OVERVIEW OF THE METAL DISTRIBUTION IN SURFACE DEPOSITS AND SEDIMENTARY RECORDS IN THE SUVA LAGOON 1J.M. Fernandez, 1G. Cadiou, 1B. Moreton, 1R. Legendre, 2R. Fichez and 3C. Badie 1IRD-Noumea, 2IRD-COM, Nouméa, NEW CALEDONIA Station Marine d'Endoume, Marseille, FRANCE 3IRSN/LESE-Papeete, FRENCH POLYNESIA ABSTRACT Human activities have impacted the natural environment, especially since the middle of the 19th century. In tropical regions the history of recent or past environmental conditions is partly recorded in the layers of the sediment reservoirs of lagoons. Geochemical approaches applied to sediments can provide information on past and present changes in heavy metal concentrations and other environmental tracers that are indicative of specific environmental modifications. These studies are combined with dating techniques such as the 210Pb method. This paper presents all the data collected from the coral reef environment of the Suva Lagoon. Superficial sediments have been studied in terms of heavy metal concentration (sequential extractions) and cores provided complementary 210Pb unsupported radioactivity. In Laucala Bay, the results showed that the inputs of terrigeneous particulate matter originating from the Rewa River has remained high and constant over time, resulting in an extremely limited bioturbation layer, in contrast with the more disturbed Suva Harbour profiles. INTRODUCTION During the past 150 years, terrestrial and coastal environments have been strongly modified by human activities. In the tropics, population growth and economic development have imposed serious constraints on lagoon ecosystems (Hatcher et al., 1989). Deforestation and mining, which are primarily responsible for hyper-sedimentation and metal pollution, are two of the major causes of disturbance in coral-lagoon environments jointly with industrial reef dredging (Carey, 1981; Naidu and Morrison, 1994; Zann, 1994, Morris et al., 2003). Evidence of these upheavals may be preserved in the sediment layers that gradually build up a history of the various successive events affecting the environment. Interpretation of these ‘sedimentary records’ is generally based on the dating of deposits, using natural clocks, such as, radionuclides, to establish a geochronology of sediment deposition. Most of the studies dealing with recent sedimentary records of environmental changes have been based on 210Pb determinations, the decrease in unsupported 210Pb permitting age determinations back to about 100 years before the present (Faure, 1986). However, deciphering both the geochemical and sedimentological data collected from these layers is not always straightforward, with results often leading to misinterpretations due to a lack of converging information. This article reports on studies of sediment samples collected during the BULA-1 cruises carried out by the IRD with the O/V ALIS in July 1998. MATERIALS AND METHODS Samples were collected in the barrier lagoon surrounding Suva, the capital of Fiji (Figure 1). Sixty one surface sediment samples were collected using a light Van-Veen grab of small capacity (approximately 1.8 L) made of stainless steel and able to sample the surface sediments with a minimum of disturbance of the water/sediment interface. The upper surface layer consisting of oxidized sediments was stored in one use only plastic bags (Whirl-pack), frozen then freeze-dried. Two aliquots were stored for each sampling site for the sedimentiological and geochemical analyses respectively. Two cores were also sampled in Suva Lagoon (Figure 1). The first was collected in Suva Harbour (S14) and the second in Laucala Bay (S31). Suva Harbour is known to have experienced a large increase in industrial and urban development in the coastal zone over the past 30-40 years (Naidu et al, 1991). Laucala Bay, where the other core was sampled, is a coral reef barrier lagoon acting as a major recipient of the discharge from the 2900 km 2 catchment of the Rewa River, the largest in Fiji. The sediment cores were sampled using a specially designed PVC corer operated by SCUBA divers (Harris et al., 2001). The corer consisted of a 1.2 m long PVC tube, 25 cm in diameter, which had been cut in half from top to bottom. The two halves were clamped together during coring and transportation to keep the core intact until sampling. The corer was forced down into the sediment by hammering on a cap placed on the top of the corer to about 0.5 m deep. The sediment surrounding it was then pumped away until the bottom could be sealed with a second cap. The corer was then removed and kept vertical. Once onboard, the core was allowed to settle vertically, the top was then removed and the overlying water carefully pumped out. The first fluid layers were sampled using a spatula and the core was then laid horizontal, and 2 cm slices taken for geochemical and sedimentological analysis. A sub-sample of each slice was used for 210Pb measurements. For the geochemical study, each sediment sample was grain sized by wet sieving in order to recover the fine material fraction (particles < 40 µm of the sediment). This fine fraction contains more than 90% of the sorbed on particles as a result of reactions taking place on their very large specific surface (e.g., Mayer and Fink, 1979; Ackerman, 1980; Deely and Fergusson, 1994; He and Walling, 1996). The sediment grain size fractionation was carried out using 25 to 30 g of bulk sediment weighted in polyethylene flasks, homogenized in 100 mL of ultra-pure water (Milli-Q, 18.6 µS.cm-1) and gently shaken during 1 hour. The samples were then sieved through a 40 µm mesh and both fractions dried at 40°C, weighed and stored in vinyl bags. Finally, to obtain water content, additional 10 mL sub-samples were taken and weighed before and after drying, in an oven at 110°C until constant weight. Results are expressed as a percentage of the initial sediment dry weight. This was subsequently used to calculate sediment accumulation in g cm-2. • 61 surface sampling sites • 2 cores (S14 & S31) Figure 1: Location of the sampling stations during the IRD Bula-1 Cruise in Suva Lagoon in July 1998 Kinoya sewage discharge Nasinu River Ndraunimbota Bay Rewa River Vunindawa River Lami Samabula River Ndaveta Levu Laucala Point Suva Point Nukulau Island Ndaveta Nukulau Ndaveta Nukimbutho PACIFIC OCEAN Sequential extraction of metals Analysis of the total metal concentrations contained in the sediments is nowadays considered a too vague method of quantification, as it includes the entire geochemical phenomena occurring in the water column. Much more useful information can be extrapolated by studying metals partitioned in different geochemical phases which can be chemically defined in the sediments and of the metal concentrations which they contain. This information relates, in particular, to the source terms of metals and the origin of the particles which transport them (geochemical signatures). To highlight the anthropogenic and terrigenous sources of metals, a sequential extraction protocol was used in this study for the determination of Ca, Cr, Mn, Fe, Co, Ni, Cu and Zn (Fernandez et al., 2005). Despite some criticism associated with sequential extraction techniques in the literature (e.g., Jouanneau et al., 1983; Nirel and Morel, 1990), this type of protocol makes it possible to assess the various inputs impacting the environmental medium (see Cornu and Clozel, 2000); the first two metal extractions (in the sequence below), in particular, reflect the conditions of sedimentation and the anthropic input influences while the two last can be comparable with geochemical signatures of the terrigenous contributions. The protocol used was based on the technique developed for river sediments (Tessier et al., 1979; Meguellati, 1982), and subsequently adapted to carbonate environments. Briefly, two grams of fine sediment were subjected to a 4 phase chemical extraction protocol to obtain metal concentrations associated with 4 geochemical fractions. The various extractions were completed as follows: Oxidisable forms - this fraction represents the metals linked to the organic matter in the sediment. This extraction was completed using strong oxidants (Oxygen peroxide) in an acidic medium (nitric acid); Acid-soluble forms - these are relatively easily extracted using a weak buffered acidic solution (acetic acid / ammonium acetate). The forms either occur in the carbonate matrix of the sediment, or are adsorbed on the particle surfaces or coprecipitated (e.g., Chester and Hugues, 1967; Perhac, 1974; Gupta and Chen, 1975). In lagoonal deposits, these metals come from the aragonitic skeleton of organisms which, when alive, ingest or assimilate metals (Brocero, 1998). Due to a lack in the chemical selectivity of the reactants, this extraction step also potentially includes the easily leachable metal, initially bound by adsorption on the poorly crystallized manganese and iron oxides (Förstner et al., 1986) and clays (Posselt et al., 1968). Reducible forms - these forms are associated with oxyhydroxides of manganese, aluminium and iron, and are extracted using reducing conditions such as hydroxylammonium chloride in acidic medium. Refractory forms - these are the metals that are strongly held by the particle matrix, e.g., in silicates and aluminosilicates. These were extracted using aqua regia mixture. In this final step of the protocol, samples were placed in Teflon vessels and subjected to high pressure and temperatures conditions (Anton Paar/Perkin Elmer mircowave oven). After each of the 4 steps of the sequential extraction protocol, the loss of mass was determined by weighing. The reagents and chemicals used were Merck, ProAnalysis grade. The containers used for the sequential extractions were made of HDPE, or Teflon. All containers were decontaminated by soaking in nitric acid (5%) for 24 hours and rinsed in ultra-pure water (Milli-Q). After each use, the Teflon vessels of the microwave oven underwent a cycle of decontamination with a strong acid solution. The quantification of Ni, Cr, Zn and Ca in each geochemical phase was carried out by ICPOES (Optima 3300 VD, Perkin Elmer). The concentrations are expressed, in mass of metal vs. mass of fine sediment, (CPel, in mg/kg) and in concentration per gram of dissolved refractory phase, (CRef, in mg/kg), defined as : CPel = (C . D . V) / Mi (1) where C is the concentration in solution (mg/L), D the dilution factor of the sample, V its volume (l) and Mi is the mass of initial fine fraction (kg), CRef = (C . D . V) / (Ma - Mb) (2) where Ma is the mass of the fine fraction before attack of the geochemical phase (kg) and Mb the mass of this fraction after solubilization of the geochemical phase (kg). The juxtaposition of concentrations CPel and CRef ensures in particular a good differentiation of the metal sources and makes it possible to specify the origin of the contribution. The analyses were carried out on triplicate to check the reproducibility of the protocol and to estimate the variability of the sample. In addition, certified reference materials (SD-M-2/TM, NRCC-BCSS-1) were analyzed for total elements content to check for total extraction efficiency. Geochronology Sediment accumulation rates were determined from the decrease in excess 210Pb activity. 210 Pb was analysed on triplicate samples by measuring its granddaughter 210Po, considered to be in secular equilibrium with 210Pb (Teksöz et al., 1991). Each sample was spiked with 208Po in order to appraise possible losses incurred during application of the digestion protocol. The 210 Po measurement was performed in a NUMELEC gridded-chamber (NU 114B model) by alpha counting, following the standardised methods of Flynn (1968), modified by Nittrouer et al. (1979) and further adapted to carbonate rich sediments by Serra et al. (1991). Excess 210Pb was determined as total 210Pb minus 226Ra supported 210Pb. Sediment supported 226Ra activity was measured using gamma spectrometry. Sediment accumulation rates were determined according to Faure (1986). Mass accumulation of sediment has been commonly used to derive independent depth scale based upon the cumulative weight per unit area (g.cm-²) and plotting excess 210Pb activity versus mass accumulated sediment (g.cm-2) rather than simple depth (cm) therefore compensating for compaction effects (Bollhöfer et al., 1994). Mass accumulation was determined according to Buesseler and Benitez (1994) after measuring particle density on each sediment sub-sample according to Boyd (1995). The age of each sediment level was derived from the linear regression of excess 210Pb versus accumulated sediment mass that allowed an average accumulation rate to be calculated (Robbins and Herche, 1993). However, carbonate, which contains little or no 210Pb, may be responsible for a significant dilution of the 210Pb signature. In order to compensate for this bias, the linear regression was conducted on non-carbonate accumulated sediment mass instead of bulk accumulated sediment mass. In doing this, only the accumulation rate of terrestrial sediment was calculated, from which a more reliable age-depth relationship could be established and subsequently was used to calculate bulk sediment accumulation rates and interpret changes in sediment deposition and composition. An additional determination of 137Cs was attempted despite the low activity levels commonly recorded in the southern hemisphere (Hancock et al., 2002). Direct 210Pb determination using gamma spectrometry at 46.54 keV, was also conducted on the two Fijian cores. These measurements were carried out on an ORTEC X beryllium window Diode with a relative efficiency of 80%. The unsupported 210Pb results were subsequently plotted for the 2 cores. RESULTS Superficial sedimentary layer The distribution of the fine terrigeneous fractions (Figure 2) makes it possible to demonstrate the extreme influence of the Rewa River as mentioned by Morrison et al. (2001); the sediments found in the eastern half of Laucala Bay containing also more than 80% of fine material. A second source of fine particulate material can be highlighted in the sheltered Draunibota Bay. The lowest values (4%) were recorded in the channel which separates Laucala Bay from Suva Harbour. This spatial distribution was remarkably consistent with the distribution of the refractory geochemical phase that constitutes a characteristic of the terrigenous contributions (Figure 3). In addition, particularly in the eastern part of Laucala Bay, the sediments contained a considerable proportion of oxyhydroxides that are good tracers of the terrigenous contributions resulting from the natural inputs (Rewa River). Conversely, Figures 4, 5 and 6 underline the existence of sediments of marine origin which are prevalent in the fringing reef area of Suva Point with more than 40% of carbonate and/or of organic matter. 2 2 2 Figure 2: Map of the fines content (%<40µm) in the Suva Lagoon sediments. Figure 3: Map of the refractory forms extracted from fines (%<40µm) in the Suva Lagoon sediments (% of total mass). Figure 4: Map of the reducible forms extracted from the fines (%<40µm) in the Suva Lagoon sediments (% of total mass). 2 2 Figure 5: Map of the acid-soluble forms extracted from the fines (%<40µm) in the Suva Lagoon sediments (% of total mass). Figure 6: Map of the oxidisable forms extracted from the fines (%<40µm) in the Suva Lagoon sediments (% of total mass). Terrigenous and Anthropogenic Influences The analysis of the labile forms of metals extracted in acidic soluble fractions (carbonates) and oxidisable fractions (organic matter) made it possible to highlight various sources with respect to the different metals. These results lead to detail the different contributions of anthropogenic metals suspected in a previous study (Morrison et al, 2001). Actually, the commercial activities of Suva Harbour contribute in an obvious way to the introduction of metals, in particular zinc (Figure 7), copper, nickel, chromium and cobalt (Figures 8 and 9), in considerable quantities, the latter being also present in front of the mouth of the Rewa River in slightly lower levels of concentrations. 2 2 2 Figure 7: Map of the Zn extracted from the oxidisable forms of the fines (%<40µm) in the Suva Lagoon sediments (mg/kg). Figure 8: Map of the Cu extracted from the oxidisable forms of the fines (%<40µm) in the Suva Lagoon sediments (mg/kg). Figure 9: Map of the Cr and Ni extracted from the oxidisable forms of the fines (%<40µm) in the Suva Lagoon sediments (mg/kg). The other potential sources of metals identified were the Lami rubbish dump, the Tamavua River (principally cobalt and zinc), the Nasinu River (principally cobalt), as well as the liquid wastes discharged from the Kinoya sewage treatment plant which introduces metals such as copper, nickel, chromium and cobalt into the Lagoon. The lowest metal contaminated sediments were located in the channel zone at the end of Suva Peninsula which separates the two bays (Suva Harbour and Laucala Bay) of the Suva Lagoon. Sedimentary records From the point of view of granulometry, the fine fraction (70-75 %) remains constant throughout the entire length of the core S31 extracted in the zone of influence of the Rewa River (Figure 10). This profile shows that the particulate material supply has not undergone major modifications during the past 100 years as the mean rates of accumulation is estimated at 0.37 g/cm²/year (210Pb) even though two sedimentary events could be distinguished (Figure 11). Moreover, the values of 234Th and 40K showed a light inflection in the top 20-25 cm of the sediments, suggesting the possibility of a sudden modification in the particulate supply. The two sedimentary layers identified in the core contain on average 20 4 Bq/kg in the lower layer and 15 3 Bq kg-1 in the upper layer for the 234Th and approximately 297 25 Bq kg-1 then 281 26 Bq Kg-1 for the 40K. The reverse was observed in the zone of the Tamavua River discharge and in the vicinity of the commercial port (Suva Harbour), where the contribution of fine particles seems to have appreciably changed during about the last 20 years since they constitute 70% in the deepest layers and approximately 60% in the top 20 cm of the core S14 (Figure 12). The rates of accumulation decreased in parallel by approximately 0.29 g/cm²/year to 0.17 g/cm²/year in the most recent levels of the core (210Pb); the values of 234Th also underlined a possible change in the nature of the contributions since the measured radioactivity is notably more important in the top layer (14 3 Bq Kg-1) than in the underlying layer of the core (10 2 Bq Kg-1). The profiles of the geochemical phase content along the core did not show any real gradients except for the refractory phase (residue) where an important decrease in the top 20 cm of the sediment was observed. The metal concentrations were constant in the sediments sampled in the area of influence of the Rewa River (Figure 11). In Suva Harbour, a low gradient of increasing metal concentrations towards the top of the core for chromium in the reducible phase and almost the all of the metals analyzed in the refractory phase (residue) was noted. In general, calcium was more highly concentrated in the top of the core, and, in particular, a strong gradient was noted in the top 10 cm (S14) highlighting a recent input to the Lagoon (Figure 12). Core S14 0 20 Core S31 Pelitic fraction (%) 40 60 80 50 0 0 10 10 20 20 60 70 0 80 20 40 60 80 0 5 10 30 Depth (cm) Depth (cm) Depth (cm) 15 30 Figure 10: Profiles of the fines fraction and geochemical phases in cores S14 and S31. 20 25 30 35 % Org Matter 40 40 S31 S14 %Carbonate 40 % Org Matter % Oxide % Residue 50 %Carbonate 45 50 %Oxide % Residue 50 [Ni] in oxidable forms (ppm) 0,0 0,5 [Cr] in acidosoluble forms (ppm) 1 1,0 3 [Cr] in reducible forms (ppm) 5 0 5 [Cr] in pelites (ppm) 10 20 40 60 Pelitic fraction %(<40 µm) 80 60 70 80 90 Ln(Po-210) (mBq/g) -5 -3 -1 0 0 0 0 0 0 5 5 5 5 5 5 10 10 10 10 10 10 15 15 15 15 15 15 20 20 20 20 20 20 25 25 25 25 25 25 30 30 30 30 30 30 Co, Cr, Fe, Zn ! Mn : 15 -47 ppm Zn : 2.2-5.9 ppm Mn : 139-259 ppm [Co] in oxidable forms (ppm) 0 5 [Ca] in acidosoluble forms (ppm) 10 0 Mn : 80-112 ppm 0 10000 [Cr] in reducible forms (ppm) 20000 0 2 [Cr] in pelites (ppm) 4 0 0 [Cr] in residue (ppm) 0 10 Accumulation rate: R = 0.37 g/cm²/y Pelitic fraction %(<40 µm) 20 0 Figure 11: Metals profiles in different geochemical phases of the core S31. Accumulation rates assessment 40 60 80 Ln(Po-210) (mBq/g) -5 -4 -3 -2 0 0 = 0,9686 RR22 = 0,97 5 5 5 5 5 10 10 10 10 10 10 15 15 15 15 15 15 5 2 =2 0,9471 RR = 0,95 20 20 20 Other metal : Same gradient Other metals : No gradient 20 [Ca]: 4600-26500 Other metals : No gradient [Cr] in residue (ppm) 20 20 Accumulation rates: R1 = 0.17 ; R2 = 0.29 g/cm²/y Figure 12: Metals profiles in different geochemical phases of the core S14. Accumulation rates assessment CONCLUSIONS The two constituent bays of the Suva Lagoon, Laucala Bay and Suva Harbour, are different in the nature and the amounts of particulate matter which has been deposited. The first core (Laucala Bay) is clearly influenced by the Rewa River inputs which are distributed over the whole of the eastern half of Laucala Bay. The second area (Suva Harbour), of more reduced size, seems to be principally influenced by the anthropogenic activities, centring mainly on the commercial port and the industrial areas, but also the rivers bordering Suva Harbour. These various influences as regards sources are found in the distribution of metals which provide geochemical signatures in the recent sedimentary layers. In Suva Harbour, evidence of significant anthropogenic activities in recent times is visible through profiles of metal concentrations, but this is not so clear in Laucala Bay. 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