Composition and distribution of seabed and suspended sediments in north and central Torres Strait, Australia

ARTICLE IN PRESS Continental Shelf Research 28 (2008) 2174– 2187

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Composition and distribution of seabed and suspended sediments in north and central Torres Strait, Australia Andrew D. Heap , Laura Sbaffi Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia

a r t i c l e in f o

a b s t r a c t

Article history: Accepted 10 March 2008 Available online 3 April 2008

Widespread seagrass dieback in central Torres Strait, Australia has been anecdotally linked to the delivery of vast quantities of terrigenous sediments from New Guinea. The composition and distribution, and sedimentological and geochemical properties, of seabed and suspended sediments in north and central Torres Strait have been determined to investigate this issue. In northern Torres Strait, next to Saibai Island, seabed sediments comprise poorly sorted, muddy, mixed calcareous– siliciclastic sand. Seabed sediments in this region are dominated by aluminosilicate (terrigenous) phases. In central Torres Strait, next to Turnagain Island, seabed and suspended sediments comprise moderately sorted coarse to medium carbonate sand. Seabed sediments in this region are dominated by carbonate and magnesium (marine) phases. Mean Cu/Al ratios for seabed sediments next to Saibai Island are 0.01, and are similar to those found in New Guinea south coastal sediments by previous workers. Mean Cu/Al ratios for seabed sediments next to Turnagain Island are 0.02, indicating an enrichment of Cu in central Torres Strait. This enrichment comes from an exogenous biogenic source, principally from foraminifers and molluscs. We could not uniquely trace terrigenous sediments from New Guinea to Turnagain Island in central Torres Strait. If sediments are a factor in the widespread seagrass dieback in central Torres Strait, then our data suggest these are marine-derived sediments sourced from resuspension and advection from the immediate shelf areas and not terrigenous sediments dispersed from New Guinea rivers. This finding is consistent with outputs from recently developed regional hydrodynamic and sediment transport models. Crown Copyright & 2008 Published by Elsevier Ltd. All rights reserved.

Keywords: Sediment geochemistry Sediment sources Terrigenous sediment Marine sediment

1. Introduction The extensive, shallow tropical continental shelves of the western Indo-Pacific region receive up to 25% of all the terrigenous sediment delivered to the world’s oceans. Rivers on the south coast of New Guinea annually deliver 4800 MT of terrigenous sediment, or approximately 4% of the total annual global terrigenous sediment yield, to the surrounding shallow shelf, including the northern Australian margin (Milliman, 1995; Milliman et al., 1999; Walsh et al., 2004). Our understanding of sedimentation on these shallow tropical shelves is incomplete because of relatively few studies of terrigenous sediment dispersal and accumulation in the narrow passages between the major land masses. Situated between the low relief and relatively arid continent of Australia to the south and the high relief (up to 5000 m) and tropical island of New Guinea to the north, Torres Strait represents a unique opportunity to examine the

 Corresponding author.

E-mail address: [email protected] (A.D. Heap).

dispersal and accumulation of terrigenous sediments. Here, a very-shallow, narrow tropical shelf links two land masses that deliver disproportionate amounts of terrigenous sediment to the same region. In north-eastern Australia, the potential detrimental effect of increased terrigenous sediment yield from local catchments has received significant recent attention, with most work focussed on the effect on reefs (e.g., Frank and Jell, 2006; Devlin and Brodie, 2005; Wolanski et al., 2005; Fabricius and De’ath, 2004). Several widespread seagrass dieback events have been recorded in central Torres Strait (Long and Poiner, 1994; Long et al., 1997). These events are of concern to the local indigenous population, who traditionally supplement their diet with dugong and turtle, which eat the seagrasses and whose numbers have reportedly been declining. Several anecdotal hypotheses have been proposed to explain the causes of the seagrass dieback, namely: (1) increased terrigenous sediment produced from mining activities in the New Guinea catchments is accumulating on the seabed and smothering the seagrasses; (2) increased terrigenous sediment produced from mining activities in the New Guinea catchments is causing elevated turbidity throughout Torres Strait and reducing light at

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ARTICLE IN PRESS A.D. Heap, L. Sbaffi / Continental Shelf Research 28 (2008) 2174–2187

and Gulf of Carpentaria creates strong tidal currents that can attain 4 m s1 at the surface (Amin, 1978; Wolanski et al., 1988; Bode and Mason, 1994). These strong currents maintain vertical homogeneity of temperature and salinity in the water column and result in elevated turbidity throughout the year, although the nature of the material in suspension across Torres Strait is not well known (Wolanski, 1994). Rainfall and prevailing winds are profoundly seasonal. During the southern hemisphere trades (May–November) strong (45 m s1) southeast winds dominate and total rainfall is o350 mm. In contrast, during the summer monsoon (December– April) winds are slight (1–2 m s1), more variable and from the north and northwest, and total rainfall exceeds 1500 mm (Australian Bureau of Meteorology, unpublished data). Although relatively weak, wind-driven currents associated with the strongly seasonal climate are sufficient to induce reversals in bedload and the orientation and migration direction of bedforms (Harris, 1989, 1991). Tropical cyclones are relatively infrequent in Torres Strait with a return period of approximately 10 years (Lourensz, 1981). Numerous rivers drain the southern New Guinea margin. The Fly River, in Papua New Guinea (PNG), is the largest single sediment source with an annual terrigenous sediment load estimated to be 4120 MT (Harris et al., 1993) and a site for the NSF-funded MARGINS source-to-sink continental margin study (e.g., Dickens et al., 2006 and references therein). Despite the strongly seasonal rainfall, the rate at which the sediment is delivered to the south coast of New Guinea is relatively invariant throughout the year (Walsh et al., 2004; Wolanski et al., 1995). Sand is principally trapped at the coast and the mud fraction is deposited in coastal deltas and on the shelf (cf., Harris et al., 1993; Brunskill et al., 2004; Walsh et al., 2004). Regional hydrodynamic modelling studies (Hemer et al., 2004; Keen et al., 2006) indicate

the bed to levels unsuitable for seagrass survival and (3) changed regional circulation has caused widespread bedform migration that has smothered the seagrasses. In this paper, we investigate the possible role of sediments as a factor in widespread seagrass dieback in central and northern Torres Strait. We determine the major components of the seabed and suspended sediments, and their variability over time and space, based on their sedimentological and geochemical properties. We then offer an explanation of the sediment sources (terrestrial or marine) based on these properties and existing regional sedimentation models.

2. Regional setting 2.1. Background Torres Strait is a complex shelf region containing a discontinuous chain of granitic islands and mobile bedforms in the west, high islands and fringing coral reefs in the centre, and isolated volcanic islands and well-developed coral reefs in the east (Fig. 1). Water depths in Torres Strait are o20 m, and for most of the last 150,000 years the strait formed a land bridge that connected Australia to the island of New Guinea. Presently, Torres Strait is a shallow oceanographic boundary between the Coral Sea/Gulf of Papua to the east and the Arafura Sea/Gulf of Carpentaria to the west. The numerous reefs and islands form a semi-impermeable barrier to tides such that only 30% of the tidal wave from the east propagates fully through Torres Strait to the west (Wolanski, 1986). The combination of narrow channels between the islands and reefs and the dissimilar tidal regimes between the Coral Sea

138°

140°

8°

142°

New

2175

Turama R. Bamu R.

Guinea Fly R.

146°

Purari R.

Gulf of Papua 00

50

0

800 MT yr-1

144°

1500

Sea

lf B

re

ak

10

Fly River delta

Arafura

10°

2000

Sh e

Figure 2

Torres Strait

km 0

100

INDONESIA

Coral Sea

PN PNG Gulf of Carpentaria

2000 2500 3000

AUSTRALIA

3500

AUSTRALIA N Reef 0

1000 km

Fig. 1. Map showing location of Torres Strait. Torres Strait is a shallow (o20 m) narrow seaway separating Australia and New Guinea, and represents a sediment-mixing zone that predominantly receives terrigenous sediment from the north. Approximately 800 MT of terrigenous sediment is delivered to the south coast of New Guinea each year (Milliman, 1995). Terrigenous inputs from Australia are negligible by comparison. The strait is characterised by complex morphology, containing granitic islands, coral reefs and bed sediments fashioned into bedforms by predominant strong E–W tidal currents (dashed arrows). The present extent of the Fly River delta is also shown. Shelf break is about 100 m. Water depths are in metres.

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(Long et al., 1997). Shelf regions near Saibai Island, located 5 km from the south coast of PNG, were occupied because they are a probable area of terrigenous sediment accumulation (Harris and Baker, 1991). Surveys were conducted in April 2004 at the end of the monsoon season (Geoscience Australia survey 266; Heap et al., 2005) and in October 2004 at the end of the trade wind season (Geoscience Australia survey 273; Daniell et al., 2006). Samples collected during the surveys were considered to reflect the cumulative effects of each season on the sediments of central and northern Torres Strait. All samples and assays can be found in Geoscience Australia’s national marine samples database (www.ga.gov.au/oracle/mars).

that up to 11% of the total annual mud load from the Fly River may be transported into Torres Strait during the trade wind season. Terrigenous sediment inputs from the northern margin of Australia are negligible by comparison due to the absence of large river systems and prevalence of small, low relief catchments. River-borne terrigenous sediments eroded from New Guinea catchments are dominated by aluminosilicate clays (montmorillonite, kaolinite, chlorite and illite) and quartz, biotite, and feldspar silts and sands, with negligible carbonate minerals (Irion and Petr, 1983; Harris and Baker, 1991). Elemental analyses indicate that terrigenous sediments can be characterised by direct correlation between their terrestrial elements (e.g., Al, Fe, Ti) and Si (e.g., Brunskill et al., 1995; Haynes and Kwan, 2002). Terrigenous sediments are enriched with heavy metals, including Cu and As from mining activities in the local catchments (Brunskill et al., 2004), and elevated concentrations of these metals have been reported in local seafood (Gladstone, 1996). Seabed sediments in Torres Strait are generally partitioned into poorly sorted, terrigenous-dominated muddy sands in the north and poorly sorted, calcareous muddy gravelly sands in the centre and south (Harris 1988; Woodroffe et al., 2000). Calcareous sediments in the centre and south of the strait have been fashioned into numerous well-developed and mobile bedforms (e.g., Harris 1988; Daniell et al., 2006). Seagrass beds occur throughout Torres Strait with the largest stands between sandy bedforms in central and western regions (Bridges et al., 1982).

3. Methods 3.1. Sediment texture and composition A total of 27 seabed samples collected using a van Veen grab were analysed at Geoscience Australia’s sedimentology laboratory to determine the texture and composition (Fig. 2). The bulk composition of the o2 mm fraction was described by examining 100 randomly picked grains under a binocular microscope. Approximately 10–20 g of bulk sediment was sieved through 2 mm and 63 mm meshes with distilled water to separate the mud, sand and gravel fractions. The mud fraction was also centrifuged at 3500 rpm for 10 min to aid the separation of water. All fractions were oven dried at 40 1C for 24 h, allowed to cool to room temperature and weighed to 70.01 g. Grain size distributions were also determined on the o2 mm fraction using a Malvern Mastersizer 2000 laser particle size analyser. Repeat analyses indicate that this procedure produces grain size modes to within 70.05 mm. Calcium carbonate concentrations were determined on the bulk sample, sand and mud fractions using the ‘‘carbonate bomb’’ method of Muller and Gastner (1971).

2.2. Study area This study area is situated next to Turnagain Island, 50 km south of PNG, in central Torres Strait and extends across to the northern margin of Saibai Island in northern Torres Strait (Fig. 2). The shelf region next to Turnagain Island was occupied because it corresponded to the area of most significant seagrass dieback

km 0

24-hr Station

142°30’

2

142°45’

9°15’

Turnagain Is.

PAPUA NEW GUINEA

24-hr Station

Fly R. delta

10

(Vol. %)

273/11GR11

%

Saibai Is. 10

0 0.01

(Vol.%)

90%

80

Carbonate fraction

10000 (µm)

Terrigenous fraction

0 0.01

(µm)

Carbonate fraction

10000

9°30’ km 0

1

273/08GR08

0

km

10

Saibai Is.

Fig. 2. Map showing the study area, which extends from Turnagain Island in north-central Torres Strait to the north coast of Saibai Island located o5 km from the island of New Guinea. Locations of the seabed sediment samples and 24-h stations (suspended sediments) are also shown. Seabed sediments in the vicinity of Turnagain Island are characterised by unimodal, moderately sorted carbonate sands. Seabed sediments in the vicinity of Saibai Island are characterised by bimodal mixed calcareous/siliciclastic muddy sands. The fine fraction is dominated by terrigenous grains and the coarse fraction is dominated by carbonate grains.

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A total of 139 water samples collected over a 24 h period at two stations near Turnagain Island were analysed in order to investigate temporal variations in the texture and composition of near-bed suspended sediments (Fig. 2). Samples were collected within 0.5 m of the bed every hour using a Niskin bottle. Both stations were occupied twice, once during a neap tide and again during a spring tide. Near-bed suspended sediment was obtained by passing 1 l of water through a 0.45 mm mesh cellulose-acetate filter, which was then oven dried at 60 1C. 3.2. Sediment elemental geochemistry The elemental composition of the bed and suspended sediments was determined by energy dispersive X-ray (EDX) using a JEOL 6400 scanning electron microscope (SEM) equipped with an Oxford Instruments light element EDS detector. Samples were mounted on metal stubs and coated with a thin film of carbon in a glass vacuum vessel using a DYNAVAC CS300 Coating Unit. The EDX analyses were then carried out at 15 kV and 1 nA and the elemental compositions determined using the Link ISIS SEMquant software and a Cameca SX100 electron probe microanalyser (Brink et al., 2004). Atomic number, absorption and fluorescence (ZAF) corrections were minimised using the stoichiometric method (Reed, 1996). The distribution and abundance of select elements in the mud and sand fractions of a seabed sample collected from next to Turnagain (273/08GR08) and Saibai (273/ 11GR11) Islands were obtained from the EDX analysis using a scanning X-ray analytical microscope (SXAM) attached to the SEM. The distribution and abundance of each element were determined from pixel intensities across the map as compared to a known standard for that element. SXAM maps for the mud and sand fractions were obtained using magnifications of  120 and  50, respectively.

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4. Results 4.1. Sediment composition and texture Seabed sediments next to Turnagain Island comprise moderately sorted, coarse to medium carbonate sand. On average, the sediments are principally composed of carbonate sand and gravel (440%), with mud comprising o10% (Table 1). Grain size distributions are unimodal with the mean grain size, occurring prominently in the sand fraction and ranging from 96 to 886 mm (Fig. 2). Microscope observations show that the dominant constituents include: benthic foraminifera, molluscs, corals and pteropods, as well as numerous smaller fragments that could not be identified. Fusulinida and Amphistegina spp. together comprise almost 50% of the total foraminiferal assemblage, followed by Textularia (20%) and Miliolida (12–15%). Most specimens and fragments are relict or present different degrees of abrasion and boring (cf., Post et al., 2007); however, the Miliolida group (mostly Quinqueloculina spp.) is generally well preserved. Seabed sediments southwest and north of Saibai Island comprise poorly sorted, muddy medium calcareous sand. On average, the sediments are principally composed of sand and gravel (440%), with a sizeable mud fraction (o35%; Table 1). Grain size distributions are bimodal, containing modes in both the mud and sand fractions (Fig. 2). On average, quartz sand and silt comprise 70–75% of the bulk sediment. The dominant biogenic constituents include: benthic foraminifera, molluscs and pteropods, as well as numerous smaller unidentified biogenic fragments. The dominant foraminiferal groups are Miliolida (35%), Textularia (20%), Elphidium spp. (12–15%) and Cibicides spp. (o10%). Biogenic grains are lightly to moderately abraded, and are generally better preserved than those observed in bed sediments at Turnagain Island.

Table 1 Texture and composition of seabed sediments in Torres Strait Sample

Latitude

Turnagain Island 266/05GR04 266/07GR06 266/08GR07 266/09GR08 266/10GR09 266/11GR10 266/12GR11 266/24GR20 266/26GR22 266/31GR27 266/34GR30 266/40GR36 273/01GR01 273/02GR02 273/03GR03 273/04GR04 273/06GR06 273/07GR07 273/08GR08 273/09GR09

91 91 91 91 91 91 91 91 91 91 91 91 91 91 91 91 91 91 91 91

Saibai Island 273/10GR10 273/11GR11 273/12GR12 273/13GR13 273/17GR17 273/18GR18 273/23GR23

91 91 91 91 91 91 91

a

Longintude

Gravel (%)

Sand (%)

35.508 35.502 33.984 32.292 32.298 34.572 35.394 35.604 35.472 34.020 34.986 34.308 47.796 34.188 35.610 35.670 35.406 35.508 33.546 30.000

1421 19.754 1421 21.956 1421 20.411 1421 20.032 1421 19.184 1421 15.261 1421 15.497 1421 21.090 1421 21.258 1421 13.705 1421 13.556 1421 13.621 1421 26.783 1421 13.753 1421 21.699 1421 21.423 1421 21.203 1421 21.433 1421 23.708 1421 28.722

94.66 16.27 51.45 32.72 61.58 62.14 71.59 64.39 55.24 58.77 21.18 37.18 36.19 23.12 58.84 18.38 53.79 38.66 43.62 34.81

4.94 81.54 45.49 65.48 38.29 37.23 28.21 35.59 44.02 40.91 75.61 61.76 56.96 67.90 40.91 81.42 43.86 61.20 55.91 63.87

25.050 22.206 20.832 21.198 21.756 21.714 21.690

1421 1421 1421 1421 1421 1421 1421

37.34 12.75 42.83 14.41 6.40 2.87 0.47

42.35 40.64 34.68 52.35 63.70 53.99 43.82

34.589 34.872 37.678 39.166 39.152 39.196 39.236

Carbonate concentration calculated by acid digestion.

CaCO3a (%)

Mean grain size (mm)

Std dev. (mm)

0.41 2.19 3.06 1.80 0.13 0.64 0.20 0.01 0.74 0.31 3.21 1.06 6.85 8.98 0.25 0.19 2.35 0.14 0.48 1.32

80.1 88.2 81.1 87.2 88.2 87.2 85.7 88.2 87.2 89.7 80.1 88.2 78.1 85.2 93.3 79.1 86.7 79.6 59.8 78.1

96.2 430.2 628.8 515.5 747.6 634.8 706.2 886.2 486.1 733.6 453.6 394.7 219.3 274.8 614.7 531.3 328.7 566.3 435.2 443.7

313.4 370.4 422.6 357.7 391.9 382.3 419.7 353.5 328.3 388.6 335.3 254.3 296.2 248.5 478.4 313.2 327.5 294.7 310.7 331.4

20.31 46.61 22.49 33.25 29.90 43.15 55.71

45.6 28.8 38.5 39.0 26.8 29.3 22.2

102.8 63.4 113.2 69.7 37.4 18.6 28.3

319.2 130.8 231.7 222.3 115.3 149.8 47.4

Mud (%)

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4.2. Sediment geochemistry

Comparison of the grain size distributions for sediment in the vicinity of Turngain and Saibai Islands indicates that the mud fraction (o63 mm) is dominantly siliciclastic grains and the sand fraction (63 mm–2 mm) is primarily carbonate grains (Fig. 2).

30

4.2.1. Seabed sediments Seabed sediments around Turnagain Island are composed principally (438%) of phases containing Ca and Mg, although Si,

6

r2 = 0.98 Gulf of Papua (Fly R. delta)

25

5 4 Mg (%)

Si (%)

20 15 10

3 2

Torres Strait

1

5

0

0 0

10

20

30 Ca (%)

12

40

50

0

Gulf of Papua (Fly R. delta)

20 30 Ca (%)

New Guinea south coast

Fe (%)

Torres Strait

4

50

Gulf of Papua (Fly R. delta)

5

6

40

r2 = 0.73

6

New Guinea south coast

8

10

7

r2 = 0.66

10

Al (%)

r2 = 0.77

4 Torres Strait

3 2

2

1

0

0 0

5

10

15 Si (%)

100

20

25

30

0

5

10

15 Si (%)

3500

r2 = 0.94

20

25

30

r2 = 0.71

90

3000 80

2500

60 50

Sr (ppm)

CaCO3 (%)

70

Sabai Is. Sand fraction Mud fraction Turnagain Is. Sand fraction Mud fraction

40 30 20

10

20 30 Ca (%)

40

1500

500 Gulf of Papua (Fly R. delta)

0 0

2000

1000

Nth Gulf of Carpentaria Bulk sediment Torres St-Gulf of Papua Bulk sediment

10

Torres Strait

50

0 0

10

20

30 Ca (%)

40

50

Fig. 3. Graphs showing the relationships between major element concentrations of the seabed sediments. The elemental concentrations of the seabed sediments collected for this study are shown with concentrations determined for sediments on the south coast of New Guinea (Brunskill et al., 2004), Torres Strait-Gulf of Papua (Haynes and Kwan, 2002) and northern Gulf of Carpentaria (Cox and Preda, 2003; Preda and Cox, 2005). For samples collected on the New Guinea south coast, dotted lines represent maximum, mean and minimum concentrations and the shaded area represents one standard deviation from the mean. Samples collected from the Fly River delta are separated out from samples collected from Torres Strait; r2 values refer to samples collected in this study.

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Mud Fraction

Sand Fraction 273/11GR11

273/08GR08

273/08GR08

273/11GR11

Bulk

100 µm

100 µm

150 µm

150 µm

Ca

Si

Fe

Al

Turnagain Island

Saibai Island

Turnagain Island

Saibai Island

Fig. 4. SEM photographs of: (a) the sand and (b) the mud fraction of seabed sediments collected next to Turnagain (273/08GR08) and Saibai (273/11GR11) Islands. Bulk sediment is shown in the top panels. Light regions represent grains of each element. The patterns indicate that the bed sediments next to Turnagain Island contain much higher concentrations of marine phases (Ca) compared to terrigenous phases (Si, Fe, Al). The opposite pattern is shown for seabed sediments next to Saibai Island, which show much higher concentrations of Si and Al.

Al and Fe occur in relatively small quantities (o10%; Figs. 3–5; Table 2). Concentrations of Ca are between 27.00% and 40.11% with a mean of 34.72%, and concentrations of Mg are between 2.67 and 5.08% with a mean of 3.45%. Cu and Ti occur in the smallest quantities, with concentrations of between 0.07% and 0.79% (mean ¼ 0.16%) and 0% and 0.10% (mean ¼ 0.06%), respectively. Seabed sediments around Saibai Island are composed principally (435%) of phases containing Si, Ca, Al and Fe, with relatively minor quantities of Mg and K (o3%; Figs. 3–5; Table 3). Concentrations of Si are between 18.21% and 23.06% with a mean of 20.38%, and concentrations of Ca are between 4.94% and 11.27% with a mean of 7.98%. Al concentrations are between 3.77% and 5.76% with a mean of 4.60%, and concentrations of Fe are between 2.23% and 3.85% with a mean of 2.80%. Ti and Cu occur in the smallest quantities, with concentrations of between 0.13% and 0.22% (mean ¼ 0.17%) and 0.04% and 0.12% (mean ¼ 0.08%), respectively. Ca and Mg, which are mineral phases dominant in marine sediments, are positively correlated (r2 ¼ 0.77), but are both

negatively correlated with Si, Al and Fe, which are phases dominant in terrigenous sediments (Figs. 3a–d). Positive correlations between Ca and Mg, and Si and Al (r2 ¼ 0.66) for all samples indicate that most of the Ca is hosted in carbonate and that most of the Si is hosted in aluminosilicate phases, with smaller amounts of quartz and biogenic opal. The major element chemistry of the seabed sediments around Turnagain and Saibai Islands define mixing trends between the compositions of calcium carbonate and common aluminosilicate clay minerals (Figs. 3a–d), consistent with the sedimentology observations. The elemental concentrations and mixing trends of the mineral phases are similar to those observed for mixed carbonate–terrigenous sediment found elsewhere in Torres Strait and from the south coast of New Guinea, including the Fly River delta in the Gulf of Papua (Haynes and Kwan, 2002; Brunskill et al., 2004), and northern Gulf of Carpentaria (Cox and Preda, 2003; Figs. 3c, d). Essentially, sediment in Torres Strait is composed of variable proportions of marine carbonates (CaCO3) and terrigenous aluminosilicate clays, and minor amounts of quartz. The spatial distributions of the

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elemental concentrations reveal that the marine carbonates are dominant near Turnagain Island and terrigenous clays (similar in composition to those from New Guinea) and quartz are dominant near Saibai Island, with a zone of mixing in between (Figs. 5a, b). There is a strong positive correlation (r2 ¼ 0.94) between Ca content determined by EDX and CaCO3 content determined by acid digestion (Fig. 3e). Because sediments in Torres Strait are composed of mostly CaCO3 and aluminosilicate clays, simple acid digestion determines the abundance of both terrigenous sediment

9°15’

142°15’

Ca (%) <10 10-20 20-30 30-40 >40

142°30’

and CaCO3. Carbonate comprises between 59.8% and 93.3% (mean ¼ 83.5%), and 22.2% and 45.6% (32.9%) of the sediment around Turnagain and Saibai Islands, respectively (Table 1). In the only other regional study of the bed sediments in central Torres Strait, Harris and Baker (1991) found that carbonate concentrations increased with distance from PNG and attained 480% near Saibai Island and 490% near Turnagain Island (Fig. 2). Although our carbonate concentrations at Saibai Island are much lower (o45%), concentrations near Turnagain Island are similar to those found by Harris and Baker (1991).

142°45’

PNG Fly R. delta Saibai Is.

Si (%) <5 5-10 10-15 15-20 >20

Saibai Is.

9°30’ Turnagain Is.

Mg <1.0 1.0-2.0 2.0-3.0 3.0-4.0 >4.0

Al (%) <2.0 2.0-3.0 3.0-4.0 4.0-5.0 >5.0

Fe (%) <1.0 1.0-1.5 1.5-2.0 2.0-2.5 >2.5

Ti (%) <0.05 0.05-0.10 0.10-0.15 0.15-0.20 >0.20

Cu (%) <0.05 0.05-0.10 0.10-0.15 0.15-0.20 >0.20

Sr (ppm) <1000 1000-1400 1400-1800 1800-2200 >2200

Fig. 5. Maps showing the elemental concentrations in seabed sediments in Torres Strait for: (a) the sand and (b) mud fraction. Marine sediment phases (Ca, Mg, Sr) occur in greater concentrations near Turnagain Island and terrigenous sediment phases (Si, Al, Fe, Ti) occur in greater concentrations near Saibai Island. Cu concentrations show enrichment near Turnagain Island. Despite the vast quantities of terrigenous sediment being delivered by the rivers on the south coast of New Guinea, the dispersal of terrigenous sediments in Torres Strait appears to be restricted to within 5–10 km of the coast.

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9°15’

142°15’

Ca (%) <10 10-20 20-30 30-40 >40

2181

142°45’

142°30’

PNG FlyR. delta Saibai Is.

Si (%) <10 10-12 12-14 14-16 >16

Saibai Is.

9°30’

Turnagain Is.

Mg (%) <1.4 1.4-1.8 1.8-2.2 2.2-2.4 >2.4

Al (%) <3.0 3.0-4.5 4.5-6.0 6.0-7.5 >7.5

Fe (%)

Ti (%) <0.1 0.1-0.2 0.2-0.3 0.3-0.4 >0.4

<1.5 1.5-2.0 2.0-2.5 2.5-3.0 >3.0

Cu (%) <0.1 0.1-0.2 0.2-0.3 0.3-0.4 >0.4

Sr (ppm) <500 500-1000 1000-1500 1500-2000 >2000

Fig. 5. (Continued)

A strong positive correlation between Ca and Sr (r2 ¼ 0.71; Fig. 3f) permits the abundance of different carbonate phases in the bed sediments to be determined because corals and algae precipitate Sr-rich aragonite whereas molluscs and foraminifera precipitate Sr-poor calcite (Milliman, 1974). Strontium concentrations range between 1826 and 3134 ppm (Fig. 3f; Tables 2 and 3), which suggests sediment with clay and multiple carbonate components (e.g., Dunbar et al., 2000). Assuming Sr concentrations of 8000 ppm for coral and algae, 2500 ppm for molluscs and foraminifera, and 200 ppm for terrigenous clays in Torres Strait (Milliman, 1974), the percentages of aragonite (%A) and calcite

(%C) can be estimated from bulk carbonate and Sr concentrations as follows: Srbulk ¼ Srclay ð100  %CaCOn3 Þ þ Sraragonite ð%AÞ þ Srcalcite ð%CÞ,

(1)

and %CaCOn3 ¼ %A þ %C,

(2)

where Srbulk, Srclay, Sraragonite and Srcalcite are the measured and assumed Sr concentrations of appropriate phases. Strontium

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Table 2 Element and metal concentrations for seabed sediments in the vicinity of Turnagain Island Sample

Ca (%)

Sand fraction (0.63 mm– 2 mm) 266/05GR04 38.50 266/07GR06 33.49 266/08GR07 33.39 266/09GR08 33.81 266/10GR09 36.06 266/11GR10 33.05 266/12GR11 27.00 266/24GR20 36.67 266/26GR22 35.97 266/31GR27 38.91 266/34GR30 33.42 266/40GR36 38.98 273/01GR01 34.15 273/02GR02 32.72 273/03GR03 36.77 273/04GR04 40.11 273/06GR06 33.71 273/07GR07 39.81 273/08GR08 28.63 273/09GR09 29.25 Mud fraction (o0.63 mm) 266/05GR04 21.81 266/07GR06 28.63 266/08GR07 27.68 266/09GR08 26.60 266/10GR09 22.57 266/11GR10 27.19 266/12GR11 25.82 266/24GR20 25.69 266/26GR22 26.54 266/31GR27 28.60 266/34GR30 27.24 266/40GR36 26.36 273/01GR01 26.73 273/02GR02 23.21 273/03GR03 24.17 273/04GR04 26.21 273/06GR06 22.26 273/07GR07 27.02 273/08GR08 22.08 273/09GR09 16.67 a b

Si (%)

Sr (ppm)

CaCO3a (%)

CaCO3b (%)

1826 – 1911 2056 2226 1983 1985 2457

87.2 91.2 89.2 90.2 90.2 90.2 89.2 91.3 90.2 93.3 91.3 92.3 82.1 94.3 94.3 86.2 93.3 82.6 77.0 80.1

96.1 83.6 83.4 84.4 90.0 82.5 67.4 91.6 89.8 97.2 83.4 97.3 85.3 81.7 91.8 100.0 84.2 99.4 71.5 73.0

– 0.17 0.19 – – – – – – – 0.22 – – – 0.36 0.45 – – 0.07 0.18

3134 – – – – – – 2030

– – – – – – – – – – – – 76.0 65.9 – – 69.9 – – –

54.5 71.5 69.1 66.4 56.4 67.9 64.5 64.1 66.3 71.4 68.0 65.8 66.7 58.0 60.4 65.4 55.6 67.5 55.1 45.6

Al (%)

Fe (%)

Mg (%)

Ti (%)

K (%)

Cu (%)

3.21 5.07 5.24 5.28 3.02 5.32 8.17 2.55 4.38 2.55 5.24 3.27 5.23 5.69 2.25 1.74 5.27 2.39 5.10 6.79

1.77 2.54 2.47 2.06 1.75 2.50 2.11 2.78 2.18 1.33 2.78 1.08 2.62 2.39 2.48 1.07 2.57 1.18 6.24 4.05

0.56 0.79 0.75 0.97 0.78 0.77 3.87 0.63 0.61 0.43 0.81 0.46 1.06 0.93 0.71 0.64 0.97 0.42 1.64 1.50

2.67 3.94 3.67 3.59 5.08 3.58 2.85 4.13 3.00 4.01 3.35 3.01 2.68 3.13 3.70 4.40 3.14 3.14 2.82 3.04

0.09 – 0.05 0.05 0.05 0.07 0.10 0.04 0.09 0.08 0.07 0.07 0.00 0.08 0.05 0.02 0.05 0.03 0.04 0.07

0.22 0.34 0.34 0.34 0.26 0.32 0.24 0.17 0.29 0.25 0.34 0.18 0.34 0.35 0.20 0.13 0.32 0.20 0.32 0.38

0.17 – 0.09 0.12 0.14 0.17 0.07 0.08 0.08 0.07 0.11 0.09 0.17 0.21 0.79 0.18 0.11 0.12 0.27 0.07

11.60 8.60 9.59 9.83 11.36 9.24 10.15 8.62 9.29 8.20 8.97 9.63 9.28 10.82 8.98 8.19 10.84 8.13 10.39 13.68

4.49 3.12 2.84 3.36 3.74 3.27 3.31 3.18 3.71 2.96 3.25 3.45 3.64 4.02 2.62 3.40 4.50 2.79 3.96 5.26

1.70 1.04 1.15 1.18 1.45 1.22 1.28 1.59 1.32 1.42 1.21 1.49 1.58 1.54 1.83 1.57 1.87 1.46 1.68 1.94

1.85 2.29 1.95 2.10 2.41 2.16 2.06 2.34 2.24 2.37 2.53 2.33 2.32 2.28 2.40 2.96 2.24 2.37 2.28 2.06

0.14 0.10 0.08 0.10 0.11 0.12 0.06 0.11 0.08 0.10 0.13 0.09 0.10 0.12 0.08 0.10 0.14 0.05 0.13 0.17

0.67 0.48 0.46 0.52 0.63 0.53 0.52 0.60 0.46 0.45 0.45 0.53 0.51 0.62 0.48 0.49 0.67 0.50 0.62 0.88

Carbonate concentration calculated by acid digestion. Carbonate concentration calculated from elemental chemistry (Ca).

Table 3 Element and metal concentrations for seabed sediments in the vicinity of Saibai Island Sample

Ca (%)

Sand fraction (0.63 mm– 2 mm) 273/10GR10 8.87 273/11GR11 7.17 273/12GR12 11.27 273/13GR13 7.32 273/17GR17 4.94 273/18GR18 7.77 273/23GR23 8.52 Mud fraction (o0.63 mm) 273/10GR10 4.59 273/11GR11 4.69 273/12GR12 3.69 273/13GR13 5.00 273/17GR17 5.48 273/18GR18 5.77 273/23GR23 4.83 a b

Si (%)

Al (%)

Fe (%)

Mg (%)

Ti (%)

K (%)

Cu (%)

Sr (ppm)

CaCO3a (%)

CaCO3b (%)

19.95 19.57 18.21 19.36 23.06 21.30 21.24

4.62 5.76 4.30 5.26 4.38 4.13 3.77

2.41 3.16 2.74 3.85 2.84 2.39 2.23

1.54 1.64 1.48 1.75 1.56 1.42 1.39

0.16 0.17 0.16 0.19 0.22 0.17 0.13

0.64 0.83 0.65 0.70 0.54 0.58 0.60

– 0.09 0.12 0.04 – 0.07 0.09

1116.0 883.7 945.0 1009.0 708.0 795.1 822.5

46.1 29.3 39.0 42.5 31.4 27.3 24.3

22.1 17.9 28.1 18.3 12.3 19.4 21.3

18.71 19.19 19.16 19.00 19.21 19.60 19.78

8.05 7.83 8.58 7.91 7.43 6.95 7.13

3.64 3.19 3.01 3.00 2.97 2.92 2.87

1.54 1.36 1.30 1.97 1.31 1.22 1.16

0.31 0.33 0.40 0.34 0.31 0.32 0.29

1.31 1.33 1.33 1.30 1.22 1.18 1.26

– 0.12 0.19 – 0.13 0.11 0.20

609.5 941.8 622.6 773.2 863.7 – 798.7

13.1 19.2 15.1 18.2 18.2 19.2 19.2

11.5 11.7 9.2 12.5 13.7 – 12.1

Carbonate concentration calculated by acid digestion. Carbonate concentration calculated from elemental chemistry (Ca).

ARTICLE IN PRESS A.D. Heap, L. Sbaffi / Continental Shelf Research 28 (2008) 2174–2187

concentrations indicate that biogenic sources of Ca are mostly from foraminifers and molluscs, which dominate the sediment at the two sites. Reefal sources (Sr-poor aragonite) contribute o16% of the Ca contributions to Turnagain Island, although the concentrations are relatively uniform across the study area (Table 6). This pattern probably reflects the contributions from fringing reefs around the islands. Low non-reefal and reefal carbonate concentrations in the mud fraction from the Saibai Island samples indicate that the fine fraction is dominated by terrigenous grains. Overall, the Sr concentrations reinforce our observations from the binocular microscope.

4.2.2. Suspended sediments Bulk sediments in suspension next to Turngain Island are principally (410%) composed of phases containing Ca and Si, with

2183

Al, Mg and Fe present in smaller amounts (o5%; Tables 4 and 5). Elemental concentrations reveal that there is no significant difference in the composition of the suspended sediments at each of the sites, between spring and neap tides, and different times of the year (Fig. 6). The composition of the suspended sediments is very similar to those on the seabed recorded in our study, and elsewhere in Torres Strait (Haynes and Kwan, 2002) and northern Gulf of Carpentaria (Preda and Cox, 2005). Interestingly, the Turnagain Island samples show concentrations of Ca that are relatively higher and Si that are relatively lower in the seabed sediments than the suspended sediments. This pattern reflects resuspension of the fine fraction, which is composed principally of terrigenous grains. Elemental concentrations reveal that the suspended sediments in the vicinity of Turnagain Island are similar in composition to those of the local seabed (cf., Harris 1988; Harris and Baker, 1991).

Table 4 Elemental concentrations for suspended sediments observed on the Monsoon Survey (Geoscience Australia Survey 266) Date

Time

Filter

Ca (%)

Si (%)

Al (%)

Fe (%)

Mg (%)

Ti (%)

K (%)

Date

SW Turnagain Island-Spring 02/04/2004 11:00 6 12:00 9 13:00 11 14:00 14 15:00 17 16:00 20 17:00 23 18:00 – 19:00 26 20:00 29 21:00 32 22:00 35 23:00 38 03/04/2004 00:00 41 01:00 44 02:00 47 03:00 50 04:00 53 05:00 56 06:00 59 07:00 62 08:00 65 09:00 68 10:00 71 11:00 74

20.13 20.62 22.74 18.27 20.39 19.44 21.64 – 22.00 23.23 21.74 20.30 20.53 21.23 21.46 22.29 21.23 20.31 21.49 22.56 22.34 22.38 21.63 22.54 18.50

10.81 10.62 10.49 10.43 10.85 10.05 11.71 – 11.43 11.48 12.52 10.61 11.10 11.23 10.59 11.13 10.24 11.11 12.76 12.03 11.69 11.84 12.52 12.10 11.54

3.36 3.42 3.53 3.64 3.71 3.65 4.19 – 4.08 3.81 3.81 3.42 3.59 3.74 3.41 3.56 3.61 3.92 3.78 3.75 3.76 3.91 3.84 3.78 3.98

1.52 1.24 1.35 1.34 1.45 1.28 1.48 – 1.45 1.40 1.37 1.35 1.37 1.31 1.24 1.39 1.37 1.31 1.33 1.38 1.43 1.43 1.49 1.28 1.26

2.17 2.34 2.20 2.51 2.25 2.59 2.16 – 2.24 2.13 2.06 2.11 2.18 2.21 2.28 2.10 2.27 2.39 2.07 2.12 2.10 1.99 2.04 2.11 2.20

0.08 0.13 0.09 0.11 0.46 0.12 0.16 – 0.17 0.10 0.12 0.09 0.10 0.13 0.11 0.13 0.13 0.08 0.10 0.12 0.10 0.13 0.14 0.08 0.09

0.63 0.66 0.65 0.69 0.67 0.68 0.65 – 0.62 0.54 0.54 0.68 0.63 0.77 0.61 0.66 0.65 0.69 0.54 0.57 0.57 0.54 0.55 0.53 0.75

SE Turnagain Island-Spring 03/04/2004 16:00 79 17:00 82 18:00 85 19:00 88 20:00 91 21:00 95 22:00 98 23:00 100 04/04/2004 00:00 – 01:00 104 02:00 107 03:00 110 04:00 113 05:00 116 06:00 119 07:00 123 08:00 126 09:00 129 10:00 132 11:00 135 12:00 138 13:00 141 14:00 144 15:00 147 16:00 150

20.50 23.00 22.74 23.51 24.91 24.98 25.63 20.37 – – 22.93 19.21 23.39 22.69 22.55 21.82 25.85 22.61 21.37 19.27 21.65 21.48 20.52 20.84 23.89

9.10 11.62 11.75 10.89 10.45 10.32 10.14 10.37 – – 9.43 10.66 11.71 11.95 12.14 11.76 10.22 11.53 11.96 10.43 10.77 10.09 9.60 10.74 11.14

3.28 3.77 3.41 3.57 3.68 3.05 3.14 3.60 – – 3.06 3.49 3.49 3.64 3.60 3.36 3.36 4.02 4.42 3.04 3.28 3.45 3.24 3.58 3.58

1.17 1.41 1.29 1.35 1.32 1.35 1.27 1.19 – – 1.15 1.23 1.25 1.23 1.24 1.18 1.18 1.43 1.25 1.15 1.21 1.20 1.20 1.29 1.32

2.32 2.13 2.08 2.09 2.13 2.02 2.09 2.18 – – 2.09 2.38 2.10 2.19 2.16 2.12 2.24 2.17 2.11 2.27 2.26 2.47 2.45 2.24 2.13

0.11 0.11 0.13 0.08 0.09 0.13 0.08 0.11 – – 0.10 0.10 0.13 0.10 0.15 0.11 0.11 0.14 0.11 0.08 0.10 0.13 0.07 0.14 0.10

0.60 0.56 0.57 0.60 0.56 0.53 0.55 0.62 – – 0.56 0.64 0.47 0.54 0.49 0.49 0.44 0.58 0.73 0.67 0.60 0.61 0.65 0.64 0.54

Time

Filter

Ca (%)

Si (%)

Al (%)

Fe (%)

Mg (%)

Ti (%)

K (%)

SW Turnagain Island-Neap 09/04/2004 09:00 157 10:00 160 11:00 163 12:00 167 13:00 170 14:00 – 15:00 175 16:00 178 17:00 181 18:00 183 19:00 186 20:00 189 21:00 192 22:00 195 23:00 – 10/04/2004 00:00 200 01:00 203 02:00 206 03:00 209 04:00 212 05:00 215 06:00 218 07:00 221 08:00 224 09:00 227

19.78 20.26 17.05 20.36 18.18 – 22.54 21.96 18.15 20.36 20.74 19.00 18.86 16.43 – 17.74 15.98 21.11 21.78 20.92 20.79 21.93 21.79 20.87 19.45

10.78 11.21 9.99 10.40 9.45 – 11.90 11.86 10.60 12.11 12.12 12.54 11.78 10.38 – 11.46 10.16 12.31 11.95 12.52 13.04 12.54 12.25 12.85 11.40

3.18 3.64 3.34 3.49 2.86 – 3.67 3.90 3.48 3.97 4.20 4.12 3.67 3.46 – 3.70 3.65 3.73 3.94 4.06 3.60 3.56 3.76 3.87 3.54

1.13 1.22 1.24 1.34 1.02 – 1.40 1.46 1.28 1.39 1.42 1.40 1.35 1.24 – 1.55 1.12 1.37 1.40 1.47 1.35 1.38 1.54 1.38 1.38

2.28 2.33 2.70 2.33 2.60 – 2.01 2.26 2.45 2.91 2.27 2.28 2.26 2.57 – 2.47 2.60 2.14 2.24 2.18 2.07 2.03 2.03 1.97 2.25

0.10 0.14 0.07 0.08 0.11 – 0.14 0.12 0.08 0.11 0.12 0.13 0.13 0.10 – 0.10 0.09 0.12 0.13 0.14 0.13 0.15 0.19 0.10 0.12

0.66 0.67 0.79 0.66 0.68 – 0.55 0.59 0.65 0.63 0.65 0.69 0.69 0.73 – 0.70 0.80 0.60 0.59 0.62 0.55 0.54 0.61 0.62 0.68

SE Turnagain Island-Neap 12:00 230 13:00 233 14:00 236 15:00 239 16:00 242 17:00 245 18:00 248 19:00 251 20:00 254 21:00 257 22:00 260 23:00 263 11/04/2004 00:00 266 01:00 269 02:00 272 03:00 – 04:00 275 05:00 278 06:00 281 07:00 284 08:00 287 09:00 290 10:00 293 11:00 296 12:00 299

13.78 16.02 13.94 19.05 18.06 18.84 20.94 20.72 16.37 15.54 19.01 16.44 17.31 17.73 17.38 – 19.26 19.13 19.56 20.36 17.31 14.79 17.21 18.02 18.51

11.68 13.15 11.60 13.27 14.42 14.24 13.11 12.80 10.67 10.37 11.50 10.30 11.40 12.01 10.82 – 13.28 13.11 12.24 12.60 14.49 12.54 10.99 11.61 11.54

3.03 3.08 2.52 3.31 3.59 3.68 3.28 3.66 2.77 2.80 3.10 3.08 3.24 3.33 3.13 – 3.29 3.66 3.21 3.55 3.57 3.30 3.25 3.68 3.67

0.93 1.12 0.82 0.77 1.20 1.22 1.12 1.22 0.96 0.97 1.12 1.08 1.16 1.31 1.20 – 1.11 1.32 1.25 1.22 1.35 1.05 1.17 1.16 1.21

2.32 2.06 2.15 2.00 2.06 1.90 1.96 2.05 2.54 2.31 2.11 2.35 2.38 2.07 2.17 – 2.03 2.03 1.99 2.11 1.88 2.26 2.20 2.41 2.15

0.15 0.11 0.11

0.69 0.68 0.67 0.56 0.60 0.58 0.57 0.56 0.68 0.63 0.58 0.72 0.67 0.71 0.71 – 0.53 0.66 0.55 0.58 0.61 0.65 0.71 0.66 0.65

0.13 0.11 0.12 0.09 0.08 0.08 0.11 0.13 0.11 0.10 0.10 – 0.09 0.11 0.12 0.11 0.11 0.09 0.10 0.10 0.10

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Table 5 Elemental concentrations for suspended sediments observed on the Trade Wind Survey (Geoscience Australia Survey 273) Date

Time

Filter

SW Turnagain Island-Neap 15/10/2004 20:00 312 21:00 314 22:00 316 23:00 318 16/10/2004 00:00 320 01:00 322 02:00 324 03:00 326 04:00 328 05:00 330 06:00 332 07:00 334 08:00 337 09:00 339 10:00 341 11:00 344 12:00 346 13:00 348 14:00 350 15:00 352 16:00 354 17:00 356 18:00 358 19:00 360 20:00 362

Ca (%)

Si (%)

Al (%)

Fe (%)

Mg (%)

Ti (%)

K (%)

Date

17.57 18.05 18.31 20.06 19.87 18.33 22.39 18.90 20.74 17.42 21.54 21.16 19.59 19.81 14.60 24.35 22.13 23.18 23.87 16.82 20.15 22.04 16.53 16.54 19.92

12.13 12.71 13.01 12.16 12.37 12.33 12.15 10.70 11.97 10.54 11.36 12.10 13.02 12.84 7.77 10.46 11.88 11.30 11.00 10.04 11.27 11.27 12.79 11.26 12.47

4.11 3.49 4.04 4.04 3.81 3.94 3.33 3.76 3.80 3.72 4.03 4.13 3.88 3.86 2.72 3.79 3.83 3.81 3.44 3.47 3.72 3.90 3.70 3.52 4.11

1.47 1.36 1.39 1.48 1.53 1.39 1.38 1.35 1.64 1.52 1.46 1.59 1.38 1.38 1.06 1.36 1.51 1.47 1.70 1.21 1.35 1.62 1.33 1.12 1.34

2.23 1.98 2.06 2.12 1.97 2.18 1.89 2.08 1.58 2.62 2.32 1.97 1.92 1.97 3.23 2.26 2.05 2.10 2.15 2.36 2.28 2.26 2.07 2.21 2.03

0.15 0.02 0.11 0.14 0.13 0.14 0.10 0.14 0.11 0.15 0.15 0.13 0.15 0.14 0.12 0.08 0.21 0.12 0.11 0.10 0.11 0.13 0.13 0.16 0.17

0.69 0.64 0.66 0.69 0.68 0.63 0.56 0.84 0.69 0.70 0.66 0.68 0.62 0.65 0.79 0.56 0.62 0.71 0.54 0.72 0.69 0.65 0.73 0.65 0.73

SE Turnagain Island-Neap 17/10/2004 00:00 366 01:00 368 02:00 370 03:00 372 04:00 374 05:00 376 06:00 378 07:00 380 08:00 382 09:00 384 10:00 386 11:00 388 12:00 391 13:00 393 14:00 395 15:00 397 16:00 399 17:00 401 18:00 403 19:00 405 20:00 407 21:00 409 22:00 411 23:00 413 18/10/2004 00:00 415

In general, marine phases (Ca, Mg, Sr) increase and non-marine phases (Si, Al, Fe, Ti) decrease with distance from the island of New Guinea (Figs. 5a, b). This trend is supported by the overall composition of the bed sediments as determined by microscopy. Seabed and suspended sediments next to Turnagain and Saibai Islands are quantitatively similar in composition and texture, and exhibit the same regional trends as the mixed carbonate–terrigenous seabed sediments found elsewhere in Torres Strait (e.g., Harris and Baker, 1991), and on the south coast of PNG, including the Gulf of Papua (e.g., Harris et al., 1993; Walsh et al., 2004) and northern Gulf of Carpentaria (Preda and Cox, 2005).

5. Discussion Testing the hypothesis that increased terrigenous sediments influence seagrasses in Torres Strait implies that the terrigenous sediments around Saibai and Turnagain Island lie on a sediment pathway from the source (New Guinea) to the sink (Turnagain Island). Identifying unique conservative geochemical tracers of terrigenous sediments in our samples from Saibai and Turnagain Islands is one way of identifying and characterising this presumed sediment pathway. Previous geochemical studies of coastal sediments on the south coast of the island of New Guinea (Brunskill et al., 2004) and seabed sediments in the Gulf of Papua (Brunskill et al., 1995, 2003) indicate that Cu is a reliable tracer of terrigenous sediments. Elevated Cu concentrations in coastal sediments are the result of mining operations in the New Guinea catchments and previous studies have shown that Cu/Al ratios for these sediments have a mean of 0.01 (Brunskill et al., 1995, 2003). The mean Cu/Al ratio for seabed sediments at Saibai Island is also 0.01, indicating that the seabed sediments in northern Torres Strait are part of a sediment pathway from New Guinea, and that terrigenous sediments from New Guinea rivers are being dispersed as far as Saibai Island. Cu/Al ratios for seabed sediments

Time

Filter

Ca (%)

Si (%)

Al (%)

Fe (%)

Mg (%)

Ti (%)

K (%)

21.40 20.78 20.04 22.76 20.36 21.00 17.56 – – 19.51 22.83 20.73 21.58 19.35 22.57 – 19.91 20.52 – 15.51 – 16.43 20.43 20.84 22.96

12.03 12.26 11.29 11.46 12.58 12.10 13.74 – – 13.11 11.40 12.83 12.20 13.32 12.14 – 12.82 12.62 – 15.94 – 11.72 12.28 13.00 11.85

3.98 4.10 3.88 3.85 3.83 4.01 3.95 – – 3.78 3.97 3.92 4.05 4.30 3.33 – 4.21 3.85 – 3.80 – 3.97 3.97 3.71 3.52

1.48 1.41 1.36 1.45 1.48 1.44 1.51 – – 1.31 1.62 1.41 1.40 1.44 1.25 – 1.36 1.42 – 1.16 – 1.29 1.34 1.32 1.15

2.10 2.21 2.13 2.03 2.03 2.17 1.93 – – 2.11 2.01 2.25 2.10 2.12 1.91 – 2.07 1.98 – 1.69 – 2.40 2.22 2.10 2.07

0.23 0.13 0.15 0.14 0.14 0.14 0.14 – – 0.10 0.16 0.12 0.10 0.21 0.08 – 0.14 0.15 – 0.09 – 0.13 0.12 0.10 0.13

0.62 0.65 0.70 0.66 0.71 0.65 0.67 – – 0.62 0.65 0.55 0.60 0.62 0.57 – 0.63 0.62 – 0.83 – 0.73 0.61 0.55 0.54

next to Turnagain Island have a mean of 0.02, indicating an enrichment of Cu in these more distal sediments. If the seabed sediments next to Turnagain Island are located on this pathway, it should be possible to calculate the amount of terrigenous phases (Al, Si) and trace elements (Cu) located in the sediments next to Turnagain Island that have come from Saibai Island (and ultimately New Guinea rivers). Of the terrigenous phases, Al contents in seabed sediments next to Turnagain Island are on average 43730% of the Al content recorded in the sediments next to Saibai Island (Tables 2 and 3). Similarly, Si contents in sediments at Turnagain Island are on average 32725% of the Si content in sediments next to Saibai Island. Cu contents in sediments next to Turnagain Island are on average 1387100% of the Cu contents recorded in New Guinea coastal sediments (Brunskill et al., 1995, 2003). Two potential processes could result in the enrichment of Cu in seabed sediments next to Turnagain Island and both require an exogenous source of Cu. Firstly, the enrichment of Cu in sediments next to Turnagain Island could be the result of diagenetic remobilisation of Cu from the sediments at Saibai Island, which is then transported to Turnagain Island and subsequently scavenged into biogenic particulates, and finally incorporated into the seabed sediments. Alternatively, the exogenous biogenic Cu could be sourced from elsewhere and be similarly incorporated into the seabed sediments next to Turnagain Island. Our Sr data indicate that Cu in the sediments has been scavenged into mostly molluscs and foraminifers, with reefal components contributing a very-low proportion. Given that Al and Si are not conservative tracers of New Guinea sediment and that Cu is enriched in the distal regions, we cannot uniquely trace terrigenous sediments from New Guinea coastal sediments to Turnagain Island from our data. Instead the concentrations of Al, Si and Cu suggest that, despite the vast quantities of terrigenous sediment being delivered by the rivers on the south coast of New Guinea, the dispersal of terrigenous sediment in Torres Strait appears to be restricted to within 5–10 km of the coast, in the vicinity of Saibai Island (Figs. 5a, b).

ARTICLE IN PRESS A.D. Heap, L. Sbaffi / Continental Shelf Research 28 (2008) 2174–2187

40

2185

Si

Ca

Torres Strait

35 30

Saibai Island

Atomic %

Turnagain Island

25 20

Nth Gulf of Carp.

Turnagain Island

15 10

Saibai Island

Nth Gulf of Carp.

Torres Strait

5 10

Al

Fe

Mg

Ti

8

6

4

2

5

1 = Turnagain Island A - Spring Tide (Monsoon Survey) 2 = Turnagain Island B - Spring Tide(Monsoon Survey) 3 = Turnagain Island A - Neap Tide (Monsoon Survey) 4 = Turnagain Island B - Neap Tide (Monsoon Survey) 5 = Turnagain Island A - Neap Tide (Trade Wind Survey) 6 = Turnagain Island B - Neap Tide (Trade Wind Survey) 7 = Turnagain Island seabed sediments 8 = Saibai Island seabed sediments 9 = Northern Gulf of Carpentaria seabed sediments 10 = Torres Strait seabed sediments

4

No data

3

No data

1

No data

2

9

10

0 1

2

3

4

Suspended Sediments

5

6

7

8

9

10

Seabed Sediments

1

2

3

4

Suspended Sediments

5

6

7

8

Seabed Sediments

Fig. 6. Box and whisker plots showing the major elemental concentrations of suspended sediments collected from the 24-h stations and the seabed sediments. Suspended and seabed sediments next to Turnagain Island are very similar in composition, but distinct from seabed sediments next to Saibai Island. Suspended sediments in the vicinity of Turnagain Island are more similar in composition to the seabed sediments in central Torres Strait and northern Gulf of Carpentaria, indicating a dominant marine (local) source.

Other geochemical tracers such as lanthanoids and rare earth elements are needed to more clearly define the sources of the terrigenous sediment present in our samples. It is clear that sediments at Turnagain Island differ significantly in their sedimentology and geochemical properties than those on the south coast of PNG, Gulf of Papua and Saibai Island. Indeed, our study shows that suspended and seabed sediments in the vicinity of Turnagain Island are identical, and that they are more characteristic of shelf sediments in other areas of central and southern Torres Strait (Haynes and Kwan, 2002) and the northern Gulf of Carpentaria (e.g., Cox and Preda, 2003; Preda and Cox, 2005; Figs. 3–6). Our results indicate that if sediments are associated with widespread seagrass dieback in central Torres Strait they are likely to be from the resuspension and advection of marine-derived sediments from the local shelf area and not from the dispersal of terrigenous sediments from New Guinea rivers.

Hydrodynamic and sediment transport models for the region (Keen et al., 2006; Margvelashvili et al., this issue; Saint-Cast, this issue) show that the predominant water and sediment movements in Torres Strait are oriented east–west and disperse sediments parallel to the coast. These currents would form an effective hydraulic barrier to sediment transport southwards from New Guinea to central and southern Torres Strait, and account for the sedimentological and geochemical patterns recorded in our study.

6. Conclusions Sedimentological and geochemical properties of seabed and suspended sediments in central and northern Torres Strait indicate that modern terrigenous sediments are restricted to

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Table 6 Composition of carbonate fraction contained in bed sediments Sample

Sr (ppm)

Calcite: nonreefal (%)

Aragonite: reefal (%)

Turnagain Island-sand fraction 273/01GR01 1791.24 273/03GR03 1880.54 273/04GR04 2029.56 273/06GR06 2198.67 273/07GR07 1954.56 273/08GR08 1944.78 273/09GR09 2424.47

54.18 54.81 54.21 50.67 54.53 48.52 43.84

4.87 5.77 7.81 11.03 6.78 8.58 16.01

Turnagain Island-mud fraction 273/01GR01 3075.22

18.22

32.24

Saibai Island-sand fraction 273/10GR10 1005.80 273/11GR11 740.12 273/12GR12 820.80 273/13GR13 883.99 273/17GR17 566.60 273/18GR18 653.71 273/23GR23 674.61

28.86 16.16 24.87 23.30 20.46 18.88 15.09

3.24 4.00 2.22 3.50 0.48 2.06 3.53

Saibai Island-mud fraction 273/10GR10 438.43 273/11GR11 782.59 273/12GR12 454.72 273/13GR13 611.22 273/17GR17 704.32 273/23GR23 639.66

7.16 7.18 8.59 8.85 8.52 9.87

3.14 7.39 2.88 4.74 6.00 4.76

within 5–10 km of the south coast of New Guinea, where mixing of terrigenous and carbonate grains occurs. The remainder of Torres Strait is characterised by carbonate-dominated, unimodal marine sands, which comprise both the suspended and seabed sediments. Suspended and seabed sediments in the region of repeated widespread seagrass dieback are mostly composed of carbonate-dominated (marine) sediment. Seabed sediments next to Turnagain Island in central Torres Strait contain only a very small and variable terrigenous fraction and enrichment of Cu, which does not allow tracing of terrigenous sediments from New Guinea to Turnagain Island. If sediment is a factor in the widespread seagrass dieback in central Torres Strait, then our data suggest that the source of this sediment is from resuspension and advection of marine-derived shelf sediments and not from riverine sediments being dispersed from New Guinea. Acknowledgements This study was funded by Geoscience Australia and the Torres Strait CRC. Fieldwork support was provided by James Cook University, School of Earth Sciences. Preparation of the filter papers and EDX interpretations were undertaken by Dr Frank Brink and Dr Cheng Huang of the Australian National University. XRD and XRF analyses were performed by Dr John Pike and Dr Elizabeth Webber of Geoscience Australia. The original manuscript benefited from discussions with Dr David Heggie of Geoscience Australia, and critical reviews by Drs Alan Orpin and Lynda Radke. This manuscript is published with permission of the Chief Executive Officer, Geoscience Australia. References Amin, M., 1978. A statistical analysis of storm surges in Torres Strait. Australian Journal of Marine and Freshwater Research 29, 479–496. Bode, L., Mason, F., 1994. Tidal modelling in Torres Strait and Gulf of Papua. In: Bellwood, O., Choat, H., Saxena, N. (Eds.), Recent Advances in Marine Science

and Technology ’94. PACON International and James Cook University, Townsville, pp. 55–65. Bridges, K.W., Phillips, R.C., Young, P.C., 1982. Patterns of some seagrass distributions in the Torres Strait, Queensland. Australian Journal of Marine and Freshwater Research 33, 273–283. Brink, F.J., Nore´n, L., Withers, R.L., 2004. Electron diffraction evidence for continuously variable, composition-dependent O/F ordering in the ReO3 type, V IV Nb1x Nbx O2x F1þx , 0pxp0.48, solid solution. Journal of Solid State Chemistry 177, 2177–2182. Brunskill, G.J., Woolfe, K.J., Zagorskis, I., 1995. Distribution of riverine sediment chemistry on the shelf, slope and rise of the Gulf of Papua. Geo-Marine Letters 15, 160–165. Brunskill, G.J., Zagorskis, I., Pfitzner, J., 2003. Geochemical mass balance for lithium, boron, and strontium in the Gulf of Papua, Papua New Guinea (Project TROPICS). Geochemica et Cosmochimica Acta 67, 3365–3383. Brunskill, G.J., Zagorskis, I., Pfitzner, J., Eillison, J., 2004. Sediment and trace element depositional history from the Ajkawa River estaurine mangroves of Irian Jaya (West Papua), Indonesia. Continental Shelf Research 24, 2535–2551. Cox, M.E., Preda, M., 2003. Trace metal distribution and relation to marine sediment mineralogy, Gulf of Carpentaria, Northern Australia. Marine Pollution Bulletin 46, 1615–1629. Daniell, J., Hemer, M., Heap, A.D., Mathews, E., Sbaffi, L., Hughes, M., Harris, P.T., 2006. Biophysical Processes in the Torres Strait Marine Ecosystem II: Survey Results and Review of Activities in Response to CRC Objectives. In: Geoscience Australia, Record 2006/10. Canberra. 210pp. Devlin, M.J., Brodie, J., 2005. Terrestrial discharge into the Great Barrier Reef Lagoon: nutrient behaviour in coastal waters. Marine Pollution Bulletin 51, 9–22. Dickens, G.R., Droxler, A.W., Bentley, S.J., Peterson, L.C., Opdyke, L.N., Beaufort, L., Daniell, J., Febo, L.A., Francis, J., Harris, P.T., Jorry, S., Mallarino, G., McFadden, M., Muhammad, Z., Carson, B., Patterson, L., Tcherepanov, E., Zarikian, C.A., 2006. Sediment accumulation on the shelf edges, adjacent slopes, and basin floors of the Gulf of Papua. MARGINS Newsletter 16, 1–5, p. 38. Dunbar, G.B., Dickens, G.R., Carter, R.M., 2000. Sediment flux across the Great Barrier Reef Shelf to the Queensland Trough over the last 300 ky. Sedimentary Geology 133, 49–92. Fabricius, K.E., De’ath, G., 2004. Identifying ecological change and its causes: a case study on coral reefs. Ecological Applications 14, 1448–1465. Frank, T.D., Jell, J.S., 2006. Recent developments on a nearshore, terrigenousinfluenced reef: Low Isles Reef, Australia. Journal of coastal research 22, 474–486. Gladstone, W., 1996. Trace scientific metals in sediments, indicator organisms and traditional seafoods of the Torres Strait. Great Barrier Reef Marine Park Authority Report Series, Report No. 5a, 184pp. Harris, P.T., 1988. Sediments, bedforms and bedload transport pathways on the continental shelf adjacent to Torres Strait, Australia–Papua New Guinea. Continental Shelf Research 8, 979–1003. Harris, P.T., 1989. Sandwave movement under tidal and wind-driven currents in a shallow marine environment: Adolphus Channel, northeastern Australia. Continental Shelf Research 11, 981–1002. Harris, P.T., 1991. Reversal of subtidal dune asymmetries caused by seasonally reversing wind-driven currents in Torres Strait, northeastern Australia. Continental Shelf Research 11, 655–662. Harris, P.T., Baker, E., 1991. The nature of sediments forming the Torres Strait turbidity maximum. Australian Journal of Earth Sciences 38, 65–78. 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. Haynes, D., Kwan, D., 2002. Trace metals in sediments from Torres Strait and the Gulf of Papua: concentrations, distribution and water circulation patterns. Marine Pollution Bulletin 44, 1309–1313. Heap, A., Hemer, M., Daniell, J., Mathews, E., Harris, P., Kerville, S., O’Grady, L., 2005. Biophysical processes in the Torres Strait Marine Ecosystem. Geoscience Australia Marine Survey 266, Post-cruise Report, GA Record 2005/11, 111pp. Hemer, M.A., Harris, P.T., Coleman, R., Hunter, J., 2004. Sediment mobility due to currents and waves in the Torres Strait–Gulf of Papua region. Continental Shelf Research 24, 2297–2316. Irion, G., Petr, T., 1983. Clay mineralogy of selected soils and sediments of the Purari River basin. In: Petr, T. (Ed.), The Purari—Tropical Environment of a High Rainfall River Basin. Dr. W. Junk Publishers, The Hague, pp. 87–107. Keen, T.R., Ko, D.-S., Slingerland, R.L., Riedlinger, S., Flynn, P., 2006. Potential transport pathways of terrigenous material in the Gulf of Papua. Geophysical Research Letters 33, L04608. Long, B.G., Poiner, I.R., 1994. Infaunal benthic community structure and function in the Gulf of Carpentaria, Northern Australia. Australian Journal of Marine and Freshwater Research 45, 293–316. Long, B.G., Skewes, T., Thomas, M., Isdale, P., Pitcher, R., Poiner, I., 1997. Torres Strait seagrass dieback. Final report to TSFSAC 26, CSIRO Division of Marine Research, Cleveland, 23pp. Lourensz, R.S., 1981. Tropical cyclones in the Australian region: July 1909 to June 1980. Bureau of Meteorology, Melbourne, 94pp. Margvelashvili, N., Saint-Cast, F., Condie, S., this issue. Numerical modelling of the suspended sediment transport in Torres Strait. Continental Shelf Research, doi:10.1016/j.csr.2008.03.037. Milliman, J.D., 1974. Recent Sedimentary Carbonates Part 1: Marine Carbonates. Springer, New York, 375pp.

ARTICLE IN PRESS A.D. Heap, L. Sbaffi / Continental Shelf Research 28 (2008) 2174–2187

Milliman, J.D., 1995. Sediment discharge to the ocean from small mountainous rivers: the New Guinea example. Geo-Marine Letters 15, 127–133. Milliman, J.D., Farnsworth, K.M., Albertin, C., 1999. Flux and fate of fluvial sediments leaving large islands in the East Indies. Journal of Sea Research (Netherlands) 41, 97–107. Muller, G., Gastner, M., 1971. The ‘‘Karbonate-Bombe,’’ a simple device for the determination of the carbonate content in sediments, soils, and other materials. Neues Jahrbuch fuer Mineraliogie 10, 466–469. Post, A.L., Sbaffi, L., Passlow, V., Collins, D.C., 2007. Benthic foraminifera as environmental indicators in Torres Strait—Gulf of Papua. In: Todd, B.J., Greene, H.G. (Eds.), Mapping the Seafloor for Habitat Characterization: Geological Association of Canada, Special Paper 47, pp. 329–347. Preda, M., Cox, M.E., 2005. Chemical and mineralogical composition of marine sediments, and relation to their source and transport, Gulf of Carpentaria, Northern Australia. Journal of Marine Systems 53, 169–186. Reed, S.J.B., 1996. Electron Microprobe Analysis and Scanning Electron Microscopy in Geology. Cambridge University Press, Cambridge, 201pp. Saint-Cast, F., this issue. Multiple time-scale modelling of the circulation in Torres Strait-Australia. Continental Shelf Research, doi:10.1016/j.csr.2008.03.035.

2187

Walsh, J.P., Nittrouer, C.A., Palinkas, C.M., Ogston, A.S., Sternberg, R.W., Brunskill, G.J., 2004. Clinoform mechanics in the Gulf of Papua, New Guinea. Continental Shelf Research 24, 2487–2510. Wolanski, E., 1986. Water circulation in a topographically complex environment. In: van de Kreeke, J. (Ed.), Physics of Shallow Estuaries and Bays. Springer, Berlin, pp. 154–167. Wolanski, E., 1994. Physical Oceanographic Processes of the Great Barrier Reef. In: CRC Marine Science Series. CRC Press, Boca Raton, Florida, 194pp. Wolanski, E., Ridd, P., Inoue, M., 1988. Currents through Torres Strait. Journal of Physical Oceanography 18, 1535–1545. Wolanski, E., King, B., Galloway, D., 1995. Dynamics of the turbidity maximum in the Fly River Estuary, Papua New Guinea. Estuarine, Coastal and Shelf Science 40, 321–337. Wolanski, E., Fabricius, K., Spagnol, S., Brinkman, R., 2005. Fine sediment budget on an inner-shelf coral-fringed island, Great Barrier Reef of Australia. Estuarine, Coastal and Shelf Science 65, 153–158. Woodroffe, C.D., Kennedy, D.M., Hopley, D., Rasmussen, C.E., Smithers, S.G., 2000. Holocene reef growth in Torres Strait. Marine Geology 170, 331–346.