Trace metals in lacustrine and marine sediments: A case study from the Gulf of Carpentaria, northern Australia

Chemical Geology, 82 (1990) 299-318 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

299

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Trace metals in lacustrine and marine sediments: A case study from the Gulf of Carpentaria, northern Australia MARC D. NORMAN and PATRICK DE DECKKER Research School of Earth Sciences, The Australian National University, Canberra, A.C.T. (Australia) Department of Geology, The Australian National University, Canberra, A.C.T. (Australia) (Received May 12, 1989i revised and accepted December 15, 1989)

Abstract Norman, M.D. and De Deckker, P., 1990. Trace metals in lacustrine and marine sediments: A case study from the Gulf of Carpentaria, northern Australia. Chem. Geol., 82: 299-318. Sediment cores taken from the shallow ( < 70-m depth), epicontinentalGulf of Carpentaria record a late Quaternary marine transgression over a well-established lacustrine sequence. Bulk sediment samples taken along the length of a single core have been analysed for their Ca, A1, Fe, Ti, Mn, Mg, Na, K, Sr, Ba, V, Ni, Co, Cr, Cu, Sc, Zr, Y, La and Ce contents to examine compositional changes across a continuous sedimentary sequence in which the depositional environment evolved from freshwater lacustrine to saline lacustrine to open marine over the past 40 ka. The data form remarkably linear covariation trends suggesting physical mixing of two sedimentary components as the predominant process controlling the compositions of both lacustrine and marine sediments. One component has high Ca, Sr and Mn, suggestive of biogenic carbonate. The other component contributes virtually all of the sedimentary budget of the other elements, and is probably terrigenous clay, either water-borne or aeolian. Minor contributions from heavy minerals such as zircon may account for the higher values of Zr and Y for some horizons in the core. Anomalously high concentrations of zircon may have been brought into the Gulf as dust particles during episodes of increased aeolian activity. Carbonate is most abundant in the marine sediments, resulting in the higher observed Ca and Sr concentrations. V and Mn seem to be the metallic elements most sensitive to depositional environment and water chemistry. Environmental conditions had little or no effect on sedimentation of the other metals. V and Mn were delivered to the lacustrine sediments more efficiently than to the marine sediments, probably reflecting changes in solubility as slightly acid lacustrine waters gave way to more neutral oceanic conditions. Metal-organic complexing was not an important process in these oxidised sediments.

1. Introduction The Gulf of Carpentaria is a shallow epicontinental sea located between Australia and Papua New Guinea (Fig. 1 ). It is an asymmetric basin with a gently sloping western shelf and a more steeply sloping eastern shelf, with the deepest portion of the basin offset to the east of its physiographic center at a maximum depth

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of 69 m below sea-level. Bathymetric studies show that the depth contours of the Gulf of Carpentaria are closed, and that the Gulf is connected to the Arafura Sea to the west by a sill now 53 m below sea level (Nix and Kalma, 1972). The Carpentaria basin must have been lacustrine at times of low sea-level during the Quaternary, and seismic profiles have identified the shoreline of an ancient lake (referred

© 1990 Elsevier Science Publishers B.V.

300

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301

TRACE METALS IN LACUSTRINE AND MARINE SEDIMENTS

to here as lake Carpentaria) dating to at least 35 ka ago (Torgersen et al., 1983, 1985). The catchment area of the Carpentaria basin drains predominantly Mesozoic and Proterozoic sedimentary rocks. During 1982, two cruises collected 35 sediment cores from the Gulf. From these, five laterally continuous units have been recognised which preserve a sedimentological record of the marine transgression that changed freshwater lake Carpentaria to an open-ocean setting (Torgersen et al., 1985, 1988). One key core (GC-2) from the deepest portion of the Gulf (Fig. 1) has been the focus of continuing detailed palaeontological and geochemical studies with the aim of deciphering the palaeoenvironmental history of the area. The present paper reports the results of a trace-element study of bulk sediment samples from core GC-2. The purpose of this work was to examine the variation of major and trace elements across a continuous sedimentary sequence in which the depositional environmental varied from lacustrine to open marine, and to determine what processes controlled the chemical and mineralogical composition of the sediment.

G C - 2 CORE 12°31.18'S 140o21.14,E it C STRATIGRAPHIC depth ages UNIT ..... 0 (cml---:'~-:---~ 5,170-+130 a B.P. I '

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Five lithologic units have been recognised and described by Torgersen et al. (1985, 1988) (Fig. 2). The uppermost unit (I) is grey-green, siliciclastic to bioclastic sediment containing abundant remains of molluscs, bryozoans, ostracods and foraminifers. This unit was deposited under open marine conditions in the modern Gulf beginning ~ 7.5 ka ago. Unit I is ~ 30 cm thick in this core. Unit II is firm, dark grey, and siliciclastic with ostracods and foraminifers typical of slightly saline conditions in a lacustrine environment. Unit II is the thickest unit in this core (120 cm). 14C dates bracket the time of Unit-II deposition as between ~ 22.5 and ~ 7.5 ka ago. Unit III is finely laminated with layers rich in small (10-20 /zm long) authigenic calcite

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302

Freshwater conditions with a high biological productivity have been suggested for this unit. Unit V is mottled, dark grey and siliciclastic. Ostracods and foraminifers characteristic of freshwater to slightly saline conditions are most abundant although a few (probably reworked) open-marine types are found. Evidence of soil formation suggests periods of subaerial exposure during deposition. A single 14C date from another core suggests an age of > 36 ka for this unit, which is ~ 10 cm thick in GC-2. This is the most consolidated unit and probably limited the depth of coring (Torgersen et al., 1988).

3. Previous geochemical studies Oxygen and carbon isotopic ratios, and Mg/ Ca and Sr/Ca ratios for calcites of Unit III sampled at the mm scale are reported by Torgersen et al. (1988). The nearly constant (~lSOpD s of -- 3.0 + 0.29%0 for these laminae implies authigenic precipitation of calcite from either dilute, tropical meteoric waters or tropical marine-derived waters. Meteoric waters are favoured because the Sr/Ca and Ca/Mg ratios of authigenic calcite in this unit imply water compositions that are significantly different from those of seawater, suggesting precipitation from a continental water mass and not from diluted seawater (Torgersen et al., 1988). The (~13CpD B of --0.09__+0.33%0 is consistent with equilibrium of the water with atmospheric C02 (Torgersen et al., 1988). The Sr/Ca, Mg/Ca and STSr/S6Sr of single ostracod shells from each of the lithologic units have been used as an indicator of water composition and salinity {De Deckker et al., 1988; McCulloch et al., 1989). Ostracods are bivalved microcrustaceans commonly found in a wide rang~ of environments. They are potentially useful for palaeoenvironmental studies (see De Deckker, 1988, for review) because they secrete low-Mg calcite shells from material taken directly from the water column, and so reflect the ambient aquatic conditions at the time of moulting and new valve formation (Chivas et

M.D. NORMAN AND P. DE DECKKER

al., 1986). These studies determined the distribution coefficient for Sr between related genera of ostracod and the water in which they grew, and found systematic compositional variations in ostracods with stratigraphic depth which provide detailed palaeoenvironmental information on water conditions in the Carpentaria basin at the time the major lithologic units were deposited. Ostracods from Units IV and V indicate water compositions with Sr/Ca close to seawater (i.e. 0.0086; Kinsman, 1969) but Mg/ Ca closer to fresh water values (i.e. ~ 1 vs. ~ 5 for seawater, Renard, 1985). De Deckker et al. (1988) interpret this as indicating deposition of these units from basically freshwater that contained some marine-derived solutes. Unit III is characterised by authigenic calcite and ostracods that record an increase in Sr/Ca and Mg/ Ca in the water column as a result of this precipitation. Ostracods from the base of Unit II up to near 80-cm depth in the core record nearly constant water composition with high Sr/Ca ( ~ 0.018) relative to seawater and Mg/Ca about the same as seawater, but no connection to oceanic waters was postulated (see De Deckker et al., 1988, for further details). Above 80 cm a connection to seawater is inferred from the presence of open-marine fauna, but between 55and 65-cm depth the ostracods also record a definite dilution with freshwater as Mg/Ca drops to values comparable to those determined for Units IV and V. These Mg/Ca values are well below that of seawater even when the substantial effect of temperature on Mg uptake into the calcite lattice is considered. Ostracods from Unit I show some evidence for reworking, but open-marine species, such as those found in the modern marine environment, have fairly constant compositions which are consistent with growth in seawater. The Sr isotopic data on single ostracod shells are also consistent with a palaeoenvironmental sequence in which a freshwater to slightly saline lacustrine environment existed from ~ 36 to ~ 12 ka ago, followed by a marine transgression which re-established the Gulf of Carpentaria ~ 12 ka ago (Mc-

303

TRACE METALS IN LACUSTRINE AND MARINE SEDIMENTS

Culloch et al., 1989). These palaeoenvironmental studies form a basis from which the bulk sediment trace-element analyses presented here can be discussed.

4. S a m p l i n g and analytical methods Twelve bulk sediment samples were obtained at various depths from core GC-2 (Fig. 2 ). Special care was taken to sample horizons that correspond to changes in environmental conditions established in previous studies. Approximately 1-2 g of material were taken for each sample from near the middle of the core using a polypropylene syringe. The samples were numbered according to their depth in cm from the top of the core. Two samples were taken from Unit I (GC2-2, GC2-13), seven samples were taken through Unit II (GC2-48, -63, -83, -96, -114, -139, -148), and one sample from Units III (GC2-159), IV (GC2-184) and

V (GC2-205). The samples were dried at l l 0 ° C for ~ 4 hr., and dissolved in HF-HNO3-HC104-HCI acids. Concentrations of Ca, Mg, Mn, Fe, A1, Ti, Na, K, Sr, Ba, V, Ni, Cr, Co, Cu, Sc, Zr, Y, La and Ce were determined in these solutions by inductively coupled Ar plasma emission spectroscopy (ICP), calibrated against international rock standards. Analytical precision based on replicate sample solutions and repeat analyses of calibration solutions is ~< + 5% of the amount present for all elements. Results for Ca, Mg, Fe, A1, Ti, Na and K are reported as wt.% oxide, and for the other elements as ttg g- 1 (ppm). Small portions of the dried bulk sample powders were prepared for X-ray powder diffraction by grinding t h e m with acetone and depositing the slurry on a glass plate. These were analysed with Cu-K~ radiation on a Siemens ® D 501 X-ray diffractometer using a scanning rate of 2.4°C (28) per minute and a curved graphite crystal monochromator.

5. Results The data are presented in Tables I and II and Figs. 3 and 4. Two important aspects of the chemical data are considered. One is the covariation of element concentrations relative to each other, and the other is compositional variation of the sediments with depth through the core. All elements display considerable ranges in concentrations (e.g., Ca varies by a factor of 100, Sr by a factor of 10 and V by a factor of 4) with excellent linear correlation between most elements (Figs. 3 and 4). Three general groups of elements can be distinguished by their relative behaviour. One group includes Sr, Ca and Mn which tend to be positively correlated between each other and negatively correlated against virtually every other element determined. The correlation between Ca and Sr is very strong, whereas Mn concentrations rise with increasing Ca and Sr to a maximum and then drop in the samples with the highest Ca and Sr contents (Figs. 3 and 4). A second group includes all of the other elements determined here except Mg and Na. All elements in this second group vary positively against each other to greater or lesser degrees (Figs. 3 and 4). The covariations within this group are strikingly linear for A1, K, Zr and the ferromagnesian elements (i.e. Fe, Ti, V, Ni, Co, Cr), and somewhat less well-defined for Cu, Ba, Y, and the rare-earth elements (REE) La and Ce. Mg and Na display poor correlations with the other elements, especially when all samples are considered (Figs. 3 and 4 ). Some systematics in the Na data do appear when they are considered in terms of the lithologic units; for example, Na and Ca are well correlated (negatively) within the central part of the core (Units II-IV), but inclusion of Units I and V considerably weakens the correlation. Compositional variability across the section can be investigated using both element concentrations and key trace-element ratios. Fig. 5 shows the variations in element concentrations

M.D. NORMANAND P. DE DECKKER

304 TABLE I Major- and trace-element compositions of sediment samples from core GC-2 Sample ( = depth in cm) Unit

2

13

48

63

83

96

114

139

148

159

184

205

I

I

II

II

II

II

II

II

II

III

IV

V

CaO (wt.%) MgO MnO A1203 Fe203* TiO2 Na20 K20

26.61 2.57 0.028 5.83 2.43 0.30 1.52 0.99

24.78 2.77 0.028 6.70 2.72 0.34 1.50 1.05

9.79 3.57 0.033 14.43 5.65 0.61 1.83 2.40

3.73 4.26 0.031 16.58 6.33 0.69 1.12 2.87

13.45 3.84 0.050 12.25 5.06 0.54 1.60 2.32

14.75 3.67 0.045 12.48 4.92 0.52 1.58 2.24

13.35 3.57 0.041 13.26 5.33 0.55 1.46 2.34

11.76 3.47 0.041 14.21 5.85 0.58 1.53 2.51

9.85 2.52 0.035 12.14 4.47 0.64 1.48 2.12

14.64 1.65 0.038 8.51 2.94 0.54 1.39 1.34

19.51 2.01 0.049 10.12 3.87 0.44 0.62 0.91

0.26 2.01 0.018 17.72 6.14 0.91 1.66 3.41

Mn (ppm) Ti Sr Ba V Ni Co Cr Cu Sc Y Zr La Ce

216 214 261 241 387 351 322 317 270 295 377 140 1,783 2,023 3,676 4.169 3,240 3,148 3,333 3,452 3,854 3,235 2,623 5,467 1,016 948 352 177 541 600 520 468 372 394 708 98 82 89 149 205 193 193 193 185 179 152 174 287 35 39 93 127 92 91 95 112 103 60 84 145 13 14 27 35 25 23 24 26 25 14 18 39 4 6 11 12 10 9 10 9 9 6 6 14 30 35 59 74 51 49 56 56 60 42 44 87 7 7 12 17 14 15 18 20 17 12 16 17 5.5 6.1 12.8 15.3 10.9 10.9 11.7 12.6 11.7 7.4 8.9 16.2 16.1 16.0 22.1 21.8 20.1 20.1 21.9 21.9 23.7 19.5 18.0 23.0 64 71 112 131 105 99 103 101 110 85 72 125 13.7 15.2 24.3 25.5 23.3 22.1 23.8 24.1 26.2 23.7 21.1 38.0 35.4 37.2 54.0 56.3 50.6 49.5 52.9 55.6 61.1 56.0 49.4 86.4

Fe203* =total Fe. T A B L E II Mineralogical c o m p o n e n t s recognised by X-ray diffraction for several of the sediment samples from core GC-2 Sample No. ( = d e p t h in cm) Unit Quartz Clay (halloysite) Calcite

High-Mg calcite Aragonite Feldspar Halite

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Conversely, the sample from the lowermost unit (Unit V, sample 205) has the lowest observed concentration of Ca (0.26 wt.% C a • ) and Sr (98 ppm), and generally the highest content of metals, alkalies and REE. Samples from the intervening portion of the section tend to have

compositions intermediate between these two samples, with Ca increasing in the order: Unit V < Unit II < Unit III-Unit IV < Unit I Mn, Mg and Na are notable exceptions to these general trends. These elements tend to rise to a

306

M.D. NORMAN AND P. DE DECKKER

maximum in the mid-section of the core. The negative covariation of Ca with the second group of elements (i.e. all those except Na, Mg, Ca, Sr and Mn) is well illustrated by the mirror image trends of Ca and A1 contents across the section (Fig. 5 ). Trace-element ratios can also yield useful information about the nature of the sedimentary components and environmental conditions of deposition. We have normalised the concentration data to the corresponding Sr and Sc contents of the same sample to remove potential

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Sc Fig. 4. Variation diagrams of AI2Oa ( w t . % ) and trace-element concentrations (ppm) vs. Sc (ppm). Labelled points a n d fields refer to the five lithologic units of core GC-2.

aspects of the data which are important to the interpretation. Along the length of the core, the ratios of Fe, A1, K, Ni, Co, Cr, Ba and Cr relative to Sc are all remarkably constant (Fig. 6). Ti/Sc is virtually constant except for a rise in Unit III. Ni, Co and Cr do display some variation relative to Sc, and although some of these differences may be real, we are hesitant to ascribe interpretive

significance to these variations as these are among the less precise elements determined here. The other elements show some systematic stratigraphic variation relative to Sc. Na/Sc, Y/ Sc, Ce/Sc and Zr/Sc all have nearly parallel patterns with the highest values occurring in Units I and III and nearly constant values throughout the rest of the core. Samples 48 and

308

M.D. NORMAN AND P. DE DECKKER

0

5O

!

50

Depth

(cm)

II

II

100

100

150

15(]

1 20C

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III ,,

200 ....

i

10

,

,

,

Concentration

,

i

,

,

20 (wt. %)

,

30

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0

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2

.y'i

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3

Concentration

4 (wt.°/o)

0

5O

I

50

Depth

(cml lOO

~i'~

Sr

"

15G

,,,

10

1 O0

15

1

5

0

~

2

0

0 50

~ 100

IV"

200 , ~ , .~ . . . . . . . . . . . v 20 0 200 400 600 800 1000 1200 0 Concentration

(ppm)

10

20

Concentration

30 (ppm)

40

0

Concentration

150

200

(ppm)

Fig. 5. Element concentrations plotted as a function of their stratigraphic position along core GC-2. Approximateboundaries of the five principal lithologic units are shown as dark horizontal lines. The stippled horizontal line marks the position of an inferred freshwater incursion within Unit II. Symbols for the different elements are used for clarity and have no special meaning. 63 within Unit II often show slightly lower values for these and some of the other elements relative to Sc. V / S c generally declines upsection except for a trough in Unit III. M g / S c tends to increase upsection except for an excursion to lower values in the upper part of the Unit II. Ca/Sc, S r / S c and M n / S c are quite variable

across the section, possibly in cycles. Each of these ratios start from a low value in Unit V, rise to a higher value in Units III and IV, then decrease at the base of Unit II. Across the lower portion of Unit II, these ratios increase before dropping again in sample 63. From this sample to the top of the core, these ratios rise sharply.

TRACE METALS IN LACUSTRINE AND MARINE SEDIMENTS

309

I

i

5O

Depth

(cm) 00

100

150

;oo ,p~,. ~

200 0.0

0.5

1.0

1.5

2.0

!'i!,

2.5

3.0

0

10

CI

100

. C. r . . .

'

,m~r~,

200 0

40

50

iO . . . .

)

4

30

V iI

~r

150

20

Element/Sc

Element/So

50

.................

i

6

8

Element/Sc

10

)oI 0

2

4

6

8

10

12

Element/Sc

Fig. 6. Element/Sc ratios plotted a s a f u n c t i o n o f their stratigraphic position along core GC-2. A s in Fig. 5, b o u n d a r i e s o f

lithologic units are shown as horizontal lines, the stippled line represents the approximate location of an inferred freshwater incursion, and symbols for each element are used only for clarity.

The general correlation of element concentrations and certain ratios with previously recognised lithologic units suggests that changes in either the environment of sedimentation or the supply of sedimentary components controlled the bulk compositions of these sediments. These processes will be explored in more

detail in the following sections. The linear trends observed in the concentration data also extend to the trace-element ratios. When normalised to Sr (Fig. 7), linear trends with positive slopes are observed for elements of the second group identified previously (i.e. A1, Fe, Ti, K, Ba, V, Ni, Co, Cr, Cu, Sc, Y,

310

M.D.NORMANANDP.DEDECKKER

10 a

0.1

0.01

0.001 0.01

0.1

1

10

3

Zr, La, Ce). Most of these trends pass through the origin, although a few (e.g., those for Zr and Y) have residual intercepts. These data suggest the importance of mixing processes for producing the observed compositional variations in core GC-2. Qualitative mineral identification by XRD analyses of the dried samples shows that quartz, clay (halloysite), calcite, high-Mg calcite, aragonite and halite are the major minerals present in these samples (Table II). Minor amounts of plagioclase were also identified in a few samples. Quartz and clay are present in every sample. Calcite was detected in every sample but one (GC2-20, Unit V). Aragonite and high-Mg calcite were found only in the upper portion of the core. Halite was found in every sample except GC2-159 (Unit III) and possibly GC2-13 (Unit I). 6. Discussion

.0 0"51.

0.4

1.0

/li

. . . . . . . . . . . .

Ic

0 O 0

0.5

~

2.0

. . . . . . . . . .

•

. 0.0

1.5

.

1 ,

0.1

~

.

.

.

~ ,

.

.

.

.

0.2

,

0.3

.

.

.

.

,

0.4

,

,

,

,

0.5

V/St

Fig. 7. Mixing diagrams plotting V/St vs. other element/ Sr ratios. The excellent linear correlations on these diagrams provide strong support for physical mixing of two sedimentary components (i.e. high-V/St terrigenous clay and low-V/Sr carbonate) as the predominant process controlling the compositions of these sediments. The lack of V enrichment relative to the other metals rules out organic complexing as an important process in these sediments. a. Log-log plot showing the covariation of V/Sr vs. Ba/Sr, Cr/Sr and Sc/Sr on an expanded scale.

6.1. Processes controlling the sediment composition Physicochemical processes which can affect the composition of sediment have been discussed by numerous authors including Calvert (1976), Chester and Aston (1976), Curtis (1977) and Taylor and McLennan (1985). The sedimentary cycle of weathering, physical and chemical erosion, deposition, diagenesis and ultimately lithification involves a continuum of chemical and physical processes which can be conveniently considered in terms of source effects, processes that occur during transport and b. Rectilinear plot showing the full range of various element/Sc ratios. The lines are best-fit linear correlations. c. Expanded view of the box in the lower left-hand corner of (b). The trends for Ba/Sc, Ce/Sr and Sc/Sr pass through the origin, suggesting that the carbonate component contains virtually no metals, alkalies, or REE. The residual Zr and Y at V / S t = 0 suggests the presence of a minor third component, probably zircon.

TRACE METALS IN LACUSTRINE AND MARINE SEDIMENTS

deposition, and post-depositional processes. Source effects could include variations in the composition of the parent material (e.g., igneous vs. sedimentary rock), in the conditions under which the weathering and erosion occurred (e.g., humid vs. arid, acid vs. alkaline), and in the nature of the chemical and mechanical agents of erosion (e.g., water-borne vs. aeolian, dissolution vs. physical abrasion). The ultimate composition of a sediment can also be influenced during transport and deposition by the chemical composition and mechanical conditions of the transporting agent (e.g., salinity, pH, flow rate, presence of organic matter). Water-rock partition coefficients and residence times for elements in the transport medium can be strongly controlled by changes in these parameters. Physical mixing of sediment from two or more different sources can occur during deposition. In particular, phases precipitated from seawater, especially skeletal material may add a highly reactive component. Postdepositional changes to the sediment during diagenesis, lithification and later alteration can be controlled by redox conditions and dissolved sulphate concentrations within the sediment, the presence and nature of organic matter within the sediment, and the circulation of fluids within the sediment. The restricted drainage of the Carpentaria basin (Torgersen et al., 1985) and the limited age range of the GC-2 core sediments (from ~ 40 ka ago to present) suggests that source effects should be of minimal importance in controlling the compositions of these sediments. Similarly, the relatively small stratigraphic interval covered by these sediments ( ~ 200 cm), and the systematic variations in ostracod geochemistry (De Deckker et al., 1988; McCulloch et al., 1989) suggest that diagenetic effects are slight. In addition, examination of shells by scanning electron microscopy reveals the lack of recrystallisation of the microfossils. Thus, although we cannot discount source effects and post-depositional processes entirely, it seems most likely that the compositional variations

311

observed in these sediment samples predominantly reflect processes that occurred during transport and deposition. The striking linear correlations among a diverse group of elements over the broad range of observed concentrations suggest that physical mixing of two sedimentary components may be the predominant process controlling the compositions of these sediment samples. If so, one end-member would be rich in Ca, Sr and Mn, suggestive of carbonate. The high Ca contents of Unit-I and -IV samples are beyond what could reasonably be expected from variations in clay composition, and are most readily interpreted as the result of a large carbonate fraction. This is supported by the visually observed abundance of shell material in these units, as well as by the X-ray mineralogical analyses which indicate considerable calcite and aragonite. The other end-member is less easily characterised, but would likely be a clay-rich detrital terrigenous component that carries most of the metals, alkalies and REE. Unit V, which has such a low Ca content that it must be virtually carbonatefree, is probably close to this end-member composition. X-ray determinations support the inference that Unit V is composed predominantly of clay and quartz, with minor halite and only negligible carbonate (Table II). The few ostracods that were recovered from this unit by De Deckker et al. (1988) show signs of dissolution suggesting that carbonate was not stable after deposition of Unit V, possibly due to leaching during pedogenesis (Torgersen et al., 1988). The two-component mixing model can be examined more closely by trace-element ratio plots using a common denominator for the ratio pairs (Fig. 7). Metal/Sr ratios are a convenient choice in this case because of the range of values inferred for the two end-members (near zero for the carbonate; relatively large for the terrigenous component). On such plots mixtures of these two components will form a linear trend passing through the origin. Fig. 7 shows that such a process adequately accounts for the range of metal (V, Ni, Cr, Co, Sc), alkali (Na, K, Ba)

312

and light rare-earth element (LREE) (La, Ce ) abundances. There is no obvious evidence in these data for the presence of other Ba- or LREE-bearing phases such as sulphates or phosphates in these sediments. Similar arguments also suggest that metal enrichment by organic complexes was not an important process controlling the compositions of these sediments. Enrichment of certain metals, principally V, Ni and Mo, relative to average shales in a well-known feature of some organic-rich shales deposited from epicontinental seas under anoxic conditions (Vine and Tourtelot, 1970; Coveney and Martin, 1983; Wenger and Baker, 1986). Unit III records dysaerobic conditions and is somewhat enriched in organic carbon relative to the other units of this core (0.7-1.1% TOC for Unit III vs. _<0.3% TOC for the other units; Torgersen et al., 1988), so it might be expected that this unit would also be anomalously enriched in some metals relative to the other units. Fig. 7 shows that Unit III is not anomalously rich in metals relative to the other units in the core, and in fact actually seems to have lower concentrations of many metals for its Sr content compared to the trend for the other samples. This may actually be due to a lower than expected Sr content in Unit III, as discussed below. The relatively low TOC contents for all of these samples may indicate that the organic matter is oxidised and essentially inert in its ability to provide sites for metallation. Of the elements determined here, only the trends for Zr/Sr and Y / S t do not pass through the origin on the mixing diagrams (Fig. 7). This residual fraction of Zr and Y could result from a third component, from a Zr- and Y-bearing carbonate composition, or from analytical uncertainty. The excellent correlation for Zr vs. the other metals tends to discount analytical uncertainty as a cause for the non-zero intercept, and low-temperature carbonates containing significant amounts of Zr and Y are unknown. The most likely explanation for this excess Zr and Y seems to be the incorporation

M.D. NORMAN AND P. DE DECKKER

of a small amount of zircon into the sediment as a third component. Zircons are often enriched in Y and the heavy REE, and only a small amount of this phase would be needed to influence the bulk-rock composition for these elements. A study of aeolian dust particles in this same core (GC-2) by De Deckker et al. (1990) has independently identified several zircon grains in acid-leached samples. The good linear correlations among the trace elements imply a nearly constant composition for the two predominant end-members (i.e. carbonate and clay) along the length of the core. Can the compositions of the carbonate and terrigenous end-members be further constrained? When plotted against Sr, the negative trends of most of the metals and other elements extrapolate to zero concentration at ~ 1400-1600 ppm Sr (Fig. 8). Assuming the carbonate contains negligible amounts of metals such as V, Ni and Sc, this zero intercept gives the Sr concentration in the carbonate. Similar abundances of Sr in biogenic carbonates have been found elsewhere (Calvert, 1976; Renard, 1985). The lower than expected Sr content in Unit III may be due to incorporation of low-Sr authigenic carbonate into Unit III compared to high-Sr biogenic carbonate in the other units. Authigenic carbonate would be expected to have a lower mineral-water distribution coefficient for Sr compared to biogenic carbonate, based on the wellknown ability of calcareous organisms to concentrate Sr in their shells beyond equilibrium concentrations. Support for this can be found in data presented by Torgersen et al. (1988), which indicate Sr/Ca ratios of 0.0021-0.0033 for calcite from Unit III. These calcites have low Mg contents, so assuming 56% CaO for a pure calcite yields estimated Sr contents of 840-1320 ppm in the carbonate of Unit III, somewhat lower than the 1400-1600 ppm estimated for the principal carbonate component in the other units. (Only Sr/Ca ratios were presented by Torgersen et al., 1988. ) Unit V (sample 205) is virtually carbonatefree, so it can be taken as representative of the

313

TRACE METALS IN LACUSTRINE AND MARINE SEDIMENTS

300

•

.

.

.

.

.

i

.

.

.

=

.

.

.

.

i

ppm 200

1

0

OI 0

-" "-

0

.~

c,=

~

~

. . . .

r- , 1000

500

1500

Sr ppm

Fig. 8. Variation diagram showing Sr vs. Ba, V, Cr, Ni and Sc concentrations. Best-fit linear correlation lines extrapolate to metal concentration = 0 at ~ 1500 ppm Sr. This gives the Sr composition of the carbonate assuming it contains none of the other elements (see Fig. 7).

clay-rich terrigenous end-member. This is supported by the nearly flat, unfractionated element abundance pattern of Unit V relative to an average post-Archaean terrigenous shale (Fig. 9). The major difference between Unit V and "average terrigenous shale" are the lower Ca, Sr, Mn, Ba and Cu in the former. Compared to "average shale", Unit V is depleted in these elements by roughly factors of 2-5. In part this may reflect the natural variability of these elements; for example, in a suite of "typical" postArchaean shales from Australia, Sr varies from 34 to 269 ppm, Ba varies from 240 to 1051 ppm, and Cu varies from 3.2 to 110 ppm (Taylor and McLennan, 1985). Similar variability for these elements has been found in particulate matter from major rivers of the world (Martin and Meybeck, 1979). In contrast, La in the Australian shales varies only from 27 to 52 ppm, and in the rivers from 42 to 50 ppm. The reasons for this natural variability in Ca, Sr, Mn, Ba and Cu are complex and not well understood, but may be related to source effects, the relatively high solubility and correspondingly long aqueous residence times for Ca and Sr, formation of insoluble Ba-sulphate, or the tendency for Cu to form organic complexes with humic

10

. . . . . . . . . .

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\~ \~

~} ® O)

- .... .......

~

"

Unit II (139) unit =v (184) Unit III (159)

_~

0.1

,

,

,

,

,

,

,

10 ' > ~ 1

,

'

,

,

,

,

,

Un;t V ( oo)

- ........ ....... .......

,

,

,

,

,

'

'

'

. . . .

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Unit II (139) Unit IV (184) Unit ill (159)

Unit I (2) A

',',,,"

0.1

,

. . . . . . . . .

~k •,k~'~ , ~ ~A~ " _

m

,

CaSr MnMgNa K "1i AI FeSc Y Ni CfCo 7x Y LaCeCuBa

'

'

..,

'

'

'

'

~.-.

'

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,,"

'

'

",. . . . .

'

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Ca Sr MnMuNa K 13 AI Fe Sc V Ni Cr Co Zr Y LaCeCu Ba

Fig. 9. Diagram showing element abundances in representative samples normalised to: (a) average post-Archaean shale (Taylor and McLennan, 1985); and (b) concentrations in Unit V.

314

acids in near-shore environments (Aston, 1978). The variability of this terrigenous end-member can be investigated further by the stratigraphic variation of key trace-element ratios which would be unaffected by the carbonate component, for this we have normalised the data to Sc concentrations because Sc is a well-determined element that is immobile under a wide variety of conditions, has a very short residence time in seawater ( ~ 2 5 a; Taylor and McLennan, 1985), and is unlikely to be affected by enrichment in carbonate, organic complexes or accessory mineral phases. This effectively allows consideration of the terrigenous-dominated elements on a carbonate-free basis. The limited variation of Fe/Sc, A1/Sc, Ni/Sc, Co/ Sc, Cr/Sc, K / S c an Ti/Sc ratios along the length of the core (Fig. 6) and the parallel shalenormalised patterns of all lithologic units (Fig. 9) suggest that a single component of nearly constant chemical composition contributes nearly the entire sedimentary budget of Fe, A1, Ni, Co, Cr, K and Ti to this core. The recycled terrigenous sediment that was delivered to the Carpentaria basin over at least the last 40 ka appears to have been broadly homogeneous and probably reflects the average upper-crustal composition of the catchment area. Ca, Sr, Zr, Y, Ti, Na, V, Mn and the REE show considerable variation of Sc-normalised ratios across the section. The variation of Ca/ Sc, Sr/Sc and M n / S c are related to the proportions of carbonate. However, it is not immediately obvious why Zr, Y, Ti and the REE should have Sc-normalised patterns that are parallel to each other and that differ from those of the other elements associated with the terrigenous component. All of these elements are very insoluble with short aqueous residence times (Taylor and McLennan, 1985), are unlikely to form organic complexes or substitute into the carbonate lattice, or have their valence affected by changes in redox conditions. Ce can be affected by redox conditions, but the La/Ce ratio of these sediments is constant ( ~ 2 . 1 ) and

M.D. NORMAN AND P. DE DECKKER

identical to that of average post-Archaean shale. These factors also make it difficult to fractionate these ratios simply by changing water volume or sedimentation rate. Changes in solubility can be discounted as the most important factor controlling the Zr, Y and REE abundances in these sediments because the relatively small water volume of lake Carpentaria would have contained insufficient amounts of these very insoluble elements to account for the observed changes in Ti/Sc, V/Sc, Zr/Sc, Y/Sc and R E E / S c ratios, even if all the dissolved material was delivered instantaneously to the sediment. Solubilities of Ti, Zr, Y, Sc and the REE typically range from 10-13 to 10 -11 g g-1 (Taylor and McLennan, 1985), so that delivering all of these elements dissolved in the waters of former lake carpentaria to a layer of sediment 1 cm thick would add only 0.001-0.1 p p m of the individual element to the sediment composition. A change in adsorptive capacity of the particulates related to salinity seems unlikely because of the similar element/ Sc ratios in freshwater units (IV and V) compared to those from more saline lacustrine waters (Unit II). Changes in ion-exchange equilibria probably would have affected other elements like K as well. Peaks in the Zr/Sc, Y/ Sc and R E E / S c trends occur in units inferred to have been deposited under both well-oxygenated (Unit I, open marine) and dysaerobic (Unit III) conditions, suggesting that redox potential was not a controlling factor, which is supported by the constant La/Ce ratio throughout the section. The non-zero intercepts for Zr and Y on the mixing diagrams indicates that at least some of these elements are contained within a separate heavy mineral phase, probably zircon, and it may be that Units I and III contain an excess of this phase. Excess heavy-mineral phases might be related to the comparatively low sedimentation rates for Units I and III (both ~ 40 m m k a - 1) compared to that for Unit II ( ~ 80 mm k a - 1). However, Unit IV, which has a lower apparent sedimentation rate ( ~ 35 m m k a - 1),

TRACE METALS IN LACUSTRINE AND MARINE SEDIMENTS

does not show similar enrichments of these elements. Excess monazite and magnetite in the heavy-mineral assemblage could account for the higher REE/Sc and Ti/Sc, respectively, in Unit III, although evidence for such phases is not apparent from the mixing diagrams (Fig. 7). V is often strongly associated with magnetite, so the decoupling of Ti and V across the section also argues against control of these particular elements by detrital heavy minerals. The high Na/ Sc values in Units I and III are also difficult to account for by heavy minerals, but may indicate the presence of a small amount of halite or plagioclase in these units, which have been detected by X-ray diffractometry (XRD). Alternatively, environmental factors or minor source effects may have contributed to the variations in Ti, Na, Zr, Y, REE and V along the core.

6.2. Environmental implications Although sediment supply to the Carpentaria basin has been predominantly from northern Australia since ~ 36 ka ago (Torgersen et al., 1988), environmental factors and minor source effects are the most likely possibilities to explain the observed changes in Zr, Y, REE, V and Mn relative to the other terrigenous-dominated elements along the length of core GC-2. We believe the changes in Zr/Sc and Y/Sc discussed in the previous section relate to the supply of air-borne zircons which may have been delivered during major episodes of aeolian activity coupled with the expansion of Australian desert (De Deckker et al., 1990). The work of De Deckker et al., (1990) identifies a major contribution of silt-sized aeolian dust to Unit III and an important sandy component in Unit I. Aeolian dust often is enriched in Zr relative to average post-Archaean shales, probably due to preferential entrainment of zircons in the siltsized fraction (Taylor et al., 1983). The sandy material in Unit I reflects sediment reworking during the marine transgression, which also

315

tends to concentrate heavy minerals such as zircon. Water compositions contemporaneous with the formation of Units I and III may also have been somewhat different from those under which the other units were deposited, although the fossil biota (mainly ostracods) recovered from core GC-2 indicate that during both the lacustrine and marine episodes the water was Na-C1 dominant even if it was close to fresh (i.e. low salinity) (see Anadon et al., 1986; De Deckker et al., 1988). Thus the broadly similar elemental composition of both marine and lacustrine sediments in this core probably reflects the same Na-Cl-dominated geochemical pathway in both environments (i.e. pathway II of Eugster and Hardie, 1978), as well as broadly similar sources for the terrigenous components. Nonetheless, variations in parameters such as pH, Eh, or temperature may have affected the delivery of some elements to the sediment. Specifically, the unique patterns for V/Sc and Mn/Sc may be related to changes in water chemistry. Shiller and Boyle (1987) showed that the solubility of V was strongly pH dependent and interpreted this in terms of different hydrolysed forms of V(V) (e.g., VO +, VO(OH) °, VO2(OH)2-). In their experiments, V solubility decreased sharply between pH 2 and 4, then increased between pH 5 and 9. Under the acid to slightly alkaline conditions inferred from the Fe-Mn relations (Fig. 10). V solubility would increase with increasing pH, reducing its delivery to the sediment. Slightly acid lacustrine waters giving way to more neutral oceanic waters would explain the general trend of decreasing V/Sc near the top of the section. Hydrolysed V is considerably more soluble than Sc or the REE, so changes in water conditions is a more viable option for affecting V concentrations in the sediments. Alternatively, V also can be sensitive to changes in source compositions which may explain the slightly lower V/Sc in Unit III, despite the bioactive nature of V in estuarine environments (Shiller and Boyle, 1987).

316

M.D. NORMAN AND P. DE DECKKER 1.5 1 .0 m

Oxides

0.5Eh

0 -

(v)

Fe 2+

2+

I

i

Mn

T. . . . . . .

-0.5-

0 "- L_

Fe ° -1.0-

~ -1.5

m "x Q

O

D "r"

I

I

I

I

4

6

8

10

2

pH

Fig. 10. S i m p l i f i e d p H vs. E h d i a g r a m o v e r l a p p i n g s t a b i l i t y fields for v a r i o u s M n (solid lines) a n d Fe (dashed lines) species, a f t e r M a r s h a l l ( 1 9 7 9 ) . Stippled area i n d i c a t e s region o f soluble M n 2+ a n d i n s o l u b l e Fe 3+.

Mn appears to behave more like soluble Sr and Ca than insoluble Fe (Figs. 3, 4 and 6). Is the covariation of Mn with Ca and Sr caused by substitution of Mn into the carbonate lattice or to coprecipitation of discrete Mn-oxide phases with the carbonate in an oxidising environment? Mn-oxides usually contain a significant fraction of Fe and can also accept Cu, Ni, Co and a number of other ferromagnesian elements (Calvert, 1976; Elderfield and Greaves, 1981). The lack of a correlation between Mn and other ferromagnesian elements (e.g., Fe, Ni, Co ), and the positive correlations between Mn, Ca and Sr across most of the core strongly suggest that Mn was being incorporated into carbonate phases rather than forming discrete oxide phases. This suggests that for much of the evolution of the Carpentaria basin, Mn was in the soluble 2 + oxidation state and available for fixation from the water column into the carbonate lattice. Fe, on the other hand, was apparently insoluble, judging by the virtually constant Fe/Sc ratios across the entire section of the core (Fig. 6). This combination of soluble Mn and insoluble Fe (probably as Fe ~+ ) requires acid to neutral and moderately oxidising water conditions (Fig. 10; see also Jones and

Bowser, 1978). As pointed out by Marshall (1979), the solubility of Mn will exceed that of Fe by as much as 6-7 orders of magnitude at a given Eh and acid pH conditions. The drop in Mn in the uppermost unit (Unit I) may be due to a change in biota as open-marine conditions developed, or possibly to development of more oxidising or pH neutral conditions in the water column under the open-marine conditions so that insoluble Mn a+ was deposited near shore as oxides. These data suggest that the water column above Unit III was also oxidising or else Fe would have been more soluble, resulting in a decrease in Fe/Sc, which is not observed (Fig. 6). Thus the dysaerobic conditions inferred for Unit III developed within the sediment, most likely at the sediment-water interface, rather than in the overlying water column. Alternatively, the change in Mn behaviour many reflect a change in biota as open-marine conditions replaced the lacustrine environment. A major change in sediment composition from the lacustrine to the marine environment is the strong fluctuation in the relative proportions of Ca, Sr and Mn. These elements are controlled largely by the carbonate component, and are naturally affected by the change from lacustrine to marine biota, some of which (principally the molluscs) have an aragonitic test in marine environments which strongly favours incorporation of Ca and Sr over Mn into the carbonate lattice. The work of De Deckker et al. (1988) on the chemistry of ostracod shells recovered at close intervals from core GC-2 allowed the calculation of Sr/Ca ratios for the Carpentaria waters over the last 40 ka. By comparing the Sr/Ca of the whole-rock samples with the inferred water compositions, it is possible to show that the higher Sr/Ca values in the upper part of the core (i.e. for samples 263) reflect the presence of aragonite as well as calcite, whereas the lower part of the core contains principally biogenic calcite. XRD analyses confirm the presence of aragonite in the upper portion of the core (Table II). Sample 205 in Unit V has an unusually high Sr/Ca ratio

317

TRACE METALS IN LACUSTRINE AND MARINE SEDIMENTS

because of its low Ca concentration. A change in the composition of the water column (as opposed to a simple increase in salinity) may also have accompanied the increased bioactivity that induced dysaerobic conditions in Unit III (Torgersen et al., 1988). Biological productivity is largely limited by the supply of nutrients, principally phosphorus, and a change in nutrient supply could indicate a change in source composition or mode of delivery (e.g., an increased aeolian component). Phosphate minerals commonly carry fairly large amounts of REE and Zr, so increased weathering of phosphates could also increase the delivery of REE and Zr. Although in situ precipitation of authigenic phosphates such as francolite could be possible under the dysaerobic conditions inferred for Unit III, this is unlikely under the oxidising conditions of Unit I. However, phosphatic fish bones and teeth may be slightly more abundant in this open-marine unit.

7. Conclusions (1) Late Quaternary sediments from the Gulf of Carpentaria in northern Australia record a marine transgression over a lacustrine sequence. Major- and trace-element analyses of bulk sediment samples taken along the length of a core allow examination of compositional changes related to depositional environments which evolved from lacustrine to open marine over the past 40 ka. (2) Physical mixing of two sedimentary components (biogenic carbonate and terrigenous clay) is the predominant process controlling the compositions of these sediments. Aeolian activity coupled with the expansion of Australian deserts may have preferentially added a small amount of zircon to some horizons in the core. Metal-organic complexing was not an important process in this relatively oxidising environment. (3) The abundance of carbonate primarily controls the concentrations of Ca, Sr and Mn in these sediments. Units deposited under open-

marine conditions contain the highest observed Ca and Sr concentrations, reflecting the highest proportions of carbonate. The recycled terrigenous clay component contributes virtually all of the sedimentary budget of the metallic (A1, Fe, Ti, V, Ni, Co, Cr, Sc, Cu), alkali (Na, K, Ba), and rare-earth (La, Ce, Y) elements. Zircon apparently contributes to anomalously high Zr and Y values for some horizons in the core. (4) Among the metallic elements, V and Mn appear to be most sensitive to depositional environment and water chemistry. These elements were delivered to the sediment more efficiently under lacustrine conditions than under open-marine conditions, possibly reflecting changes in their solubilities as oxic, slightly acid lacustrine waters shifted to a more neutral oceanic environment. Under these conditions, the solubility of hydrolysed V would have increased, resulting in longer aqueous residence times and lower proportions of V in the marine sediments. Mn may have become less soluble and precipitated near shore. Alternatively, the aragonite produced by open-marine biota may have incorporated less Mn than the freshwater biogenic calcite. (5) Environmental conditions apparently had little or no effect on sedimentation of the other metallic elements which are associated with the terrigenous component. Thus carbonate-free marine and lacustrine shales deposited under oxic conditions should have broadly similar trace-element compositions if the lacustrine catchment area drains recycled sedimentary rocks or regions of average upper-crustal composition.

Acknowledgements We thank Chris Foudoulis for the XRD analyses, and Mike Shelley for technical assistance with the ICP. Constructive reviews by Scott McLennan, Kenneth Walker, Lloyd Wenger, and an anonymous reviewer, and helpful discussions with Allan Chivas and Malcolm McCulloch are appreciated. Clementine Krayshek provided expert assistance with the draft-

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