Composition and Behaviour of Trace Metals in Post-oxic Sediments of the Gulf of Papua, Papua New Guinea

Estuarine, Coastal and Shelf Science (1996) 42, 197–211

Composition and Behaviour of Trace Metals in Post-oxic Sediments of the Gulf of Papua, Papua New Guinea

D. M. Alongi, S. G. Boyle, F. Tirendi and C. Payn Australian Institute of Marine Science, PMB No 3, Townsville M.C. Qld 4810, Australia Received 21 March 1994 and in revised form 10 November 1994

Keywords: behaviour; composition; diagenesis; trace metal; Gulf of Papua; Papua New Guinea Trace metal composition and behaviour were examined in post-oxic muds of the Gulf of Papua in January 1993. Porewater profiles and flux measurements indicate significant Fe and Mn reduction in these coastal muds. Vertical profiles, rates of sediment–water exchange, and correlation analyses imply that these silt-dominated muds mobilize Al and are a sink for Mo. Undetectable or very low porewater concentrations and flux rates of other metals (Ni, Cu, Cr, Co, Pb, Cd) indicate immobilization into solid-phase minerals. Solid-phase metal concentrations are consistent with those in average shale, suggesting pristine conditions in the gulf. Low (<1–2 ìM) free sulphides, low clay content, and bioturbation are partly responsible for trace metal composition and concentrations, but several lines of evidence imply non-steady-state diagenesis in these muds: (1) the lack of a clear zonational sequence of solutes with sediment depth; (2) lack of agreement between vertical profiles of dissolved and solid-phases of most metals; (3) significant, but weak, correlations between dissolved metals and dissolved organic carbon, and between solid-phase metals and total organic carbon; and (4) lack of correlation between conservative elements (e.g. dissolved Mo and Cl " ; dissolved Al and SiOH). Intense, massive physical reworking is cited as the major factor fostering apparent non-steady-state behaviour of trace metals in the surface muds of the Gulf of Papua. This scenario is very similar to that occurring on the Amazon shelf, and may be typical of other wet tropical regions with high-energy coastlines receiving large amounts of highly weathered, river-borne material. ? 1996 Academic Press Limited

Introduction The flux of trace metals to sediments is influenced by the rate of transport from the water column (detrital flux), dissolution rate of biogenic material and rate of pelagic productivity. The accumulation and diagenesis of metals within sediments is also greatly dependent upon sediment redox conditions and microbial activity (Chester, 1990; Shaw et al., 1990). In most coastal and shelf deposits, redox potential decreases with sediment depth due to bacterial decomposition of organic matter and limited diffusion of oxygen across the 0272–7714/96/020197+15 $12.00/0

? 1996 Academic Press Limited

198

D. M. Alongi et al.

sediment–water interface. In surface oxic layers, trace metals adsorbed onto particulate organic matter may be released upon aerobic mineralization (Klinkhammer et al., 1982; Sawlan & Murray, 1983). A transitional zone called the ‘ post-oxic ’ (or ‘ sub-oxic ’) layer (Froelich et al., 1979; Berner, 1981) forms between the oxic surface layers and deeper anoxic sediments where sulphide formed from bacterial reduction of sulphate permits the formation of insoluble trace metal sulphides. The post-oxic zone is characterized by low oxygen and sulphide concentrations in the porewater and by high Fe and Mn concentrations and reduction rates. Trace metals having a high affinity for Mn and Fe oxides are frequently found in high concentration in post-oxic porewaters due to release when these oxides are reduced and dissolved. These trace metals may re-adsorb onto other solid phases or may precipitate as sulphide minerals (Klinkhammer et al., 1982; Sawlan & Murray, 1983). Depth scales of these diagenetic sequences in shallow water muds are typically 0·1–1 cm for oxic decomposition, 1–10 cm for post-oxic reactions, and 10–100 cm for anoxic diagenesis, although these reaction sequences can overlap (Chester, 1990). In deep-sea muds, these zones are vertically expanded due to lower rates of organic matter supply and decomposition (Froelich et al., 1979). Anoxic decomposition, particularly sulphate reduction, may proceed slowly or not at all, or occur only at great (>1 m) sediment depth (Chester, 1990). Most of our knowledge of trace metal cycling in the suboxic zone comes from analysis of deep-sea deposits (Sawlan & Murray, 1983; Pedersen et al., 1986). Recent studies have revealed that the muds off the Amazon (Aller et al., 1986, 1994; Mackin & Aller, 1986; Mackin et al., 1988) and off several rivers of the southern coast of Papua New Guinea (Alongi et al., 1992, 1993; Alongi, 1994) are similarly dominated by post-oxic diagenetic processes. The Amazon shelf sediments are characterized by extensive (to 4 m depth) zones of Fe and Mn cycling with little depletion of sulphate. Diffusive ÓCO2 fluxes and porewater profiles suggest substantial loss of remineralized C to authigenic carbonate formation or flux imbalances due to non-steady-state growth (Aller et al., 1994). These characteristics, attributed mainly to massive physical reworking, are in contrast to many temperate estuarine and shelf deposits where S can dominate early diagenesis. Off the southern coast of Papua New Guinea, a similar situation exists in which deposits on the Gulf of Papua shelf are dominated by post-oxic decomposition (Alongi et al., 1992; Alongi, 1994). On the Amazon shelf, the behaviour of dissolved Fe, Mn and Al has been assessed, but the composition and behaviour of trace metals in the Papuan sediments in relation to post-oxic conditions and massive physical/biological disturbance is unknown. This information is crucial to our understanding of trace metal flux in the oceans as the diagenetic characteristics of the Amazon and Papua New Guinea sediments may be typical of other wet tropical regions with rivers draining highly weathered debris onto high-energy shelves. This study emphasizes the surficial deposits (0–20 cm depth) of the Gulf of Papua, including the sediment–water interface, in order to assess the role of these muds in the flux of trace metals to the Coral Sea. Study area The Gulf of Papua (Figure 1) covers a broad continental shelf area of approximately 54 000 km2 in the northern Coral Sea. Sedimentary facies are influenced by the great

Trace metals in post-oxic muds

199

Papua New Guinea

Aird Delta

Coral Sea

Purari Delta

Turama R Australia

Be

bea

R

5

10 IGP3 20

8°S

IGP2 IGP1 50

D9 Fly

GP8

De

lta

100

GULF OF PAPUA

9°S 144°E

145°E

Figure 1. Map of station locations in the Gulf of Papua off the coast of southern Papua New Guinea.

rivers of the Papua New Guinea mainland to the north, the Great Barrier Reef to the south, and by live and relict Halimeda beds and coral reefs to the south-west and on the mid- to outer shelf (Alongi et al., 1992; Alongi & Robertson, 1993; Harris et al., 1993). The Fly River is the largest system draining into the Gulf of Papua, with a drainage basin area of 76 000 km2, and sediment and water discharge rates of 85#106 tonnes year "1 and 220 km3 year "1, respectively (Salomons & Eagle, 1990). Several other rivers—the Bamu, Bebea, Turama, Omati, Aird and Purari—discharge into the gulf and make up a total drainage basin area of 150 000 km2, transporting 470 km3 year "1 of water and 300#106 tonnes year "1 of sediment (MacFarlane, 1980). On the inner shelf (<60 m depth) where most of the river-derived mud deposits, water circulation is strongly dependent on the prevailing tidal regime (Wolanski & Eagle, 1991). Coastal water is well-mixed by winds and tides, and considerably diluted by river water. Station locations and sediment characteristics In January 1993, five stations were sampled in the inner Gulf of Papua (Figure 1). Three stations (IGP1, IGP2, GP8) were characterized by bioturbated muds. Sta. IGP1 (08)08·10*S, 144)37·23*E) was 48 m deep and Sta. IGP2 (07)57·9*S, 144)56·6*E) was 41 m deep and located c. 2 nautical miles off the Purari Delta. Sta. GP8 (08)28·5*S, 144)19·32*E) was 55 m deep and characterized by intensive bioturbation (Alongi et al., 1992; Alongi & Robertson, 1993). The other two stations (IGP3, D9) were characterized by sedimentary facies of laminated mud and were at shallower depth. Sta. IGP3 (08)08·35*S, 144)16·75*E) was 15 m deep, and Sta. D9 (08)20·3*S, 143)51·9*E) was 8 m deep and located at the Fly Delta–Gulf of Papua boundary. Bottom-water salinity was lowest at the latter two stations (Sta. IGP3, 33·8; Sta D9, 30·3) with higher salinity at the other three stations (Sta. IGP1, 35·2; Sta. IGP2, 35·3; Sta. GP8, 35·1). The sediments at all five stations are silt dominated, with low CaCO3 content (Table 1). Bulk concentrations of total organic carbon and total nitrogen and phosphorus are similar among stations (Table 1) with little or no change with sediment depth. Total

200

D. M. Alongi et al.

T 1. Mean granulometric and element characteristics at the Gulf of Papua stations (from Alongi & Robertson, 1993; Alongi, 1994)

Porosity (ml cc "1) CaCO3a Grain size (ìm) Percent sand Percent silt Percent clay Total organic carbona Total nitrogena Total extractable phosphorusa Total sulphura

IGP1

IGP2

IGP3

GP8

D9

0·59 1·5 11·0 1·2 90·4 8·4 0·98 0·066 0·083 0·173

0·60 1·7 9·8 2·0 85·5 12·5 1·16 0·077 0·079 0·207

0·63 0·7 7·7 0·5 86·7 12·8 1·13 0·107 0·080 0·144

0·55 3·1 12·1 2·4 89·1 8·5 1·00 0·095 0·075 0·178

0·67 1·4 8·0 5·0 70·6 24·4 0·97 0·091 0·073 0·168

a

Expressed as percent of sediment dry weight.

sulphur concentrations are low (<0·21% by dry weight) indicating low net S precipitation. Sulphate concentrations in the porewater showed no significant depletion and no free sulphides were detected (detection limit: 1–2 ìM) at any of the stations. Rates of sulphate reduction were low (Alongi, 1994) ranging from 3·6 to 6·8 mmol Sm "2 day "1, with little (18–25%) of the total reduced 35SO4 recovered as acid-volatile sulphide. In contrast, surface oxygen consumption was high (17·8–46·8 mmol O2 m "2 day "1,) as were bacterial numbers and rates of total carbon production (Alongi, 1994). Redox levels ranged from "20 to 120 mV and declined with sediment depth only at Stas. IGP1 and IGP3 (Alongi, 1994). Vertical profiles of porewater ammonium and phosphate concentrations increased with depth at most stations, but NO2 " +NO3 " were present to 20 cm depth at all sites in low (0·1–4·0 ìM) concentration. Measured fluxes of dissolved nutrients demonstrated rapid rates of release from the sediments, but dissolved organic nitrogen, phosphorus and carbon (DON, DOP and DOC) showed moderate to rapid rates of uptake implying nutrient-limiting conditions on the shelf (Alongi, 1994). Sediment accumulation rates are not known for these stations, although Harris et al. (1993) measured accretion rates of up to 4 cm year "1 in prodelta deposits at water depths of 17–45 m. Methods Field sampling and sample handling Replicate cores were taken at each site using a modified 0·027 m2 Bouma boxcorer with a maximum depth of penetration of 20 cm. Boxcores were subsampled for dissolved and solid-phase metals by inserting three 7-cm diameter cores into the middle of each boxcore. Each 7-cm diameter core is made of an outer stainless steel jacket containing a recessed inner PVC tube subdivided into 2-cm rings. Each subcore sample was sectioned at 2-cm intervals under a N2 atmosphere with each slice placed immediately into acid-washed Petri dishes. The porewater was extracted immediately using a Teflon porewater extractor (Robbins & Gustinis, 1976). Porewaters were squeezed through 0·4 ìm Nuclepore filters under an applied N2 pressure of 100 kPa for 10–15 min to collect c. 2–5 ml of sample. Each sample was stored cooled (2–5 )C) until analysis in acid-washed Teflon-capped glass test tubes containing 100 ìl of 25% (w/v) HCl. Blanks were run by passing distilled water through the porewater squeezing procedure.

Trace metals in post-oxic muds

201

The remaining squeezed cake was frozen immediately for later analysis of solid-phase metals. Flux measurements of dissolved metals across the sediment-water interface were made from each station using opaque glass bell jars (n=3), placed into undisturbed boxcores and incubated under in situ temperature conditions for 3 h in a continuously flowing water bath. A 3-h incubation time was chosen because previous experience and concurrent measurements of oxygen flux (Alongi, 1994) indicated that a longer time period would cause O2 to become depleted (<2 mg l "1 from an initial concentration of c. 5–6 mg l "1) leading to artifacts (Sundby et al., 1986). The speed used to stir the bell jars was a compromise between efficient stirring for representative sampling and avoidance of disturbing the sediment surface. The bell jars (1 l volume; 0·007 m2 surface area) were gently fitted into the boxcores and pushed 2–3 cm into the sediment. Syringe samples (10 ml) were taken from a sampling port immediately and at 45-min intervals over the 3-h period. Samples were filtered, acidified and processed later as described below for porewater metals. Analytical procedures Solid-phase metal concentrations (except Mo, Cd and Pb) were determined on a Varian Liberty 200 ICP Atomic Emission Spectrometer following a total HF digestion procedure modified from Loring and Rantala (1992). Briefly, 0·5 g of dried (80 )C for 24 h) and ground sediment was weighed into a 50-ml Teflon beaker, digested with 5 ml each of aqua regia (HNO3 +HCl), HClO4 and HF (Merck) at 200 )C to insipid dryness, redigested in 5 ml of HClO4 and made up to 50 ml in a volumetric tube. Analytical performance was monitored with standard reference materials NBS 1646 (estuarine sediment) from National Institute of Standards Technology and BCSS-1 (marine sediment) obtained from the National Research Council of Canada. Values were always within the certified range. Dissolved metal concentrations were measured on a Varian Spectra 400 Zeeman atomic absorption spectrometer. Due to salt interference, the method of Apte and Gunn (1987) was followed in which the metals were chelated with ammonium pyrolidine dithiocarbamate and extracted into 1,1,1-trichloroethane. The resulting metal dithiocarbamate complex was analysed sequentially. Standard seawater reference materials CASS-1 and SLEW-1 (National Research Council of Canada) were used to verify the analysis. Porewater Fe, Al and Mn were analysed on the ICP Emission Spectrometer. Solid-phase Mo, Cd and Pb were determined on the Zeeman atomic absorption spectrometer following strong acid (aqua regia+HClO4) digestion as initial analyses on the emission spectrometer indicated metal concentrations below instrument detection limits. Detection limits for the solid-phase metals on the ICP Emission Spectrometer were 5 ìg g "1 sediment dry weight. Detection limits for the dissolved metals were: Mn (18 nmol l "1), Al (370 nmol l "1), Cd (1·8 nmol l "1), Ni, Co, Cu (32 nmol l "1), Fe (90 nmol l "1), Mo (52 nmol l "1), Pb (24 mol l "1) and Cr (96 nmol l "1). Dissolved V and Zn were not determined due to low V sensitivity and Zn contamination in the 1,1,1-trichloroethane. All field blanks were below detection limits for the other metals. Detection limits for solute metals were high because of the small sample volume. Values below detection limits were excluded from any statistical tests. Analysis of variance (ANOVA) and Student–Newman–Keuls (SNK) tests were used to examine for differences in metal concentrations among stations and depths. Linear

202

D. M. Alongi et al.

Fe (µM) 50

100

0.1–0.6 0

Mn (µM) 100

200

1–1.6 0

Al (µM) 10 20 30 //

0.1–0.4 0 4

//

cm

8 12 16 20 Figure 2. Mean vertical depth profiles of porewater Fe, Mn and Al expressed as ìM. Arrows represent mean range of overlying water column concentrations. -, IGP1; ,, IGP2; /, IGP3; ., GP8; 4, D9.

regressions were performed on the flux data to estimate rates of release/uptake. Best-fit regression models were used to examine possible relationships of metals with nutrients and measures of microbial activity (e.g. sulphate reduction). All tests were performed using methods described in Sokal and Rohlf (1981). Results Porewater solutes Interstitial concentrations of trace metals were dominated by Mn, Fe and, to a much lesser extent, Al (Figure 2). Fe and Mn concentrations were significantly (P<0·01) greater at Stas. IGP3 and D9; station differences for Al in porewater were: IGP2>IGP3>GP8>IGP1>D9 (SNK test; P<0·05). Porewater concentrations were significantly greater than in the overlying water column at all five stations (see arrows, Figure 2). Mn concentrations declined significantly with sediment depth at Stas. IGP1, IGP2 and GP8 whereas Al porewater concentrations only declined significantly (P<0·01) at Sta. IGP2. The vertical profiles of Fe and Mn at Stas. IGP3 and D9 exhibited subsurface maxima. All other profiles were either monotonic or irregular. Concentrations of Cr, Pb, Co and Cd were below detection limits at all five stations. Mo, Ni and Cu concentrations exhibited monotonic or irregular profiles with sediment depth (Figure 3), with porewater concentrations at or below detection limits at a few depth intervals or in all replicate cores at a given station (IGP3 and GP8). Sediment–water exchange Dissolved metal flux across the sediment–water interface was dominated by Al, Fe and Mn (Table 2). Al flux was high when measurable, as were Fe and Mn fluxes at Stas. IGP1 and D9. Dissolved Mo was taken up by the sediment at Stas. IGP1, IGP3 and GP8. Ni and Co fluxes were measurable only at Stas. IGP2 and IGP1, respectively (Table 2). Cr, Pb and Cd concentrations were always below detection limits, even after 3 h incubation. Cu fluxes were not significant at the only sites (IGP2 and D9) where concentrations were above detection limits. Solid-phase metals The order of abundance of metals in these deposits, on average, was: Al, Fe, Mn, V, Zn, Cr, Ni, Cu, Co, Pb, Mo, Cd [Figure 4(a,b)]. Solid-phase Al concentrations were

Trace metals in post-oxic muds

Mo (nM) 40 0

60

80

203

Ni (nM) 100

0

100

Cu (nM)

200

300

0

100

200

4

cm

8 12 16 20 Figure 3. Mean vertical depth profiles of porewater Mo, Ni and Cu expressed as nM. Arrows represent mean ranges of overlying waters. Dashed lines represent detection limits. Values below detection limits at some stations are not presented (see text). -, IGP1; ,, IGP2; 4, D9. T 2. Trace metal fluxes (ìmol m "2 day "1) at Gulf of Papua stations Metal Al Fe Mn Mo Ni Cu Co

IGP1

IGP2

381&207 310&81 112&17 "4&3

421&198 a

a

IGP3

GP8

D9

a

a

"41&18

a

982&408 585&199 606&46

42&11

a

a

"37&14

23&7 "45&11 b

a

33&4

b

a

b

a

b

b

a

16&6

b

b

b

b

a

No significant flux; bconcentrations below detection limit. Values are means&95% confidence interval. Negative fluxes denote sediment uptake. Concentrations of Cr, Pb and Cd were below detection limits.

equivalent at Stas. IGP1, IGP3, GP8 and D9, but significantly lower at Sta. IGP2. Station differences for Fe were: IGP3>IGP1=IGP2>D9>GP8. Vertical profiles of Al and Fe were either irregular with depth (no increasing or decreasing trend) or monotonic. In contrast, Mn levels at Stas. IGP1, IGP2 and GP8 were significantly enriched in surface layers [Figure 4(a)] with no clear pattern with depth at Stas. IGP3 and D9. Station and sediment depth differences were both highly significant (P<0·01) for Ni, Zn, V and Cr (Figure 4(b)) but with no clear trend with sediment depth or separation among stations (station#depth interaction terms were very highly significant, 2-way ANOVA; P<0·001). However, there were clear station (but only one or two depth) differences for Co, Cu and Pb. Station differences were: for Co, IGP3=IGP2>D9=IGP1>GP8; for Cu, IGP3=IGP2=D9>IGP1>GP8; for Pb, D9=IGP3>IGP1>GP8>IGP2 [Figure 4(b)]. Cd concentrations were at detection limits (0·08 ppm) at all depths at all five stations. Mo concentrations declined significantly (P<0·01) with sediment depth at all five stations [Figure 4(b)]. Concentrations at Sta. GP8 were significantly less than Mo levels at the other four stations.

204

D. M. Alongi et al.

(a) 8 0

Al 8.5

9

9.5

Fe 5.0

4.6

5.4

Mn 0.08

0.06

0.10

4

cm

8 12 16 20

(b) 30 0

Ni 35

40

V 170

200 70

18

Co 20

22

Cu 40

20

60

100

Zn 120

4 8 12 16

cm

20 140 0

Cr 90

0.2

Mo 0.6

1.0

0

Pb 10

20

4 8 12 16 20 Figure 4. Mean vertical depth profiles of (a) solid-phase Al, Fe and Mn (all expressed as percentage of sediment dry weight), (b) solid-phase Ni, Zn, Cu, Cr, Co, V, Mo and Pb. Cadmium is not plotted. Values are in ppm. -, IGP1; ,, IGP2; /, IGP3; ., GP8; 4, D9.

Metal–nutrient relationships Dissolved Fe, Mn, Al [Figure 5(a)] and Mo and Ni [Figure 5 (b)] correlated significantly with porewater DOC. The relationship with DOC was inverse for Al [Figure 5(a), bottom] but positive for the other metals [Figure 5 (a,b)]. Although significant, these relationships were weak; the strongest correlation of DOC (r=0·75) was with Ni and the weakest (r=0·35) was with Mo [Figure 5 (b)]. There were no clear distinctions of metals between laminated and bioturbated stations. There was only one other significant relationship between a dissolved metal and another porewater constituent: a weak, inverse relationship between molybdenum and rates of sulphate reduction (Figure 6).

Trace metals in post-oxic muds

210

(a)

205

96

y = –13.2 + 0.095x r2 = 0.35***

64

0

0

300

1050

200

700

y = 31.7 + 0.118x 2 r = 0.23**

100

y = 60.6 + 0.026x r2 = 0.12*

80

70

Ni

Mn

Fe

Mo

140

(b)

y = –139 + 1x 2 r = 0.57***

350 0

0

150 300 450 600 750 900 Dissolved organic carbon (mM)

75

Al

50

y = 465e – 0.008x r2 = 0.39***

25

300 600 900 1200 1500 1800 Dissolved organic carbon (mM) Figure 5. (a) The relationship between dissolved Fe, Mn and Al with porewater DOC (data from Alongi, 1994), all stations and depths. Metal concentrations are ìM. (b) The relationship between dissolved Mo and Ni with porewater DOC (data from Alongi, 1994), all stations and depths at which concentrations were above detection limits. Metal concentrations are nM.

96 y = 86.6 – 0.36x 2 r = 0.38** Mo (nM)

0

80

64

15

30

45

60

75

90

Sulphate reduction rate (nmol S cm–3 day–1) Figure 6. The inverse relationship between porewater Mo (nM) and rates of sulphate reduction (data from Alongi, 1994), all stations and depths at which Mo concentrations were detectable.

7.5 7 y = 5.67 – 1.08x 2 r = 0.21**

6.5

4.4 4.2 y = 3.78 – 0.39x 2 r = 0.17**

4 3.8

0.75

0.9

1.05

1.2

1.35

1.5

Cr/Fe (×10–4)

D. M. Alongi et al.

Cu/Fe (×10–4)

Co/Fe (×10–4)

Ni/Fe (×10–4)

206

18.0 16.5 y = 13.9 – 2.62x 2 r = 0.14**

15.0

12 9 y = 2.77 – 4.5x 2 r = 0.15**

6 0.75

0.9

1.05

1.2

1.35

1.5

Total organic carbon (% dry weight of sediment) Figure 7. The relationship of solid-phase Ni/Fe, Co/Fe, Cr/Fe and Cu/Fe ratio with total organic carbon (percent dry weight of sediment), all stations and depths.

There were significant relationships of solid-phase metals and nutrients (Fe vs. C, r=0·35; Fe vs. P, r=0·60), but the relationships of metal/Fe ratios with total organic carbon content were either not significant or were weak (Figure 7). Distinctions between laminated and bioturbated muds were not clear due to the high scatter among stations (Figure 7). Discussion Decomposition of organic matter in the bioturbated and laminated muds of the Gulf of Papua are dominated by oxic and post-oxic diagenetic processes rather than by sulphate reduction (Alongi et al., 1992, 1993; Alongi, 1994). Only the laminated muds reveal vertical profiles of porewater solutes consistent with the zonation model of successive mineralization processes that are predicted to occur in post-oxic sediments (Froelich et al., 1979; Klinkhammer et al., 1982; Sawlan & Murray, 1983; Shaw et al., 1990; Burdige, 1993). The bioturbated muds show no vertical progression of Fe mobilization, but do reveal a decline in dissolved Mn with increasing sediment depth. The solid-phase Mn profiles at these bioturbated sites show depletion with depth indicating distinct zones of manganese oxide reduction and oxidation. Although the zones of Fe and Mn reduction are clear in the laminated muds, the solid-phase profiles do not show the expected surface enrichment and depletion with depth due to reductive processes. Moreover, the pattern of sediment–water exchange of Fe and Mn do not agree with the expected pattern based on the porewater profiles. For instance, at the bioturbated site, IGP1, significant release of Fe and Mn was measured despite very low concentrations and the lack of a depth gradient in the porewater. It is difficult to reconcile the lack of agreement among the porewater and solid-phase profiles, and the flux rates. One possible explanation is that trace metal (and organic matter) diagenesis is not in steady-state in these silt-dominated muds. Pedersen et al. (1986) came to a similar conclusion for metals in intensively-bioturbated, hemipelagic sediments on the East Pacific Rise. They found that the distribution of Fe and Mn in the sediments were in disequilibrium with their respective porewater profiles;

Trace metals in post-oxic muds

207

upward-diffusing Mn and Fe precipitated below the horizon expected assuming steadystate diagenesis, producing ‘ perched ’ or a series of ‘ perched ’ enrichments. They attributed diagenetic disequilibrium to a geologically recent decrease in productivity. In the Gulf of Papua, non-steady-state diagenesis may be due to similar variations in productivity and subsequent deposition of particulate material, as well as to seasonal changes in physical reworking of the benthos. Analysis of vibrocores (1–4 m length) taken by Harris et al. (1993) in close proximity to the stations used in the present study indicate historical variations in the deposition of material probably due to seasonal changes in river runoff. Physical reworking is likely to be a more significant factor affecting trace metal behaviour in the Gulf of Papua than bioturbation. Biogenic mixing is intense, but only in the upper 5–7 cm at most of the stations—above the zone of release of most trace metals. X-radiographs of sediment cores from the Gulf of Papua reveal episodes of physical disturbance (Alongi et al., 1992). The muds at Stas. D9 and IGP3 consist of subsurface bands of silt–clay disrupted by erosional contacts normally produced by erosion–deposition events. Even the bioturbated muds have a disrupted subsurface fabric that cannot be fully explained by infaunal activity. Thus, both the bioturbated and laminated surface muds are probably dominated, in the long term, by a sequence of quiescent–disturbed–quiescent conditions. Harris et al. (1993) suggested that strong tidal currents from the many rivers lining the gulf, and surface waves originating from the Coral Sea, cause reworking and resuspension of surface muds in the Gulf of Papua. The frequency of such events is not known, but is likely to be on time-scales longer than can be observed for reabsorption–precipitation and mobilization reactions to occur for most trace metals (Kerner & Wallmann, 1992). Nevertheless, such disturbance events may occur frequently enough to promote oxidant recharge and favour oxic and post-oxic diagenesis rather than more reductive processes, thus affecting trace metal behaviour. This scenario is similar to that observed off the Amazon where the diagenetic sequence is vertically expanded, favouring coating of sand particles by reactive Mg–Fe–Al (Aller et al., 1986). Indeed, Fe-coated sand grains are a common feature in the Fly Delta (Alongi et al., 1993)—the largest river emptying into the Gulf of Papua. The notion of diagenetic disequilibrium being due to physical reworking in the Gulf of Papua is supported by a comparison of the present data taken in January 1993 with the authors’ earlier data of July–August 1989 (Sta. D9) and February 1990 (Sta. GP8). The earlier profiles of Fe and Mn (see Alongi et al., 1993) for these two stations are considerably different than those presented here (Figure 2). At Sta. D9 in July–August 1989 there was no evidence of Fe or Mn mobilization. Indeed, porewater concentrations were much less (0·2–4 ìM for Fe, 2–45 ìM for Mn) and the only measurable flux was for Fe into the sediment ("233 ìmol m "2 day "1). At Sta. GP8 in February 1990, the behaviour of Fe was similar to the present data, but Mn exhibited mobilization (up to a concentration of 100 ìM) over the 2–12 cm depth interval. The drastic differences for Sta. D9 can be partly explained by seasonal variations in massive physical reworking in the Fly Delta. Sea conditions were rough for several weeks preceding the July–August 1989 cruise. Such conditions may have resulted in disturbance of porewater and solid-phase profiles, and may, over time, maintain diagenesis of organic matter and trace elements in a disequilibrium phase. Disturbance-induced changes in reduction–oxidation reactions of metals have been similarly observed off the Amazon (Mackin et al., 1988; Aller et al., 1994). Vertical profiles of solid phase Fe +2 undergo seasonal patterns of net oxidation and reduction in

208

D. M. Alongi et al.

T 3. Mean metal composition of Gulf of Papua sediments in comparison with average shale (Turekian & Wedepohl, 1961; Broecker, 1974)

Gulf of Papua Shale

Al

Fe

Mn

Cd

Co

Cr

Cu

Ni

Zn

Pb

Mo

V

8·7 8·2

5·0 4·7

721 850

0·08 0·3

20 19

83 90

33 45

34 68

108 95

13 20

0·47 2·6

174 120

Values are ppm or % dry weight (Fe, Al).

the upper 1 m or more of sediment. During rising or high flow periods, the most oxidized Fe is found; the most reduced Fe is found during falling or low flow conditions. These alternating patterns of exposure, oxidation, burial and reduction relate to changes in sediment mobility/stability and maintain non-steady-state diagenesis. The behaviour of Al in sediments is complicated by several reactions, including biological uptake, aluminosilicate formation and weathering, and complexation with organic matter (Stoffyn-Egli, 1988; Mackin & Aller, 1984a,b). In the Gulf of Papua muds, porewater Al concentrations were particularly elevated at Sta. IGP2. Possible explanations are release during Fe oxyhydroxide reduction, release during dissolution of aluminosilicates, and complexation with organic matter. Release via dissolution of mineral phases is plausible considering the high rates of Al release across the sediment– water interface (Table 2), but there is no evidence of release at depth in the sediment. One would expect Al profiles to agree with profiles of Fe and SiOH release, but this was not the case (correlation coefficients were 0·12 and 0·31 for Al with SiOH and Fe, respectively). It is possible that acidification of samples caused leaching of Al from the walls of the borosilicate glass tubes, or dissolution of colloids <0·4 ìm in size (Hydes, 1979). Absolute concentrations are not certain, but there is a realistic possibility that the high values of this study are due to complex reactions with organic matter. There was a significant inverse relationship between dissolved Al and porewater DOC [Figure 5(a), bottom]. This plot is similar to that of Al vs. Si in cores from the East China Sea (Mackin & Aller, 1984a). Mackin and Aller attributed the inverse Al-Si relationship to: (1) unrelated reactions or transport factors producing opposite trends in concentrations; or (2) equilibrium between porewater and mixed phases containing Al and Si. A similar equilibrium may exist between dissolved and particulate organic matter containing Al. Whereas the dissolved phases correlated inversely, the solid phases related positively (POC vs. Al; r=0·61; P<0·001). The vertical profiles of porewater, solid-phase Mo, and sediment–water exchange indicate geochemical and possible biological cycling of Mo in these deposits. First, dissolved Mo concentrations are low compared with average seawater concentrations possibly due to the low solid-phase Mo levels (see comparisons with shale, Table 3). Second, porewater concentrations are below the detection limits (52 nmol l "1) at Stas. IGP3 and GP8, but measured fluxes were into the sediment, implying that these sites are a sink for Mo. Several mechanisms have been proposed to explain molybdenum behaviour in anoxic sediments, including association with iron and manganese oxides (Shimmield & Price, 1986) complexation with organic matter (Brumsack & Gieskes, 1983), and remobilization/uptake during organic matter decomposition (Contreras et al., 1978; Emerson & Huested, 1991). In the Gulf of Papua porewaters, Mo correlated

Trace metals in post-oxic muds

209

(albeit weakly) with DOC [Figure 5 (b)] and inversely with rates of sulphate reduction (Figure 6). The association with DOC would explain the uptake of dissolved Mo by these sediments as DOC is rapidly taken up at these sites (Alongi, 1994). Molybdenum may be utilized by sulphate-reducing bacteria (Widdel, 1988) but it is equally likely that the association is indirect as Mo may be coprecipitated with iron sulphides during their formation via sulphate reduction. In fact, solid-phase Mo and S were significantly correlated (r=0·62; P<0·001) as were Mo and Mn (r=0·43; P<0·01) suggesting some absorption/incorporation into minerals containing these two elements. The behaviour of the other trace metals (Ni, Co, Cr, Cu, Cd, Zn, V, Pb) may be regulated by several factors such as complexation with organic matter or association with sulphide minerals (Heggie & Lewis, 1984; Westerlund et al., 1986; Shaw et al., 1990; Wallmann, 1992). Dissolved Ni related strongly to DOC indicating significant complexation with organic matter, particularly at Sta. D9. Resolution of porewater metals was low in this study due mainly to small sample size, but porewater concentrations of solutes other than Fe, Mn and Al were very low, and the general lack of mobility across the sediment–water interface suggest that solutes were primarily removed to solid-phase minerals. Free sulphides were low (1–2 ìM) in these Papuan silts, but thermodynamic calculations (Wallmann, 1992) indicate that metals such as Cd and Zn are precipitated as sulphide minerals at dissolved sulphide concentrations of 10–100 nM in sub-oxic sediments. Thus, even at very low sulphide concentrations, some metals can be immobilized into sulphide minerals. The experiments of Klinkhammer et al. (1982), Gerringa (1990), and Kerner and Wallman (1992) have shown that Zn, Cd, Pb, Cu, Ni were not mobilized under post-oxic conditions when nitrate was present. The lack of (or very low) concentrations of dissolved Co, Cr, Pb, Cu, Ni and Cd in this study would indicate immobilization to solid-phase minerals. The low abundance of clay particles may also help to explain the composition and concentrations of trace metals in this region. Dissolution of clays is an important pathway for flux of trace metals in sediments as the clay fraction contains the bulk of solid-phase metals (Chester, 1990). The extent to which sedimentary clays contribute to the bulk metal pool is dependent upon clay type as well as abundance. In the Gulf of Papua, the inner shelf sediments are classified as silts (Table 1). Clay content is low and composed mainly of smectite and kaolinite, with low illite content (Salomons & Eagle, 1990). Trace metal–clay mineral associations are not well understood (Chester, 1990), but studies from other tropical coastal areas indicate that trace metals do not show any association with smectite and kaolinite—the dominant clays in the Gulf of Papua and frequently dominant in other tropical estuarine habitats (Schorin et al., 1991; Pandarinath & Narayana, 1992). Finally, a comparison of the metal concentrations obtained in this study with those in average shale indicates that the Gulf of Papua is pristine (Table 3). This region has been the focus of controversy because of recent mining and shipping activities, both of which are expected to increase into the next century (Alongi et al., 1991; Baker & Harris, 1991). The Gulf of Papua silts are enriched with Zn and V compared with shale (Table 3), probably due to their association with the relatively high Fe and P content, but Cd and Mo levels are very low. These low concentrations may reflect the low S content of the gulf muds (Table 1) as well as the mean composition of metals in soils on the island of New Guinea. Many topsoils of New Guinea are deficient in S, Cd and Mo—particularly alluvial deposits typically used for tree crops, pastures and arable crops

210

D. M. Alongi et al.

(Bleeker, 1983). These deficiencies are typical of wet tropical soils that are highly weathered (Petr & Irion, 1983) and undoubtedly contribute to the composition and behaviour of trace metals in the Gulf of Papua. Acknowledgements We thank P. Christoffersen for field and lab assistance and the crew of the RV Lady Basten for help at sea. Liz Howlett typed the manuscript, and Mike Susic, Jack Middelburg and an anonymous referee provided helpful comments on an earlier draft. Ok Tedi Mining Ltd provided information and logistics. This project was supported by AIMS and by a grant from Ok Tedi Mining Ltd. Contribution No. 707 from the Australian Institute of Marine Science. References Aller, R. C., Mackin, J. E. & Cox, R. T. 1986 Diagenesis of Fe and S in Amazon inner shelf muds: apparent dominance of Fe reduction and implications for the genesis of ironstones. Continental Shelf Research 6, 263–289. Aller, R. C., Blair, N. E., Xia, Q. & Rude, P. D. 1994 Remineralization rates, recycling, and storage of carbon in Amazon shelf sediments. Continental Shelf Research 15, (in press). Alongi, D. M. 1994 Decomposition and recycling of organic matter in muds of the Gulf of Papua, northern Coral Sea. Continental Shelf Research 15, 1319–1337. Alongi, D. M. & Robertson, A. I. 1993 The influence of fluvial discharge in pelagic and benthic ecology and biogeochemistry of the Fly Delta and Gulf of Papua. Final Report to Ok Tedi Mining Limited, Tabubil, Papua New Guinea, 123 pp. Alongi, D. M., Tirendi, F. & Robertson, A. I. 1991 Vertical profiles of copper in sediments from the Fly Delta and Gulf of Papua (Papua New Guinea). Marine Pollution Bulletin 22, 253–255. Alongi, D. M., Christoffersen, P., Tirendi, F. & Robertson, A. I. 1992 The influence of freshwater and material export on sedimentary facies and benthic processes within the Fly Delta and adjacent Gulf of Papua (Papua New Guinea). Continental Shelf Research 12, 287–326. Alongi, D. M., Tirendi, F. & Christoffersen, P. 1993 Sedimentary profiles and sediment-water solute exchange of iron and manganese in reef- and river-dominated shelf regions of the Coral Sea. Continental Shelf Research 13, 287–305. Apte, S. C. & Gunn, A. M. 1987 Rapid determination of copper, nickel, lead and cadmium in small samples of estuarine and coastal waters by liquid/liquid extraction and electro-thermal atomic absorption spectrometry. Analytica Chimica Acta 193, 147–156. Baker, E. K. & Harris, P. T. 1991 Copper, lead and zinc distribution in the sediments of the Fly River Delta and Torres Strait. Marine Pollution Bulletin 22, 614–618. Berner, R. A. 1981 A new geochemical classification of sedimentary environments. Journal of Sedimentary Petrology 51, 359–365. Bleeker, P. 1983 Soils of Papua New Guinea. Australian National University, Canberra, 352 pp. Broecker, W. S. 1974 Chemical Oceanography. Harcourt Brace Jovanovich, New York, 214 pp. Brumsack, H. J. & Gieskes, J. M. 1983 Interstitial water trace metal chemistry of laminated sediments from the Gulf of California, Mexico. Marine Chemistry 14, 89–106. Burdige, D. J. 1993 The biogeochemistry of manganese and iron reduction in marine sediments. Earth-Science Reviews 35, 249–284. Chester, R. 1990 Marine Geochemistry. Unwin Hyman, London, 698 pp. Contreras, R., Fogg, T. R., Chasteen, N. P., Gaudette, H. E. & Lyons, W. B. 1978 Molybdenum in porewaters of anoxic marine sediments by electron para-magnetic resonance spectroscopy. Marine Chemistry 6, 365–373. Emerson, S. R. & Huested, S. S. 1991 Ocean anoxia and the concentrations of molybdenum and vanadium in seawater. Marine Chemistry 34, 177–196. Froelich, P. N., Klinkhammer, G. P., Bender, M. L., Luedtke, N. A., Heath, G. R., Cullen, D., Dauphin, P., Hammond, D., Hartman, B. & Maynard, V. 1979 Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis. Geochimica et Cosmochimica Acta 43, 1075–1091. Gerringa, L. J. A. 1990 Aerobic degradation of organic matter and the mobility of Cu, Cd, Ni, Pb, Zn, Fe and Mn in marine sediment slurries. Marine Chemistry 29, 355–374.

Trace metals in post-oxic muds

211

Harris, P. T., Baker, E. K., Cole, A. R. & Short, S. A. 1993 A preliminary study of sedimentation in the tidally dominated Fly River Delta, Gulf of Papua. Continental Shelf Research 13, 441–472. Heggie, D. & Lewis, T. 1984 Cobalt in porewater of marine sediments. Nature 311, 453–455. Hydes, D. J. 1979 Aluminium in seawater: control by inorganic processes. Science 205, 1260–1262. Kerner, M. & Wallmann, K. 1992 Remobilization events involving Cd and Zn from intertidal flat sediments in the Elbe Estuary during the tidal cycle. Estuarine Coastal Shelf Science 35, 371–393. Klinkhammer, G., Heggie, D. T. & Graham, D. W. 1982 Metal diagenesis in oxic marine sediments. Earth Planet Science Letters 61, 211–219. Loring, D. H. & Rantala, R. T. T. 1992 Manual for the geochemical analyses of marine sediments and suspended particulate matter. Earth-Science Review 32, 235–283. MacFarlane, J. W. 1980 Surface and bottom currents in the Gulf of Papua and western Coral Sea. Department of Primary Industries Research Bulletin No. 27, 128 pp. Mackin, J. E. & Aller, R. C. 1984a Dissolved Al in sediments and water of the East China Sea: implications for authigenic mineral formation. Geochimica et Cosmochimica Acta 48, 281–297. Mackin, J. E. & Aller, R. C. 1984b Diagenesis of dissolved aluminium in organic-rich estuarine sediments. Geochimica et Cosmochimica Acta 48, 299–313. Mackin, J. E. & Aller, R. C. 1986 The effects of clay mineral reactions on dissolved Al distributions in sediments and waters of the Amazon continental shelf. Continental Shelf Research 6, 245–262. Mackin, J. E., Aller, R. C. & Ullman, W. J. 1988 The effects of iron reduction and non steady-state diagenesis on iodine, ammonium and boron distributions in sediments from the Amazon continental shelf. Continental Shelf Research, 8, 363–386. Pandarinath, K. & Narayana, A. C. 1992 Clay minerals and trace metal association in the Gangolli estuarine sediments, west coast of India. Estuarine, Coastal and Shelf Science 35, 363–370. Pedersen, T. F., Vogel, J. S. & Southon, J. R. 1986 Copper and manganese in hemipelagic sediments at 21)N, East Pacific Rise: Diagenetic contrasts. Geochemica et Cosmochimica Acta 50, 2019–2031. Petr, T. & Irion, G. 1983 Geochemistry of soils and sediments of the Purari River basin. In The Purari—tropical environment of a high rainfall river basin (Petr, T., ed.). Dr W. Junk Publishers, The Hague, pp. 325–339. Robbins, J. A. & Gustinis, J. 1976 A squeezer for efficient extraction of porewater from small volumes of anoxic sediment. Limnology and Oceanography 21, 905–909. Salomons, W. & Eagle, A. M. 1990 Hydrology, sedimentology and the fate and distribution of copper in mine-related discharges in the Fly River System, Papua New Guinea. Science of the Total Environment 97/98, 315–334. Sawlan, J. J. & Murray, J. W. 1983 Trace metal remobilization in the interstitial waters of red clay and hemipelagic marine sediments. Earth Planetary Science Letters 64, 213–230. Schorin, H., de Benzo, A., La Brecque, J., Velosa, M., Rosales, P., Gomez, C. & Mejias, G. 1991 Contribution to the geochemistry and mineralogy of the sediments of the Tigre River, States of Anzoategui and Monagas, Venezuela. Acta Cientifica Venezolana 42, 247–256. Shaw, T. J., Gieskes, J. M. & Jahnke, R. A. 1990 Early diagenesis in differing depositional environments: the response to transition metals in pore water. Geochimica Cosmochimica Acta 54, 1233–1246. Shimmield, G. B. & Price, N. B. 1986 The behaviour of molybdenum and manganese during early sediment diagenesis—offshore Baja California, Mexico. Marine Chemistry 19, 261–280. Sokal, R. R. & Rohlf, F. J. 1981 Biometry. Freeman, San Francisco, 859 pp. Stoffyn-Egli, P. 1982 Dissolved aluminium in interstitial waters of recent terrigenous marine sediments from the North Atlantic Ocean. Geochimica et Cosmochimica Acta 46, 1345–1352. Stoffyn-Egli, P. 1982 Dissolved aluminium in interstitial waters of recent terrigenous marine sediments from the North Atlantic Ocean. Geochimica Cosmochimica Acta 46, 1345–1352. Sundby, B., Anderson, L. G., Hall, P. O. J., Iverfeldt, A., Rutgers van der Loeff, M. & Westerlund, S. F. G. 1986 The effect of oxygen on release and uptake of cobalt, manganese, iron and phosphate at the sediment-water interface. Geochimica Cosmochimica Acta 50, 1281–1288. Turekian, K. K. & Wedepohl, K. H. 1961 Distribution of elements in some major units of the earth’s crust. Bulletin of the Geological Society of America 72, 175–192. Wallmann, K. 1992 Solubility of cadmium and cobalt in a post-oxic or sub-oxic sediment suspension. Hydrobiologia 235/236, 611–622 Westerlund, S. F. G., Anderson, L. G., Hall, P. O. J., Iverfeldt, A., Rutgers van der Loeff, M. M. & Sundby, B. 1986 Benthic fluxes of cadmium, copper, nickel, zinc and lead in the coastal environment. Geochimica Cosmochimica Acta 50, 1289–1296. Widdel, F. 1988 Microbiology and ecology of sulfate- and sulfate-reducing bacteria. In: Biology of Anaerobic Microorganisms (Zehnder, A. J. B., ed.). Wiley & Sons, New York, pp. 469–585. Wolanski, E. & Eagle, M. 1991 Oceanography and Fine Sediment Transport, Fly River Estuary and Gulf of Papua. 10th Australasian Conference on Coastal Ocean Engineering, Auckland, pp. 453–457.