Major- and trace-element composition of suspended matter in the north-east gulf of Alaska: Relationships with major sources

Marine Chemistry, 00 (1981) 10 (1981) 431--453

431

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

MAJOR- AND TRACE-ELEMENT COMPOSITION OF SUSPENDED MATTER IN THE NORTH-EAST GULF OF ALASKA: RELATIONSHIPS WITH MAJOR SOURCES* RICHARD

A. F E E L Y and G A R Y

J. M A S S O T H

Pacific Marine Environmental Laboratory, Environmental Research Laboratories, National Oceanic and Atmospheric Administration, 3 711-15th Avenue N.E., Seattle,

WA 98105 (U.S.A.) WILLIAM M. LANDING**

Department of Oceanography, University of Washington, Seattle, WA 98195 (U.S.A.) (Received August 10, 1979; revision accepted February 23, 1981)

ABSTRACT Feely, R.A., Massoth, G.J. and Landing, W.M., 1981. Major- and trace-element composition of suspended matter in the north-east Gulf of Alaska: relationships with major sources. Mar. Chem., 10: 431--453. Seasonal variations of the distributions and chemical compositions of suspended particulate matter in the north-east Gulf of Alaska were studied during 1975--1976. Selected samples were analyzed for total suspended matter by gravimetry; particulate C and N by dry combustion gas chromatography; and particulate AI, Si, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu and Zn by thin-film X-ray secondary emission spectrometry. The results showed that suspended material from the Copper River and the coastal streams which drain the Bering, Guyot, and Malaspina glaciers was carried westward along the coast and deposited in nearshore environments. However, near Kayak Island, significant quantities of suspended material of terrestrial origin were deflected to the south-west, past the edge of the outer shelf, by an anticyclonic gyre. The distribution patterns of the major and trace elements in the particulate matter and their elemental ratios with aluminum indicated that: K, Ti, Mn and Fe were primarily associated with aluminosiUcate material and C and N with organic material in all samples; and Si, Ca, Cr, Ni, Cu and Zn were primarily associated with aluminosilicate material in near-shore surface and near-bottom samples and with organic material in offshore surface samples. Only C, N, Ca, Ch and Zn showed significant seasonal variations which appeared to be related to biological production of organic matter. INTRODUCTION

Suspended particles play a major role in regulating the chemical form, distribution, and deposition of many oceanic constituents. This is particularly true in coastal waters where dissolved and particulate runoff from rivers interact with seawater. Some elements in particulate form are transported to * Contribution No. 410 from the NOAA/ERL Pacific Marine Environmental Laboratory. Contribution No. 1092 from the Department of Oceanography, University of Washington, U.S.A. ** Present address: Chemistry Board, Thimann Labs, University of California, Santa Cruz, CA 95064, U.S.A. 0304-4203/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

432

the oceans, via river runoff, where they are desorbed at the freshwater--seawater interface (Turekian, 1971; Martin et al., 1971; Evans and Chatshall, 1973; Fukai et al., 1975; and Grieve and Fletcher, 1977). Others are adsorbed onto the surfaces of suspended particles and are subsequently removed to the sediments as the particles settle (Kharkar et al., 1968; Shokovitz, 1976; and Grieve and Fletcher, 1977). Many trace elements occur in seawater it~ association with one or more solid phases (i.e. terrigenous particles from river runoff and winds, biogenic tests of marine organisms, detrital organic matter, etc.), or in various chemical forms (i.e. as an adsorbed cation, as an exchangeable cation, as a structural c o m p o n e n t of the particle, or as part of a coating on the particle). Some important prerequisites to developing an understanding of the physical, chemical, and biological processes that control the distribution of major and trace elements in particulate matter of coastal waters include: (1) determination of the spatial and temporal variations of particulate matter and associated elements; (2) identification of the most abundant solid phases within the particulate matter and their sources; (3) determination of the chemical forms and concentrations of trace elements within the major solid phases; {4) determination of the physical--chemical processes affecting the distribution and composition of the solid phases. In this paper the spatial and temporal variations of major and trace elements associated with suspended matter from the north-east Gulf of Alaska are described with a view to understanding their relation to major sources. Subsequent reports will describe the fluxes and biogeochemical reactions affecting the trace elements and associated solid phases in the suspended matter.

Background There is very little published information a b o u t trace-element distributions in particulate matter of coastal waters, nor is there much information about the relationships between the trace elements and the major solid phases that contain the elements. Spencer and Sachs (1970) presented data on the distributions of particulate A1, Fe, Mn, Cu, Ni and Zn in suspended matter from the Gulf of Maine. The surface concentration and element ratios of particulate A1 and Fe indicated that these elements were primarily associated with aluminosilicate minerals, whereas the elevated concentrations of particulate Cu and Zn indicated that these elements were probably concentrated by phytoplankton. Chester and Stoner (1975) studied the distributions of particulate Mn, Cu, Co, V, Ba and Zn in suspended matter from the north and south Atlantic Ocean, Indian Ocean, and China Sea. From a comparison of their results with published data for marine plankton and sediments, they concluded that

433 Mn, Co and V were probably present primarily in continentally derived material and authigenic precipitates, that Pb and Zn were primarily located in phytoplankton; and that Cu and Ba appeared to be partitioned between the two components. Although only a few of their samples were taken from coastal waters, their results are probably of some limited application to coastal regions. Wallace et al. (1977) determined the concentrations of particulate C, N, A1, Mn, Fe, Ni, Cr, Cu, Zn, Pb and Cd in surface samples from the northwest Atlantic Ocean along a transect from Rhode Island to Bermuda. From a comparison of element/A1 and element/C ratios, the authors concluded that, with the possible exception of particulate Fe, organic matter was the probable regulator of particulate trace element concentrations in continental shelf and slope waters as well as open-ocean surface waters. THE STUDY REGION The north-east Gulf of Alaska (Fig. la) is bordered by a mountainous coastline containing numerous glaciers, rivers, and streams which deliver large quantities of suspended material to the Gulf during the summer months when maximum discharge occurs. The major sediment discharge is from the Copper River. Reimnitz (1966) estimated that approximately 107 x 106t of finegrained material are discharged annually to the gulf via the Copper River system. Additional inputs into the gulf occur along the coastline east of Kayak Island where coastal streams containing high concentrations of suspended material drain the Bering, Guyot, and Malaspina glaciers (Feely and Cline, 1977). Coastal circulation in the gulf is dominated by the westward-flowing Alaska Current, which generally parallels the coastline (Dodimead et al., 1963). This current is characterized by a core of relatively warm (;> 5.0°C), saline (> 33.0°/00) water located in the shelf break region at a depth of approximately 150 m (Gait and Royer, 1975). Royer (1975) discussed seasonal changes in hydrographic properties across the continental shelf which can be related to wind forcing. In winter the Aleutian Low influences weather systems. Severe stormg with southeasterly winds are c o m m o n (Danielson et al., 1957). This wind pattern produces onshore surface Ekman transport, a setup of sea level along the coast and downwelling. During summer, winds are weak, the system relaxes, and offshore Ekman transport can occur. West of Kayak Island, the influence of the Alaska Current is apparent primarily at the shelf edge. Over the large shelf area bordered by Middleton Island, Kayak Island, the Copper River delta, Hinchinbrook Island, and Montague Island, circulation responds to a great extent to seasonally varying forcing by wind and river discharge. Circulation is characterized by two gyres (Gait, 1976; Royer and Muench, 1977; and Hansen, 1977). The inner gyre is less well defined and is a cyclonic loop flow. It is located south of the Copper River delta between Kayak Island and Hinchinbrook Island. The outer gyre is characterized by anticyclonic circulation extending seaward to the vicinity of the shelf break and approximately 60 km to the west of Kayak Island.

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Fig. 1. The study region: (a) physiography of the north-east Gulf of Alaska; (b) locations of suspended matter stations in the north-east Gulf of Alaska (October 21--November, 1975). For the July and April cruises, stations 10--16 were relocated 70 km east and stations 49 and 50 were relocated 40 km north-east. EXPERIMENTAL METHODS

Sampling me thods

Particulate matter samples were collected from 51 stations (Fig. lb) in the north-east Gulf of Alaska during each of thine cruises (October 21--

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436 TABLE I Analytical accuracy (XRF) and sampling precision ( l o ) for parameters determined from Test

TSM (mg1-1 )

C (wt.%)

N (wt.%)

Accuracy Reference, W-1a Measured, W-1b Sampling precision Replicate c ( l a range) Short-term d variability ( l o range)

Al (wt.%)

Si (wt,%)

K ~wt.%)

7.9 8.2 -+0.3

24.6 25.3 +0.4

0.53 0.56 -+0.02

0.04--0.16

1--2

0.1--0.3

0.4--1.5

1.5--2.8

0.1-0,2

0.07--0.24

6--23

1--3

0.4--2.0

2.0--4.5

0.1--0.2

a Data from Flanagan (1976). b Sample prepared by passing a suspension of ground USGS W-1 rock through a 37 pm to those used for sample acquisition. Ten replicate XRF analyses of this sample were ranbility monitor. c Replicate sampling precision determined from five or more sets (n = 3--10) of surface d Short-term variability determined from t i m e series sampling of surface and near-bottom for periods ranging from 12--60 h.

November 10, 1975; April 13--30, 1976; and July 19--31, 1976) aboard the NOAA ship "Discoverer". Nine standard depths extending from the surface to 500 m were sampled for suspended matter loading at all stations, except those limited by shoaling where the deepest sample was positioned within 5 m of the bottom relative to pinger response. Samples for elemental analysis were collected from the surface and near-bottom depths only. In addition, a sample from the m o u t h of the Copper River was obtained in June, 1976 by extending a bucket sampler from a helicopter. Water samples were collected in General Oceanics Model 1070 10-L PVC Top-Drop Niskin bottles. In order to avoid the loss of rapidly settling particles (Gardner, 1977) and the resultant bias in elemental composition due to fractionation (Calvert and McCartney, 1979), aliquots from each sampler were quickly trarmferred (within 10--15 min of collection) to enclosed, graduated holding columns where parallel vacuum filtration was effected using three separate filters: a 47-ram diameter, 0.4-#m pore size Nuclepore polycarbonate membrane for suspended matter concentration determination; a 25-mm diameter, 0.4-#m pore size Nuclepore polycarbonate membrane for particulate elemental (atomic number greater than 12) analysis; and a 25-mm diameter, 0.45-t~m pore size Selas Flotronics silver membrane (pre-combusted) for total particulate C and N analyses. All columns and samples were rinsed with three liNml aliquots of deionized, membrane-filtered water. The loaded filters were placed in individual plastic pet~i slides with lids ~ l h t t y ajar for vacuum desiccation (24h over NaOH) and then sealed and stored (silver filters frozen) for subsequent laboratory analysis.

437

n o r t h e a s t Gulf of Alaska samples Ca (wt.%)

Ti (wt.%)

Cr (ppm)

Mn (ppm)

Fe (wt.%)

Ni (ppm)

Cu (ppm)

Zn (ppm)

7.8 7.9 +0.1

0.64 0.65 +0.01

114 114 +4

1278 1290 +20

7.8 7.8 -+0.1

76 79 +2

110 129 -+12

86 71 +2

0.3--0.9

0.02--0.05

3--20

50--110

0.3--0.7

3--11

6-11

13--64

0.2---0.6

0.04--0.09

8--20

120--160

0.6--0.9

10--37

11--23

23--144

nylon mesh followed by collection of suspensate (353/~g) on a Nuclepore filter identical domly chosen from 53 sequential days of analysis during which this f'flter served as a stasamples collected from near-shore stations. particulate matter at stations 15 and 46. The samples were collected at 2--4 h intervals

Analytical methods The Nuclepore filters were weighed on a Cahn Model 4700 Electrobalance before and after filtration with the suspended matter masses being determined by difference. The analytical error in total suspended matter (TSM) concentration is approximately 0.01mg1-1 , based on a minimal filter loading of 0.02 rag, a volume of 2.01 (-+0.01), and a 2o weighing precision of +0.011 rag. This error is overshadowed, however, by the sampling precision in the range 0.04--0.24 mg 1-1 as indicated in Table I. Fortunately, the diverse range of TSM concentrations encountered in the Alaskan coastal waters allows us to contour our data at a level ( 1 . 0 m g l -I ) well above the sampling variability (Figs. 2 and 3). The major (Al, Si, K, Ca, T i and Fe) and trace (Cr, Mn, Ni, Cu and Zn) elements in the particulate matter were determined by energy dispersive X-ray secondary emission (fluorescence) spectrometry, utilizing a Kevex Model 0810A-5100 EnergySpectrometer and the thin-film technique (Baker and Piper, 1976). The inherent broad-band radiation from an Ag-anode X-ray tube was used to obtain a well-characterized di-energetic line structure from a Se secondary target which provided selective analyte excitation and enhanced peak-to-background ratios relative to the direct tube excitation mode. Standards were prepared from suspensions of finely ground (90% by

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439

volume less than 15 #m in diameter as determined by scanning electron microscopy) USGS Standard Rocks (W-I, AGV-1, GSP-1 and G-2; concentration data from Flanagan, 1976) collected on Nuclepore filters identical to those used for sample acquisition. Linear and quadratic (for elements A1, Si, K and Ca) least-squares regression curves which bracketed the sample elemental mass ranges were obtained to effect calibration (r ~ 0.98 in all cases). An example of the X-ray fluorescence (XRF) analytical accuracy and ranges for the XRF-determined elemental sampling precisions are given in Table I. The sampling variability is the limiting factor for interpretation of the elemental data. Subsequent considerations of these data are based on grouped mean concentration values for which the associated averaging variability (Table II) is generally greater than or equal to the sampling precision. Determinations of total particulate C and N in the suspended matter were performed with a Hewlett--Packard Model 185B C--H--N analyzer. In this procedure, particulate C and N compounds captured by the silver filters were combusted to COx and N 2 (micro Pregl--Dumas method), chromatographed on Poropak Q, and detected sequentially with a thermal conductivity detector according to the method outlined by Sharp (1974). NBS acetanilide was used for standardization. The C and N concentrations were determined on a mass/volume of seawater filtered basis. These values were then compared to the TSM loadings to yield weight percentages of C and N. Inherent differences in the filtering characteristics of the polycarbonate and silver filters and the propagation of errors when combining themass/volume values probably adds to the natural variability of these elements. The sampling precisions for particulate C and N are given in Table I. The large short, term variabilities for these elements were determined from samples of relatively high organic content. The magnitude of these variabilities never exceeded 50% of the mean value for the respective sample groups. RESULTS AND INTERPRETATION

Particulate matter distributions and transport

Since the details of the distribution and transport of suspended matter have been adequately described elsewhere (Feely et al., 1979), only a brief summary will be provided below. Figures 2 and 3 show the distributions of suspended matter at the surface and 5 m above the bottom for the fall, spring, and summer cruises, respectively. The surface distributions showed significant variations which can be related to fluctuations in sediment flux from coastal rivers, formations of eddies, and local variations in current patterns and transport processes; whereas near-bottom distributions appeared to be affected primarily by local variations in bottom currents and secondarily by regional sources. East of Kayak Island, surface particulate matter distributions were dominated by the discharge of sedimentary material from the coastal streams

T A B L E II

1

No. samples

Surface (Group t) Surface (Group ll) Surface (Group lI1) 5 m above the b o t t o m

July, 1 9 7 6

(Group I) Surface (Group II) Surface (Group Ill) 5 m above the b o t t o m

Surface

April, 1 9 7 6

Surface {Group I) Surface (Group II } Surface (Group Ill} 5 m above the b o t t o m

5.5 +2.2

21.4 +6.8

28.5 ±7.7

48-+36

10

15

4.81° " 1 7

7

23.7 s ~2.1

6

11

3

1B3; :59

6

6 9:3~

I 1 3 l° - 5 . 8

l1

6.2 ±1.6

4 8 . 8 +-81

2

12

0 71a -'.0.6

4.69 +2.0

3.86 *1.8

1.0 +0.5

0.8 m ~6.4

t 3 4 *0.1

3 4 s =1.0

1 2 lr~ -+0 7

1.4 *0 -*0.7

4.7 s " 1 . 2

11.49 -+4.7

33.8 ~ : 5 . 9

1.69 + 0 . 8

0 1 *0.1

N (wt%)

6

1.0+05

C (wt.%)

11

October-November, 1975

Copper River

Sample location

10.8:0.7

1.1 '~ + 0 7

5.5 +1.3

10.3 ~ 1 0

10.2 "0.6

0.54 + 0 3

4.8 ±1.9

10.3 ~1.3

9.4 -+17

1 5 *0 7

4.1 "1.',

102 "13

93*02

AI (wt %)

31.3.-1.6

15.8 + 6 7

28.4 +7.1

3 1 3 +1.0

30.6 "1 5

19.0 +14.0

19.0 ±7.2

3 0 8 +2.2

30.7:3.6

] 3.9 ÷1 0

18.0 ~5.6

29.7 "2.1

279*05

Si (wt ~ )

1.6 i 0 . 2

0.4 ~ ÷0.5

0.9 ±0.2

1.7 40.2

1.5 +0.1

0.I ~O.1

0.6 +-0.3

1.5 ÷0.2

1.3 -+0.3

0.4 -+0.3

0.6 +0.2

1.5 ~0.2

1.8+0.1

K (wt.%)

2.8 +0.6

0,1 -+0.2

1 3 +0.3

3 1 *0.7

2 9 ~0.5

0.5 ~0.3

1.1 +-0.3

2.5 ~:0,8

2.3 -+07

2.4 +1.1

2 . t +0.9

3.3 e l . 3

4.4 ~0.1

Ca (wt.%)

0.58±005

0 . 1 0 -+0.03

0 . 2 8 -+0.5

0.59 -0.10

0 . 5 8 +0.04

0 . 0 8 ~0.06

0 . 2 8 *-0.09

0.54 ~0,08

0 . 5 0 -+O.10

0 . 1 6 +0.13

0 . 2 7 ~0.07

0.57 ~0.06

0.64±0.01

Ti (wt.%)

119+15

687 +-45

78 +24

103 +18

1 1 6 ~8

354 ±14

67 ±14

113 *20

104 +-16

47 +6

75 ~30

119 " 1 8

126+13

Cr (ppm)

1330±180

340 +200

9 3 0 ~160

] 260 +81

1 1 6 0 *70

260 *-210

7 4 0 ~180

1 1 8 0 ~80

1170 +210

4 1 0 +113

6 6 0 +140

1 2 0 0 -+120

1 2 1 0 ~50

Mn (ppm)

68±06

1 4 ~0.6

3 7 +0.5

6.5 !1.0

6.6 +0.3

1 3 +-0.~

3 1 *0.9

6 3 *0.9

6.1 ~ 1 2

2u -0 7

3.1 ~0.7

6.5 +0 8

6.7 +0.2

Fe (wt.%)

+20

67 5 ! : ,

~,73420

55 +12

65 19

79 *16

393 ±7

46 *1~

78 -1,1

79 +22

~

62 ±23

81 *19

61 +5

Ni (ppm)

9~

1;~

¢,: :31

!25 ~3~

97 ~ ;

56 ~" ~9

303 ~4

26 *10

55 m "11

100 t 1 8

t 16 %1

195 :~67

109 +20

6 3 !2

Cu (ppm)

-

2i1"58

~32 1 1 2

26t 152

.:1~ ~:,~

!9 ~, ~36

237 ~67

307 ~15~.

292 ~ ] 5 t

184 *30

l 7fi

27(1 6 0

2 1 0 +58

133 ' 5

Zn (ppm)

S u m m a r y of the elemental c o m p o s i t i o n of suspended p a r t i c u l a t e m a t t e r from t h e Copper River and t h e north-east Gulf o f Alaska. The Copper River p a r t i c u l a t e m a t t e r was collected during high dir,chs.rge conditiorts (June, 1 9 7 6 ) from a position 10 km l a n d w a r d of the river m o u t h The ItJ precision values are from replicate analyses of a near surface .~,ample For t h e n o r t h - e a s t Gulf of Alaska data groups, the 1 a averaging precision is g i v e n Superscripk, indicate, the n u m b e r of e l e m e n t a l values averaged when different from the n u m b e r of samples in the respective group

C:~

441 which drain the Bering, Guyot and Malaspina glaciers. As this material was discharged into the gulf, coastal along-shelf currents quickly advected it to the west along the coast. Comprehensive analyses of LANDSAT imagery for this region (Sharma et al., 1974; Carlson et al., 1975) have indicated that most of the material discharged from the rivers east of Kayak Island remains relatively close to the coast (within 40 km) until it reaches Kayak Island, where it is deflected offshore. Surface particulate matter distributions for the cruises in October and April (Figs. 2a and 2b) followed this pattern. Along the transect southeast of Icy Bay (Stations 10--13), particulate matter concentrations in fall and spring decreased from ~ 1.0 mg1-1 near the coast to ~ 0.5 mg1-1 approximate.ly 40 km off the coast. During July, however, a plume of turbid water was observed extending outward from the coast (Fig. 2c). From careful analysis of LANDSAT imagery for this area, Burbank (1974) observed that occasionally counterclockwise eddies were formed which t r a n s p o r t e d plumes of terrigenous material offshore. Similar lowfrequency motions were observed in current meter records from stations located southwest of Icy Bay (Hayes and Schumacher, 1976). During fall and spring, plumes of turbid water (> 1.0mg1-1 ) extended to the southwest from Kayak Island. From an analysis of LANDSAT-1 sateUite photographs taken on August 14, 1973, Sharma et al. (1974) postulated that terrigenous debris discharged from the coastal rivers east of Kayak Island is carried to the west around the island's southern tip (Cape St. Elias) and trapped by a quasi-permanent anticyclonic gyre. Our data from the October and April cruises support their hypothesis (Figs. 2a and 2b). During October, a turbid plume extended to the west about 100 km from Kayak Island to Station 33 (Fig. 2a). Particulate concentrations within the plume were high, averaging about 1.5mg1-1 . North and south of Station 33, particulate concentrations dropped below 1.0 mg 1-1 , suggesting that the plume had an eastern origin. In April, a similar plume extended 50 km southwest of Kayak Island {Fig. 2b), with particulate concentrations decreasing from east to west. These similarities in the suspended matter distribution patterns suggest that, at the time of the cruises, similar hydrographic processes were operating to cause offshore t~ansport of suspended matter. In July, particulate distributions were significantly different from the preceding cruises. Near Kayak Island, there was no evidence of plumes extending offshore (Fig. 2c). A zone of turbid water extended only about 4 km from the eastern coast of Kayak Island, around the southern tip of the island and northward along the western coast. Near the m o u t h of the Copper River a plume of highly turbid water extended as far as 40 km offshore. Suspended matter concentrations within the plume were the highest of the three cruises, averaging 6.7 mg 1-1 , reflecting the increased sediment discharge during July. As with the previous two cruises, sedimentary material from the Copper River was carried west along the coast until it reached Hinchinbrook Island. A portion of the material passed into Prince William Sound from either side

442 of the island and the remaining material was carried southwest along the southern coast of Montague Island. The distribution of suspended matter 5 m above the b o t t o m for the three cruises is shown in Figs. 3a, 3b and 3c. The concentration data must be considered with some caution because the sampling was conducted with respect to the b o t t o m and actual depths vary with the topography. Nevertheless, the data show a consistent pattern of decreasing concentrations away from the coast. Near-bottom concentrations were highest in the region south of the Copper River delta and on either side of Kayak Island, where particulate concentrations ranged from 1.1--10.4 mg1-1 . At the edge of the continental shelf, near-bottom concentrations were generally below 0.5 mg 1-". Similar near-bottom distributional patterns have been described previously for the continental shelf of the eastern U.S.A. (Meade et al., 1975; Biscaye and Olsen, 1976) and have been attributed to resuspension of fine-grained sediments. The near-bottom turbid plumes in the north-east gulf were primarily located over regions dominated by modern accumulations of clayey silts and silty clays (Carlson et al., 1977) and showed little resemblance to the surface plumes in space and time. These data suggest that b o t t o m sediments were being resuspended locally to form near-bottom nepheloid layers in the gulf, either by the actions of b o t t o m currents, waves or benthic organisms.

Particulate C, N, Al, Si, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu and Zn Table II provides a comparison of averaged elemental composition data for suspended matter from the Copper River and the north-east Gulf of Alaska. Some data for the underlying sediments are given in Table III, and Table II values expressed as element/A1 ratios are found in Table IV. The data for the gulf surface suspended matter samples have been arranged into three groups. Group I contains all samples in which the sum of the major element concentrations (expressed as oxides and exclusive of C and N) is greater than 75% of the total suspended load; samples containing oxide sums ranging between 31% and 74% comprise Group II; and samples containing less than 30% oxide sum are in Group III. Excess SiO2, relative to the Copper TABLE III Summary of the elemental composition of sediment samples from the north-east Gulf of

Alaska* Sample description

No. samples

Al (wt.%)

Ca (wt.%)

Fe (wt.%)

Mn (ppm)

Cr (ppm)

Ba (ppm)

Sediments

15

7.24 -+0.85

4.10 -+2.20

4.64 -+0.88

813 -+111

126 -+22

503 -+118

* Data from Robertson and Abel as reported in Burrell (1977).

TABLE IV

0.11 -+0.05

C/AI

Surface ( G r o u p I) Surface ( G r o u p II) Surface ( G r o u p III) 5 m above the b o t t o m

July 1976

Surface ( G r o u p I) Surface ( G r o u p If) Surface ( G r o u p III) 5m above the b o t t o m

April 1976

Surface ( G r o u p I) Surface ( G r o u p II) Surface ( G r o u p III) 5 m above the bottom

1.2 -+0.6

0 . 4 4 -+0.33

0 . 0 6 -+0.06

4.2 -+3.2

0 . 6 9 -+0.37

25 -+18

0 . 1 0 +-0.05

3.9 -+1.5

0 . 0 8 -+0.04

0.53 +0.22

0.47 +0 .1 7

8.6 -+5.2

0 . 7 1 -+0.35

47 +29

0.12 +0.07

3.8 +2.1

0 . 1 5 -+0.08

0 . 6 7 -+0.33

1.2 -+0.7

4.1 -+2.2

8.2 -+3.7

33 -+16

0 . 1 6 -+0.08

0 . 0 1 "+0.01

N/AI

1.1 -+0.5

October--November 1975

C o p p e r River

Sample description

2.9 -+0.2

1 4 "+11

5.2 -+1.8

3.0 -+0.3

3.0 "+0.2

3 8 "+36

4.0 -+2.2

3.0 -+0.4

3.3 -+0.7

9 . 3 "+4.4

4.4 "+2.3

2.9 -+0.4

3,0 -+0.1

Si/AI

0 . 1 5 -+0.02

0 . 3 4 -+0.49

0 . 1 6 -+0.05

0 . 1 7 -+0.03

0 . 1 5 -+0,01

0 . 2 2 -+0.21

0 . 1 3 -+0.07

0.15 +0.03

0 . 1 4 +-0.04

0 . 2 3 -+0.23

0 . 1 5 -+0.08

0 . 1 5 +-0.03

0 . 1 9 -+0.01

K/AI

0 . 2 6 -+0.06

0 . 4 0 -+0.34

0 . 2 4 -+0.08

0 . 3 0 -+0.07

0 . 2 8 -+0.06

1 . 0 0 "+0.81

0 . 2 3 -+0,11

0.24 +0.08

0 . 2 4 -+0.09

1.6 -+1.1

0 . 5 1 -+0.31

0 . 3 2 -+0.13

0 . 4 7 -+0.01

Ca/Al

0 . 0 5 -+0.01

0 . 0 9 "+0.06

0 . 0 5 +-0.02

0 . 0 6 -+0.01

0 . 0 6 +-0.01

0 . 1 6 "+0.15

0 . 0 6 -+0.03

0 . 0 5 +0.01

0 . 0 5 -+0.01

0 . 1 1 -+0.10

0 . 0 7 -+0.03

0 . 0 6 -+0.01

0 . 0 7 -+0.1

Ti/Al

1.1 -+0.2

6.2 -+5.7

1.4 -+0.6

1.0 -+0.2

1.1 +0.1

7.0 "+5.1

1.4 -+0.6

1.1 +0.2

1.1 -+0.3

3.1 -+1.5

1.8 +-1.1

1.2 -+0.2

1.3 +-0.1

Cr/AI × 1 0 -3

12 +-2

31 -+27

17 +-5

12 -+1

11 +1

5 2 "+52

15 -+7

12 -+2

1 3 -+3

2 7 -+15

1 6 -+8

1 2 -+2

1 3 -+1

Mn/Al × 1 0 -3

0 . 6 3 -+0.07

1.3 -+1.0

0 . 6 7 -+0.18

0 . 6 3 +0. 11

0 . 6 5 -+0.05

2.6 "+2.2

0 . 6 5 -+0.32

0 . 6 1 -+0.12

0 . 6 5 -+0.17

1.3 -+0.8

0 . 7 6 "+0.36

0 . 6 4 -+0.11

0 . 7 2 -+0.03

Fe/Al

0 . 6 2 -+0.14

5.2 "+3.8

1.0 +-0,3

0 . 6 3 -+0.11

0 . 7 7 -+0.16

7.8 -+4.8

0 . 9 6 -+0.44

0 . 7 6 +0.17

0 . 8 4 -+0.28

3.9 -+2.2

1.5 "+0.8

0 . 7 9 -+0.21

0.66 +0.02

Ni/AI × 10 -3

0 . 8 8 -+0.18

7.9 -+5.6

2.3 +-0.9

0 . 9 4 -+0.11

0 . 5 5 -+0.09

6.0 -+3~7

0 . 6 5 -+0.49

0 . 5 3 -+0.13

1.1 -+0.3

7.7 -+3.6

4.8 +-2.6

1.1 "+0.2

0 . 6 8 -+0.03

Cu/AI × 10 -3

--

2.0 -+0.5

21 -+17

4.8 -+3.0

2.4 -+0.6

1.9 -+0.4

47 -+31

6.4 -+4.1

2.8 -+1.5

2.0 -+0.5

11

6.6 -+3.1

2.1 -+0.6

1.4 -+0.1

Zn/ A1 × 1 0 -3

S u m m a r y o f e l e m e n t / A l r a t i o s for p a r t i c u l a t e m a t t e r s a m p l e s f r o m t h e C o p p e r R i v e r a n d t h e n o r t h - e a s t G u l f o f A l a s k a . T h e l o p r e c i s i o n values w e r e d e t e r m i n e d b y p r o p a g a t i o n o f e r r o r s f r o m d a t a given in T a b l e II

~o

444 River Si/A1 ratio, was omitted from the oxide summations. Geographicalty~ the Group I samples are from nearshore stations and those immediately offshore from Kayak Island. The Group II and III samples are generally from stations located near or seaward of the shelf break. SEM micrographs of selected filters from these groups show that Group I samples are primarily composed of clay-sized terrestrial particles, Group II samples are mixtures of terrestrial and biogenic matter, and Group III material is mostly biogenic. The elemental compositions and element/Al ratios illustrate some compositional differences between near-shore (Group I) and offshore (Groups II and III) suspended matter. Since most of the A1 in marine particulate matter is located in aluminosilicate material (Sackett and Arrhenius, 1962) and because marine plankton contains only about 500 ppm A1 by weight (Martin and Knauer, 1973), the A1 concentrations in the suspended matter can be used to estimate aluminosilicate percentages in the particulate matter (A1 ~:10}. Similarly, Gordon (1970) suggested that particulate carbon may also be used to estimate the amount of organic matter in the suspended matter by multiplying the particulate carbon content by a factor of 1.8. Based on the particulate A1 and C concentrations, approximately 103(-+13)% and 15(-+6)%, respectively, of the suspended matter from Group I is aluminosilicate material and organic matter. Within the statistical limits of the measurements, nearly all the elemental concentrations of the Group I samples are the same as the river samples, indicating that the coastal rivers are the major source of the inorganic material. Only particulate C and N show significant enrichments over the river-suspended matter, this is probably due to production of organic matter in near-shore waters (Larrance et al., 1977). In contrast to the near-shore samples, the offshore samples (Groups II and III) are significantly depleted in particulate A1, Si, K, Ca, Ti, Cr, Mn and Fe; and are enriched in particulate C and N. These depletions are attributed to a drop in the relative amount of aluminosilicate material in the suspended matter ( ~ 15% by weight for the Group III samples) and an increase in the proportion of organic matter ( ~ 40% by weight), which is depleted in these elements relative to aluminosilicate material (Martin and Knauer, 1973), Particulate Cu is relatively depleted in the samples taken during the spring cruise and enriched in samples taken during the summer cruise. This may be due to selective uptake of Cu by some planktonic organisms during a particular phase of their growth cycle, as suggested by the data of Morris (1971) and again by Martin and Knauer (1973). Table IV shows the average elemental concentration ratios to A1 for the samples from the various groups. As indicated in the table, the Si/Al and C/A1 ratios from Group III are considerably elevated over ratios for the river sample. This is due to the presence of biogenic Si and biogenic C in the suspended matter. Price and Calvert (1973) and Feely (1975) demonstrated that the amount of biogenic Si and biogenic C can be estimated by assuming a constant Si/Al and C/A1 ratio due to suspended aluminosilicates and terrestrial C, respectively. Any excess Si and C is assumed to be of biogenic origin. Using the Si/A1 and C/A1 ratios of the suspended material from the Copper

445

River, values of approximately 14(+12)7o and 29(+19)% by weight of the Group III samples are estimated to be composed of biogenic Si and biogenic C, respectively. In contrast, it is estimated that on the average, the Group I samples only contain about 8% by weight biogenic C and ~ 1% biogenic Si. In similar fashion, examination of the element/A1 ratios for the other elements reveal that: N/AI, Cr/A1, Ni/A1, Cu/A1, and Zn/A1 ratios from Group III are considerably elevated (~ 2 × ) over ratios for the river samples; Ca/A1, Mn/A1, and Fe/A1 ratios are moderately (1--2 × ) elevated over ratios for the river samples; and K/AI and Ti/A1 ratios are virtually the same as the ratios for the river samples. These data are taken as evidence for concentration of N, Si, Ca, Cr, Mn, Fe, Ni, Cu and Zn in biogenic phases in offshore waters. These results are consistent with the general conclusion of Wallace et al. (1977) that biogenic matter regulates the concentrations of particulate metals in offshore surface waters where aluminosilicate concentrations are low. Table II also summarizes the elemental composition of suspended matter samples taken from 5 m above the bottom. With the exception of particulate C and N, the major and trace elements in the near-bottom suspended matter have about the same concentration as the river samples. These data suggest that the coastal rivers, which are the major source of the near-shore surface suspended matter, are also the major source for the near-bottom material. As stated by Feely et al. (1979), this is caused by a number of processes, including: offshore transport and subsequent sinking of near-shore surface material captured by gyres; downwelling and offshore transport of near-shore surface material in winter; and resuspension and offshore transport of previously deposited sediments. DISCUSSION

Factors controlling the chemical composition of the suspended matter in surface waters In order to develop an understanding of the relationships between elements in suspended matter and the major phases which contain them, several authors have examined interelement scatter diagrams (Spencer and Sachs, 1970; Price and Calvert, 1973; Feely, 1975; Baker and Feely, 1978). Usually A1 is used as the independent variable because: (1) it is relatively abundant in terrestrially derived aluminosilicate material and is present in minor amounts in biogenic phases; and (2) no chemical evidence to date suggests that particulate A1 is reactive in seawater. We assume, therefore, that elements which are strongly correlated with A1 are probably associated with aluminosilicate material; and conversely, elements which are weakly associated with A1 are probably regulated by other particulate phases, such as biogenic matter or authigenic minerals. Figure 4 shows particulate C, N, Si, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu and Zn plotted as a function of the A1 concentration for all the samples used in preparation of Tables II and IV. The

446

• ~e

Y

...~;. ..... id

C 4:

.

i 2O

"f

:i: •

:I" ~ ' ~ "

4~.g

F

'

,{ 2J""

~

YI'

~ii

" 6c

•

•

t J

"

)

;

5

;

~um,num

i~

~''~;" wr

o

5

~,

2

:

5

{

'

;

-~

''

•

~

Fig. 4. Interelement scatter diagrams (versus AI) of the elemental data used to prepare Tables II and IV. The linear least-squares regression lines are representative o f the fit for total data set o f each element. The vertical bars indicate the standard errors for the regressions. (Symbol key: o, Group I samples; E~, Group II samples; A, Group III samples; and e, samples from 5 m above the bottom.)

447 strong correlations between particulate K, Ti, Mn and Fe and particulate A1 suggest that these elements are primarily associated with the structural components of the terrestrial aluminosilicate material that is discharged from the coastal rivers and]or a component of resuspended bottom sediments. The fact that offshore samples show only small enrichments in these elements relative to A1 means that: (1) these elements are not appreciably concentrated in biogenic phases, and (2) their concentration in the suspended matter of the northeast gulf will be dependent upon the amount of aluminosilicate material that is present in suspension. Particulate C shows a slight negative correlation with A1, indicating that C and A1 are present in distinctly different phases in the suspended matter. The same situation exists for particulate N. Particulate Ca, on the other hand, appears to be weakly correlated with A1 in near-shore surface samples and near-bottom samples and poorly correlated with A1 in offshore samples. Furthermore, some offshore samples contain more Ca than near-shore samples. These data mean that Ca is present in more than one phase in the suspended matter, i.e. an aluminosilicate phase and one or more biogenic phases. Similarly, particulate Cr and Ni also appear to be weakly associated with A1 in near-shore surface and near-bottom samples and poorly associated with A1 in offshore samples. These data are taken as evidence for partitioning of Cr and Ni between aluminosilicate and biogenic phases, with aluminosilicate phases being dominant in near-shore waters and biogenic phases being dominant in offshore waters. Particulate Cu and Zn show almost no correlation with A1. These data suggest Cu and Zn are also partitioned between aluminosilicate and biogenic phases. However, in the case of Cu and Zn the enrichments of these elements in the biogenic phases are more significant, and therefore, these phases play a more dominant role in controlling their distributions in the particulate matter. Since it has been established that the presence and concentrations of particulate Cr and Ni are a function of the varying amounts of inorganic aluminosilicate material and biogenic material in the suspended matter, the data in Table II may be used to estimate the relative fractions of any solid phase present in the suspended matter and the trace element concentrations within that solid phase. Taking the Si/A1 ratio for the Copper River samples as a constant for all aluminosilicate material in the surface waters of this region, the concentration of excess si, primarily due to the presence of diatom frustules, has been estimated. The same procedure has been applied to estimate the excess CaCO 3 fraction of the particulate matter within the various groups, which is also shown in Table V. The organic matter fraction was estimated by multiplying the particulate C concentration by a factor o f 1.8 (Gordon, 1970) and the aluminosilicate fraction was estimated by taking the difference between the total and the sum of the above three fractions. Since A1 has not been observed to be present in marine particulate matter as a major constituent of anything but inorganic aluminosilicates, this method for estimating the relative fractions of the various solid phases present seems

448 TABLE V Calculated fractions of the total suspended matter represented by aluminosilicate minerals, biogenic silica, calcium carbonate, and organic matter for average Copper River Group I, Group II and Group III surface samples and near-bottom samples Sample location and description

Aluminosilicate fraction

Excess SiO2 fraction

Excess CaCO3 fraction

Organic matter fraction

Copper River Surface (Group I) Surface (Group II) Surface (Group III) 5 m above bottom

0.98 0.85

0.00 0.00

0.00 0.00

0.02 0.15

0.41

0.16

0.00

0~43

0.12

0.30

0.05

0.53

0.87

0.01

0.00

0.12

feasible. It may also be reasonable to assume that the solid phases SiO2 and CaCO3 do not contain appreciable amounts of the trace elements of interest (Martin and Knauer, 1973; Turekian and Imbrie, 1966), allowing us to calculate the trace element composition of the remaining solid phases using two simultaneous equations of the form:

1=1 n

=

Z

",j,

f,j

(2)

j=l

Where C~ and C~Ix = total concentration of trace element i in Groups I and III, respectively; C~.1 = concentration of trace element i in particular solid phase, either aluminosilicate matter or particulate organic matter; and 7j = fraction of solid phase j in the suspended matter for Groups I and III. We have assumed that the aluminosilicate material or particulate organic matter, present in the Copper River samples, the Group I samples, and the Group III samples is essentially the same material (i.e. C~.1 = ,~m~ ,~i.j J, only proportioned differently within the groups; thus, the trace element concentration in the aluminosilicate phase and the. particulate organic matter phase may be calculated as shown in Table VI. Note the relatively close agreement between the concentrations of Cr, Mn, Fe and Ni observed for the Copper River samples (Table I) and those calculated for the alumL,msilicate phase. The agreement is not as good for Cu and Zn. The reason for this is that Cu and Zn show large seasonal variations which are not accounted for by this kind of data manipulation. There is also reasonably good agreement between the calculated values of the trace element concentrations in the particulate organic phase and the ranges o f accepted values for marine microplankton. The calculated values of Mn and Fe in particulate organic matter are high by a

449 T A B L E VI Comparison of trace element concentrations and partitioning among the solid phases for Group I, Group II, and Group III,and near-bottom samples with the range of accepted values for marine microplankton*

Description

Cr

Mn

Fe

Ni

Cu

Zn

Aluminosilicate phase (ppm) Particulate organic phase (ppm) Group I trace element fraction: aluminosilicate phase particulate organic phase Group II trace element fraction: aluminosilicate phase particulate organic phase Group III trace element fraction: aluminosilicate phase particulate organic phase 5 m above bottom trace element fraction: aluminosilicate phase particulate organic phase

129

1361

77 500

84

134

382

31

287

9 700

46

76

228

0.96 0.04

0.96 0.04

0.99 0.01

0.92 0.08

0.76 0.24

0.77 0.23

0.71 0.29

0.71 0.29

0.91 0.09

0.64 0.36

0.27 0.73

0.33 0.67

0.28 0.72

0.51 0.49

0.63 0.37

0.20 0.80

0.11 0.89

0.12 0.88

0.92 0.03

0.96 0.04

0.98 0.02

0.99 0.01

0.75 0.25

0.99 0.01

Marine microplanktona

1--21

2--33

500--4000

0.6--70

1--300

3--840

* Data from NichoUs et al. (1959) and Martin and Knauer (1973). f a c t o r of a b o u t 2 or more. These data suggest t hat in the case of these elements the above calculations are an over-simplification and t h a t nonaluminosilicate, non-biogenic phases, possibly including Mn and Fe oxyhydroxides, m a y be contributing significant a m o u n t s of these elements to the suspended matter. Table VI also contains the fraction of the total c o n c e n t r a t i o n of trace elements for the various groups t h a t is due t o the presence of each solid phase and its accompanying trace element concent rat i on, similar t o the presentation of Gibbs (1973). T he data show t h a t aluminosilicate minerals are the major c o n t r i b u t o r ( > 75% by weight) of trace elements t o near-shore surface and n e a r - b o t t o m samples. F u r t h e r m o r e , aluminosilicate minerals and o t h e r non-biogenic phases contribute 51% and 63%, respectively, of the Mn and Fe in the offshore samples. Particulate organic matter, however, contributes mo st o f the Cr, Ni, Cu and Zn in the offshore samples, comprising a b o u t 72--89% o f the total metal content. Since these elements have been shown to be biologically active (Bacon et al., 1976; Bender and Gagner, 1976; Bruland et al., 1978}, t h e y are probabl y i ncorporat ed into the particulate organic phase f r om the dissolved state via organic productivity.

450

Factors controlling the chemical composition of suspended matter in nearbottom waters

As discussed previously, the chemical composition of the near-bottom suspended matter is both similar in composition to the riverine suspended matter and underlying sediments, and seasonally invariable with respect to concentration. This second result is somewhat surprising since the data for surface samples show some seasonal variability within groups which is presumed to be related to seasonal variations of organic productivity. The invariability of the chemical composition of the near-bottom suspended matter is due to two effects: (1) remineralization of particulate organic matter within the upper regions of the water column, leaving the non-reactive aluminosilicate material to sink and become part of the near-bottom suspended matter; and (2) resuspension of previously deposited sediments. The first mechanism is supported by the C data which show a general concentration decrease between surface and near-bottom samples. Further support for this mechanism was demonstrated by the work of Landing (1978), who determined from an analysis of suspended matter samples collected in the region south of Icy Bay that as much as 50% of the total C fixed in surface waters was remineralized in the water column between 0 and 40 m and an additional 24% between 48 and 86 m. Landing further estimated that about 20% of the total C fixed in surface waters gets buried in the underlying sediments. If these estimates are assumed to be typical for the northeast Gulf of Alaska, then remineralization could account for all the concentration decreases ~ff C between the surface and near-bottom samples. However, this process cannot explain the observed three-to-fourfold increase of near-bottom suspended matter concentrations over midwater values (Feely et al., 1979). Since there are no strong vertical gradients of temperature and salinity in near-bottom waters which would tend to retard the settling of particles (Feely et al., 1979), these results can best be explained by evoking a mechanism in which some fraction of the b o t t o m sediments were being resuspended by local b o t t o m currents. The concentration data suggest that approximately 75-80% of the near-bottom suspended matter is composed of resuspended sediments. Therefore, if this process is continuous throughout the study region, as suggested by Feely et al. (1979), then the composition of the near-bottom suspended matter should be relatively invariant and dependent upon the a m o u n t and fraction of the b o t t o m sediments being resuspended. ACKNOWLEDGEMENTS

The authors wish to express their appreciation to Captain Clinton D. Upham and the crew of the "Discoverer", without whose help this work would n o t have been possible, Ms. Jane Hannuksela and Ms. J o y c e Quan for assisting in sample preparation and data reduction, and Ms. Marilyn Lamb for preparing the figures.

451 This s t u d y was s u p p o r t e d b y t h e B u r e a u o f L a n d M a n a g e m e n t t h r o u g h i n t e r - a g e n c y a g r e e m e n t w i t h the N a t i o n a l O c e a n i c a n d A t m o s p h e r i c A d m i n istration, u n d e r w h i c h a m u l t i y e a r p r o g r a m r e s p o n d i n g t o n e e d s o f p e i x o l e u m d e v e l o p m e n t o f t h e A l a s k a n c o n t i n e n t a l shelf is m a n a g e d b y t h e O u t e r C o n t i n e n t a l Shelf E n v i r o n m e n t a l A s s e s s m e n t P r o g r a m ( O C S E A P ) office.

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