The chemistry and vertical flux of particles in the northeastern Gulf of Alaska

Vol. 28A, pp. 19 to 37 D Sea gamon eResearch, Press q Ltd p1981. Printed p in Great Britain

0198-0149/81/0101-0019 $02.00/0

The chemistry and vertical flux of particles in the northeastern Gulf of Aiaska*'~ WILLIAMM. LANDING~§and RICHARDA. FEELYII (Received 30 May 1979; in revisedform 22 April 1980; accepted 10 May 1980; final revision received 20 May 1980) Abstract--The physical and chemical compositions of settling particles and underlying sediments collected from a pristine coastal environment south of Icy Bay in the northeastern Gulf of Alaska were compared to investigate the processes affecting the compositions, flux, and early diagenesis of the materials. Settling panicles were collected with three self-closing sediment traps moored vertically approximately 30 km offshore. Sediments collected by gravity coring at the site were dated by 2~°pb geochronology. Total particulate Ai, Si, Cr, Mn, Fe, Ni, Cu, Zn, and Pb were determined on all samples by flameless atomic absorption and standard colorimetric procedures. Total C and N were determined by dry combustion gas chromatography. A dilute hydrogen peroxide oxidation dissolution treatment was used to ~tudy element associations with labile particulate organic matter. A selective biogenic SiO2 dissolution treatment also was employed to determine the biogenic SiO2 fraction. Results suggest that the sediment traps efficiently collected settling particles. The elemental accumulation rates, remineralization of particulate organic matter, and remobilization of trace elements, notably Cu, were quantified by comparing the composition of the trapped materials with the underlying sediments. The majority of the remineralization appears to occur within the sediments below the zone influenced by resuspension, presumably as a result of biological activity.

INTRODUCTION

PARTICULATEMATERIALSplay a major role in the oceanic cycles of many elements and are

important in the chemistry, biology, and geology of coastal waters. Comprehensive analysis of representative particulate materials should lead to a greater understanding of the physical, chemical, and biological processes regulating their composition and distribution. MCCAVE(1975) and GARDNER (1977) concluded that fecal aggregates may dominate the downward flux of particles while contributing only slightly to the 'standing stock' of suspended particles sampled by conventional techniques such as bottle samplers coupled with microfiltration. MAN.EIM, HATHAWAYand UCHAPI (1972) concluded that zooplankton grazing of fine-grained suspended material followed by more rapid settling of the resultant fecal particles may be a geologically significant mechanism for suspended matter deposition. The mechanism is perhaps the reason for the unusually high correlations between the composition of near-surface particles and the underlying sediments in a * Contribution No. 1078 from the Department of Oceanography, University of Washington. T Contribution No. 406 from the NOAA/ERL Pacific Marine Environmental Laboratory. ~/ Department of Oceanography, University of Washington, Seattle, WA 98195, U.S.A. § Now at: Center for Coastal Marine Studies, University of California, Santa Cruz. II Pacific Marine Environmental Laboratory, Environmental Research Laboratories, National Oceanic and Atmospheric Administration, 3711 15th Avenue N.E., Seattle, WA 98105, U.S.A. 19

20

WILLIAM M. LANDINGand RICHARDA. FELLY

variety of oceanic environments (e.g., REX and GOLDBERG, 1958; DELANEY, PARKIN, GRIFFIN, GOLDBERGand REIMANN,1967; GRIFFIN,WINDOMand GOLDBERG,1968). Adequate collection of representative samples of rapidly-settling particles can be accomplished by large-volume in situ filtration (BishoP, EDMOND, KETTEN, BACON and SILKER,1977) and by the use of particle interceptor (or sediment) traps (e.g., WIEBE, BOYD and WINGET, 1976; COBLERand DYMOND, 1977; GARDNER, 1977; SPENCER, BREWER, FLEER, HONJO, KRISHNASWAMIand NOZAKI, 1978). Recently, SOUTAR,KLING, CRILL, DUFFRINand BRULAND (1977) demonstrated that chemical analysis of particles collected with sediment traps in a coastal environment can be linked with studies of the underlying sediment to provide a more complete picture of the biogeochemical cycles of elements and biogenic solid phases in seawater. In this paper, we report on the use of self-closing sediment traps of our design to describe some of the biogeochemical processes affecting the composition of particulate materials (collected on the Alaskan continental shelf south of Icy Bay in the northeast Gulf of Alaska) as the materials fall through the water column and are buried in the underlying sediments. The study region

The study area (Fig. 1) has been described by FELLY, BAKER, SCHMACHER, MASSOTH and LANDING (19791. It is a high energy depositional environment, with the energy 144 =

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Fig. 1. Location of stations in the northeast Gulf of Alaska for collection of suspended particulate matter and deployment and recovery of sediment traps (station D).

The chemistry and vertical flux of particles in the northeastern Gulf of Alaska

21

provided by the anticyClonic flow of the Alaska Current (see FELLY et al., 1979, and references therein). Results obtained from current meters deployed at 20-m depth in spring of 1975, 1976, and 1977 (ScnMACHER, personal communication) indicated that the near-surface net drift ranged from 2 to 16 cm s -1 alongshore with an average onshore component of 7 to 11 cm s-1. Near-bottom observations from this same period yielded mean alongshore velocities between 6 and 12 cm s -1 and a small offshore component in the bottom waters at stations B and D (approximately 1 cm s-1). Strong tidal oscillations are superimposed upon the mean flow of surface and bottom water, and eddies generated by local wind and runoff conditions are responsible for episodic offshore transport of nearshore surface waters (HAYES and SCHMACHER, 1976). The current meter data are discussed more fully by LANDING(1978). The composition and morphology of the suspended particles and sediments were investigated by FELLY and CLINE (1977) and MOLN1Aand CARLSON(1976), respectively. The major sedimentary mineral phases observed included chlorite, illite, talc, qurtz, and f e l d s p a r s . CARLSON, MOLNIA, KITTLESON and HAMPTON (1977) found banded sediments extending offshore and about 80 km longshore in either direction from the study region. Sediments were significantly coarser nearshore.

EXPERIMENTAL PROCEDURES

Sampling methods

Suspended matter samples were collected from eight stations along a transect normal to the coast just south of Icy Bay (Fig. 1) during a cruise aboard the NOAA ship Discoverer (14 to 31 March 1977). Nine standard depths (1, 10, 20, 40, 60, 80, 100 and 150 m, and 5 m above the bottom) were sampled at all stations except those limited by shoaling, where the deepest sample was positioned 5 m above the bottom. Aliquots of each sample were withdrawn within 10 to 15 min of collection and filtered as outlined by FELLYet al. (1979). Assuming an aggregate density of 1.43 (PIERCE, 1975), Stokes law shows that only particles greater than 74 Bm in diameter would have time to settle completely out of the samplers in this time. Denser particles of this size (i.e., minerals) would not be expected any distance from shore due to their more rapid settling and the nature of the currents in this area. Three self-closing sediment traps were positioned on a current meter mooring at station D, approximately 30 km south of Icy Bay. The traps were deployed open at 48, 86, and 96 m (bottom depth = 103 m) (Fig. 2). The traps were modifications of those described by GARDNER (1977) (Fig. 3). A trap consists of a PVC cylinder (diameter = 152 ram; length--475 mm) with a height/diameter ratio of 3.125.* A grid (cell size= 12 x 12 x 12 mm) was placed at the mouth of the trap to reduce turbulence within the trap and to prevent the entrance of large nektonic organisms. A guillotine-type closing plate was included to avoid loss of material during recovery. The plate was actuated by a battery-operated timer in an aluminum pressure casing on the side of the trap. A PVC cylinder on the opposite side contained compressed reagent grade sodium azide, which slowly dissolved and diffused into the trap through a 47-ram, 0.4-1~m Nuclepore ® filter * GARDNER (1977) concluded, on the basis of efficiency experiments conducted with sediment traps of various height/diameter ratios in a recirculating flume, that cylinders with height/diameter ratios of about 2 to 3 provided the best agreement between the trap collection rate and the sedimentation rate.

22

WILLIAM M. LANDING and RICHARDA. FEELY

STATION D -

Design Depth IO3 Meters

SEa SURFACE

I 23M

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Fig. 2. Mooring design for the current meter-sediment trap array deployed at station D in the northeast Gulf of Alaska. The mooring was deployed 17 March 1977 and recovered 8 June.

tO retard biological activity (SPENCER et al., 1978). Before deployment each trap was sequentially washed with a mild soap solution, rinsed with a 3:1 ethanol-acetone mixture, rinsed with 0.1 N HNO3, and rinsed with deionized filtered water (Millipore ® Milli-RO system). The traps were filled with deionized filtered water immediately before deployment. A 3-in. (1.1 cm) gravity corer with a plastic core liner was used to obtain a 40-cm core at station D. The core was frozen intact for return to the laboratory, where the core was partially thawed and split. One-half was sectioned into l-cm intervals for excess 21°Pb radiometric analysis. A subsample (for elemental analysis) of the sediment from the top

The chemistry and vertical flux of particles in the northeastern Gulf of Alaska

23

.Jr

Fig. 3. Exploded isometric drawing of the sediment trap. The major components are: A, plastic grid for preventing large organisms from entering the trap; B, guillotine-type dosing plate; C, sodium azide diffusion chamber; D, clamping brackets; E, presettable timer; and F, timer brackets.

5 cm of the remaining half of the core was placed in a covered Teflon beaker and oven dried at 80°C. Hydrographic data were collected at each site (Fig. 1) with a CTD (conductivity temperature~lepth recorder) as described by FEELVet al. (1979). Analytical methods Total suspended particulate masses were determined using 47-mm, 0.4-~tm Nuclepore filters. A replicate study indicated that the variability in the particulate concentrations in these waters (_ 25~o; n = 10) far exceeded the variability involved in subsampling from the bottles or in weighing of the filters (FEELVet al., 1979). The sediment accumulation rate for the core sample was determined by 21°pb dating. One-cm subsections of the core and fractions of the sediment trap samples were analyzed by the methods described by FLYNN(1968). The particulate materials were analyzed using a variety of techniques. Total carbon and nitrogen were determined with a Hewlett-Packard 185-B CHN analyzer (SHARP, 1974). Major and trace elements (AI, Cr, Mn, Fe, Ni, Cu, Zn, and Pb) were determined by flameless atomic absorption following strong acid total dissolution in Teflon digestion bombs (EGGIMANN and BETZER, 1976). Silicon was determined in the solutions by the

24

WILLIAM M. LANDING and RICHARD A. FEELY

colorimetric technique of STRICKLANDand PARSON(1968) following dilution to avoid HF interference. To determine the biogenic Si* content of particulate samples, approximately 25-rag subsamples were extracted twice with 10 ml of 5~o Na2CO3 (J. T. BAKER,Ultrex) in acidcleaned conventional polyethylene (CPE) centrifuge tubes, following procedures by HURD (1973). The CPE tubes were placed in a heated (60°C) ultrasonic bath for 12 h, centrifuged at 500 g for 2 h, and the supernatant liquid was poured into tared, pre-cleaned CPE bottles. The second extract was collected in a separate bottle. The solutions were acidified to pH 2.0 with 2 ml quartz-distilled (Q) 6 N HC1 and diluted to 15 g total weight with quartz distilled water (Q-H20). The biogenic Si content of the original particulate matter sample can be calculated using the equation: Si (9/0) Biogenic = [(Si)l - (AI)I(Si)2/(AI)2] IO0/M,

(1)

where (Si) and (A1) are the total Si and A1 contents of the various solutions and M is the mass of material subjected to the dissolution treatment. The first solution (1) presumably contains all of the biogenic Si and a small fraction of Si and Al from the slow basedissolution of the aluminosilicates in the sample. The second solution (2) contains only Si and Al from the dissolution of aluminosilicates, and yielded Si/A1 weight ratios near those observed for the near-bottom suspended particles (which contain a minimum of quartz and biogenic Si, LANDING, 1978). TO determine directly the association of major and trace elements with the organic component of the particles, a selective oxidation~lissolution treatment was developed. CRECELIUS, BOTHNER and CARPENTER(1974) demonstrated that 30~o hydrogen peroxide could be used efficiently to oxidize particulate organic matter and solubilize certain trace elements from Puget Sound sediments. The efficiency of hydrogen peroxide for the oxidation and dissolution of fresh, easily oxidizable organic matter was studied using varying concentrations of peroxide and differing reaction conditions. Peroxide concentrations ranging between 1 and 309/0 by weight removed essentially the same amounts of carbon (91 _+ 39/0) and nitrogen (90_+ 19/o) from an organic-rich freshwater sediment collected in Portage Bay, Seattle, Washington, providing that enough active oxygen was present in the solution. With 109/o peroxide solutions 24 h of ultrasonic treatment was more than sufficient for approximately 90~ removal of carbon and nitrogen from the same sediment (Fig. 4). Clean hydrogen peroxide solutions for the trace element determinations were prepared by passing 10~ concentrations of reagent grade peroxide at 5 ml min -1 through a 25 cm 3 (25 cm x 1 cm 2) column of Amberlite 200 macroreticular cation exchange resin (Rohm and Haas, Inc.) in the H ÷ form. The particles for this study were subsampled (approximately 0.5 g), placed in CPE centrifuge tubes with 5 ml of pre-cleaned 10~o peroxide (pH = 5.5), and heated to 60°C for 24 h. The tubes were then placed in the heated ultrasonic bath for an additional 24 h. Following centrifugation at 500 y for 2 h the supernatant liquid was filtered through precleaned 25-ram, 0.4-tam Nuclepore filters. The particulate pellets were rinsed three times with 3 ml Q-H20, briefly sonified, and filtered as before, adding the rinses to the supernatant liquid. The solution was then acidified with 100 ~tl 6 N Q-HC1, diluted to 20 g total weight in a tared CPE bottle, and analyzed for major and trace element con* Because the exact molecular formula for Si-bearing species in marine plankton can vary, all data for biogenic silica will be presented as elemental concentrations or fluxes.

The chemistryand verticalflux of particles in the northeastern Gulf of Alaska

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Fig. 4. Time of soniflcation required to achieve maximum removal elficiency of particulate organic matter in sediments from Portage Bay, Seattle, Washington. Carbon and nitrogen removal was monitored by the analytical procedures as described in the text. Precision was determined by replicate analyses (+ 1 SD). centrations. The mineralization efficiency was determined by carbon and nitrogen analyses before and after treatment. Experiments were also performed with the sediment sample using precleaned 10~ peroxide solutions acidified to pH 4.0 (LANDING,1978). STUMM and MORGAN (1970) suggested that oxy-hydroxides of Fe and Mn should not form below pH 4.0, so that precipitation and concurrent trace metal scavenging should have been minimized. The results showed that acidified peroxide solutions extracted significantly greater amounts of Al and Fe only. Because of the small amount of material available from the sediment traps the results from the selective oxidation~iissolution studies are for unacidified treatment only. The carbon and nitrogen oxidation-dissolution efficiencies for the samples varied from 20 to 50~ for carbon and from 60 to 70~o for nitrogen, perhaps reflecting the contributions of refractory organic materials or inorganic carbonates, which have been shown to be present in the sediments in the region (MoLNIAand CARLSON,1976). Therefore, we believe that the determined major and trace element removal values represent associations with labile particulate organic matter, presumably of marine origin.

Particle size distributions Particle size distributions were determined for suspended matter, sediment trap, and sediment samples using a model TA-II Coulter Counter. Particulate samples were resuspended in 3 ~ NaC1, mildly sonified in an ultrasonic bath for 30 min to disperse the material (SHELDON, 1968), and analyzed for 100 s using the 100- and 400-pM apertures. Duplicate samples were sonified for 8 h to disassociate the aggregated materials and analyzed under the same conditions. The particle number distributions were overlapped, normalized, and converted to volume distributions using the mean diameter of each channel (1.6 to 203 pm).

Scanning electron microscopy To examine microscopically the morphology of the particulate samples, an ISI Super Mini-SEM II scanning electron microscope was used at × 1000. Samples were

26

WILLIAM M. LANDINGand RICHARDA. FEELY

re-suspended in deionized filtered water, deposited onto 0.4-pm Nueleopore filters, sub-sectioned, mounted on to AI stubs, and gold sputter-coated (F~LY, 1976). RESULTS AND I N T E R P R E T A T I O N

Suspended matter distributions and vertical particulate fluxes Before the sediment trap deployments, suspended matter distributions and hydrographic properties were observed along a line of eight stations from Icy Bay to just beyond the shelf break (Fig. 5). The hydrographic parameters had the expected distributions with the warm subsurface core of the Alaska current contacting the shelf below 150 m at stations E, F, G, and H. In nearshore waters (stations A to D), temperature and salinity increased with depth, reflecting the input of relatively cold freshwater runoff. Further seaward (stations E and F), temperature decreased with depth while salinity and sigma-t generally increased with depth throughout the area. The surface suspended matter concentrations generally decreased seaward from 1.3mgl -~ at station A to about 0.2 mg 1-1 at station H. At the nearshore stations (A to D) particulate concentrations were relatively constant with depth from the surface to about 10 m above the bottom. Within the bottom 10 m a steady increase in suspended matter concentrations suggested ,

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Fig. 5. Hydrographic data and suspended particulate matter distribution for stations in the northeast Gulf of Alaska (14 to 31 March 1977).

75 I

27

The chemistry and vertical flux o f particles in the northeastern G u l f of Alaska

the presence of a nepheloid layer. The layer was probably due to local resuspension and sedimentation of bottom sediments, the result of the interactions of tidal currents with sediments (FEELV et al., 1979). The nepheloid layer shows an offshore extension at stations E and F (Fig. 5). While the near-bottom current-meter record at station D indicated only a small offshore current component, DRAKE, KOLPACK and FISHER (1972) suggested that detachment of turbid water along isopycnals can result in such a feature. The sediment traps deployed on the current meter mooring at station D remained there from March to June 1977. The self-closing plates were triggered 14 days after deployment and were open between 17 and 31 March 1977. The trap at 48 m failed to close and significant amounts of material were lost. Table 1 compares the sediment accumulation rate at station D (based on 2~°pb dating of the sediment core) with the vertical particulate fluxes measured with the traps (Fig. 6). Although the trap at 48 m remained open for the entire 84-day deployment, only 4.63 g of material were collected, yielding a flux of 0.3 mg cm -2 day -~. The trap at 86 m collected approximately 87% of the underlying sediment accumulation rate, while the trap at 96 m, deployed within the bottom nepheloid layer, collected 2.7 times the accumulation rate. Table 1. Comparison of the sediment accumulation rate at station D (based upon 21°Pb dating of a sediment core sample) with sedimentation rates from each of the three self-closing sediment traps deployed at 48, 86, and 96 m on a current meter array. The sediment traps were deployed open on 7 March 1977

Sample description Sediment trap No. 1 (station D, 48 m) Sediment trap No. 2 (station D, 86 m) Sediment trap No. 3 (station D, 96 m) Sediment core (station D)

Open (days)

Total flux (mg c m - 2 d a y - l )

Total flux as percentage of sediment accumulation rate

84

0.30*

--

14

2.79

87.2

14

8.80

275,0

--

3.20

100.0

* The sediment trap from 48 m did not close and part of the sample was lost.

Particle size analysis

The size distributions of sediment trap samples (station D) (Fig. 7, Table 2) shift toward smaller particles with depth and generally show broad distribution patterns with a major fraction of the volume associated with coarse particles and aggregates greater than 8 ~tm in diameter. Following intense (8-h) ultrasonic treatment the sediment trap samples shifted toward finer sizes, while the sediment sample changed little. The shifts of particle sizes suggest that approximately 5 to 33~o of the sediment trap samples consisted of aggregates broken down by the sonification (Table 3). To substantiate the results, aliquots of the samples were examined before ultrasonic treatment with SEM (Fig. 8). The aggregates appear as complex associations of biogenic fragments and mineral grains. Similar aggregates were described by BisHop e t al. (1977) as fecal material. The aggregates were sized and tabulated using the approximate aggregate diameter to calculate the spherical volume and the percent of the total sample volume they represented (Table 3).

28

WILLIAM M. LANDING and RICHARDA. FEEL¥

15KM

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Fig. 6. Summary of results obtained from the particulate matter collections conducted in the northeast Gulf of Alaska• The upward extent of the nepheloid layer was determined from discrete suspended matter samples• Vertical fluxes were determined with sediment traps deployed at station D. The underlying sediment accumulation rate was determined by excess 21°Pb dating of the core collected at station D (see inset). Table 2.

Particle size distributions of materials collected in the northeast Gulf of Alaska calculated from the volume distributions illustrated in Fig. 7

Sample description Nearshore sediments*

~o Sand (< 64 m)

~o Silt (4 to 64 m)

~o Clay ( < 4 m)

Mean diameter (lam)

1-10

60

20-30

4-16

3 5 3 1 0-5

62 79 70 72 58

35 16 27 27 42

14 25 15 13 10

Station D

Suspended matter collected on filters Sediment trap No. 1 (48 m) Sediment trap No. 2 (86 m) Sediment trap No. 3 (96 m) Sediment sample * D a t a from CAXLSON et al. (1977).

Elemental compositions

The bulk elemental c o m p o s i t i o n s of the sediment trap a n d sediment samples (Table 4) show: (1) decreases in the c a r b o n a n d n i t r o g e n c o n c e n t r a t i o n s with depth, followed by a similar decrease between 96 m a n d the u n d e r l y i n g sediments; (2) a d r a m a t i c decrease in

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Fig. 7. Percent of total volume distributions of: A, near-surface suspended particulate at station D; B, the sediment trap sample from 48 m at station D, C, the sediment trap sample from 96 m at station D; E, the underlying sediments collected at station D. Analyses were conducted with a model TA-II Coulter Counter.

30

WZLUAM M. LANDING and RICHARD A. FEELY

the Pb concentrations between the 48- and 86-m sediment trap samples vs the 96-m trap and underlying sediments; and (3) the constancy of most other element concentrations with depth, and relative to the sediments. The few major changes in the concentrations of certain elements between the sediment traps (water column) and the underlying sediments illustrate the effects of particulate input and alteration mechanisms and can be demonstrated better through comparisons between the vertical flux and accumulation rate of certain elements. Because the sediment trap from 86 m collected material at about the same rate that material accumulates in the underlying sediments the analyses of these samples will be compared. Table 5 contains the result of the bulk elemental analyses of the materials expressed as the vertical fluxes through 86 m and the underlying sediment accumulation rates of each element. Because the total vertical flux accounted for only 87% of the sedimentation rate, the individual element fluxes were normalized by the factor 1.0/0.87 = 1.15, divided by their respective

Table 3. Results from procedures used to estimate the contributions of coarse particles and aggregates to the sediment trap and sediment samples collected at station D in the northeast Gulf of Alaska. Columns 1, 2, and 3: results from visual particle size analysis of scanning electron micrographs; column 4: results from Coulter Counter analysis of mildly and intensely sonified subsamples Percent of total Sample description

N u m b e r of

N u m b e r of

Percent of total

particles

aggregates

per field

per field

volume associated as aggregates

volume of aggregates destroyed by intense sonification

165

0.45

58

33

154

0.69

31

5

182

0.05

9

14

NA

NA

NA

3

Sediment trap sample No. 1 (48 m) Sediment trap sample No. 2 (86 m) Sediment trap sample No. 3 (96 m) Underlying sediment sample (103 m)

NA = sample not analyzed by this technique.

Table 4. Summary of the elemental composition of sediment trap and sediment samples from station D in the northeast Gulf of Alaska. Zn concentrations were determined by X-ray secondary emission spectrometry.* Precision was determined by replicate analysis (_4-1 SD) Sediment trap No. 1 Element

(48 m)

C (wt ~o) N (wt %) Si (wt ~o) AI (wt ~o) Fe (wt ~ ) M n (ppm) Cr (ppm) Cu (ppm) Ni (ppm) Pb (ppm) Zn (ppm)

1.82 + 0.21 0.20 4- 0.02 28.0 4- 1.0 6.21 __. 0.39 4.53 _+ 0.23 995 + 13 95 _+ 5 36 _4-2 45 + 1 21 _+ 4

* Data provided by G. J. MASSOTH.

Sediment trap No. 2 (86 m) 1.37 0.12 26.2 7.34 5.29 983 95 42 52 29 121

4- 0.09 _ 0.01 4- 2.0 _ 0.59 + 0.57 + 57 _+ 9 + 4 4- 6 ___7 4- 22

Sediment trap No. 3 (96 m)

Underlying sediments (0-5 cm)

1.32 4- 0.12 0.13 + 0.01 30.8 + 2.2 7.61 + 0.94 5.19 4- 0.30 964 + 14 95 + 4 42 + 3 51 _+ 4 12 _ 1 122 + 15

0.87 4- 0.08 0.05 ___0.01 28.2 + 1.0 8.14 + 0.36 5.18 4- 0.29 990 + 16 100 -I- 4 40 4- 2 51 ___1 10 4- 3 116 4- 7

Fig. 8. Scanning electron micrographs of samples collected from sediment traps deployed at station D and the underlying sediments: A and B are from the trap at 48 m; C and D are from the trap at 86 m; E and F are from the trap at 96 m; and G and H are from the underlying sediment. Large aggregates, consisting of biogenic fragments and inorganic mineral grains, were found in all traps.

33

The chemistry and vertical flux of particles in the northeastern Gulf of Alaska

Table 5. Comparison of the vertical particulate flux of material collected in the sediment trap at 86 m and the underlying sediment accumulation at station D in the northeast Gulf of Alaska. The percent of accumulation rate was calculated by normalizing the vertical fluxes by the factor 1/0.87 = 1.15 to account for the discrepancy (13%) observed between the vertical mass flux and the sedimentation rate. Precision determined by propagation of errors (_+1 SD)

Material

Flux units

Total mass flux Total carbon flux Total nitrogen flux Si flux AI flux Fe flux Mn flux Cr flux Cu flux Ni flux Pbflux Zn flux

mg c m - 2 day-~ g cm -2 day -1 g cm-2 day- a g c m - 2 day -~ g c m - 2 day -1 g c m - 2 day -~ ng cm -2 day -1 ng c m - 2 day -1 ng cm-2 day -1 ng cm-2 day -1 ng cm-2 day -1 ng cm - 2 d a y - ~

Total vertical particulate flux 2.8 38 _ 4 3.3 + 0.3 732+38 205+11 151+11 2743 + 54 265+19 118+3 145+11 82+14 361 ___45

Underlying sediment accumulation rate 3.2 28 _ 3 1.6 + 0.3 901+25 260+10 166+9 3168 + 51 321+11 128+7 163+4 33+1 371 + 33

Ratio of vertical particulate flux ( x 1.15) to accumulation in sediments (%) 100 156 + 24 237 + 54 93+6 91+6 104+9 100 ___2 95+8 106+6 102+8 284+11 111 _ 14

Table 6.

Comparison of the verticle particulate flux and underlying sediment accumulation rate of quartz, biogenic particulate organic matter. The percent of the sediment accumulation rate accounted for by the particulate flux through 86 m was calculated by normalizing the particulate fluxfrom the sediment trap at 86 m by the factor 1.15 and dividing by the sediment accumulation rate Si, and elements associated with easily-oxidizable

Element Si quartz Si biogenic AI Si Fe Mn Cr Ni Cu

Vertical particulate flux through 86 m (ng cm - 2 d a y - 1) 279,000 + 5600 9250 2.03 _+ 0.07 2 9 0 _ 14 N.D.* 0.38 + 0.01 1.51 + 0.10 0.18 _ 0.01 4.30 -t- 0.20

Percent of total particulate flux (see Table 2)

Accumulation rate in sediments (ng c m - 2 d a y - i)

Ratio of vertical particulate flux ( × 1.15) to accumulation in sediments (%)

38 1.3 0.0001 0.02 -0.02 0.22 0.05 3.20

320,000 _ 32,000 3800 2.5 _ 0.1 495 ___11 20 _ 1 0.86 + 0.01 1.06 + 0.04 0.12 + 0.01 0.16 + 0.01

100 278 92 67 -50 162 171 3091

* N.D. = not determined.

accumulation rates, and presented as percentages of those rates. Ratios less than 100~ may indicate additional sources of sediment not observed in the 14-day average trap sample. Ratios in excess of 100~ may imply that (1) additional sources contributed to the trap sample but not to the underlying sediments, or (2) mechanisms act to alter the composition of the particles in the time they require to pass through 86 m and become incorporated in the top 5 cm of the sedimentary column. There are no appreciable deficiencies in the normalized vertical fluxes, but there are significant excesses for carbon, nitrogen, and Pb (Table 5). Similar treatment of the data collected from the biogenic Si selective dissolution analysis (Table 6) shows that while the vertical flux of quartz is approximately conserved, a significant excess vertical flux of biogenic Si was calculated.

34

WILLIAM M. LANDING and RICHARDA. FELLY

Data in Table 6 illustrate the vertical fluxes and sediment accumulation rates of elements associated with easily-oxidizable particulate organic matter. Although small deficiencies were calculated for Si, A1, and Mn, and small excesses were calculated for Cr and Ni, only Cu exhibits an association with organic material that accounts for more than a few tenths of a percent of its total vertical particulate flux. The association eclipses by two orders of magnitude the small excesses calculated for Cr and Ni. Zn and Pb were not observed in any of the peroxide selective oxidation dissolution solutions, so their behavior with respect to particulate organic matter cannot be established from the data. DISCUSSION

Bulk elemental analyses of the particulate matter samples indicate that fluvial input of continentally derived inorganic material dominates the particles. The current meter results and the broad-banded sediment deposits noted in the area by CARLSONet al. (1977) indicate that alongshelf transport should carry material ofcquivalent composition into or out of the study area. The information serves to justify the direct comparison of the analyses of the suspended matter, vertically-settling material collected with the sediment traps, and underlying sediments from station D. McCAvE (1975) and BISHOP et al. (1977) demonstrated that suspended matter concentrations and particle size distributions can be coupled with Stokes settling velocities to calculate approximate vertical particle fluxes. So treating the data from the surface waters at station D (using an average total suspended matter concentration of 0.69 mg 1-1 and a particle density between 1.5 and 2.5 g c m - 3), the calculated flux falls within a factor of 2.8 of the observed flux through 86 m: 1 to 3 mg c m - 2 day -1 calculated vs 2.79 mg cm -2 day -~ observed. Because of the dependence of the calculation on factors such as the size, shape, and density of the particles the agreement is satisfactory. However, such calculations are greatly affected by fluctuations in the concentrations of surface suspended matter, such as might be caused by episodic offshore transport of particle-laden nearshore surface waters. The fluctuations are about 0.5 mg 1-~ (FELLYand CLINE, 1977) and could change the calculated flux by as much as a factor of two. The relatively good agreement between the flux monitored with the sediment trap at 86 m at station D and the sediment accumulation rate determined by the 21°pb method suggests that the traps were deployed during approximately average fluvial input. The agreement between the calculated and observed fluxes also suggests that the suspended matter concentration and particle size distribution observed in the surface waters were about average. The results in Table 3 can be used to examine the relative importance of two vertical transport mechanisms for suspended matter at station D. If aggregates are the direct result of zooplankton grazing, then grazing accounted for up to 30~ of the particle flux through 86 m, with coarse- and fine-grained individual particles accounting for the difference. Remineralization of elements associated with organic matter

The excess fluxes through 86 m of carbon, nitrogen, biogenic Si, and Cu associated with particulate organic matter are consistent with biological remineralization, which would be expected to break apart aggregates and to oxidize organic material. Release of labile nutrients and trace elements and dissolution of biogenic hard parts would follow. Because

The chemistry and vertical flux of particles in the northeastern Gulf of Alaska

35

Table 7. Summary of the element recycling rates based on the excess vertical particulate flux relative to the accumulation rates in the sediments of carbon, nitrogen, biogenic Si, and Cu associated with easilyoxidizable particulate organic matter. Precision determined by propogation of errors (+ 1 SD)

Element

Recycling rate (ng cm- 2 day- 1)

Percent of normalized total vertical particulate flux

C N Si biogenic Cu organic

16,116 + 386 1250 + 453 6838 4.8 + 0.1

37 58 0.81 3.6

the sediment trap deployed at 96 m contained large quantities of resuspended material whose composition closely resembled that of the material collected at 86 m, we conclude that remineralization and dissolution occur primarily within the sediments below the shallow zone influenced by resuspension. The absolute differences between the particle fluxes and sediment accumulation rates of the remineralization materials (carbon, nitrogen, biogenic Si, and Cu associated with particulate organic matter) can be used to calculate rates of remineralization (Table 7). Only 37% of the carbon and 58% of the nitrogen flux through 86 m is remineralized, yielding a C/N ratio of 7.2 by weight. The increase in the ratios between 86 m (11 + 1) and the underlying sediments (17 + 4) may reflect preferential remineralization of nitrogencontaining organic compounds. Dissolution of approximately 7 pg Si cm-2 day-1 is required to balance the vertical flux of biogenic Si. This remineralization accounts for 64% of the normalized biogenic Si flux through 86 m, but less than 1% of the total normalized Si flux. Remineralization of organically associated Cu accounts for regeneration of about 5 ng Cu cm-2 day-~, nearly 100% of the vertical flux of this material through 86 m and about 3.6% of the total Cu flux. The remineralization rates are clearly functions of near-surface production and subsequent vertical transport of the materials. As primary production varies seasonally and spatially by several orders of magnitude in the northeast Gulf of Zlaska (LARRANC~, TENNANT, CHSSTSRand RUFFIO, 1977) the remineralization rates estimated here are valid only for the time the sediment trap samples were taken. During maximum blooms such as those encountered in June, July and August (LARRANCEet al., 1977) the production and vertical fluxes of organic carbon and nitrogen and biogenic Si will be substantially greater and require higher remineralization rates than long-term average sediment accumulation rates. The remineralization rate proposed for Cu associated with particulate organic matter probably represents a minimum value because the samples were collected prior to periods of maximum productivity, and because the hydrogen peroxide treatment did not remove 100% of the organic material present in the trap and sediment samples. Other direct measurements of the rates of remineralization of organic material and the regeneration of trace elements are lacking, particularly in coastal areas dominated by fluvial input. HEc,oIE and BURR~LL (1978) estimated the dissolved Cu flux from the sediments of Resurrection Bay in the northeast Gulf of Alaska (13 ng cm-2 day-~) and concluded that it accounted for between 20 and 46% of the particulate Cu flux at the sediment-seawater interface. BOYLE, SCLATeRand EDMOND(1977) estimated the dissolved Cu flux from deep-sea sediments to be 0.44 ng Cu cm -2 day -~ or 71% of the particulate Cu flux at the sediment-seawater interface. Dissolution of biogenic Si below the sediment-seawater interface appears to account for

36

WILLIAM M. LANDINGand RICHARD A. FEELY

64% of its normalized particulate flux through 86 m (Table 4). HURD (1973) concluded that from 92 to 99% of the biogenic Si produced in Pacific surface waters was dissolved before reaching the sediments and that dissolution and flux out of the sediments accounted for about 320 ng Si cm-2 day-1, or from 1 to 8% of the surface production. Only 0.1% of the biogenic Si was actually preserved in the sedimentary column. EDMOND (1974) suggested, on the other hand, that rapid settling of particulate Si followed by dissolution at the sediment-seawater interface was the primary mechanism regulating the deep-water distribution of dissolved Si. The present study suggests that the latter mechanism dominates the behavior of biogenic Si in shallow water, at least during the sampling period. In summary, comprehensive analyses demonstrate that the sediment traps used during this study appear to collect material at a rate comparable to the underlying sediment accumulation rate without severe size or compositional fractionation. The elemental accumulation rates, remineralization of particulate organic matter, and remobilization of trace elements, notably Cu, were quantified by comparing the composition of the trapped material with the underlying sediments. The majority of the remineralization appears to occur within the sediments below the zone influenced by resuspension, presumably as a result of biological activity. Acknowledgements--The authors wish to express their appreciation to : Captain SIDNEY MILLER and the crew of the Discoverer without whose help this work would not have been possible; ELIZABETH A. MARTIN (USGS, Corpus Christi, Texas) for performing z~°Pb analyses; ANTHONY YOUNG for building the sediment traps after a similar design given to us by CARL LORENZEN; GARY MASSOTH,JANE FISHER, and MARILYN LAMB for assisting in sample preparation and data reduction This study was supported by the Bureau of Land Management through an interagency agreement with the National Oceanic and Atmospheric Administration, under a multi-year program responding to needs of petroleum development of the Alaskan continental shelf which is managed by the Outer Continental Shelf Environmental Assessment Program (OCSEAP) office. The majority of the major and trace element analytical data were collected and utilized in partial fulfillment of the requirements for the MS degree by W. M. LANDING at the Department of Oceanography, University of Washington, Seattle. The authors are also grateful to H. CURL, JR, J. CLINE, A. J. CHESTER and M. C. GLENDENING for providing critical reviews of this manuscript.

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