ARTICLE pubs.acs.org/est
Laser Ablation Inductively Coupled Plasma Mass Spectrometric Analyses of Base Metals in Arctic Char (Salvelinus alpinus) Otoliths Collected from a Flooded Base Metal Mine Lisa A. Friedrich*,† and Norman M. Halden‡ † ‡
Department of Fisheries and Oceans Canada, Winnipeg, Manitoba, Canada, R3T 2N6 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 ABSTRACT: Otoliths from arctic char recovered from the water body formed from an abandoned open-pit nickel copper mine contain a trace element record related to the geology of the immediate watershed, past mining activity in the area, and the fish’s diet. Laser ablation inductively coupled plasma mass spectrometric analyses across the annular structure of the otoliths detected trace amounts of nickel, copper, and chromium believed to be related to the metal-bearing, mafic ultramafic minerals in the pit. Oscillatory strontium, barium, and zinc profiles may reflect changing water temperature, diet, or fish metabolism. Lead was detected in very low concentrations and may be related to anthropogenic influence. This closed lake system provides a unique opportunity to study an introduced exotic species in a setting where neither migration nor recruitment have been possible. The fish have successfully occupied the lake and continue to breed despite the influence of the surrounding rocks and local contamination. The chemical record retained within otoliths provides a method of monitoring trace elements affecting fish on a yearly basis and may be regarded as a useful assessment tool for examining the exposure of wild organisms to trace elements.
’ INTRODUCTION Environmental effects monitoring (EEM) for Canadian metal mines was initiated in 1993 to assess the health of fish and their habitats potentially impacted by mining effluents under terms of the Canadian Fisheries Act.1 All mines regulated under the Metal Mining Effluent Regulations are required to conduct EEM, the objective of which is to evaluate the effects of mining effluent on the aquatic environment, specifically fish, fish habitat, and the use of fisheries resources. Although these general criteria have been established, consensus on which methods provide the most practical and reliable results for a given exposure scenario is a subject of discussion.2 4 In particular, considerable debate has occurred with respect to appropriate fish sampling methods for the analysis of effects. To evaluate effects on the use of fisheries resources, fish tissue analyses are required.1 However, muscle and visceral tissues are good indicators of recent contaminant exposure only, due to metabolic transformation and tissue recompartmentalization of trace elements.4 Otoliths are metabolically inert and, therefore, contain a complete chemical history of exposure. In recent years, much microbeam analytical research has focused on trace element distributions in biominerals, such as otoliths, and how they may aid in reconstructing past environments.4 9 Otoliths are calcium carbonate structures in the inner ear of teleost fish that assist in detecting sound and are used for balance and orientation.10 They typically consist of aragonite deposited r 2011 American Chemical Society
in daily and yearly increments in a protein matrix11 and have been used to determine age and life history events of fish and fish populations. During formation, trace amounts of numerous elements may be incorporated into either the organic or inorganic portion. In contrast to muscle or visceral tissues, otoliths are metabolically inert, and therefore, only ontogenetic and environmental factors should cause changes to their chemical composition.12 As such, otoliths retain a complete chemical record of the fish’s life. Coupling this record to the annular structure of otoliths adds a time scale to the chemistry, affording a unique opportunity to provide information on environments the fish have occupied, changes in those environments, and any history of exposure to pollutants. Specifically, if a link exists between the microchemistry of otoliths and the geochemistry of the surrounding environment, otoliths can provide an assessment tool for the impact of historic or active mining activities on fish populations, either as part of background geochemical or environmental surveys or in the form of ongoing monitoring programs and fisheries management. Several studies have pointed to a link between the environment, particularly watershed geochemistry, and the microchemistry Received: December 3, 2010 Accepted: March 27, 2011 Revised: February 25, 2011 Published: April 13, 2011 4256
dx.doi.org/10.1021/es1040514 | Environ. Sci. Technol. 2011, 45, 4256–4261
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Figure 1. Location of the Maskwa nickel deposit in southeast Manitoba, Canada.
of otoliths;4,6,7 these studies were done in natural, open systems. The purpose of this study was to examine whether there is a link between otolith microchemistry and the geochemistry of the watershed through analysis of otoliths obtained from fish in a closed system. It is hypothesized that the watershed will impart a chemical signature to the arctic char otoliths. The Maskwa pit is the result of open-pit nickel copper mining activities that have since ceased. Currently, the closed mine is water-filled and persons unknown introduced a population of arctic char, the timing of which is also not known. Arctic char are not native in this region and there is no connection to a seaway; therefore it is reasonable to conclude they are exotic. The rocks in the area have been extensively studied as a potential source of Ni, Cu, Cr, and platinum group elements; therefore, much is known about the geochemistry of the rocks of the area. Ongoing environmental monitoring of the pit has provided information about the water chemistry. A suite of elements (Cr, Ni, Cu, Zn, Sr, Ba, and Pb) was analyzed across the annular structure of char otoliths (i.e., life history of the fish) to delineate chemical signals from the geology.
’ STUDY AREA AND GEOLOGIC SETTING The Maskwa nickel deposit is located in southeast Manitoba, Canada, 160 km northeast of Winnipeg (Figure 1). It consists of marcasite-pyrite (FeS2), violarite (FeNi2S4), pentlandite ((Fe, Ni)9S8), pyrrhotite (Fe1 xS), and chalcopyrite (CuFeS2), in addition to layers of chromite (FeCr2O4).13 An open-pit mine was developed and operated in the mid-1970s at Maskwa.13 Over 30 million pounds of Ni and nearly 1.5 million pounds of Cu were mined from Maskwa and the adjacent Dumbarton property.14 The mine was closed with appropriate reclamation methods for the time and gradually filled naturally with water to form the current lake (Ian Ward, Mustang Minerals Corp., personal communication, 2009). The closed pit is 300 by 100 m and is approximately 52 m deep. A dissolved oxygen profile completed in the fall of 2006 indicated that the lake was anoxic (less than 5 mg/L) below 11.5 m depth, and concentrations dropped to less than 1 mg/L below 13 m depth (Dave Tyson, Wardrop Engineering Inc., personal communication, 2009). ’ MATERIALS AND METHODS Sixteen arctic char (Salvelinus alpinus) were collected from the Maskwa pit by gill netting in August, 2007 as part of an environmental survey of the area by Wardrop Engineering Inc. The sample set contained mature and immature, male and female
specimens that ranged from 5 to 9 years old. Fork lengths ranged from 34 to 56 cm, and wet weights ranged from 400 to 1650 g. To prepare for microbeam analysis, sagittal otoliths from all 16 fish were embedded in epoxy resin and cut transverse to create a dorso-ventral cross section through the core of the otolith, exposing all annuli. The posterior half of each cut otolith was re-embedded in a 25-mm Lucite microprobe mount, ground, and polished. Prior to analysis, samples were rinsed with distilled deionized water and allowed to air-dry. Sectioned otoliths were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), which has micrometer-scale resolution that can resolve the annular structure of otoliths. LA-ICP-MS analyses were done on a Thermo Finnigan Element 2 ICP-MS instrument coupled to a Merchantek LUV 213 Nd:YAG laser. Running conditions were set similar to those used by Friedrich and Halden6 to minimize noise and optimize sensitivity and resolution of the annular growth zones (typically 80 100 μm wide) and included a 30-μm-diameter beam traveling 2 μm 3 s 1. Calcium (56 wt % CaO) was used as an internal standard, and external calibration was done with NIST glass 610.15 Line scans were run from core to edge, at a high angle to the annuli in scanning mode. Standard analyses were completed at the beginning and end of each program and at least once every hour for longer programs. Measured trace element concentrations, standard deviations, and detection limits were processed by use of GLITTER software.16
’ RESULTS Chromium, Ni, and Cu were analyzed to determine if the otoliths contained a chemical signature that may be related to the geochemistry of the surrounding environment. Chromium and Ni were detected in all samples, with concentrations ranging from 3 to 42 μg/g and 1 to 6 μg/g, respectively (Table 1). Copper was detected above background values in only some of the samples and, where resolved, Cu ranged from 0.5 to 3 μg/g. Zinc, Sr, and Ba were analyzed for comparison with other suites of otoliths as well as for signatures relating to diet. All three were detected in all samples. Zinc concentrations ranged from 5 to 175 μg/g, Sr from 113 to 523 μg/g, and Ba from 1.2 to 43 μg/g. Lead was analyzed as an indicator of either Pb-bearing minerals or anthropogenic influence. It was detected in most samples at concentrations up to 2.2 μg/g. A typical chemical signature from the Maskwa pit arctic char includes Cr, Ni, and Pb profiles that were relatively flat with 4257
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Table 1. Summary of Annulus-to-Annulus Trace Element Concentration Variations in Arctic Char Otoliths from the Maskwa Pit 53
Cr
60
Ni
65
66
Cu
88
Zn
Sr
137
Ba
208
Pb
concn range (μg/g)
3.44 41.73
1.05 6.31
0.56 3.43
5.08 174.75
113.29 523.48
1.21 43.33
0.07 2.18
avg detection limit (μg/g)
0.46
0.53
0.56
4.35
43.80
0.20
0.07
typical 1σ error (μg/g)
6.76
1.18
0.97
30.01
37.20
1.29
0.04
Figure 2. Representative LA-ICP-MS spectra showing trace element concentrations (micrograms per gram) versus distance (micrometers) across the otolith from core to rim.
approximate means of 13, 1.3, and 0.1 μg/g, respectively (Figure 2). Copper, if present, had uniform concentrations around 0.5 μg/g. Zinc, Sr, and Ba were ubiquitous and have oscillatory patterns. Zinc concentrations were highest in the primordia and first 1 2 years, between 100 and 150 μg/g, after which values oscillated on a yearly basis with an overall decrease in Zn concentration with age. A typical Sr profile was oscillatory and ranged from 200 to 300 μg/g. Oscillations in Ba concentrations were not uniform among the fish but typically stayed within the range of 3 10 μg/g.
’ DISCUSSION Several recent studies have demonstrated a link between otolith microchemistry and proximity to mineral deposits. Friedrich and Halden17 reported elevated concentrations of Cu, Pb, and Zn in northern pike and walleye otoliths collected from lakes adjacent to mine tailings piles of Lynn Lake, Manitoba. They concluded the sudden increase in concentration is a record of fish movement into areas affected by tailings effluence. Friedrich and Halden6 demonstrated a link between local geology near rare element pegmatite mining activity and the microchemistry of otoliths from fish of the surrounding area. Otoliths obtained from water bodies adjacent to or downstream from the mine contained Li, Cs, and Rb in elevated concentrations, elements that are known to be in abundance in the pegmatite. Otoliths obtained from water bodies upstream from the mine and from lakes in a different watershed, however, did not contain Li or Cs and contained lower concentrations of Rb, suggesting a direct correlation
between the geology and mining activity of the area and the microchemistry of otoliths. Palace et al.4 provided the first determinations of selenium in the otoliths of rainbow trout captured from a site receiving runoff with elevated selenium from a coal mine operation. Annular concentrations of selenium in the otoliths indicate that fish from the mine-impacted system are recent immigrants from nearby reference streams not receiving selenium-bearing effluent. Otoliths are considered to serve as natural markers, provided that the chemical environment influences the rate of trace element incorporation into the growing otoliths.12 For certain elements, such as Ni and Cr, there is no evidence that fish physiology and/ or biological processes influence their uptake, and therefore, their incorporation into the otolith structure is dependent mostly on the concentrations available in surrounding water.18 Most previous investigations of Ni in otoliths were whole-otolith studies that focused on marine species and reported concentrations from 0.02 to 9.5 μg/g.18,19 Similarly, the majority of previous studies on Cr in otoliths have been on whole otoliths from marine fish, reporting concentrations in the range of 0.23 1.8 μg/g.20,21 One study used laser spot analyses across the life history transect of chum salmon otoliths and reported a range of Cr values from 3.8 to 68.1 μg/g, concluding that element ratios have the potential to aid in distinguishing between salmon spawning sites and habitats.21 Uncertainty regarding residence in the study area and extent of exposure are major concerns where measurements in wild organisms are used as part of EEM programs.4 The present study provides a unique opportunity to study the microchemistry of otoliths from fish living within a closed system exposed to the effects of base metal mining, and it is the first to analyze this group of base metals, namely, Ni, Cu, and Cr, across the entire life history transect of fish from freshwater. Concentrations of Ni and Cu in the Maskwa pit arctic char otoliths were higher than those found in arctic char otoliths from pristine lakes in their native habitats, where Ni and Cu concentrations were well below detection limits (T. Loewen, unpublished data, 2010). Given that the fish in this study were obtained from a well-constrained setting where much is known about the geochemical character of the watershed, it is most likely that the metal-bearing, mafic ultramafic rocks are the ultimate source of the base metals in the otoliths, in particular sulfide minerals bearing Ni (pentlandite and violarite) and Cu (chalcopyrite) and layers of chromite. Oxidation of these minerals exposed at the surface is a likely mechanism that would release these elements into the water in the pit. Although the route of uptake of these metals into otoliths is still unclear, this environmental link implies that, in cases where the geology of habitats is sufficiently different as to provide the environment with a unique suite of elements, the trace element chemistry of otoliths may be used in distinguishing between such habitats. Studies of Pb in otoliths in both natural and laboratory settings have found that elevated concentrations do not correlate with pH 4258
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Table 2. Water Chemistry Data for the Maskwa Pita Ca
Cr
Ni
Cu
Zn
Sr
Ba
Pb
dissolved metal (mg/L)
56.6
