouth of Delaware Bay (station 23) and lower concentrations ( < 0 . 1 5 - 0 . 4 nM) at the other stations. Since the exact size cutoff is not known particularly when the filter becomes clogged, the 0 . 2 - 0 . 4 # m Fe may represent only a small portion of the total colloidal Fe ( 1 - 1 0 0 0 nm), which includes inorganic/organic colloids and bacteria. High concentrations of 0 . 2 - 0 . 4 # m Fe at station 23 (Fig. 9) could be due to the input of estuarine Fe colloids. The Fe-salinity curve in Fig. 9 indicates that the 0 . 2 - 0 . 4 # m Fe is more variable than the other size fractions and may undergo transformation into other size ranges during the mixing of water masses. The decreased 0 . 2 - 0 . 4 # m Fe fraction relative to salinity occurred at stations and salinities different from those for the other Fe size fractions (Figs. 3, 6, 9), indicating that there were different mechanisms controlling the different Fe fractions. Since < 0 . 2 # m Fe fraction decreased with increasing salinity (Fig. 3), the major process for 0 . 2 - 0 . 4 # m Fe transformation should be aggregation and/or biological uptake into larger size fractions, rather than the disaggregation of 0 . 2 - 0 . 4 # m Fe fraction into the smaller size fraction ( < 0 . 2 #m).

4.2. Temporal Variation The concentrations of Fe varied not only spatially but also temporally. In inshore waters (stations 2 0 - 2 3 ) , <0.2 /~m Fe fraction was much higher in March 1993 (14 nM) than in the other cruises ( 5 - 7 nM) (Fig. 3 and Tables 1 - 4 ) . Similar trends were also observed for >0.2 # m Fe (11.1 # M in the March and 0 . 6 - 0 . 8 # M in May and July, Fig. 6, Tables 1 - 4 ) . The elevated Fe concentration in March 1993 appeared related to the enhanced Delaware Bay river discharge, vertical mixing and bottom resuspension at this time. According to the U.S. Geological Survey, the river discharge of Delaware Bay is highest in March 1993 ( 18710 Ft3/S as compared to 11180 Ft3/S in May 1993, 5809 Ft3/S in July 1992, and 6809 Ft3/S in October 1991). The increase in river runoff can decrease the water residence time in the estuary, leading to a decrease in estuarine Fe removal and, hence, increasing Fe input to the coastal waters. Increasing river discharge also contributed to an enhanced mixing and bottom sediment resuspension at the mouth of the bay, which should correspond to an increase in the concentration of Fe containing particles and small colloids as observed at station 23. During winter surface cooling, there was enhanced vertical mixing as indicated by the upward inclination of isopynal

Geochemistry of iron in Atlantic surface water

2739

N ........O ........ Si

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FIG. 8. Nutrients vs. salinity plots for cruises in March 1993, July 1992, May 1993, and October 1991. Station numbers for phosphate are shown for May 1993 and July 1992 cruises.

2-

2May 18-20, 1993

March 7-9, 1993

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FIG. 9. Fe (0.2-0.4 gm) vs. salinity plots for cruises in March 1993 and May 1993. Station numbers are shown.

2740

J. Wu and G. W. Luther IIl

surfaces (Fig. 5) and elevated nitrate concentrations in offshore waters (Stations 9 - 1 7 , Fig. 8). The bottom shelf sediment resuspension and advection along the isopycnal surface ( 2 3 . 0 - 2 6 . 0 ot in Fig. 5) into the surface water may lead to the observed high Fe concentrations in inshore waters in March. In the offshore waters ( stations 9 - 1 7 ), detectable changes of Fe concentration were observed for all cruises (Tables 1 - 4 ) . The < 0 . 2 # m Fe in these waters decreased in the following sequence: M a r c h 1993 ( F e = 0.92 _+ 0.10) > M a y 1993 ( F e = 0.42 _ 0.07), July 1992 ( F e = 0.5 ± 0.06), and October 1991 ( F e = 0.30 ± 0.05). The > 0 . 2 p m Fe followed a similar sequence: M a r c h 1993 ( 2 . 2 - 1 0 . 1 n M ) > M a y 1993 ( 0 . 7 - 2 . 8 n M ) and July 1992 ( 0 . 1 - 3 . 4 n M ) . Since river-derived Fe can be r e m o v e d rapidly onto sediments as discussed above, the enhanced riverine Fe input may, thus, affect Fe distributions at nearshore stations but not offshore stations. However, shelf sedimentary Fe upwelled and advected offshore along the isopycnal surface (26.5 ~r, in Fig. 5 ) , resulting in higher Fe concentrations in the offshore waters (Stations 9 - 1 7 ) in M a r c h 1993 than in the other cruises. Other processes such as atmospheric deposition and mixing between surface and deep waters in the winter cannot maintain the high Fe concentrations ( < 0 . 2 # m Fe = 1.0 nM, > 0 . 2 # m Fe = 10 n M ) that we observed in these waters. Iron data from a precipitation time series station at the m o u t h of Chesapeake Bay (Scudlark et al., 1994) show that atmospheric wet deposition reached its m a x i m u m in M a r c h - A p r i l (1833 # g / m 2 on average) and J u n e - J u l y ( 1 7 5 0 # g / m 2) and was relatively low from August to February ( 3 0 0 ¢zg/m2). This eolian Fe input should correspond to 0.16, 2.08, and 0.07 n M Fe in the surface mixed layer in March, July, and October, respectively. These calculations assume that the mixed layer depths are 200, 15, and 80 m for March, July, and October respectively (estimated from our vertical ~7t distributions) and that there is no Fe loss to below the mixed layer. This result suggests that despite m a x i m u m eolian Fe input in March, the enhanced vertical mixing tends to decrease the atmospheric signal. The mixing of deep water into the euphoric zone in winter can not account for the elevated Fe concentration in M a r c h either, because these Fe concentrations were higher than those observed in the Atlantic deep waters ( < 0 . 2 # m Fe = 0 . 6 - 0 . 9 nM, > 0 . 2 / ~ m Fe = 1 . 2 - 1 . 5 nM, Sherrell and Boyle, 1992; W u and Luther, 1994). Thus, the observed elevated Fe concentration offshore in M a r c h relative to the other cruises was likely due to the seaward advective transport of Fe from the continental shelf. 5. CONCLUSION Iron concentrations in the surface water of the northwest Atlantic ocean showed considerable spatial and temporal variations. Iron in < 0 . 2 #m, 0 . 2 - 0 . 4 #m, and > 0 . 2 # m fractions appeared to b e h a v e differently from the lower estuary to the open ocean. The removal of > 0 . 2 # m Fe was m u c h more significant than that of < 0 . 2 / z m Fe, which may be controlled by particle scavenging, phytoplankton uptake, shelf sediment input, and atmospheric deposition. E n h a n c e d

vertical mixing, upwelling, and convergence circulation resulted in concomitant m a x i m u m concentrations of major nutrients and > 0 . 2 # m Fe at the shelf water/slope water fronts. There was temporal variation in both < 0 . 2 # m and > 0 . 2 / z m Fe fractions, which appeared related to seasonal variations of river discharge, eolian Fe input, and water column stability. Acknowledgments--We especially thank G. Gawarkiewicz for hy-

drographic data analysis and helpful discussions. We appreciate T. Church for critical review of an earlier revision of the manuscript. This paper benefited from reviews by John Donat and two anonymous reviewers. This work has been supported by NOAA office of Sea Grant under contract number NAI6RG0162-01. Ship time was provided by NSF grant OCE89-16804 to G. W. Luther. Editorial handling: L. S. Balistrieri REFERENCES

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2741

aluminum in the Pacific Ocean. Earth Planet. Sci. Lett. 78, 397410. Reid R. T., Live D. H., Faulkner D. J., and Butler A. (1993) A siderophore from a marine bacterium with an exceptional ferric ion affinity constant. Nature 366, 455-458. Rue E. and Bruland K. W. (1995) Complexation of iron(Ill) by natural organic ligands in the central north Pacific as determined by a new competitive ligand equilibration/adsorptive cathodic stripping voltammetric method. Mar. Chem. 50, 117-138. Saager P. M., De baar H. J. W., and Burkill P. H. (1989) Manganese and iron in Indian Ocean waters. Geochim. Cosmochim. Acta 53, 2259-2267. Schaule B. K. and Patterson C. C. ( 1981 ) Lead concentration in the Northeast Pacific: Evidence for global anthropogenic perturbations. Earth Planet. Sci. Lett. 11, 155-174. Scudlark J. R., Conko K. M., and Church T. M. (1994) Atmospheric wet deposition of trace elements to Chesapeake Bay: CBAD study year 1 results. Atmos. Environ. 28, 1487-1498. Sherrell R. M. and Boyle E. A. (1992) The trace metal composition of suspended particles in the oceanic water column near Bermuda. Earth Planet. Sci. Lett. 111, 155-174. Sholkovitz E. R. (1976) Flocculation of dissolved organic and inorganic matter during the mixing of river water and seawater. Geochim. Cosmochim. Acta 40, 831-845. Sick L. V., Johnson C. C., and Engel R. (1978) Trace metal enhancement in the biotic and abiotic components of an estuarine tidal front. J. Geophys. Res. 83, 4659-4667. Skrabal S. A. (1994) The marine geochemistry of titanium. Ph.D. dissertation, Univ. Delaware. Steele J. H. (1981) Some varieties of biological oceanography. In Evolution of Physical Oceanography (ed. B. A. Warren and C. Wunsch), pp. 112-139. MIT Press. Strickland J. D. and Parsons T. R. (1972) A Practical Handbook of Seawater Analysis (2nd ed); Bull. Fish. Res. Board. Can. 167. Symes C. J. and Kester D. R. (1985) The distribution of iron in the northwest Atlantic. Mar. Chem. 17, 57-74. Trick C. G. (1989) Hydroxamate-siderophore production and utilization by marine eubacteria. Curr. Microbiol. 18, 375-378. van den Berg C. M. G. (1995) Evidence for organic complexation of iron in seawater. Mar. Chem. 50, 139-157. van den Berg C. M. G., Nimmo M., Abollino O., and Mentasti E. ( 1991 ) The determination of trace levels of iron in seawater using adsorptive cathodic stripping voltammetry. Electroanalysis 3, 477-484. Wu J. and Luther G. W., III(1994) Size-fractionated iron concentrations in the water column of the Northwest Atlantic Ocean. Limnol. Oceanogr. 39, 1119-1129. Wu J. and Luther G. W., III (1995) Complexation of Fe(III) by natural organic ligands in the northwest Atlantic Ocean by a competitive ligand equilibration method and a kinetic approach. Mar. Chem. 50, 159-177.