Antarctic Marine Sediments: Distribution of Elements and Textural Characters

MICROCHEMICAL JOURNAL ARTICLE NO.

59, 77– 88 (1998)

MJ981586

Antarctic Marine Sediments: Distribution of Elements and Textural Characters L. Ciaralli,1 R. Giordano, G. Lombardi,* E. Beccaloni, A. Sepe, and S. Costantini Applied Toxicology Department, Istituto Superiore di Sanita`, Viale Regina Elena, 299-00161 Rome, Italy; and *University ‘‘La Sapienza,’’ P.le Aldo Moro, 5-00185 Rome, Italy The analysis of trace metals and their distribution in marine sediments is a very important step toward understanding geochemical and environmental processes and their possible changes due to anthropogenic activities. The present work reports results concerning the distribution of some major (Al), (Fe), and trace (Be, Cd, Cr, Mn, Ni, Pb, Sn, Zn) elements in Antarctic marine sediments. Chemical data were evaluated before and after extraction with 0.5 N HCl, taking into account the textural character of the sediments. The mean values of total metals in the ,2-mm granulometric fraction were (mg/kg) (Al) 56,100 6 3900; (Be) 2.04 6 0.25; (Cd) 0.26 6 0.16; (Cr) 20.3 6 8.3; (Fe) 16,400 6 4800; (Mn) 359 6 108; (Ni) 6.31 6 3.5; (Pb) 20.7 6 2.8; (Sn) 2.12 6 0.71; and (Zn) 42.3 6 10.4. With the exception of Be, all the elements showed higher concentrations in the fraction ,63 mm. The efficiency of extraction with 0.5 N HCl was generally low, as expected for unpolluted sediments. The relative percentage extraction of various elements was similar in the two fractions, with the exception of lead which presented a sensibly increased value in the fraction ,63 mm. The results indicate that our samples are typical of areas not affected by anthropogenic inputs of elements and that potential polluting compounds are not bound within the structure of clay minerals. © 1998 Academic Press Key Words: Antarctica; marine sediments; elements.

INTRODUCTION Antarctica is an ecosystem that can be regarded as mostly free of anthropic influences. For this reason, the distribution of trace metals within the various components should reflect interactions and equilibria only among naturally occurring elements. Nevertheless, the presence of some logistic bases and other human activities can bring about local or regional environmental changes (1). Moreover, a low-level anthropogenic input from the rest of the world could derive from oceanic mixing and atmospheric transport (2). In our contribution to the Antarctica Project (PNRA)—Environmental Impact Sector, we have undertaken a study on the determination of some major and trace metals in superficial sediments from Terra Nova Bay, near the Italian base (Gerlache Inlet); one sample was collected at Wood Bay. The study was performed with the purpose of increasing the baseline information with further data potentially useful in detecting future environmental changes. As the concentrations of elements in sediments depend on particle size distribution, grain-size analysis was also performed on our samples. In fact, particle-size distribution is one of the most significant factors influencing the capacity of sediments to concentrate metals, as their concentrations generally increase with a decrease in grain size owing to the tendency of metals to bind the finest fraction (3). 1

To whom correspondence should be addressed. 77 0026-265X/98 $25.00 Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.

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CIARALLI ET AL.

FIG. 1. Italian expedition, 1993/1994: locations of the sampling stations.

In this work, total element concentrations and their evaluations after extraction with hydrochloric acid were measured on two different granulometric fractions, one smaller than 2 mm and the other smaller than 63 mm. We performed the leaching with hydrochloric acid with the aim of evaluating the performance of this extraction technique on potentially unpolluted samples. This procedure usually provided a rapid way of establishing a potential contrast between anomalous and background levels of elements in environmental monitoring (4, 5). The elements we studied were beryllium, cadmium, chromium, manganese, nickel, lead, tin, zinc, iron, and aluminum. MATERIALS AND METHODS Sampling During the Italian expedition to Antarctica in 1993/1994, nine sediment samples were collected with a stainless-steel grab along the coast of Terra Nova Bay in the Ross Sea; one sample was collected at Wood Bay by drilling the sea ice. The sampling locations are shown in Fig. 1, and the geographic parameters are indicated in Table 1, where it is possible to see that samples are characterized by a minimum depth of 32 m in the western side of Terra Nova Bay and by a maximum of

79

ANTARCTIC MARINE SEDIMENTS TABLE 1 Geographic Position and Depth of Sampling Points Station

Depth (m)

South latitude

East longitude

BTN1a BTN2a BTN3a BTN4a BTN5a BTN6a BTN13b BTN14b BTN15b EMSc

64 105 32 256 135 170 250 200 404 310

74°43.659 74°42.899 74°42.589 74°42.589 74°41.889 74°41.289 74°41.039 74°40.709 74°41.829 74°22.129

164°07.759 164°08.099 164°07.709 164°09.649 164°08.349 164°07.979 164°24.889 164°25.309 164°28.409 165°24.069

a

Western side of Terra Nova Bay. Eastern side of Terra Nova Bay. c Wood Bay. b

404 m in the eastern side. Immediately after collection, sediments were stored and kept at 220°C in precleaned polyethylene containers until treatment. Before analysis, they were left to thaw out and air-dry, and all shell fragments and other visible marine organisms were removed by hand. Sample manipulations were performed under controlled conditions to minimize contamination. Grain-Size Analysis The grain-size analysis was carried out by means of a Retsch Model AS-200 Control vibro sieving apparatus, using inox steel sieves of Wentworth scale from 4 mm down to 63 mm. Finer sizes were analyzed by means of a Sympatec laser diffraction Helos system. Chemical Analysis The reagents used in the study were of Suprapur type (Merck, Germany). To prepare reagents and standards, ASTM type I high-purity water was used, with a background resistivity not lower than 16.6 MV-cm (#0.06 mS/cm) (Labconco). The dissolution procedure for total element determinations has been widely described elsewhere (6). Briefly, it consists of acid digestion in a microwave oven (Milestone) using high-pressure Teflon vessels. Aliquots of 0.2 g of sample were used for Be, Cd, Ni, Mn, Pb, Sn, and Zn determinations; 0.05 g of sample was weighed for Al, Cr, and Fe determinations. After digestion, the final volume of the solution was taken up to 50 or 100 ml with water, respectively. As far as the 0.5 N HCl dissolution is concerned, 1 g of sample was treated with 20 ml of acid for 24 h in a mechanical shaker at room temperature. Afterward, the mixture was centrifuged at 24,000 rpm. The element concentrations were determined by flame (Al, Fe, Mn, Zn) and flameless (Be, Cd, Cr, Ni, Pb, Sn) atomic absorption spectrometry. The spectrometers used were a Perkin–Elmer 5100 Z (HGA-600 graphite furnace, AS-60 autosampler) and a Varian

80

CIARALLI ET AL. TABLE 2 Accuracies and Precisions for Total Element Concentrations MESS-1 (mg/kg)

PACS-1 (mg/kg)

MURST-ISS-Al (mg/kg)

Element

Certified

Found

Certified

Found

Certified

Found

Ala Be Cd Cr Fea Mn Ni Pb Sn Zn

5.83 6 0.2 1.9 6 0.2 0.59 6 0.10 71 6 11 3.05 6 0.17 513 6 25 29.5 6 2.7 34.0 6 6.1 3.98 6 0.44 191 6 17

5.79 6 0.05 2.0 6 0.1 0.60 6 0.02 69 6 4.1 3.09 6 0.09 508 6 12 28.1 6 1.3 36.6 6 1.2 4.1 6 0.1 187 6 8

6.47 6 0.15 — 2.38 6 0.2 113 6 8 4.87 6 0.09 470 6 12 44.1 6 2.0 404 6 20 41.1 6 3.1 824 6 22

6.41 6 0.13 — 2.23 6 0.08 109 6 8 4.93 6 0.06 466 6 12 43.4 6 1.7 402 6 18 40.3 6 2.7 825 6 13

6.71 6 0.33 — 0.54 6 0.03 42.1 6 3.4 2.44 6 0.07 446 6 18 9.56 6 0.04 21.0 6 2.9 — 53.3 6 2.7

6.57 6 0.11 — 0.54 6 0.06 39 6 1.4 2.42 6 0.05 455 6 6 10.3 6 0.4 22.2 6 0.1 — 53.8 6 1.7

a

Values expressed in g%.

SpectrAA 300 Z (GTA-96 graphite furnace, PDS-96 autosampler). Thermal programs for flameless determinations were experimentally chosen for each element. Suitable matrix modifiers were used and the quantifications were done against calibration curves. To verify the validity of the calibration procedure the standard addition method was, at intervals, also employed. Accuracy and precision of total concentrations were evaluated through the use of suitable reference materials. The results are summarized in Table 2. Three materials were employed; in particular, MURST-ISS-A1 is a new reference material of Antarctic sediment prepared at the Istituto Superiore di Sanita` within the framework of the PNRA project. For the extraction method, precision was evaluated directly using a real sample; the results, expressed as coefficients of variation, are reported in Table 3. For total element analyses, the quantification limits, calculated as 10 3 s (standard deviation of 10 measurements of a standard solution having a concentration close to blank) and referred to the entire procedure, were (mg/kg) (Al) 50, (Be) 0.05, (Cd) 0.02, (Cr) 0.5, (Fe) 10, (Mn) 10, (Ni) 0.4, (Pb) 0.5, (Sn) 0.4, and (Zn) 0.5. In relation to a lower solution/sample ratio, the quantification limits for the HCl extraction were better than those obtained for the total concentration analysis (mg/kg): (Al) 1.0, (Be) 0.008, (Cd) 0.004, (Cr) 0.010, (Fe) 0.2, (Mn) 0.2, (Ni) 0.1, (Pb) 0.05, (Sn) 0.03, (Zn) 0.1. Statistical Analysis The statistical evaluation involved analysis of variance, the multiple regression method, and the Mann–Whitney U test. RESULTS AND DISCUSSION Table 4 shows that all samples may be classified as sands with a very small clay fraction (,1%), but there are significant differences in their sorting, as revealed by the range of the

81

ANTARCTIC MARINE SEDIMENTS TABLE 3 HCl Leaching: Coefficients of Variation (%) (Performed on Five Analyses) Fraction Element

,2 mm

,63 mm

Al Be Cd Cr Fe Mn Ni Pb Sn Zn

2.2 10.2 3.1 8.9 5.1 5.5 4.9 4.6 10.2 6.0

1.5 8.5 3.2 7.4 5.2 4.3 4.0 3.8 7.8 5.8

.2-mm fraction (from 0.51 to 42.3%), with a reasonably direct relationship between amount, depth, and distance from the coast. Sample EMS from Wood Bay, dredged at 2310 m, has a greater than 3%.2-mm fraction. Among the samples from the Western side of Terra Nova Bay, BTN3, sampled close to the coast at 232 m, has more than 42% of the .2-mm fraction, whereas the other five samples, taken further away from the coast and at depths down to 2256 m, have a maximum of 3.09% of the .2-mm fraction. On the eastern side of the Bay, the two samples closer to the coast and to the Campbell Glacier have 7.13 and 18.65% of the .2-mm fraction, in contrast with only 1.81% for sample BTN13, away from the coast. Seven of ten samples (BTN1, BTN2, BTN4, BTN5, BTN6, BTN13, BTN14) are

TABLE 4 Grain-Size Characteristics

Samplea

Gravel (.4–2 mm)

Coarse sand (2–0.71 mm)

Medium sand (710–250 mm)

Fine sand (250-63 mm)

Silt (,63 mm)

Clay (,63 mm)

BTN1 BTN2 BTN3 BTN4 BTN5 BTN6 BTN13 BTN14 BTN15 EMS

1.79 2.25 42.30 0.51 1.48 3.09 1.81 7.13 18.65 31.48

2.07 1.74 40.23 2.22 0.65 2.76 2.28 1.41 11.60 18.59

13.75 26.10 13.62 30.19 12.16 14.02 26.55 20.58 19.06 14.37

81.31 67.20 3.75 64.41 83.99 77.54 62.48 64.46 38.63 19.80

1.05 2.66 0.10 2.61 1.70 2.52 6.61 6.24 11.70 15.02

0.03 0.05 0 0.06 0.02 0.07 0.27 0.18 0.33 0.74

a

BTN 5 samples from Terra Nova Bay, EMS 5 sample from Wood Bay.

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CIARALLI ET AL.

FIG. 2. Grain-size cumulative curves of the analyzed samples.

texturally similar, as may be seen from their cumulative curves (Fig. 2) and descriptive parameters (Table 4): more than 91% of their granules are between 2 mm and 62.5 mm (the field of sands); their sorting index (Sg) is a maximum of 1.04 (indicative of the existence of small ‘‘tails’’ toward smaller and larger sizes); their median sizes (Md) are very similar, between 2.37 and 2.67. Altogether they may be classified as unimodal sediments derived from one major transporting energy output. The other three samples (BTN3, BTN15, EMS) differ significantly: the larger-sized fractions are more abundant; the mean size is larger; the sorting indexes are as high as 2.91; they have significantly large ‘‘tails.’’ They may be classified as polymodal sediments, generated from diverse energy moments. Chemical analyses were performed on two different granulometric fractions: the ,2-mm and ,63-mm fractions (hereafter indicated as F2 and F63, respectively). F2 was chosen because grain-size analysis showed it to be representative of most samples; F3, although present in our sediments in a very low percentage, was considered because it is a fraction suggested for environmental investigations (7–10) because of its known capability to bind elements by complex mechanisms of absorption/adsorption that occur at the water/sediment interface. Thus, this fraction also has significant implications for filter-feeding organisms and is a valid aid in recognizing concentrations indicative of enrichment. Total element concentrations are reported in Table 5 together with the data on HCl extraction. In general, the concentrations of toxicologically interesting elements appear to be characteristic of unpolluted areas (11) and sensibly lower than those reported for Antarctic sediments collected around McMurdo Station (1), an area that has been heavily influenced by the largest human settlement in Antarctica (U.S.A. Base). With a few exceptions (Al, Be), all the metals showed a remarkable dispersion of

83

ANTARCTIC MARINE SEDIMENTS TABLE 5 Total Element Concentrations in Sampling Stations and Percentage Extraction in 0.5 N HCl Fraction ,63 mm

Fraction ,2 mm

Station

Total (mg/kg)

BTN1 BTN2 BTN3 BTN4 BTN5 BTN6 BTN13 BTN14 BTN15 EMS

2.40 1.90 1.50 2.30 2.30 2.00 2.00 2.00 2.00 1.50

Extracted (mg/kg)

% Extraction

Total (mg/kg)

Extracted (mg/kg)

% Extraction

a. Beryllium 0.04 0.04 0.03 0.05 0.06 0.06 0.15 0.13 0.15 0.41

1.67 2.11 2.00 2.17 2.61 3.00 7.50 6.50 7.50 27.33

1.60 2.00 1.20 1.50 2.20 1.70 1.70 2.00 1.60 1.30

0.10 0.10 0.09 0.13 0.12 0.14 0.16 0.16 0.20 0.44

6.25 5.00 7.50 8.67 5.45 8.24 9.41 8.00 12.50 33.85

1.27 0.73 12.91 0.53 0.54 0.48 0.41 0.39 0.23 0.14

1.17 0.73 10.41 0.50 0.53 0.45 0.42 0.35 0.20 0.07

92.13 100.00 80.17 94.34 98.15 93.75 102.44 89.74 86.96 50.00

55.90 44.80 41.30 39.30 56.40 50.80 65.80 35.80 43.00 77.70

3.93 4.25 1.03 4.30 4.02 7.10 8.70 4.41 5.65 5.90

7.03 9.49 2.49 10.94 7.13 13.98 13.22 12.32 13.14 7.59

b. Cadmium BTN1 BTN2 BTN3 BTN4 BTN5 BTN6 BTN13 BTN14 BTN15 EMS

0.37 0.23 0.66 0.13 0.18 0.27 0.22 0.16 0.16 0.16

0.34 0.23 0.61 0.10 0.16 0.22 0.18 0.14 0.15 0.07

BTN1 BTN2 BTN3 BTN4 BTN5 BTN6 BTN13 BTN14 BTN15 EMS

22.20 13.10 4.10 18.40 15.20 25.60 27.90 22.90 33.50 55.20

1.06 0.98 0.81 1.07 0.99 2.94 2.84 1.96 2.62 3.54

91.89 100.00 92.42 76.92 88.89 81.48 81.82 87.50 93.75 43.75 c. Chromium 4.77 7.48 19.76 5.82 6.51 11.48 10.18 8.56 7.82 6.41 d. Manganese

BTN1 BTN2 BTN3 BTN4 BTN5 BTN6 BTN13 BTN14 BTN15 EMS

421.0 333.8 107.1 341.0 286.0 514.0 405.0 402.0 423.4 1018.0

21.2 27.7 19.0 17.9 22.0 46.1 47.2 48.4 49.2 155.2

5.04 8.30 17.74 5.25 7.69 8.97 11.65 12.04 11.62 15.25

1497.0 1201.0 498.7 766.0 1134.0 1292.0 707.0 661.6 478.4 832.0

58.9 62.4 53.6 53.5 56.0 65.6 63.5 56.8 62.3 166.1

3.93 5.20 10.75 6.98 4.94 5.08 8.98 8.59 13.02 19.96

84

CIARALLI ET AL. TABLE 5—Continued Fraction ,63 mm

Fraction ,2 mm

Station

Total (mg/kg)

Extracted (mg/kg)

BTN1 BTN2 BTN3 BTN4 BTN5 BTN6 BTN13 BTN14 BTN15 EMS

4.20 3.90 2.20 3.70 3.70 7.50 9.30 8.70 13.60 32.70

0.72 0.93 0.67 0.94 0.98 1.58 2.56 2.30 3.01 6.18

BTN1 BTN2 BTN3 BTN4 BTN5 BTN6 BTN13 BTN14 BTN15 EMS

21.50 24.50 27.70 20.50 23.10 20.40 17.20 16.30 15.30 11.60

0.59 0.50 1.33 0.64 0.59 0.87 1.85 1.81 2.46 1.75

BTN1 BTN2 BTN3 BTN4 BTN5 BTN6 BTN13 BTN14 BTN15 EMS

2.8 1.3 0.9 2.3 2.4 3.3 2.5 1.6 2.0 2.3

0.13 0.13 0.33 0.13 0.13 0.17 0.13 0.13 0.13 0.19

% Extraction

Total (mg/kg)

Extracted (mg/kg)

% Extraction

15.10 12.00 20.20 15.70 15.20 14.70 19.70 13.10 18.80 46.40

3.09 3.03 7.52 5.22 3.61 5.20 6.51 5.37 5.98 13.88

20.46 25.25 37.23 33.25 23.75 35.37 33.05 40.99 31.81 29.91

17.70 22.40 79.50 14.10 19.80 15.70 14.50 14.60 13.60 11.00

4.00 4.58 26.7 3.57 5.40 3.60 3.52 3.43 3.76 5.20

22.60 20.45 33.55 25.32 27.27 22.93 24.28 23.49 27.65 47.27

6.90 6.00 10.60 3.40 5.00 4.20 3.30 3.00 3.70 4.80

0.47 0.84 0.33 0.47 0.77 0.48 0.49 0.49 0.63 0.77

6.81 14.00 3.11 13.82 15.40 11.43 14.85 16.33 17.03 16.04

e. Nickel 17.14 23.85 30.45 25.41 26.49 21.07 27.53 26.44 22.13 18.90 f. Lead 2.74 2.04 4.80 3.12 2.55 4.26 10.76 11.10 16.08 15.09 g. Tin 4.64 10.00 36.67 5.65 5.42 5.15 5.20 8.13 6.50 8.26 h. Zinc BTN1 BTN2 BTN3 BTN4 BTN5 BTN6 BTN13 BTN14 BTN15 EMS

44.30 38.00 20.60 34.20 37.80 49.30 52.50 46.60 57.10 142.30

10.8 11.7 8.9 9.7 10.9 17.1 20.5 21.7 24.4 31.8

24.38 30.79 43.20 28.36 28.84 34.69 39.05 46.57 42.73 22.35

125.60 152.20 94.00 92.20 111.40 98.90 76.60 74.80 75.00 154.80

39.8 34.9 44.8 34.4 38.0 27.4 34.2 34.4 34.4 41.0

31.69 22.93 47.66 37.31 34.11 27.70 44.65 45.99 45.87 26.49

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ANTARCTIC MARINE SEDIMENTS TABLE 5—Continued Fraction ,63 mm

Fraction ,2 mm

Station

Total (g/kg)

Extracted (g/kg)

BTN1 BTN2 BTN3 BTN4 BTN5 BTN6 BTN13 BTN14 BTN15 EMS

62.80 59.60 58.60 51.50 58.30 56.80 53.00 51.00 52.90 58.50

1.02 1.26 0.76 0.97 1.05 1.74 3.64 2.96 3.86 7.52

BTN1 BTN2 BTN3 BTN4 BTN5 BTN6 BTN13 BTN14 BTN15 EMS

20.50 16.30 6.80 12.90 11.30 21.10 18.20 18.70 21.60 55.70

1.86 2.59 1.86 1.49 1.89 4.30 3.67 3.70 4.10 8.75

% Extraction

Total (g/kg)

Extracted (g/kg)

% Extraction

45.0 50.0 31.3 36.2 51.4 42.3 41.6 43.0 40.7 49.4

2.58 2.15 2.52 2.70 2.51 3.50 3.70 3.39 3.85 9.45

5.73 4.30 8.05 7.46 4.88 8.27 8.89 7.88 9.46 19.13

63.50 54.60 25.40 30.60 44.60 45.60 25.80 23.90 21.60 50.90

4.68 6.38 4.00 4.05 4.77 5.10 5.20 5.20 4.85 9.56

7.37 11.68 15.75 13.24 10.70 11.18 20.16 21.76 22.45 18.78

i. Aluminum 1.62 2.11 1.30 1.88 1.80 3.06 6.87 5.80 7.30 12.85 j. Iron 9.07 15.89 27.35 11.55 16.73 20.38 20.16 19.79 18.98 15.71

values, probably caused by the poor grouping of the sampling stations. Samples BTN3 and EMS showed very different behavior, as previously observed for the granulometric data. This occurred for several elements with respect to both concentration and efficiency of extraction. In consideration of the different sampling area, the single Wood Bay sample was not included in the statistical evaluations. Table 6 lists mean values calculated for total elements in both F2 and F63 together with their coefficients of variation and the percentages of extraction. Beryllium levels were comparable to those reported for the Earth’s crust (12) and exhibited a distribution quite regular throughout the examined area. The mean concentrations in the two fractions were slightly different (P , 0.05), with the highest value observed in F2. On the contrary, a greater efficiency of extraction with 0.5 N HCl occurred in F63 (P , 0.002). Similar behavior was noted for aluminum, but the differences were of higher statistical significance. In any case, both Al and Be were scantily soluble with very low extraction values. These observations, together with the presence of some significant correlations between these two elements (AlTot vs BeTot, r 5 0.91, P , 0.001; AlF2Ext vs BeF2Ext, r 5 0.98, P , 0.0001; AlF63Ext vs BeF63Ext, r 5 0.90, P , 0.001; Tot 5 total concentration; Ext 5 concentration after extraction) could mean that beryllium follows the same pattern of aluminum.

86

CIARALLI ET AL. TABLE 6 Mean Concentrations of Elements in Sediments and Percentage Extraction in 0.5 N HCl Mean 6 SD (mg/kg)

CV%

Be , 2 mm

2.04 6 0.25

12.4

Be , 63 mm Cd , 2 mm

1.72 6 0.29 0.26 6 0.16

16.6 58.8

Cd , 63 mm Pb , 2 mm

1.94 6 3.89 20.7 6 2.8

200 18.4

Element

Pa

,0.05 ,0.005

% Extraction in 0.5 N HCl 3.9 6 2.4 7.9 6 2.2 88.3 6 6.8

23.5 6 20.1 359 6 108

84.9 30.2

Mn , 63 mm Zn , 2 mm

915 6 350 42.3 6 10.4

38.3 24.6

Zn , 63 mm Ni , 2 mm

100 6 24.5 6.31 6 3.5

24.5 55.4

Ni , 63 mm Sn , 2 mm

16.1 6 2.7 2.12 6 0.71

17.0 33.5

Sn , 63 mm Cr , 2 mm

5.12 6 2.30 20.3 6 8.3

44.9 40.7

Cr , 63 mm Fe , 2 mm

48.1 6 9.2 16,400 6 4,800

19.1 29.1

Fe , 63 mm Al , 2 mm

37,300 6 14,400 56,100 6 3,900

38.6 6.9

Al , 63 mm

42,400 6 5,900

13.9

a b

,0.0001 ,0.0001 ,0.0001 ,0.0001 ,0.0001 ,0.0001 ,0.0001

,0.002

NS 93.1 6 6.5 6.4 6 4.7

NSb Pb , 63 mm Mn , 2 mm

Pa

25.3 6 3.6 9.8 6 3.7

,0.0001

NS 7.5 6 2.9 35.4 6 7.4 NS 37.6 6 8.5 25.4 6 3.7 31.2 6 6.4 9.7 6 9.6

,0.01

NS 12.5 6 4.4 9.2 6 4.2 NS 9.9 6 3.6 17.8 6 5.0 NS 14.9 6 5.1 3.5 6 2.3 0.001 7.2 6 1.7

Statistical difference between the two granulometric fractions, Mann–Whitney U test. Not significant.

Cadmium showed an uneven distribution, particularly in the fine fraction, where some spikes were observed at certain stations (BTN1, BTN2, BTN3). This aspect is probably correlated with the presence of numerous fragments of shells noted in several samples from Terra Nova Bay; in fact, Cd tends to be enriched in biogenic calcareous debris. Concentrations of Cd ranging from 8 to 13 mg/kg measured in some small pieces of shells found in our samples support this opinion. In consideration of this, some of the high levels we observed should not be surprising. According to this hypothesis of natural enrichment, the HCl leaching extracted Cd almost fully from all samples collected in Terra Nova Bay; a significantly larger percentage of the element was resistant to the acid in the sediment from Wood Bay, which would indicate that in this sample a large amount of Cd is residual and, consequently, its bioavailability is low.

ANTARCTIC MARINE SEDIMENTS

87

With respect to lead, total element concentrations did not differ between the two fractions but the percentage extracted was statistically higher (P , 0.0001) in F63. A higher level of Pb in the BTN3 sample yelded a high coefficient of variation in the fine fraction. In general, the efficiency of extraction of Pb was quite low; however, despite a lower total concentration in F2, the samples collected on the eastern side of the bay (BTN13, BTN14, BTN15) showed greater solubility in 0.5 N HCl. As far as the remaining elements are concerned (Cr, Fe, Mn, Ni, Sn, Zn), total concentrations were generally higher in F63 than in the coarse fraction (P , 0.0001); this is in agreement with the general opinion that metals in sediments are often associated with the finest particles. However, the percentages of elements leached by the HCl did not differ significantly in the two fractions, with the exception of nickel, which gave a higher percentage extraction in F63 (P , 0.01). In the light of the results, the efficiency of extraction was generally low, as expected for unpolluted sediments. In the two fractions, the solubilities of the leached elements were as follows: Fraction , 2 mm:

Al > Be , Pb , Cr > Sn > Mn , Fe , Ni , Zn , Cd

Fraction , 63mm:

Al > Mn > Be , Cr , Sn , Fe , Pb , Ni , Zn , Cd

As is possible to see, the relative percentages extraction of various elements were similar in the two fractions with the exception of lead, which presented a sensibly increased value in F63. Significant correlations were found for both total and extractable elements and this probably indicates that the metals come, in general, from similar sources and that their geographic distribution patterns are similar as well. Some strong correlations among total metal content (P , 0.001) can be attributed to the mineralogical composition, as generally expected for unpolluted sediments in relation to their geochemical affinities: MnF2 vs FeF2, r 5 0.95, P , 0.0001; NiF2 vs CrF2, r 5 0.85, P , 0.003; ZnF2 vs FeF2, r 5 0.94, P , 0.0002; ZnF63 vs FeF63, r 5 0.91, P , 0.0006. Moreover, the comparison between the group of samples from the western side of the Bay and that from the eastern area showed appreciable differences for some variables in F2 (Pb, Ni: P , 0.005; Cr, Zn, Al: P , 0.05) and in F63 (Zn, Fe: P , 0.05). Other, statistically significant differences were found among the elements extracted by the acid; since these differences cannot be attributable to a grain-size effect, they are probably due to compositional parameters that may be different in the two groups of samples. A larger number of samples are required to validate this finding. With respect to the average, sample BTN3 showed higher concentrations of Cd, Pb, Ni, and Sn coupled with lower values of Al, Fe, Mn, and Zn; this may be related to a grain-size effect. In fact, this sample has a very small amount of the fine fraction whereas it has a very large percentage of coarse components; this context may bring about a random shift in metal contents. Some element concentrations and percentages extraction were higher in the only Wood Bay sample than in the samples from Terra Nova Bay; this is likely to be due to a different watershed lithology and sedimentation basin. A comparison of our data with those of previous Antarctic campaigns in the same area is not fit because the elements analyzed and the granulometric fractions are not always the same. However, the levels of Cd, Cr, Ni, and Pb found in F2 were similar to those obtained by Hieke Merlin et al. (1987/1988) (13), but lower than the values reported by Cosma et

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al. (14) for the 1989/1990 campaign. The values we found in F63 were higher than those of the other authors, but this is probably due to differences in the grain size of the fractions analyzed. CONCLUSIONS The total element concentrations we obtained indicate that our sediment samples are typical of areas not affected by anthropogenic inputs of elements. The grain size analysis showed that none of the samples presented a significant clay fraction. We may therefore reasonably conclude that potential polluting compounds are not bound within the structure of clay minerals, but instead adhere to the surface of the sand and silt granules. This hypothesis is strengthened by the observation that even in the fine fraction examined, the percentage of elements extracted by 0.5 N HCl was low except for Cd as a consequence of the low solubility expected. ACKNOWLEDGMENTS The financial support of the Italian Antarctic Project (PNRA) is greatly acknowledged. The authors also thank all of the members of the Italian Environmental Impact Group for collecting samples in Antarctica (Expedition 1993/1994).

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