the geochemistry of manganese in the northeast pacific ocean off washington

The Geochemistry of Manganese in the Northeast Pacific Ocean Off WashingtonAuthor(s): Carolyn J. Jones and James W. MurraySource: Limnology and Oceanography, Vol. 30, No. 1 (Jan., 1985), pp. 81-92Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2836217 .

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Limnol. Oceanogr., 30(1), 1985, 81-92 ? 1985, by the American Society of Limnology and Oceanography, Inc.

The geochemistry of manganese in the northeast Pacific Ocean off Washington' 2

Carolyn J. Jones3'4 and James W. Murray School of Oceanography WB- 10, University of Washington, Seattle 98195

Abstract

Water collected along a transect normal to the coast of Washington was analyzed for dissolved (<0.4 ,im) and total dissolvable manganese (TDM). The vertical profiles exhibit the same general features observed elsewhere in the Pacific, but a horizontal section of the transect shows that concentrations increase markedly toward the continental margin. A surface maximum, ranging from 0.57 nmol kg-l in offshore waters to 5.24 over the continental slope, is probably due to fluvial inputs, especially from the Columbia River. Another possible source is the upwelling of manganese- enriched bottom water over the shelf. Horizontal gradients are also observed in the oxygen minimum as manganese concentrations decrease from 4.75 nmol kg-' near the continental slope to 0.71 in open ocean water. Porewater and solid phase data suggest that manganese is being actively remobilized under reducing conditions in the slope sediments and is diffusing into the overlying seawater at a rate sufficient to balance losses by vertical mixing and particulate scavenging. Two stations on opposite sides of the Juan de Fuca Ridge have deep concentration maxima that are almost certainly due to injections of hydrothermal vent fluid along the ridge.

Major advances in understanding the cycling of trace metals in the ocean have been made in the last few years. At the same time, studies of manganese indicate that the geochemistry of this trace metal is one of the most complicated and least understood. Re- cent work (Bender et al. 1977; Klinkham- mer and Bender 1980; Landing and Bruland 1980; Martin and Knauer 1982, 1983, 1984) suggests that the distribution of Mn is con- trolled by complex interactions between in situ physical and chemical processes and the biogeochemical cycle. In addition, the "boundaries" of the oceans, such as the continental margin and the seafloor, are thought to be important sites of mechanisms regu- lating the concentrations of Mn and other elements in seawater (Murray et al. 1983).

Although the overall distribution pattern of manganese is known quite well, the processes responsible for specific features have not been unequivocally identified. Atmo- spheric fallout, river runoff, and the diffu-

I University of Washington School of Oceanography Contribution 1387.

2 This research was supported by NSF grants OCE 78-19454 and OCE 80-18335.

3Supported by a NSERC (Canada) postgraduate scholarship.

4Present address: Department of Oceanography, University of British Columbia, Vancouver V6T I W5.

sion of dissolved Mn from reducing hemipelagic sediments have been proposed as sources of the surface maximum commonly seen in vertical profiles. A concentration maximum coinciding with the oxygen minimum appears to result from a combination of advective-diffusive transport of Mn from reducing nearshore sediments and the re- generation of Mn from sinking biogenic particles (Martin and Knauer 1984). Concen- trations are low in deep waters except where enriched by the resuspension of bottom sediments or by injections of hydrothermal fluids at spreading centers (Klinkhammer 1980; Lupton et al. 1980; Jones et al. 198 1). In general, Mn concentrations are higher in nearshore waters because of the proximity to sources on land or at the continental margin.

We chose a section of the northeast Pa- cific Ocean off Washington as an appropri- ate location in which to examine the sig- nificance of various inputs of Mn to the ocean. In particular, we wanted to assess the importance of the continental margin and the Juan de Fuca Ridge as possible sources of Mn and to determine if the horizontal concentration gradients observed elsewhere (Bender et al. 1977) also existed in this part of the ocean. The results of our investiga- tions are presented here.

81



82 Jones and Murray

i000

48'

2500 100o 2500

2 WASH

3000- 4 7 8 9 12

3<> 3 12/;8' 9 12 46' 1 -8I

Fig. 1. Station locations for TT 121 off the Wash- ington coast. The Juan de Fuca Ridge is defined by the 2,500-m contour near 1 30?W.

Many people helped with the collection and analysis of our samples. On board ship, B. J. Spell and R. E. Cranston collected the water column samples, V. Grundmanis and J. Sawlan assisted in collecting the porewater and box core samples, and J. Jennings ran the oxygen titrations. In the laboratory, V. Grundmanis, B. J. Spell, and B. Paul aided in the manganese and porewater nutrient analyses. S. E. Calvert, T. F. Pedersen, and J. H. Martin critically reviewed the manuscript at various stages in its produc- tion.

Methods Sampling- Samples for Mn analysis were

collected during cruise TT 121 of the RV Thomas G. Thompson in July 1977 at the stations shown in Fig. 1. Water column samples were collected in 30-liter acid- washed PVC Niskin bottles equipped with surgical rubber tubing closures and hung on stainless steel hydrowire. Unfiltered samples were drawn directly into 500-ml acid- cleaned polyethylene bottles and acidified with 0.5 ml of ultraclean HNO3 within 1 h. Filtered samples were collected in 500-ml acid-cleaned bottles after pressure filtration through 0.4-,um Nuclepore membrane filters housed in Teflon/polyethylene filter holders.

Interstitial water samples were collected with a sampler similar to that of Sayles et al. (1976). Porewater is drawn into the sampler by in situ filtration and is stored in Teflon capillary tubing. Sample ports are located 50 cm above the sediment-water

interface and at burial depths of 5, 15, 30, 60, 100, and 140 cm.

Sediment cores for solid phase Mn analysis were obtained by box coring. As soon as the box corer was on deck, the sediments were subsampled with PVC core barrels. These subcores were kept frozen until analyzed.

Analytical techniques-Manganese in the water column was measured by a modification of the ion-exchange method of Riley and Taylor (1968). The filtered samples were extracted on board ship at their natural pH by adding a 20-ml slurry of Chelex- 100 resin (Bio-Rad) in its NH4+ form to each sample and shaking for 36 h. These samples were returned to the University of Wash- ington for elution and analysis. The unfiltered acidified samples were extracted in the lab by readjusting the pH to 8 with ultraclean NH40H, adding Chelex- 100, and shaking for 36 h. Manganese was eluted from the resin with 2 N HNO3 and analyzed by flameless atomic absorption spectrophotometry (AAS) with deuterium arc back- ground correction. The overall precision of this method, expressed as relative standard deviation (1 a), was about 4%.

Filtered samples were collected at the in- shore stations 1, 2, and 10 (Fig. 1) because of the anticipated large particulate load. Samples were also filtered at station 5 to compare total and dissolved Mn concentrations in the open ocean. Unfiltered samples were collected at stations 5 and 7. The manganese content of the unfiltered samples is the sum of the dissolved Mn concentration and the Mn concentration in particulate fractions that is solubilized by storage at pH 2; this is the "total dissolvable manganese (TDM)" of Bender et al. (1977) and others. The filtered samples yield values re- ferred to herein as dissolved manganese.

Dissolved oxygen was determined by the Carpenter (1965) modification of the Wink- ler titration method. Salinities were measured with a Bisset-Berman inductive sali- nometer; temperatures were read from deep-sea reversing thermometers.

About 16 ml of uncontaminated sample were collected at each depth in the sediment by the in situ porewater sampler. Four 4-ml aliquots were drawn of which the first was



Mn in the NE Pacific 83

used for nutrient analyses and the last for manganese. The nutrients were measured colorimetrically with an AutoAnalyzer by the methods of Strickland and Parsons (1968) for silicate and phosphate and of So- lorzano (1969) for ammonia. Manganese was analyzed by flameless AAS by direct injection of porewater into the graphite fur- nace. The leachable Mn content of the upper 3 cm of each sediment core was determined by the method of Chester and Hughes (1967).

Results The water column manganese, dissolved

oxygen, salinity, and temperature data are given in Table 1 and the interstitial water nutrient and manganese data in Table 2. The distribution of Mn, shown in vertical profiles in Fig. 2 and as a horizontal section in Fig. 3, confirms many of the findings of previous coastal and open ocean studies. A near-surface maximum is clearly evident, especially at the two stations over the continental slope. There is, as well, a pronounced horizontal gradient in the surface concentrations; values of dissolved Mn increase by an order of magnitude between station 5 (0.57 nmol kg-') and station 10 (5.24 nmol kg-'). Middepth maxima in dissolved Mn occur at 1,000 m and 600 m (stations 2 and 5); both maxima lie within the broad oxygen minimum zone of the Cascadia Basin (Table 1). Station 7 is somewhat anomalous in that there is no obvious middepth maximum.

The behavior of Mn near the seafloor var- ies markedly from station to station. On the shelf (station 1), the concentration of dissolved Mn increases sharply with depth to a value of 39.7 nmol kg-l at the bottom. It is likely that such a near-bottom increase also occurs at our stations over the continental slope, but since the deepest samples were taken 300 m above the bottom we cannot show this enrichment.

Stations 5 and 7, on opposite sides of the Juan de Fuca Ridge, both have deep concentration maxima at the depth of the ridge crest; this feature probably indicates a hydrothermal input from the ridge as. suggested by Jones et al. (1981) and Normark et al. (1982). Below the maxima, TDM con-

centrations decrease to 0.4-0.8 nmol kg-', values typical of the deep Pacific (Bender et al. 1977; Landing and Bruland 1980).

Horizontal concentration gradients, first observed by Bender et al. (1977) off Cali- fornia, are apparent off the coast of Wash- ington as well (Fig. 3). Water enriched in Mn is confined to the continental slope, with concentrations gradually decreasing offshore. At station 2, for example, the dissolved Mn maximum has a value of 4.75 nmol kg-'; at station 5, the value of the maximum is only 0.71 nmol kg-'.

The distribution of porewater Mn is shown in Fig. 4. In most cases, there is a sharp increase in manganese concentrations below the sediment-water interface; this increase, however, is much less in the sediments of the continental shelf (stations 12 and 1) and upper slope (stations 10 and 2) than in the sediments of the central basin (stations 3, 7, and 4). The porewater ammonia and phosphate concentrations also reach their highest levels in the sediments of the shelf and upper slope, suggesting that these sediments are the most reducing.

Discussion Most trace metals can be arbitrarily placed

into two general categories. Because of their participation in the biogeochemical cycle, certain metals have depth distributions characteristic of the labile nutrients: surface depletion and deep water enrichment. Al- uminum, cadmium, nickel, and zinc are examples of this group (Sclater et al. 1976; Bruland et al. 1978; Bruland 1980; Stoffyn and Mackenzie 1982).

Manganese, on the other hand, is a trace metal with the opposite distribution. Con- centrations are typically highest at the surface and low in deep water. Although vertical profiles frequently exhibit intermediate-depth maxima similar to those of phosphate and nitrate, linear regressions between our manganese and nutrient data showed little or no correlation. This is not surprising in light of the work of Martin and Knauer (1982, 1983) who have shown that the remineralization of organic matter in the upper water column is not an important source of Mn.

Surface maximum-One characteristic



84 Jones and Murray

Table 1. Water column data for the northeast Pacific off Washington. Where more than one Mn concentration is listed for the same depth, each number represents a separate subsample taken from one Niskin bottle. For the sake of clarity, however, these data points have been averaged and plotted as a single point in the vertical profiles (-, no data).

Mn* TDMt Depth S 0 02

(in) (m1) (IC) (AM) (nmol kg-')

Station 1 (102 m) 1 24.50 32.325 12.20 - 5.86, 5.06, 4.19

10 24.86 32.290 10.02 - 6.75 20 25.47 32.696 8.13 182.5 4.78 30 25.81 32.975 7.24 215.5 2.11 40 26.01 33.196 7.08 195.0 - 50 26.35 33.609 6.99 101.5 1.64 60 26.48 33.750 6.83 118.0 - 70 26.55 33.822 6.69 97.5 13.00 80 26.60 33.858 6.60 97.5 - 90 26.62 33.879 6.54 93.5 33.13

100 26.62 33.882 6.54 85.0 41.68, 40.59, 36.77

Station 2 (1,888 m) 1 23.72 32.215 15.58 265.5 2.82, 2.55, 3.22

10 23.73 32.212 15.53 212.5 - 20 24.27 32.416 13.71 264.5 2.04 50 24.94 32.520 10.57 281.5 1.26

100 25.37 32.636 8.45 269.0 0.619, 0.692, 0.510 150 26.10 33.316 7.10 213.5 - 200 26.52 33.808 6.89 158.5 0.983 250 26.66 33.906 6.42 138.0 - 300 26.75 33.936 5.90 106.0 - 350 26.82 33.955 5.39 83.5 - 400 26.87 33.991 5.20 60.0 1.09 450 26.93 34.090 5.01 44.0 - 500 27.09 34.119 4.11 - - 600 27.18 34.208 3.99 25.5 2.57 700 27.25 34.300 4.01 15.5 - 800 27.30 34.334 3.77 12.0 3.91 900 27.36 34.379 3.50 16.0 -

1,000 27.42 34.426 3.34 14.5 4.75 1,100 27.47 34.458 3.09 20.0 - 1,200 - - 2.93 19.5 4.30 1,400 - - - - 3.49 1,600 - - - - 3.60, 3.93, 4.06

Station 5 (3,143 m) 1 24.10 32.626 15.30 272.0 0.601, 0.437, 0.673 0.673, 0.819, 0.783

10 24.35 32.609 14.05 284.0 - - 25 24.66 32.597 12.46 296.0 0.546 0.637 50 25.17 32.635 9.76 298.0 0.291 0.546 75 25.24 32.640 9.37 294.5 - -

100 25.41 32.670 8.42 289.0 0.946t 0.528 150 25.92 33.078 7.09 257.5 -- 200 26.34 33.592 6.94 220.5 - - 300 26.70 33.894 5.97 178.0 0.273 0.983 400 26.86 33.932 4.91 96.5 - - 600 27.13 34.137 3.97 27.0 0.710 0.746 800 27.28 34.286 3.62 12.0 - -

1,000 27.40 34.386 3.23 13.0 0.619 0.965 1,200 27.48 34.451 2.90 19.5 0.455 0.928 1,500 27.86 34.523 - 32.5 0.801 1.26 2,000 27.69 34.599 1.80 67.0 1.06 1.46 2,500 27.73 34.632 1.58 89.0 1.16 1.73 3,000 27.75 34.651 1.53 106.0 0.801, 0.783 1.66, 1.33, 1.12 3,100 27.76 34.656 1.40 110.5 - -




Table 1. Continued.

Mn* TDMt Depth S 0 02 (in) (m1) (IC) (jIM) (nmol kg-')

Station 7 (2,719 m) 1 24.01 32.576 15.55 269.0 0.946, 1.00, 0.910

25 24.53 32.546 12.94 306.0 0.473 50 25.01 32.554 10.32 292.5 0.473

100 25.39 32.650 8.43 279.0 0.419 200 26.53 33.867 7.10 126.5 0.801t 300 26.76 33.987 6.09 75.0 0.419 400 26.88 34.056 5.55 45.0 - 600 27.10 34.132 4.40 20.5 0.655 700 27.16 34.202 4.17 14.5 - 800 27.24 34.279 3.93 9.5 - 900 27.30 34.325 3.67 11.0 0.637

1,000 27.37 34.374 3.40 10.5 - 1,100 27.42 34.442 3.23 12.5 0.692 1,200 27.47 34.441 2.90 17.0 - 1,400 27.54 34.495 2.64 22.5 - 1,600 27.60 34.535 2.29 35.0 0.764 2,100 27.68 34.593 1.83 64.5 1.27, 2.11 2,600 27.73 34.636 1.59 62.5 0.364, 0.437, 0.400

Station 10 (672 m) 1 23.43 32.089 16.45 260.0 5.11, 5.37

10 23.48 32.106 16.30 262.5 - 25 24.79 32.537 11.52 309.0 2.40 50 - - 9.04 288.0 2.49 75 25.51 32.722 7.98 268.0 -

100 25.94 33.130 7.23 225.0 1.31 150 26.50 33.847 7.22 128.5 - 200 26.65 33.929 6.64 118.5 2.80 300 26.78 33.968 5.82 85.0 3.53 350 26.83 34.004 5.70 68.0 - 400 26.87 34.024 5.09 57.0 - 500 26.94 34.060 4.93 43.0 - 550 27.06 34.190 4.55 35.5 - 600 27.06 34.150 4.55 24.0 - 650 27.03 34.089 4.47 18.5 -

* Dissolved manganese t Total dissolvable manganese : Contamination suspected

feature of the oceanic distribution of Mn is a surface enrichment that increases toward the continental boundary (Murray et al. 1983). As mentioned earlier, we observed an increase in dissolved Mn from 0.57 nmol kg-' in open ocean water (station 5) to 5 over the continental shelf and slope (stations 1 and 10). The concentrations at our offshore stations (5 and 7) are a half to a fourth those reported by Bender et al. (1977) and Landing and Bruland (1980) for stations about the same distance from the coast. Nearshore values, however, are in good agreement with those of Landing and Bru-

land (1 980) who also noted an order of magnitude increase in dissolved manganese between the Central Pacific Gyre and coastal waters.

The surface maximum has been ascribed to many sources including river runoff, de- sorption from aerosols, and diffusion of dissolved manganese from reducing nearshore sediments (Bender et al. 1977). On the basis of a single transect, it is difficult to evaluate the manganese contributions from the Co- lumbia River which has a TDM concentration of 75 nmol kg-' (Murray unpubl. results). The Columbia River supplies over



86 Jones and Murray

Table 2. Interstitial water nutrient and manganese data for hemipleagic sediments of the northeast Pacific Ocean. Concentrations are given in ,umol kg-' (BDL, below detection limit; -, no data).

Depth (cm) NH4' Si PC43- Mn

Station 1 (102 m) 0 4.5 61.4 2.2 0.08

7.6 61.2 2.3 5 45.8 380 4.7 0.46

39.3 338 3.7 15 46.7 433 9.4 0.53

46.7 392 11.6 30 39.7 358 4.1 0.33

36.4 317 15.7 60 94.9 435 7.5 0.54

86.4 403 9.0 100 110 433 17.8 0,73

107 429 16.1 140 144 461 26.4 0.82

141 453 25.6 Station 2 (1,888 m)

0 4.3 - 3.8 BDL 1.6 - 3.4

S - _ _ _

15 57.1 461 6.1 3.1 46.0 419 6.1

30 132 523 9.9 1.7 105 515 8.2

60 227 531 14.7 1.1 223 526 12.8

100 325 528 22.0 1.2 310 529 19.0

140 400 527 29.8 0.82 394 531 28.6

Station 3 (2,636 m) 0 3.3 204 3.2 0.31

4.8 192 3.0 5 0.0 410 4.7 20.2

0.0 404 4.8 15 0.0* 232* 3.8* 1.7*

0.0* 211* 4.0* 30 20.4 437 8.8 11.5

20.4 420 7.0 60 58.2 431 10.9 26.9

57.1 432 10.6 100 107 413 14.5 32.4

101 409 12.4 140 155 399 15.8 32.2

150 400 15.7 Station 4 (2,742 m)

0 6.8 - 2.9 0.10 8.8 318 2.9

5 5.9 516 3.7 20.2 6.8 510 3.5

15 13.4 522 7.5 40.4 12.7 525 7.3

30 33.6 536 8.9 26.5 32.9 530 8.4

60 67.5 522 14.2 62.9

Table 2. Continued.

Depth (cm) NH4+ Si PO43 Mn

60.6 527 12.2 100 116 513 16.2 64.4

89.6 513 13.4 140 127 530 16.2 53.3

119 519 15.2 Station 7 (2,719 m)

0 2.5 198 2.6 BDL 2.0 215 2.1

5 1.0 518 6.8 33.3 0.0 532 4.3

15 2.7 518 5.0 39.1 1.0 515 2.7

30 5.2 492 3.0 25.6 10.7 477 5.4

60 25.8 451 6.8 48.7 29.0 465 10.0

100 57.0 410 10.5 57.6 48.5 375 10.2

140 86.9 419 13.1 58.5 63.9 419 11.3

Station 10 (672 m) 0 1.8 108 5.8 -

1.5 108 5.8 5 6.9 175 7.2 1.2

4.3 145 6.5 15 7.1 236 6.6 1.3

9.5 224 5.5 30 41.4 342 5.0 1.7

60 113 395 16.5 1.5

100 172 403 20.0 1.8

140 245 407 27.8 3.3

Station 12 (97 m) 0 1.3 75.5 4.2 0.11

1.5 69.4 4.7 5 32.0 321 9.3 2.0

27.8 301 5.4 15 32.4 386 6.4 2.0

18.3 431 3.9 30 - - - 1.0

50.3 417 6.8 60 91.4 490 20.3 1.0

92.6 484 18.5 100 104 390 17.4 1.1

104 392 14.1 140 202 431 27.3 1.1

195 427 25.5 * Contamination of the porewater sample by seawater that leaked into

the port during retrieval of the in situ sampler.

75% of the freshwater discharged into the Pacific between San Francisco Bay and the Strait of Juan de Fuca. In summer, the outflow usually moves to the southwest under




Mn (nmol kg-1)

0 10 20 30 40 0 2.0 4.0 6.0 0 I 0 1 I -.

0

STA I STA 10

100 0 * 1 00_

E

o 200 200

300 300

Mn (nmolkg-)

0 1.0 2.0 3.0 4.0 5 0 0 0.5 1.0 1.5 2.0 0 0.5 1 0 1 5 2 0 0 0 0 VW i

50 STA 2 50 * STA 7 50 r . STA 5

100 * 100 * 100 * (.)

E~~ ~~ (00|U)10 10

* ~~ ~~~~~~0 U

0~~~~

2000~~~~~ _U00F 20

1000 1 1000 1000 * U E

Z 0 ~~~~~~~~~~~~~~~~~~0 w 0 0 3 0~~~~~~~~~

2000 2000 2000 *

U~~~~~~~

3000 3000 3000 *

Fig. 2. Water column profiles of dissolved (0) and total dissolvable (B) manganese in the northeast Pacific off Washington. The stations are shown in order of increasing distance from shore. In parentheses-contaminated samples.

the influence of prevailing winds (Barnes et al. 1972). Wind speed data for July 1977 (Hickey unpubl. results) indicate that strong northerly winds blew for most of the month, so that the river water would have been driven south of our stations and thus would not have contributed to the manganese levels

at the time of sampling. However, after changes in wind direction, Hickey (pers. comm.) has noted significant changes in the position of the effluent within 3 days. While the outflow is generally localized along the coast, it can move offshore for several hundred kilometers (Barnes et al, 1972). So



88 Jones and Murray

STATION NUMBER

5 7 2 10 1 - - - - ZS~~= - - .> 7 50

5.0

-;5 3.5t|

0,75~~~~I

2.0 I

RE PBASIN FUCA 3

RIDGE

13 ~ 130' 128' 126' 124'

LONGITUDE WEST

Fig. 3. Horizontal cross-section of the distribution of Mn along the cruise track. Solid lines-dissolved manganese; dashed lines-total dissolvable manganese. Units are nmol kg-'.

our observed surface maximum may actually be a remnant from a previous period of northward flow of Columbia River water. The Fraser River (via the Strait of Juan de Fuca) and the small rivers draining Wash-

ington State may also provide some input of Mn to our survey area.

Another possible explanation for both the surface maximum and the horizontal concentration gradient is that the manganese- rich bottom water on the shelf (Fig. 2) is upwelled and advected offshore. This kind of mechanism has been suggested by Boyle et al. (1981) and Kremling (1983) to account for similar features in their trace metal profiles. As the upwelled water moves offshore, particulate scavenging and mixing will reduce the elevated nearshore manganese concentrations to the levels measured at stations 7 and 5. The source of the manganese- rich bottom water is discussed below.

Manganese in the oxygen minimum- The least understood aspect of the distribution of Mn is the association of the middepth concentration maximum with the oxygen minimum. This association has been observed consistently in the Pacific in both coastal and open ocean waters (Bender et al. 1977; Klinkhammer and Bender 1980; Landing and Bruland 1980; Murray et al. 1983; Martin and Knauer 1984). Off the

Mn ( ipmol kg-')

0 1.0 2.0 0 2.0 4.0 0 20 40 60 0 0

0~~00 * 0 O *o [ * O 0 0 0 0

0 0 0 0

40 (a) 40 (b) 40 (c)

E A @0 @ 0 0 0

I- a- 80 80 80

_ *0 _ *0 _0 0 A

120 120 120

*0 0 0 0 A EO

Fig. 4. Porewater manganese profiles for hemipelagic sediments of the northeast Pacific. a-Data from continental shelf sediments (0-Sta. 1; C1-Sta. 12); b-from slope sediments (0-Sta. 2; 0-Sta. 10); c-from abyssal plain sediments (0-Sta. 3; C-Sta. 7; A-Sta. 4). Points at 0 cm are bottom water samples taken 50 cm above the sediment-water interface by the in situ sampler.




coast of Washington, the maximum in dissolved Mn deepens toward the coast from 600 m at station 5 to 1,000 m at station 2 but still lies within the oxygen minimum zone of the northeast Pacific (Fig. 2; Table 1). A marked horizontal gradient is again evident; the Mn concentration maximum is 4.75 nmol kg-' near the continental slope but only 0.71 in offshore waters.

Because of the coinciding positions of the manganese maximum and oxygen minimum, an input of dissolved Mn from de- grading organic matter has often been in- ferred. However, after conducting leaching studies on trapped particulate material, Martin and Knauer (1983, 1984) concluded that living plankton concentrate only minor amounts of Mn and other trace metals. It now appears that Mn is taken up at the surface by passive adsorption onto biogenic particles and then released back to the water as these particles sink through the oxygen minimum. According to the measured fluxes of Martin and Knauer (1984), this mechanism of biogenic transport and in situ re- generation accounts for 30% of the dissolved Mn maximum. The remaining 70% comes from a horizontal flux of Mn derived from coastal diagenetic processes.

Our data cannot be used to test the biogenic transport mechanism of Martin and Knauer (1983, 1984), but they do provide evidence for horizontal advective-diffusive transport from the continental margin. The shape of the concentration isopleths in Fig. 3 suggests that there is a source of dissolved Mn in or near the sediments. There are two possible ways to explain this nearshore enrichment. One is by diffusive fluxes from continental slope sediments, and the other is by the resuspension of manganese-rich slope sediments.

The porewater data (Table 2) indicate that mildly reducing conditions exist in the sediments of the continental shelf and upper slope (stations 1, 12, 10, 2) and that active remobilization of Mn may also be occurring. In fact, the relatively low concentrations of Mn in these porewaters compared with those of the porewaters from the central basin suggest that much of the remobilized Mn is being lost to the overlying seawater. The solid phase manganese anal-

Table 3. Leachable manganese content (Chester and Hughes 1967) in the surface layer (0-3 cm) of cores collected during TT 121. Stations are listed in order of increasing distance from shore.

Station Leachable Mn (% dry wt)

I 0.001 10 0.001 2 0.065, 0.043, 0.073 9 0.444 3 1.495 8 2.251 4 2.935 6 3.904, 4.267

yses (Table 3) lend support to this argument. The tops of cores from the shelf and slope contain little leachable manganese. As our leaching agent dissolves, among other things, manganese oxides (Chester and Hughes 1967), this means that the sediments of the shelf and slope have only a thin oxidized surface layer which does not act as an ef- fective trap for upwardly diffusing Mn. In contrast, in the central basin where sedi- mentation rates are lower and oxidizing conditions persist somewhat deeper in the sediment (Table 2), the core tops have a much higher leachable manganese content and, therefore, a thicker surface manganese oxide layer. Most of the remobilized Mn is trapped within this oxic zone, which ex- plains the lack of a bottom water enrichment at the two offshore stations.

Trefry and Presley ( 1982) observed a similar trend in the Gulf of Mexico. In rapidly accumulating nearshore sediments, reducing conditions occur close to the sediment- water interface, and the surficial sediments are relatively manganese-poor. In the slowly accumulating sediments of the abyssal gulf, they found several centimeter thick, manganese-rich surface layers, with reducing conditions occurring much deeper in the sediments.

Diffusive fluxes of manganese from the slope sediments were estimated from Fick's first law:

F= -

_bDsC 6 bz

where 0 is porosity (0.88; measured), 0 is tortuosity (1.5; Manheim and Waterman



90 Jones and Murray

1974), D. is the diffusion coefficient of Mn2+ in free solution (Li and Gregory 1974) ex- trapolated to in situ temperatures, and 6C/ bz is the porewater manganese concentration gradient, assuming linearity between the sediment-water interface and the first porewater sample. Using the porewater data (Table 2), we calculated fluxes of 17 and 14 nmol cm-2 yr-I for stations 10 and 2. These are minimum fluxes, as the sampling inter- val in the top 15 cm is too coarse to determine the Mn gradient at the sediment-water interface. The assumed gradients therefore significantly underestimate the actual gradients. Nevertheless, our manganese fluxes are intermediate between those from oxic red clay sediments (Callender and Bowser 1980) and from highly reducing nearshore sediments (Sawlan and Murray 1983). Our benthic fluxes are also consistent with the horizontal transport fluxes of 1-5 nmol cm-2 yr-1 required by Martin and Knauer (1984) to balance Mn losses in the water column and to provide 70% of their dissolved Mn maximum. Since the value of our dissolved Mn maximum is comparable to Martin and Knauer's, we believe that most of this manganese originates from diagenetic processes in the sediments of the Washington continental slope and is then advected laterally offshore.

The resuspension of bottom sediments is also a significant source of Mn. At station 5, for example, the TDM and dissolved manganese profiles show concomitant changes over the lower part of the water column (Fig. 2). Thus, much of our measured Mn may actually be associated with the <0.4-,um particulate fraction that is not removed by filtration. Episodic resuspension of surficial sediments along the continental slope by near-bottom turbulence may inject manganese-rich particles into the water column, and particles passing through the 0.4-,um filters are then analyzed as dissolved Mn. Hydrographic studies off the Washington coast (Hickey pers. comm.) found sharp reductions in transmissivity near the sediments, indicating the presence of bottom nepheloid layers. Extensive nepheloid layers have been observed pre- viously over the continental shelves and slopes of Oregon and Washington (Pak and

Zaneveld 1977; Pak et al. 1980). It appears that these layers are caused by offshore transport of resuspended materials along isopycnal surfaces (Pak et al. 1980). At station 2, suspended matter concentrations (unpubl. results) double over the bottom 200 m of the water column, suggesting that the resuspension of sediments has been occurring over the entire continental slope.

Manganese in bottom waters-The near- bottom behavior of Mn reflects the influence of local processes. On the shelf, dissolved Mn increases rapidly with depth to a concentration of 40 nmol kg-' at the sediment-water interface. To supply the bottom 10 m of the water column with this concentration of Mn during the average res- idence time of water on the shelf (- 1.5 months), the diffusive flux from the sediments would have to be about 330 nmol cm-2 yr-'. This value is > 10 times the flux of 30 nmol cm-2 yr-I calculated from the porewater data for station 12. But to reit- erate, our calculated fluxes are minimum fluxes and could easily be in error by an order of magnitude. Furthermore, Trefry and Presley (1982) have shown that in areas of rapid sediment deposition, such as at the mouths of rivers, benthic Mn fluxes are very large (up to 15 ,umol cm-2 yr-1), leading to Mn enrichments in coastal waters.

Other sources of dissolved Mn in the bottom waters are the release of manganese from particles falling on shallow shelf and upper slope sediments and the resuspension of fine-grained sediments. Martin and Knauer (1983, 1984) have demonstrated the importance of biogenic transport as a mechanism for transferring Mn introduced into the surface waters to the sediments. Since the high latitudes of the North Pacific are regions of intense diatom productivity, Mn will be rapidly transported to the sediments in association with biogenic particles and regenerated at the sediment-water interface. Coupled with this process is the de- sorption of Mn from riverine particulate material. According to Gibbs (1977), over 90% of the manganese transported by rivers is in particulate form, and within that phase, < 40% of the Mn is carried in the crystalline form. Thus, much of the particulate Mn supplied by the Columbia River, the largest



Mn in the NE Pacific 9 1

source of modem sediment on the Wash- ington shelf (Gross et al. 1967), may go into solution shortly after introduction into coastal waters.

The resuspension of fine-grained sediments by advection of water along the seafloor may also contribute some Mn to the bottom water. Pak and Zaneveld (1977) found that bottom nepheloid layers were ubiquitous on the Oregon shelf and were especially thick during periods of weak upwelling and high stratification. As upwelling is episodic, a cessation prior to our cruise would prevent dissipation of the nepheloid layer, and any resuspended manganese-rich particles <0.4,um in size would pass through our filters and be measured as dissolved Mn.

None ofthe above mechanisms, however, can account for the deep maxima at stations 5 and 7 (Fig. 2). Duplicate samples from 2,100 m (station 7, Table 1) both show sub- stantial TDM enrichments. Because these samples were processed on different days, random contamination cannot be blamed for the elevated levels of Mn. Each maximum is situated well above the bottom so that sedimentary sources are also unlikely.

Hydrothermal circulation through the Juan de Fuca Ridge is the most probable source of the deep manganese maxima (De- laney et al. 198 1; Jones et al. 198 1; Normark et al. 1982). Manganese concentrations 10- 100 times greater than those reported here have been measured directly over the ridge axis (Jones et al. 1981; Normark et al. 1982), and 63He values of 480o in samples collected at the southern end of the Juan de Fuca Ridge confirm the presence of hydrothermal vent fluid at that location (Normark et al. 1982). The positions of the deep maxima are nearly coincident with the depth of the ridge crest and may therefore result from horizontal advection of water enriched in hydrothermally derived Mn. Below the ridge crest, scavenging and dilution reduce concentrations to levels typical of the deep Pa- cific.

Conclusions The distribution of Mn off the coast of

Washington is consistent with that observed in other parts of the Pacific Ocean. Maxi- mum concentrations were found at the sur-

face, in the oxygen minimum, and in deep water near an active hydrothermal ridge system. In addition, pronounced horizontal concentration gradients were observed throughout the water column.

The Columbia River with a TDM concentration of 75 nmol kg-' is probably the major source of the surface maximum. Dur- ing periods of northward wind stress, the river effluent moves north along the coast of Washington and is then advected offshore when the wind direction changes and upwelling resumes. When this happens, manganese-rich shelf water is also upwelled, adding to the inputs from the Columbia River.

Two processes are responsible for the manganese maximum in the oxygen minimum: the release of Mn from biogenic particles as they settle through oxygen-poor waters and the horizontal advection of Mn regenerated at the continental margins. Flux calculations indicate that sufficient Mn is produced at the margins, by diffusion from reducing slope sediments and by resuspension of Mn-rich surficial sediments, to balance particulate removal in the water column (Martin and Knauer 1984).

A hydrothermal origin is suggested for the deep concentration maxima at two stations situated on opposite sides of the Juan de Fuca Ridge. The coincidence of the depth of the ridge crest and the positions of the maxima together with other evidence point- ing to hydrothermal circulation within the Juan de Fuca Ridge system support this sug- gestion.

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Submitted: 18 January 1984 Accepted: 2 July 1984



the geochemistry of manganese in the northeast pacific ocean off washington

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