Reactive trace metals in the stratified central North Pacific*kbruland/...3172 K. W. Brufand, K. J. Orians, and J. P. Cowen external sources and have deep water concentrations in the

Geochimica et Cosmochimica Acta, Vol. 58, No. 15, PD. 3171-3182. 1994

Pergamon Copyright 0 I994 &evier Science Ltd Printed in the USA. All rights reserved

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0016-7037(94)00112-X

Reactive trace metals in the stratified central North Pacific*

KENNETH W.BRULAND,' ~ISTINJ.ORIANS,~ and JAMESP,~OWEN~

‘institute of Marine Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA *Department of Oceanography, University of British Columbia, Vancouver, BC V6T 124, Canada

‘Department of Oceanography, University of Hawaii, Manoa, Honolulu, HI 96822, USA

(Received June 16, 1993; accepted in revised form February 2.5, 1994)

Abstract-Ve~ical concent~tion profiles of the dissolved and suspended pa~iculate phases were deter- mined for a suite of reactive trace metals, Al, Fe, Mn, Zn, and Cd, during summertime at a station in the center of the North Pacific gyre. During summer the euphotic zone becomes stratified, forming a shallow (O-25 m), oligotrophic, mixed layer overlying a subsurface (25-140 m), strongly-stratified region. The physical, biological, and chemical structure within the euphotic zone during this period enhanced the effect of atmospheric inputs of Al, Fe, and Mn on mixed layer concentrations. For example, the con~ntration of dissolved Fe in the surface mixed layer was eighteen times that observed at a depth of 100 m. The observed aeolian signature of these metals matched that predicted from estimates of atmospheric input during the period between the onset of stratification and sampling.

The distributions of suspended particulate Al, Fe, and Mn all exhibited minima in the euphotic zone and increased with depth into the main thermocline. Particulate Al and Fe were then uniform with depth below 1000 m before increasing in the near bottom nepheloid layer. Average particulate phase concentrations in intermediate and deep waters of the central North Pacific were 1 .O, 0.3 1, and 0.055 nmol - kg-’ for Al, Fe, and Mn, respectively. The dist~bution of particulate Cd exhibited a maximum within the subsurface euphotic zone. Particulate zinc also exhibited a surface maximum, albeit a smaller one. Con- centrations of particulate Zn and Cd in intermediate and deep waters were 17 and 0.2 pmol . kg-‘. Substantial interbasin differences in particulate trace metals occur. Concentrations of suspended particulate Al, Fe, and Mn were three to four times lower in the central North Pacific than recently reported for the central North Atlantic gyre, consistent with differences in atmospheric input to these two regions. Con- centrations of suspended particulate Cd and Zn were enriched in the North Pacific relative to the North Atlantic, an observation consistent with their assimilation by plankton.

Reactive trace metals exhibit a range of biogeochemical behaviors that can be characterized by two endmembers, nutrient-type and scavenged-type. Nutrient-type metals, best exemplified by Zn and Cd, are primarily removed from surface waters by biogenic particles and then remineralized at depth. Internal biogeochemical cycles together with physical mixing and circulation patterns control the distributions of nutrient-type metals. Scavenged-type metals, best exemplified by Al, continue to be removed onto particles in intermediate and deep waters as well as at the surface. External inputs, such as the deposition of aeolian dust, control the concentrations and di~~butions of ~avenged-ty~ metals. Other metals, such as Fe, exhibit a mixture of the characteristic behaviors of these two endmembers.

INTRODUCI’ION

OVERTHEPASTITVODECADES ourknowledgeofthechemical behavior of trace metals in the oceans has grown dramatically. Advances in analytical methods, sample collection, and processing techniques have produced datasets allowing a fun- damental understanding of the distributions of dissolved trace metals in the major ocean basins (BRULAND, 1983; WHIT- FIELD and TURNER, 1987; BURTON and STATHAM, 1990; DONAT and BRULAND, 1994). C. C. Patterson and co-workers were irn~~nt cont~buto~ to these advances. Most notably, they pioneered the use of clean techniques of sampling and analysis to accurately determine Pb in seawater (SCHAULE and PATTERSON, 198 1 ), revolutionizing oceanographic re- search on trace metals in the process. As a result, oceano- graphically consistent profiles demonstrating the existence of

* Paper presented at the symposium “Topics in Global Ceochem- istrv” in honor of Clair C. Patterson on 3-4 December 1993 in Pas- adena, California, USA.

3I71

well-defined vertical and horizontal variations in dissolved concentrations now exist for the majority of trace metals. The distribution of each trace metal is shaped by a combi- nation of processes: external and internal input and removal processes superimposed upon physical mixing and advection in the ocean basins. WHITFIELD and TURNER (1987) classified trace metals ac-

cording to their oceanic profiles. Reactive trace metals, i.e., those existing at low concentrations in seawater relative to their crustal abundance, fall into two categories, nutrient- type (or recycled) and scavenged-type. Nutrient-type trace metals, like the major nutrients nitrate, phosphate, and silicic acid, are removed from surface waters and remineralized into intermediate and deep waters as part of the major biogeochemical cycles associated with plankton productivity. They exhibit surface water depletion and have concentrations in the relatively young deep waters of the North Atlantic which are substantially less than those observed in the older, nutrient-rich, North Pacific deep waters. In contrast, scavenged trace metals undergo net removal even from deep waters. They exhibit concentration maxima corresponding to their

3172 K. W. Brufand, K. J. Orians, and J. P. Cowen

external sources and have deep water concentrations in the North Pacific that are less than those observed in the younger deep waters of the North Atlantic.

The interaction of dissolved trace metals with particles suspended in seawater is a major control on the concentrations and distribution of trace metals in the world’s oceans (GOLDBERG, 1954; TUREKIAN, 1977; WHITFIELD and TURNER, 1987; CLEGG and WHITFIELD, 1990). An understanding of trace metal behavior requires a quantitative de- scription of solute/particle interactions. However, data on partitioning between dissolved and suspended particulate phases is scarce because few such measurements have been made in open ocean environments. Recently, a high quality dataset of suspended particulate trace metal composition in

the oceanic water column has become available from the central North Atlantic, specifically the Sargasso Sea near Ber- muda (SHERRELL and BOYLE, 1992). This allows a comparison of both the suspended particulate and dissolved concentrations of a suite of trace metals between our data for the oligotrophic central gyre of the North Pacific and pub- lished data for the North Atlantic to be made.

The principal goals of this work were to (1) determine water column profiles of dissolved and suspended particulate trace metals in the central gyre of the North Pacific, (2) ex- amine the imprint of atmospheric dust deposition on the surface water concentrations of these metals, and (3) quantify the partitioning between dissolved and particulate phases. We also compare the results from this study with data from the central gyres of other major ocean basins.

SAMPLING AND ANALYSIS

Samples were collected from depths of 20 to 5250 m at the VER- TEX-IV site in the center of the Korth Pacific subtropical gyre (28’N, I55”W, approximately 200 km North of Hawaii) during July, 1983. Seawater samples were collected and filtered using estabtished trace metal clean techniques (BRULAND et al. 1979; BRULAND 1980). Samples were collected with 30-liter, Teflon-coated, modified Go- flo bottles (General Oceanics) suspended on Kevfar hydroline. The Go-Flo samplers were modified by replacing the standard stopcock with a Teflon ball-valve and adding a Teflon tube insert extending to the bottom of the sampler to allow the complete sampling of all but a few mLs of the 28 L of sample. This was done to alleviate any potential problem due to settling of particles within the water sampler.

Samples were pressure-filtered at approximately 0.5 atm over- pressure through 142-mm, 0.3-pm Nucfepore filters. The pore size of 0.3-em (a special batch of filters with double density pores) was selected as a compromise between being able to filter out picoplankton and maintaining a reasonable flow rate during the filtration operation. While stiff within the Teflon filter holder, the 142-mm filters were rinsed with approximately 10 mL of pH 8.5 quartz distiff~ water (adjusted with NH.,OH) to rinse off excess sea salts. This rinse step removed negligible amounts of particulate trace metals, with the ex- ception of Cd for which up to 10% was observed in the rinse solution for some surface samples. Particulate samples were analyzed for the trace metal content of both the 25% acetic acid (HAc) feachate and the residual refractory fraction. Details are presented in LANDING and BRULAND (1987). Filter blanks, expressed as nmof per filter, for the HAc leachable and refractory fractions, respectively, were: Zn, 0.055 and 0.034; Fe, 0.16 and 1.2; Mn, <0.002 and <0.002: Cd, 0.003 and 0.00 I.

Samples for the determination of dissolved Fe, Mn, Cd, and Zn were processed at sea by a Chefex-100 ion exchange column technique similar to that described by BRULAND (1980) and BRULAND et al. f 1985). Two liter samples, stored in Teflon bottles, were acidified to pH < 2 with qua~z-distiff~ HCf (Q-HCf), and then adjusted to pH 5.8 with an ammonium acetate/acetic acid (Q-NH4HAc) buffer.

Within two days of collection, the samples were pumped through columns containing 7 mt of Chefex-100 resin at 1.5 mL~min_‘. Subsequent processing and analyses took place in our shore-based laboratory at UCSC. The dissolved and particulate ~uminum data are from ORIAFIS and BRULAND (1985, 1986).

RESULTS

Vertical profiles of nitrate and silicic acid at the VERTEX- IV site are presented in Fig. 1. During this sampling period, there was a 25 m surface mixed layer and a strong seasonal thermocline extending from the base of the mixed layer to the base of the euphotic zone (25- 140 m). Nitrate and silicic acid are depleted to concentrations near their detection limits within the upper 100 m. Profiles of dissolved and suspended

(a) Nitrate (umol/kg)

10 20 30 40

600 I

(W Silicic Acid (umol/kg)

0 20 40 60 80 100120140160180

FIG. 1. Vertical profiles of (a) nitrate (pmof -kg-‘), and (b) sificic acid (rmof - kg-‘) at VERTEX-IV.

Vertical concentration profiles of dissolved and suspended particulate phases 3173

particulate Al, Fe, Mn, Cd, and Zn are presented in Figs. 2

through 6.

Aluminum

Dissolved Al (Fig. 2a) exhibited a maximum concentration

of close to 5 nmol - kg -’ in the surface mixed layer, and

sharply decreased through the seasonal thermocline to values

of 1.0-l .5 nmol -kg-’ at depths of 100-300 m. In interme-

diate and deep waters (500-4000 m), dissolved Al exhibited

a broad minimum with an average concentration of 0.4

(a) Dissolved Al (nmol/kg)

(W

600 A

F Q) a00 4-J 0)

200 - .a a

0

400 - 0

. 0

600 - 0 A

if al a00 -’ -w

Particulate Al (nmol/kg)

0 1 2 3 4 5

FIG. 2. Vertical profiles of (a) dissolved aluminum (nmol kg-‘) and (b) particulate aluminum (nmol kg-‘) at VERTEX-IV. For particulate aluminum filled circles represent the acetic acid leachable fraction and open triangles represent refractory aluminum.

TABLE 1 - COMPARISON OF DISSOLVED AND PARTICULATE

TRACE METALS IN SURFACE WATERS (0 - IOOM) OF

THE CENTRAL NORTH PACIFIC GYRE

Al Fe Mtl Ztl Cd

Dissolved 2.7 0.14

Particulate 0.37 0.15 HAc Leach 0.028 0.005

% Diss. 88 48

Diss./Part. 7.3 0.9

DissJHAc Part. 96 30

nmolikg

0.76 0.23

0.0093 0.015 0.0065 0.014

99 94

82 15

120 16

0.0028

0.6015 0.0015

65

1.9

1.9

nmol - kg-’ before increasing to 2 nmol - kg-’ near the sedi-

ment interface.

The lowest concentrations of suspended particulate Al (Fig.

2b) occurred in the surface 100 m, averaging 0.37 nmol - kg-‘,

with 7.6% of the particulate Al being HAc leachable, i.e., solubilized by 25% acetic acid (Table 1). Throughout the in-

termediate and deep waters (500-4000 m), the particulate Al

averaged 1 .O nmol - kg-‘, with only 4.7% being HAc leachable. The suspended particulate Al increased in the near bottomwaters to values of 4.5 nmol - kg-‘.

Dissolved/particulate partitioning was depth dependent. Within the surface 100 m, an average of 88% of the Al occurred in the dissolved phase, while in the intermediate and deep waters, only 30% of the Al was dissolved (Tables 1 and 2).

Iron

Dissolved Fe (Fig. 3a) exhibited a maximum concentration of 0.37 nmol - kg-’ in the surface mixed layer, with a sharp gradient through the seasonal thermocline similar to that observed for Al. A minimum in dissolved Fe was observed be-

tween 70 and 100 m depth, with values of 0.02 nmol * kg-‘. Below 100 m, dissolved Fe again increased with depth to values on the order of 0.45 nmol - kg-’ by 1000 m.

The lowest concentrations of suspended particulate Fe (Fig. 3b) were within the surface 100 m, with an average concentration of 0.14 nmol. kg-‘. Only 3.3% of the surface sus-

TABLE 2 COMPARISON OF DISSOLVED AND PARTICULATE

TRACE METALS IN INTERMEDIATE AND DEEP WATERS (500 - 4tXIOM)

OF THE CENTRAL NORTH PACIFIC GYRE

Al Fe Mtl Zll Cd nmolikg

Dissolved 0.43 0.38 0.22 8.0 0.90

Partictdate I .03 0.31 0.055 0.017 HAc Leach 0.048 0.028 0.053 0.014 ::z

96 Dissolved 30 55 80 99.8 99.98

DissJPart. 0.42 1.2 4.1 470 4500

Diss./HAc Pan 9.0 14 4.2 570 4500

3174 K. W. Bruland, K. J. Orians, and J. P. Cowen

(a) Dissolved Fe (nmol/kg)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 o-. , .

-8 l .d

r X.8 ’ I *

200 - ’ l

l

l

l

l

4000

5000 I

e

l

l

l m

l *

l

m *

l

_* . I. I. I. I. t .

Particulate Fe (nmol/kg)

0.0 _. 0.2 0.4 1.2 1.4

2o”,t~ +--I 400

t.

v

V i 600

; I V

E 91 800 l

V

;;; l V

E 1000 I r V

Jz 8 $ *“a 2000

v l V

2 3000 V

V

4000

-

i

V

l V

5000 l V l

b--J v

FIG. 3. Vertical profiles of (a) dissolved iron (nmol kg-‘) and (b) particulate iron (nmol kg-‘) at VERTEX-IV. For particulate iron filled circles represent the acetic acid leachable fraction and open triangles represent refractory iron.

pended particulate Fe was HAc leachable. The average concentration in the intermediate and deep waters (500-4000 m) was 0.38 nmol - kg-‘, with 9.0% being in the HAc Ieachable fraction. As with particulate Al, there is a near bottom increase to a value of I .3 nmol - kg-‘.

Partitioning of Fe between dissolved and particulate phases varied little with depth. Within the surface 100 m, an average of 48% of the Fe was in the dissolved form, while in the intermediate and deep waters this increased slightly to 55% (Tables 1 and 2).

Manganese

Dissolved Mn (Fig, 4a) had a maximum concentration of 1 .O nmol - kg-’ within the surface mixed layer. Similar to dissolved Al and Fe, dissolved Mn exhibited a sharp gradient through the seasonal thet-mocline. Dissolved Mn had a pronounced minimum (approaching 0.1 nmol - kg-‘) in the major thermocline at 400 m depth, a secondary maximum (0.5 nmol - kg-‘) centered around the 0 minimum, and values decreasing to less than 0.1 nmol - kg-’ within the deep and near-bottomwaters.

Dissolved Mn {nmol/kg) @I

0.0 0.2 0.4 0.6 0.6 1.0 1.2 o-1,. I. I. I. ,.’ -

l * e 200 ” l l

grn

a 400 - m

0 _ 600 - l

1? e, 800 -

0

% 0

.k lOOO- a

I 0

I b

T a 2000 - i

T

2 3000 - a

. l

4000 - l

. 0 5000 - l

l I I I * 1 .

Particulate Mn (nmol/kg) 64

0.00 0.02 0.04 0.06 0.06 0.10 0.12

I

TL 2000

~

V

iii

3000

v l

4000

i *

l

0

l

l

l

l l

0

l

l

5000 0 a l 6 , 1

FIG. 4. Vertical profiles of (a) dissolved manganese (nmol kg-‘) and (b) particulate manganese (nmol kg-‘) at VERTEX-IV. For particulate manganese filled circles represent the acetic acid leachable fraction and open triangles represent refractory manganese.


Suspended particulate manganese (Fig. 4b) exhibited its lowest concentrations within the surface 100 m, averaging only 0.009 nmol - kg-‘. Values increased through the major thermocline to a broad maximum of 0.08 nmol- kg-’ at depths of 600-1000 m and then decreased again to 0.02- 0.05 nmol . kg-’ within the deep waters. Within the surface 100 m, 70% of the particulate Mn was HAc leachable, while within the intermediate and deep waters, 96% existed in the leachable fraction.

(4 Dissolved Cd (nmol/kg)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 . , . , I, I,. , .

0

400 - l

l

600 - 0

800 - a

0

lOOO-

03 Particulate Cd (pmol/kg)

_ 600

E al 000 aI

4000

5ooo lc-__ul FIG. 5. Vertical profiles of (a) dissolved cadmium (nmol kg-‘) and

(b) particulate cadmium (pmol kg-‘) at VERTEX-IV. For particulate cadmium only the acetic acid leachable fraction is presented. The refractory particulate cadmium was below detection limit (~0.05 pmol kg-‘).

Partitioning into particles increased markedly between surface and deep waters. Within the surface 100 m, 99% of the Mn was in the dissolved form, while in the deep and intermediate waters, only 80% was dissolved (Tables 1 and 2).

Cadmium

Dissolved Cd (Fig. Sa) exhibited a typical nutrient-type profile associated with the production and remineralization of organic tissue, with concentrations of 0.002-0.003 nmol + kg-’ within the surface 100 m that increased through the major thermocline to approximately 1 nmol - kg-’ at depths of 800-1000 m. The average dissolved Cd concentration in the intermediate and deep waters was 0.90 nmol . kg-‘.

The suspended particulate Cd concentrations (Fig. 5b) displayed a pronounced maximum of 0.002 nmol - kg-’ within the lower photic zone at depths (50- 100 m) corresponding to the chlorophyll maximum. It then decreased sharply to about 0.0005 nmol- kg-’ at 200 m and more gradually to values on the order of 0.0002 nmol * kg-’ at a depth of 1000 m. It showed a slight maximum at 3000-3500 m and then decreased to values less than 0.000 1 nmol - kg-’ in the near- bottomwaters. Approximately 98% of the particulate Cd existed in a form that was HAc leachable. The refractory Cd was undetectable in most samples and is not presented.

Cadmium exhibited the greatest difference in partitioning between surface and deep waters. Within the surface 100 m, an average of 65% of the Cd exists in the dissolved form, while within the deep and intermediate waters 99.98% is dissolved (Tables 1 and 2).

Zinc

Dissolved Zn (Fig. 6a) exhibited a nutrient-type distribution similar to that of silicic acid. Concentrations within the surface 100 m averaged 0.23 nmol . kg-‘, then increased through the major thermocline to an average value of 8 nmol - kg-’ within the intermediate and deep waters.

The suspended particulate Zn concentration profile (Fig. 6b) shows more scatter than the other profiles. Concentrations averaged 0.015 nmol -kg-’ within the surface 100 m, and 0.0 17 nmol - kg-’ within the intermediate and deep waters. Within the surface 100 m, 93% of the particulate Zn was HAc leachable, while in the intermediate and deep waters an average of 82% was in this fraction. Within the surface 100 m, 94% of the Zn was dissolved, while in the intermediate and deep waters this value increased to 99.8% (Tables 1 and 2).

DISCUSSION

Surface Water Distributions: The Aeolian Imprint

The euphotic zone of the central North Pacific gyre, approximately the upper 140 m of the water column, undergoes a well-defined seasonal stratification (BATHEN, 1972). In mid- March, the end of the winter cooling and storm period, the mixed layer extends to approximately 140 m. During April the mixed layer rapidly warms and shoals, developing a seasonal thermocline. From May through September the mixed layer depth is on the order of 20-30 m, with a pronounced seasonal thermocline between the base of the shallow surface

3176 K. W. Brufand, K. J. Orians. and J. P. Cowen

(4 Dissolved Zn (nmol/kg)

0

0

8

l 0 _C Z 2000 - 0 0

2 3000 - l -

a

4000 - l

0.

5000 - a l

t . I. I. La

@l Particulate Zn (nmol/kg)

0.00 0.02 0.04 0.06 0.08 0 rrr’ , * . ’ . .

200 3. v 0

400 l

600 v

- i

? v 800 V'

% ve E IOOO

3

_c

z. 2000 a, T ? l

V l l

e

V l

l

FIG. 6. Vertical profiles of (a) dissolved zinc (nmol kg-‘) and (b) particulate zinc (nmol kg-‘) at VERTEX-IV. For particulate zinc filled circles represent the acetic acid leachable fraction and open triangles represent refractory zinc.

mixed-layer and 140 m, stratif~ng the subsurface euphotic zone {Fig. ?a). In the late fall the surface layer begins to cool and with winter storms the mixed-layer begins to deepen toward its late-winter maximum depth.

This physical stratification of the lower euphotic zone leads to biological and chemical stratification as well (COALE and BRULAND, 1987). During the summer, there is a well devel- oped chlorophyll pigment maximum within the seasonally stratified region at a depth of 90 m (Fig. 7b). The presence of this eutrophic layer influences trace metal scavenging as well. COALE and BRULAND (1987) examined the distribution

of the particle reactive tracer, 234Th, to determine scavenging rates throughout the surface 300 m at our VERTEX-IV station. Dissolved 234Th (halflife = 24 d) exhibited a substantial disequilibrium with its parent 238U at depths of 60 to 100 m (Fig. 7c), indicating substantial scavenging and removal of 234Th in this part of the lower euphotic zone. Within the oligotrophic, surface mixed layer, however, 234Th is near equilibrium and little or no net scavenging occurred between stratification sometime in April and our July sampling date. COALE and BRULAND (1987) and BRULAND and BEALS

(I 994) have argued that the scavenging of 234Th is propor- tional to new production. They suggest that during summertime conditions in the central gyre of the North Pacific, the oligotrophic, surface mixed layer is dominated by the reminerahzation of organic particles and nutrient regeneration. In this type of system there is little particulate organic matter export (new production), minimal net scavenging of 234Th, and a relatively low rate of reactive trace metal scavenging. In contrast, new production rates are higher in the more eutrophic lower layers of the euphotic zone, resulting in more intense scavenging of dissolved 234Th, and by inference other reactive trace metals, via biogenic particles.

The low summertime rates of scavenging within the sufface mixed layer should allow the atmospheric input of dissolved metals to accumulate in the surface mixed-layer during this time. Since any aeolian input of trace metals is mixed relatively rapidly within the surface mixed layer, a given amount entering the 20-25 m deep mixed layer during late spring or summer would have roughly a sixfold greater impact on concentrations than an equal amount entering the 140 m deep mixed layer at the end of winter. This basic idea, the seasonal isolation of surface dissolved trace metals coupled with their aeolian input, has been previously observed for lead in the Sargasso Sea (BOYLE et al., 1986).

On the basis of this conceptual model, we can predict the seasonal variations in surface trace metal concentrations. Es- timates of the atmospheric deposition Flux to the North Pacific central gyre exist in the literature (MARI~G and DUCE, 1987; ZHUANG et al., 1990; DONAGHAY et al., 1991; DUCE and TINDALE, 1991; DUCE et al., 1991). The Asian dust input varies seasonally, with the maximum rate occurring in the spring. However, only a fraction of each metal in mineral aerosols is soluble (Table 3). From these data, we have estimated the input of dissolved metals over the three-month period between the onset of stratification and our study. The concentration increase in the surface mixed layer for each trace metal can be calculated from these estimates of aeolian input (Table 3) and compared with observed concentrations of these dissolved metals. Our calculations assume negligible effects of scavenging and physical mixing from the surface mixed layer during this period.

Aluminum

The dissolved Al concentration of the surface mixed layer is approximately 3 nmol - kg-’ greater than that observed at depths of 50-100 m. The predicted aeolian signal of -l-3 nmol -kg-’ (Table 3) can account for the elevated concentration observed in the surface mixed-layer. This close agree- ment is consistent with negligibie scavenging of dissolved Al


(a) Temperature Pigments (mg/m”3) W’)

6 8 10 12 14 16 18 20 22 24 26 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Ol-

1

Th-234/U-238

0.0 0.2 0.4 0.6 0.6 1.0 1.2

500

600

i? +A

, , I . , . I .

FIG, 7, Vertical profiles of (a) temperature (“C), (b) phytoplankton pigments, and (c) activity ratio of 234Th:238U in dissolved and particulate fractions at VERTEX-IV (COALE and BRULAND, 1987).

from the surface mixed layer under summertime conditions, as was previously observed for *“Th.

Our observations and analysis suggest that there should be a substantial seasonal variability of dissolved Al within the surface euphotic zone. At the end of winter, there should be uniform concentrations of approximately 2 nmol - kg-’ within the 140 m mixed layer. As seasonal stratification develops, the dissolved Al within the surface layer will increase due to aeohan input to the surface and the relatively low scavenging intensity within the shallow mixed layer. The lower euphotic zone, on the other hand, becomes isolated from the direct atmospheric input and has more intense scavenging (indicated from the 234Th studies). Consequently, the dissolved Al concentrations decrease to values close to 1 nmol -kg-‘. Our mid-summer data depicts this situation: the signature of the atmospheric input is amplified, with the surface mixed layer

concentration increasing to roughly four times that observed within the lower, stratified euphotic zone.

The observed suspended particulate Al profile does not display this atmospheric influence. During the same three month period, the aeolian input of mineral dust would have provided the equivalent of approximately 30 nmol * kg-’ particulate Al to the 25 m mixed-layer. The observed particulate Al concentration is approximately one hundredfold less than this and is consistent with a relatively short residence time of particulate aluminosilicates in the surface euphotic zone (ORIANS and BRULAND, 1986). A residence time for particulate Al of a couple weeks within surface waters would lead to a continual and effective removal of the aeolian particulate Al from the surface waters and its transfer to intermediate and deep waters. Although particulate organic matter within the summertime surface mixed layer is largely recycled, re-


TABLE 3 ESTIMATES OF THE ATMOSPHERIC DEWSITION FLUX To THE

NORTH PACIFIC GYRE (Donaghay et al., 1991; Duce et al., 1991;

Maring and Duce, 1987)

Element Assumed

S”‘;bility

Diss Flux

umol/m2/yr

3 MO. Seasonal Diss Input umollm*

Impact on Upper 25m

nmollkg

AI

Fe

Mll

Zll

Cd

IO 300 75 3

10 60 I5 0.6

50 9 2.3 0.1

76 1.1 0.3 0.01

83 0.04 0.01 O.tXO4

fractory particulate aluminosilicates are still effectively ex- ported from the surface waters.

Iron

The dissolved Fe concentration in the surface mixed layer also shows a strong aeolian signature. The surface value is 0.35 nmol - kg-’ greater than the concentrations observed at depths of 70-100 m. The predicted aeolian signal was 0.6 nmol - kg-‘, a value 70% greater than observed. This may reflect the greater particle reactivity of Fe than Al, or simply the range of uncertainty of the input estimates. In contrast to Al, Fe has a substantial biological demand with active assimilation by planktonic organisms (MOREL et al., 199 1; BRULAND et al., 1991). HUTCHINS et al. (1993) have dem- onstrated that Fe assimilated by plankton in such oceanic waters is recycled on a timescale of days. Thus, despite the biological demand for Fe, a substantial fraction of the dissolved Fe coming from atmospheric sources could be recycled and retained in the oligotrophic surface mixed-layer leading to the observed elevated concentrations during summertime conditions.

In the lower euphotic zone, which is isolated from direct atmospheric inputs, intensified scavenging of Fe results in the extremely low values of 0.02 nmol. kg-’ observed at depths of 70- 100 m. The surface concentrations are almost twentyfold greater than the values observed in the lower euphotic zone, a strong indicator of the atmospheric input on oceanic surface waters. At depths below 100 m, dissolved Fe exhibits a nutrient-type distribution.

The conceptual model predicts that throughout the late spring and summer, dissolved Fe concentrations would increase within the oligotrophic surface mixed-layer and si- multaneously decrease in the lower euphotic zone. MARTIN et al. (1989, 199 1) have shown that in the productive waters of the Subarctic Pacific and Equatorial Pacific, low values, on the order of 0.02 nmol - kg-‘, exist throughout the surface layer. These are regions of higher new production with intensified scavenging of 234Th occurring within the surface mixed layer (BRULAND and BEALS, 1994). In these regions, dissolved Fe concentrations are similar to what we observe at VERTEX-IV at depths of 70- 100 m.

Suspended particulate Fe concentrations in the surface layer do not show the atmospheric signal displayed by dissolved Fe. Concentrations in the surface 100 m averaged 0.15 nmol - kg-‘, a value far less than the roughly 6 nmol - kg-’ of particulate Fe that would have been supplied to the mixed- layer during this three month interval. Again, as with particulate Al, this is consistent with the relatively short residence time of particulate aluminosilicates in the surface euphotic zone.

Manganese

The dissolved Mn concentration in the surface mixed-layer was 0.3-0.4 nmol - kg-’ greater than at depths of 70-100 m. This difference is roughly three times greater than predicted from the input of aeolian dust (Table 3). Particulate Mn is well known to undergo photoreduction to dissolved Mn2+ ion within oceanic surface waters, resulting in decreased net scavenging. Such a process allows elevated levels of dissolved Mn to be maintained in near-surface waters (SUNDA and HUNTSMAN, 1988). This photoreduction of Mn leads to the low concentrations of particulate Mn in the surface waters, i.e., 99% of the Mn is in the dissolved phase. Just beneath the photic zone, however, the particulate Mn increases abruptly and the dissolved Mn decreases, indicative of scavenging to particulate phases in the absence of photoreduction. At depths below the photic zone, bacteria appear to dominate the scavenging and oxidation of dissolved Mn in the oceans (COWEN and BRULAND, 1985; NEALSON et al., 1988; COWEN, 1992).

Cadmium

Dissolved Cd is a classic example of a nutrient-type trace metal that closely mimics the distributions of nitrate and phosphate. Our results are consistent with others observed in the North Pacific (BRULAND et al., 1978a; BRULAND, 1980, BRULAND, 1992). The concentration in the surface mixed layer was between 0.002 and 0.003 nmol - kg-‘, values indis- tinguishable from those at depths of 70- 100 m. The predicted impact of the atmospheric input was only 0.0004 nmol - kg-‘, a value too small to measure. The distribution of dissolved Cd is governed almost completely by its internal cycle, rather than its external inputs. Vertical mixing provides the bulk of the Cd to the surface waters of the central North Pacific.

In contrast to particulate Al, Fe, and Mn, particulate Cd exhibited a maximum concentration within the surface euphotic zone. The distribution of suspended particulate Cd within the surface 200 m mimicked that of chlorophyll pigments, with a maximum near a depth of 90 m. Consistent with its nutrient-type dissolved distribution, dissolved Cd appears to be readily transferred from solution phase to planktonic organisms within the surface waters. PRICE and MOREL (1990) have suggested that a reason for this nutrient-type behavior is Cd substitution for the essential micronutrient Zn in oceanic phytoplankton. The concentration of dissolved free Zn is extremely low in oceanic surface waters (BRULAND, 1989) and Cd may be sequestered by planktonic organisms in its place.


Zinc

Dissolved Zn is another example of a reactive metal with a nut~ent-ty~ dist~bution, in this case mimicking silicic acid. Our VERTEX-IV results are consistent with others observed in the North Pacific (BRULAND et al., 1978; BRULAND, 1980, 1989). Dissolved Zn within the surface 100 m averages 0.2 nmol - kg-‘, with little if any discernible difference between concentrations in the surface mixed layer and the lower euphotic zone. This is consistent with the predicted aeolian signal of only 0.0 I nmol . kg-’ during this time period.

This lack of an elevated Zn concentration in the surface mixed layer provides an important additional reason for confidence in the elevated surface Fe concentrations. In our studies of trace metals, Zn and Fe contamination generally co-occur to approximately the same extent. MARTIN et al. (1993) have observed the same phenomenon. Thus, the elevated mixed-layer concentrations of dissolved Fe do not appear to be an artifact of sample contamination. Instead, the surface enrichment of dissolved Fe is consistent with that observed for dissolved Al and Mn, two metals much less prone to contamination and strongly influenced by their aeolian inputs.

In summary, based on estimates of atmospheric inputs relative to surface concentrations, we predicted atmospheric signatures in the surface mixed layer for dissolved Al, Fe, and Mn. The predicted atmospheric signatures were readily observable. External input sources and scavenging characteristics dominate the surface distributions of these reactive trace metals. The largest absolute signal is for dissolved Al, with an increase of 3 nmol- kg-’ over lower euphotic zone concentrations, while the greatest relative signature is for dissolved Fe with a twentyfold increase over those concentrations observed in the lower euphotic zone. In contrast, we did not predict measurable atmospheric signatures for Cd or Zn, nor were they observed. The distributions of these two metals are dominated by their internal biogeochemical cycles.

Interbasin Comparison of Open Ocean Data

The concentrations and distributions of both dissolved and particulate forms of the reactive trace metals in the oceans are a function of their various internal and external sources, together with their respective chemical properties and scavenging behaviors, all superimposed upon the genera1 patterns of ocean circulation and mixing. This can lead to substantiai interbasin fmctionation for these. reactive trace metals. We have examined a spectrum of reactive trace metals, ranging from strongly scavenged-type metals with short oceanic residence times, to nutrient-type or recycled trace metals with longer oceanic residence times.

Dissolved metal concen~r~~~ons

Two of these metals, Al and 221, exemplify idealized endmembers of the behavior of scavenged- and nutrient-type trace metals, respectively. The western North Atlantic basin intermediate and deep waters are dominated by relatively young, nutrient-poor waters recently formed at the surface

in the high latitude regions of the North Atlantic. The North Atlantic is a relatively small basin with its surface waters influenced by large atmosphe~c and river inputs. In contrast, the North Pacific basin is much larger. It is less influenced by atmospheric and riverine inputs, and, as a result of global ocean circulation patterns, its deep waters are characterized as old and nutrient-rich.

Dissolved Al concentrations are roughly fiftyfold higher in the deep waters of the North Atlantic relative to the North Pacific (HYDE& 1983; ORIANS and BRULAND, 1985). This is the greatest interbasin fractionation observed for any element. It is consistent with elevated external sources of Al plus its short oceanic residence time. North Atlantic deep waters are formed from North Atlantic surface waters, which are enriched in dissolved Al due to large external sources of Al to this basin (MEASURES and EDMOND, 1992). During the transit and aging of the deep water enroute to the North Pacific, the dissolved Al is continually removed at a scavenging timescale of roughly a century (ORIANS and BRULAND, 1986). Hence, the lowest concentrations of dissolved Al are found in the old, deep waters of the North Pacific. This distribution and pronounced interbasin fractionation of dissolved Al repre- sents the endmember example of a scavenged-type metal.

Zinc provides the endmember example of a nutrient-type (or recycled) trace metal. Dissolved Zn concentrations are approximately five times greater in the old, nutrient-rich deep waters of the North Pacific than they are in the young, nutrient-poor, North Atlantic deep waters. Its dist~bution in both ocean basins is similar to that of silicic acid (BRULAND and FRANKS, 1983) and is governed by an internal cycle with rapid removal from surface waters coupled with almost complete remineralization at depth. The efficiency with which Zn is recycled in the ocean leads to its relatively long oceanic residence time. Cadmium is another example of a nutrient- type or recycled metal, with dissolved concentrations in the North Atlantic deep waters approximately one-third of those found in North Pacific deep waters.

Manganese is a scavenged-type metal whose distribution is complicated because of its redox chemistry. In oxic intermediate and deep waters, Mn has an oceanic residence time on the order of 50-100 years. In suboxic or anoxic waters, reduction of oxidized particulate phases produces high concentrations of dissolved Mn, which can mix laterally into the oxygenated ocean’s interior (LANDING and BRULAND, 1987; RUE et al., 1994). At VERTEX-IV, the distribution of Mn is influenced by the lateral transport of dissolved Mn-enriched waters from suboxic boundary regions. The Mn maxima observed in both dissolved and suspended particulate profiles (Fig. 4) in the zone of the 02 minimum (500-1000 m) is evidence of this phenomena. Here, under oxic conditions, the dissolved Mn maximum is being eroded by particulate scavenging, thereby generating a particulate Mn maximum.

Iron exhibits characteristics of both scavenged- and nutrient-type trace metals. Its scavenged-type properties include its short oceanic residence time of less than a century, and its low concentrations in old, deep North Pacific waters. Its nutrient-type characteristics are reflected by its strong cor- relation with nitrate and phosphate at depths below 100 m (see also MARTIN et al., 1989).

3180 K. W. Bruland. K. J. Orians, and J. P. Cowen

Suspended particulate metal concentrations

Table 4 presents average concentrations of the five metals

of interest within the surface 100 m of the North Pacific and North Atlantic oligotrophic central gyres. The three scavenged type metals, Al, Fe, and Mn all have significant atmospheric inputs. Consistent with the higher atmospheric dust flux to the North Atlantic, they also have suspended particulate concentrations in the North Atlantic central gyre that are enriched by a factor of 2.1-3.6 relative to those observed in the central North Pacific. In contrast, the two nutrient-type trace metals, Zn and Cd, have suspended particulate concentrations enriched by three- to five-fold in surface water from the North Pacific. This is consistent with the North

Pacific being enriched in dissolved Zn and Cd and with their higher inputs into the surface waters from vertical mixing

due to the higher vertical gradients of these two dissolved metals in the North Pacific.

Table 5 presents a comparison of the average concentrations of suspended particulate trace metals in intermediate

and deep waters of the central North Pacific and North At- lantic gyres. The concentrations of particulate Al, Fe, and Mn in the North Atlantic are 2.6-3.9 times greater than those in the North Pacific. Again, this is consistent with the relative atmospheric inputs to the two ocean basins. Estimates of suspended particulate residence times in intermediate and deep waters of the oceans range from 5-10 years (BACON

and ANDERSON, 1982; CLECG and WHITFIELD, 1990); whereas, estimates of horizontal mixing times from conti- nental boundaries into the central interior regions of ocean basins are on the order of 50-100 years (ANDERSON et al., 1983a,b; ANDERSON et al., 1990). Atmospheric mineral dust enters the surface waters, where it resides for a short time prior to aggregation and vertical transport to intermediate and deep waters. During this vertical transit, it can undergo multiple fragmentation and aggregation steps. Thus, the suspended particulate concentrations in the basin interiors can reflect their overlying atmospheric inputs.

Table 6 presents suspended particulate data on Al, Fe, and Mn from intermediate and deep waters of four distinct re- gimes; the California Current, the North and South Pacific gyres, and the Sargasso Sea. The lowest concentrations of Al and Fe occur in the South Pacific, consistent with the lower

TABLE 4 COMPARISON OF SUSPENDED PARTICULATE TRACE METALS

IN SURFACE WATERS OF THE NORTH PACIFIC

AND NORTH ATLANTIC GYRES

Al Fe nmollkg

0.37 0.15

Mn

9.3

ZII pmol/kg

15

Cd

1.5

Sargasso sea 1.2 0.32 34 5.2 0.30 (Shwreltn; Boyle, 1992)

Ill

TABLE 5 - COMPARISON OF SUSPENDED PARTICULATE TRACE METALS

IN INTERMEDIATE AND DEEP WATER OF THE

NOmH PACIFIC AND NOHTH ATLANTIC GYRES

AI Fe Mll Zn Cd nmollkg pmollkg

North Pacific Gyre 1.03 0.31 55 17 0.2 (500 4OOOm)

Sargasso Sea (She&l and Boyle, 1992)

(500 - 3000m)

3.6 1.2 146 7.4 0.06

atmospheric input to this region (DUCE and TINDALE, 199 1;

DUCE et al., 199 1). The Al concentrations are a factor of 3 lower in the South Pacific than the North Pacific and a factor

of 10 lower than the Sargasso Sea. To provide a contrast between boundary regions and these central gyres, we in- cluded average values from a station in the middle of the California Current. Concentrations of suspended particulate Al in the intermediate and deep waters underlying the Cal-

ifornia Current are fifteenfold greater than they are in the central gyre of the North Pacific. Similarly, particulate Fe is eighteen-fold greater at the California Current station than

in the central gyre. The percentage of metal leachable by 25% acetic acid is

also presented in Table 6. At the lower suspended particulate concentrations observed in the central gyres, the HAc-leachable fraction increases. For example, HAc-leachable iron comprises 5.7% of the total particulate Fe concentration at the California Current station, increasing to 9.0% in the North Pacific gyre and to 27% in the remote South Pacific gyre. This behavior is consistent with the decreasing input of ter- rigenous aluminosilicates to these three areas.

Table 7 presents the elemental ratios (normalized to Al) of the various metals in suspended particles from the intermediate and deep waters of the North Pacific and Atlantic.

TABLE 6 - COMPARISON OF SUSPENDED PAKI-ICULATE TRACE

METALS IN INTERMEDIATE AND DEEP WATERS

nmollkg (96 HAc leachable) Al Fe MIl

California Current (Landing and Bruland, 1987)

(750 - 35OOm)

15.4 5.7 0.13 (4.1%) (6.8%) (73%)

North Pacific Gyre 1.03 0.31 0.055 (500 - 4000m) (4.7%) (9.0%) (96%)

South Pacific Gyre (Landing and Bruland, 1987)

(500 - 35OOm)

Sargasso Sea (Sherrell and Boyle, 1992)

(500 - 3coom)

0.35 0.22 0.063 (10.6%) (27%) (97%)

3.6 1.2 0.15


TABLE7 _ ELEMENTALRATIOSNORMALIZEDTOALFOR Deep Water Residence Times INTERMEDIATE ANDDEEPWATERSUSPEND~DPA~ICLES

Element Crustal Ratio Cent. N. Ad. Cent. N. Pac.

molimol mol/mol mollmol

_..~

Al

Fe

Mll

ZIl

Cd

1.0 1.0 1.0

0.33 0.33 0.30

5.7 x IO” 4.0 x lo” 3.2 x IO-2

3.5 x 104 2.1 x IO” 1.6 x 105

5.9 x IO-’ 1.7 x 10s 1.9 x 104

BACON and ANDERSON (1982) presented a simple, yet effective, way of estimating the oceanic residence time of metals with respect to deep water scavenging. The scavenging residence time can be estimated by the proportion of the total exchangeable metal concentration, [Me*], that exists in the particulate form, [Me&

In both basins, the particulate Fe:Af atom ratio is close to its crustal ratio. This is consistent with the refractory nature of the bulk of the Al and Fe in the deep waters. Greater than 90% of both of these elements exist in a relatively refractory fraction (not HAc-leachable). In deep waters of the central gyres, most of the particulate Fe and Al appears to be refractory aluminosilicate particles of aeolian input. Compared to crustal ratios, the particulate Mn:Al atom ratio is approximately an order of magnitude enriched in both ocean basins, with the North Pacific being 30% enriched over the North Atlantic. This is consistent with the observation that an average of 96% of the particulate Mn is in the HAc-leachable fraction, the bulk of which is formed in situ. Bacterial me- diation appears to dominate the scavenging and oxidation of dissolved Mn in intermediate and deep waters (COWEN and BRULAND, ~~~~;NEALSON et al., 1988; COWEN, 1992).

Since the refractory aluminosilicate fraction is relatively inert, we exclude it from this calculation. Thus, [Me*] should include only the HAc-leachable fraction, while [Mer] shouid include the dissolved and HAc-particulate metal fractions. Using the data in Table 2, together with a 5-10 year residence time of particles in the deep sea, we can estimate their deep water residence times. The scavenged-type metals, Al, Fe, and Mn, yield deep water residence times of 45-90,70-140, and 20-40 years, respectively. In marked contrast, the nutrient-type metals, Zn and Cd, have deep water residence times estimated to be 3,000~6,000 and 22,000-45,000 years, respectively. For both types of metals, these estimates are consistent with mean oceanic residence times based upon box models or vertical adve~on/di~usion models (BR~LAND, 1983).

Particulate Zn:Al and Cd:Al ratios both exhibit a dramatic enrichment over average crustal values. Both Zn and Cd also show an order of magnitude en~~hment in suspended particulate concentrations from the North Pacific relative to the North Atlantic (Table 7). This enrichment is consistent with the nutrient-type or recycled behavior of these two elements. The major component of the particulate Cd and Zn appears to be associated with plankton phases and is part of the internal biogeochemi~l cycle of biogenic particle production and reminerahzation. The enrichment of Cd and Zn in suspended particles in the North Pacific relative to the North Atlantic is consistent with the observed fractionation of dissolved Cd and Zn between the two ocean basins and their involvement in uptake and regeneration associated with plankton pr~u~tivity.

Each of these five trace metals is reactive. They are all scavenged on relatively short timescales from oceanic surface waters, The nutrient-type or recycled metals, Zn and Cd, undergo many internal cycles of removal onto particles in surface waters, with some of the particles sinking into intermediate and deep waters, followed by remineralization at depth and eventual mixing back into the surface. The scavenged-type metals, Al, Fe, and Mn, have substantial net scavenging, even in the deep waters. This type of chemical behavior, together with global circulation patterns, and the magnitude of their external inputs, leads to their respective interbasin fractionation.

Acknowledgments-We thank R. M. Sherrell and R. J. M. Hudson for critical reviews of this manuscript. Grant support was provided by NSF OCE 90-000 I5 1 and ONR NO001 4-92-J- 1304.

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Reactive trace metals in the stratified central North Pacific*kbruland/...3172 K. W. Brufand, K. J. Orians, and J. P. Cowen external sources and have deep water concentrations in the