DISTRIBUTION OF HEAVY METALS IN SURFACE SEDIMENTS FROM AN ANTARCTIC MARINE ECOSYSTEM SANTIAGO ANDRADE1 , ARMANDO POBLET2 , MARCELO SCAGLIOLA3 , CRISTIAN VODOPIVEZ4 , ANTONIO CURTOSI4 , ADÁN PUCCI1 and JORGE MARCOVECCHIO1,2∗ 1 Instituto Argentino de Oceanografía, Bahía Blanca, Argentina; 2 Universidad Nacional de Mar del Plata, Mar del Plata, Argentina; 3 OSSE, Mar del Plata, Argentina; 4 Instituto Antártico
Argentino (IAA), Buenos Aires, Argentina (∗ author for correspondence, e-mail:
[email protected])
(Received 16 June 1998; accepted 28 October 1999)
Abstract. The concentrations of lead, cadmium, copper, chromium, iron, manganese and zinc in surface sediments collected from Potter Cove, in the 25 de Mayo Island (King George Island), Antarctica, and its drainage basin, were measured by atomic absorption spectroscopy. The obtained results were use to determine the areal and vertical distribution of the metals of in the Cove and potential sources of these metals to this environment. The geochemical data suggest that most of the metals found in Potter Cove constitute a redistribution of autochthonous materials within the ecosystem. Therefore, the metal concentrations can be considered to be present at natural background levels in surface sediments. Keywords: Antarctica, marine ecosystem, surface sediments
1. Introduction Potter Cove is an Antartic marine ecosystem located within the 25 de Mayo Island in The Southern Shetland Islands archipelago. This ecosystem is stable and has a large percentage of indigenous species. Thus, its structure is simple in comparison with other ecosystems. The Antarctic area has been recognized as a comparatively closed environment (Honda et al., 1987), basically sustained by the very slow atmospheric mixing between northern and southern zones of the Antartic Convergence. These natural barriers may prevent the atmospheric deposition of global pollutants which affect many areas of the planet. However, when toxic compounds are releaved in the area, e.g., from Research Stations such as Jubany Station, in Potter Cove, the contaminants cannot be easily dispersed, and the pollutants become localized, concentrated and accumulated in the sediments. The geographical distribution of heavy metals in surface marine sediments is regulated not only by their concentrations, but also by sediment physico-chemical characteristics, mineralogic composition, grain size distribution, organic matter content, etc. as well as several environmental conditions (marine currents, wind, continental runoff, etc.) (Salomons and Förstner, 1984). Environmental Monitoring and Assessment 66: 147–158, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.
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Figure 1. Location of sampling stations.
Moreover, within this abiotic compartment, the transport and deposition of various natural and man-made substances are dependent on the nature of the particles with which they are associated. In addition, and looking for an integral understanding of trace metal processes occurring in surface sediments, several conditions might be included as has been opportunely pointed out by different authors: i.e., physical and chemical characteristics of surface sediments (Salomons and Förstner, 1984), metals bio-geochemical cycles involved within the considered environment (Barcellos and Lacerda, 1996), as well as the role of sediments as a dynamic site for biological and chemical reactions and as a food source for marine organisms (Muir et al., 1999). Even though numerous papers have been written on the occurrence, concentration and distribution of heavy metals within marine sediments (Katz and Kaplan, 1981, Ryan and Windom, 1988), data portaining to antarctic environments is scarce, and in the specific case of the Potter Cove, only two papers are available (Alam and Sadiq, 1993; Scagliola et al., 1994). This paper includes preliminary results on the concentration and spatial distribution of lead, cadmium, manganese, copper, iron and zinc of surface sediments of Potter Cove, 25 de Mayo Island, Antarctica.
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2. Materials and Methods This study was conducted in Potter Cove, which is a small bay located between 62◦ 140 and 62◦ 150 S. and 58◦ 390 and 58◦ 430 W., in the ‘25 de Mayo Island’ (King George Island), on the Southern Shetland Archipelago, Antarctica (Figure 1). The Cove is a small fiord which exhibits an outer and internal zone separated by a rise in the bottom topography to a depth of 30 m. The internal area has a maximum depth of 50 m, and its bottom sediment is predominantly composed of mud (< 63 µm). Its northern and eastern coasts are limited by several glaciers, while the southern boundary is formed by a sandy beach. Observations of surface waters suggest a wind-controlled circulation pattern exists within the cove which generates a cyclonic gyre promoting the input of shelf water to the cove, and delaying the corresponding output (Kloser, 1994). Field work and sampling were carried out during an Argentine Antarctic Summer Cruise in 1994–95. Three marine transects were designed, including depths from 5m, 10m, 20m and 30m. Samples of surface sediments were collected at fourteen stations, using plastic sledges following the removal of an upper layer of gravel. Surface sediment samples were also collected from the ‘The Lagoon’ and from two streams which flow between the lagoon and Potter Cove (Figure 1). All sediment samples were placed in plastic bags, and stored in a freezer (at – ◦ 20 C) until their analytical treatment at the laboratory. These samples were dried, at 40 ± 5◦ C for 48 h until developing constant weight. Subsequently, they were carefully sifted through different iron steel meshes to determine their grain size distribution. Samples of seawater and freshwater were collected at the same stations, using polypropilene sampling bottles. These samples were vacuum-filtered through a 0.45 µ pore size cellulose acetate filter. The retained material on the filter (suspended particulate matter) was kept in a freezer, at –20 ◦ C, until its analysis at the laboratory. The analytical determination of heavy metals (Pb, Zn, Cu, Fe, Mn and Cd) in the sediments and suspended particulate matter (SPM) follow the method previously reported by Marcovecchio et al. (1988). Subsamples of 500 ± 50 mg of sediment were removed, and digested with a perchloric and nitric acids mixture (1:3) in a thermostatic bath (at 90 ± 10 ◦ C), up to minimum volume (less than 1 ml). Solutions were made up to 10 ml with 0.7% nitric acid, and the absorbances of each metal were measured by atomic absorption spectrophotometry. Membrane filters with SPM samples were digested using the same procedure. A Shimadzu AA-640-13 atomic absorption spectrophotometer was utilized for all the analyses, working with air/acetylene flame and deuterium background correction, and the data were processed with a personal computer using appropiate software. Reagents of analytical grade were utilized for the blanks and calibration curves, and the analytical quality (AQ) was checked against a reference material (Pond Sediment, R.M. #2), provided by The National Institute for Environ-
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TABLE I Percentages of recovery in the analysis of reference materials to assess analytical quality Metal analyzed
Percentage of Recovery (range)
Cd Cu Cr Fe Mn Pb Zn
91.4 – 99.3% 93.1 – 99.5% 92.8 – 99.2% 95.6 – 101.7% 91.2 – 97.9% 94.7 – 98.8% 96.5 – 102.3%
mental Studies (NIES) from Tsukuba, Japan (Table I). The obtained results were copmpared through one-way analysis of variance (ANOVA).
3. Results and Discussion 3.1. T RACE
METALS : MARINE SURFACE SEDIMENTS
The concentration (mean value ± standard deviation for each studied transect) of selected trace metals bulk sediment samples of the surface sediments from Potter Cove are presented in Table II and Figure 2. Most of the evaluated heavy metals exhibit a similar distribution trend along the studied area, even though their concentration ranges are completely different. The spatial distribution of lead, chromium, copper, zinc and manganese concentrations suggests that they are controlled by similar processes and that these metals exhibit a similar behaviour in the evaluated environment; this fact was verified through an analysis of co-variance (Table III). Unlike the above mentioned description, iron possesses a specific spatial pattern of distribution, considering that its concentration was higher (in a different order of magnitude) than the other metals along the whole studied area (Table II; Figure 2). This fact is related with iron condition of ‘abundant element’ (Salomons and Förstner, 1984), and provides a good start-point to interpret the natural background of metals in surface sediments (Schropp et al., 1990; Amín et al, 1996). The concentrations of cadmium as determined in the sediments of the Potter Cove were – in every case – below the detection limit of the utilized analytical method (0.25 µg g−1 ). These results did not agree with those previously reported
Transect #1 n = 4 Transect #2 n= 5 Transect #3 n= 5
Zn (µg g−1 )
Cu (µg g−1 )
Cr (µg g−1 )
Pb (µg g−1 )
Mn (mg g−1 )
Fe (µg g−1 )
53.69 ± 6.09 {45.98–63.02} 51.49 ± 5.79 {45.53–59.12} 52.23 ± 4.78 {44.96–56.66}
140.97 ± 9.95 {128.5–156.3} 115.30 ± 20.43 {83.64–137.81} 94.11 ± 16.29 {73.37–110.61}
6.73 ± 0.63 {6.01–7.72} 6.42 ± 1.16 {5.09–8.11} 5.66 ± 1.22 {4.11–7.29}
3.92 ± 0.97 {3.06–5.52} 3.68 ± 0.77 {2.94–4.84} 3.40 ± 0.95 {2.29–4.64}
1.05 ± 0.05 {0.99–1.13} 0.94 ± 0.02 {0.92–0.98} 0.88 ± 0.05 {0.79–0.95}
15.51 ± 3.79 {12.01–21.39} 13.26 ± 4.83 {6.55–18.63} 10.86 ± 3.71 {5.15–14.25}
DISTRIBUTION OF HEAVY METALS IN SURFACE SEDIMENTS
TABLE II Mean, standard deviation and ranges of heavy metals concentrations detected in surface marine sediments from Potter Cove. (Expressed in µg g−1 , dry weight)
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Figure 2. Heavy metals concentrations in surface marine sediments from Potter Cove.
by Alam and Sadiq (1993) for sediments collected close to Jubany Station, in which cadmium residues (19.2 ± 2.8 µg g−1 dry wt.) were evaluated in surface sediments. Higher contents of zinc, copper, lead and manganese were recorded in the Station beneath 20 m of water along Transect #1 (T1 20 m) (Figure 1). In this area of Potter Cove seawater circulation is slower than in any other area resulting in an increase in the deposition of the suspended particulate matter. Moreover, in summer, particulate matter deposition is enhanced by the influx of material from Potter Stream (B). The highest heavy metal levels within the other transects have
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TABLE III Correlation matrix showing the coefficients of correlation between different pairs of variables measured in bottom sediment. S + C: silt + clay (< 63 µm)
Cu Cr Fe Mn Pb Zn S+C
Cu
Cr
Fe
Mn
Pb
Zn
S+C
1.00 0.67a 0.53b 0.89a 0.64b 0.62b 0.89a
– 1.00 0.57b 0.65b 0.63b 0.69a 0.79a
– 1.00 0.62b 0.29 0.35 0.47
– 1.00 0.53b 0.56b 0.80a
– 1.00 0.76a 0.70a
– 1.00 0.62b
– 1.00
a Significant at the 95% level (P < 0.05) b Significant at the 99% level (P < 0.01)
been recorded with the Station at 10 m depth along Transect #2 (T2 10 m) and at 20 m depth along Transect #3 (T3 20 m). Zinc, iron, lead and chromium contents in surface sediments have shown non significant differences when a test for comparison of mean values (ANOVA) – in a 25% significance level – were performed between considered transects. Unlike this, copper and manganese have exhibited high significant differences (p < 0.01) in the same test. The heavy metal concentration reported in this paper are similar to those reported as natural background values for different regions all over the world by several authors (Table IV). However, copper levels measured during this study are significantly higher than those reported for other environments. These high copper concentrations could be explained keeping in mind the nature of the studied sediments which were produced by glacier erosion of volcanic rocks (mainly basaltic and andesitic) composed primarily of olivine and pyroxene and by plagioclase and pyroxene, respectively (Fourcade, 1960). Salomons and Förstner (1984) have reported that during magmatic differentiation, copper is incorporated – among others – into olivine, pyroxene and plagioclase with mean concentrations of 115 ppm, 120 ppm and 62 ppm, respectively. Even though, in order to understand the geochemical distribution of the studied metals in the surface sediments, a study of geochemical partitioning of these elements (meaning the separation of the metal geochemical fractions which could be present in sediments: (i) exchangeable adsorbed metals; (ii) oxidisable metal complexes; (iii) metals in carbonates; (iv) reducible compounds; (v) residual metals) is necessary, which will be developed in the near future. Among other authors, Förstner and Wittmann (1983) have opportunely reported the occurrence of an increase of heavy metal concentrations linked with decreasing
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TABLE IV Natural levels of heavy metals detected in surface sediments from different systems worldwide compared with those measured in The Potter Cove Zn (µg g−1 )
Cr (µg g−1 )
Pb (µg g−1 )
Mn (mg g−1 )
Fe (mg g−1 )
Cd (µg g−1 )
76.7
48.6
1.1
73.1
0.59
34.47
19.2
46.5
52.1
2.6
120.9
0.28
23.73
10.1
38.68
9.01
–
7.87
4.35
ND
20
15
–
10–93
241.6
39.2
–
44.4
9.02
25.5
52.47
116.79
6.27
– 0.05–2.5
–
–
74
–
–
–
10.5
–
–
0.43
3.66
0.96
13.20
ND
Alam and Sadiq, 1993 Alam and Sadiq, 1993 Amin et al. 1996 Dassenakis et al. 1996 French, 1993 Katz and Kaplan, 1981 This study
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Jubany Station King George Is Beagle Channel Mediterranean background (UNEP, 1993) Severn Estuary (UK) Southern California Potter Cove
Cu (µg g−1 )
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Figure 3. Percentage of grain size < 63 µm in surface marine sediments.
grain size of the sediment. Heavy metal concentrations as presented in this study were determined in total sediment. Even though, the percentages of sediment smaller than 63 µm in size within samples collected from the sampling stations have been compared in Figure 3. In a first approach, it could be observed that sediments in 5 m of water allong Transect #3 (T3 – 5 m), which have the lowest percentage of sediment size
